RELATED APPLICATIONS

[0001] This patent application claims priority to U.S. Provisional Patent Application No. 62/369,397, filed on August 1 , 2016, and entitled "Rapid, Non-Destructive Dynamic Modulus Measurements for Quality Control," which is hereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

[0002] The present disclosure relates generally to material measurements and, more particularly, to methods and apparatus to perform non-destructive dynamic modulus measurements of materials.

BACKGROUND

[0003] Maintaining a high level of quality control is an important aspect of

manufacturing processes. For example, different manufactured components have different material parameters that affect the quality of those manufactured components. Some material parameters can be monitored using different techniques. For example, some monitoring techniques involve an off-line manual visual inspection to identify any visibly obvious defects in the materials. Other monitoring techniques involve using machine-based measurements.

[0004] For a number of years, ultrasonic methods have been used to measure the complete set of elastic and engineering constants of friction materials. Friction materials exhibit transversely isotropic symmetry. Among other uses, friction material is used in the manufacture of brake pads. The elastic modulus in the plane of a brake pad is 3 to 5 times that of the modulus perpendicular to the brake pad (out of the plane of the brake pad). The measurement methods follow directly from those applied to measure elastic constants in single crystals. Specific measurement techniques have been formulated which can be applied to automotive friction materials and are described in SAE specification J2725. To obtain a complete set of elastic properties needed for noise, vibration, and harshness (NVH) simulation for braking applications, prior techniques analyze brake pads using destructive measurement methodologies. For example, during the manufacturing process of a brake pad, a friction material is cured onto a steel backing. To analyze the brake pad using prior techniques, the friction material is removed from the steel backing member, and small rectangular and trapezoidal pieces are extracted or cut from each friction material pad such that the brake pad is destroyed and no longer useful for a braking application. By propagating ultrasonic signals or waves as a combination of longitudinal/compression and shear wave modes along various directions in these extracted pieces, all elements of a stiffness matrix for the brake pad can be generated. From the stiffness matrix, relevant engineering constants are determined (e.g., Young's modulus, shear modulus, and Poisson's ratios).

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is an example brake pad.

[0006] FIG. 2 depicts dimensional properties of the example brake pad of FIG. 1 .

[0007] FIG. 3 is an example measurement configuration to perform non-destructive dynamic modulus measurements of the example brake pad of FIGS. 1 and 2 in accordance with the teachings of this disclosure.

[0008] FIG. 4A is an example line graph representative of dynamic modulus versus load for discrete measurements.

[0009] FIG. 4B is an example line graph representative of percent dynamic modulus change versus load for continuous measurements.

[0010] FIG. 5 is an example line graph representing the spatial extent of an ultrasonic beam used for a dynamic modulus measurement.

[0011] FIG. 6 shows a plurality of measurement regions across the example brake pad of FIGS. 1 -3 for measuring spatial uniformity of dynamic modulus across the brake pad.

[0012] FIG. 7 is an example line graph of intra-pad dynamic modulus variations of a brake pad. [0013] FIG. 8 is an example bar graph of average inter-pad dynamic modulus variations across multiple brake pads.

[0014] FIG. 9 is an example graph of data plots showing a correlation between longitudinal/compression wave-based dynamic modulus and dynamic Young's modulus determined using prior techniques of destructive testing for friction material.

[0015] FIG. 10 is an example graph of data plots showing a correlation between shear wave-based dynamic modulus and dynamic Young's modulus determined using prior techniques of destructive testing for friction material.

[0016] FIGS. 1 1A-1 1 C are example waveform plots representing time domain measurements of a 1 megahertz (MHz) longitudinal/compression wave ultrasound signal emitted through a brake pad along an out-of-plane axis under different applied loads.

[0017] FIG. 12 depicts example waveform plots representing time domain

measurements of a 2.25 MHz shear wave ultrasound signal emitted through a brake pad along an out-of-plane axis under different applied loads.

[0018] FIG. 13 is an example in-process material inspection station that may be implemented to perform non-destructive dynamic modulus measurements of brake pads during a manufacturing process in accordance with the teachings of this disclosure.

[0019] FIG. 14 is an example brake manufacturing process with in-process nondestructive dynamic modulus testing using the in-process material inspection station of FIG. 13.

[0020] FIG. 15 is an example apparatus that may be used to perform non-destructive dynamic modulus measurements of materials in accordance with the teachings of this disclosure.

[0021] FIG. 16 is a flow diagram representative of example machine readable instructions that may be executed to implement the example apparatus of FIG. 15 to perform non-destructive dynamic modulus measurements of materials in accordance with the teachings of this disclosure.

[0022] FIG. 17 is another flow diagram representative of example machine readable instructions that may be executed to implement the example apparatus of FIG. 15 to perform non-destructive dynamic modulus measurements of materials in accordance with the teachings of this disclosure.

[0023] FIG. 18 is a flow diagram representative of example machine readable instructions that may be executed to determine a dynamic modulus per unit density of a measured material in accordance with the teachings of this disclosure.

[0024] FIG. 19 is a flow diagram representative of example machine readable instructions that may be executed to control a manufacturing process based on dynamic modulus per unit density measures determined using examples disclosed herein.

[0025] FIG. 20 is an example processor platform capable of executing the example machine readable instructions represented by FIG. 16, FIG. 17, FIG. 18, and/or FIG. 19 to implement the apparatus of FIG. 15 to perform non-destructive dynamic modulus measurements of materials and/or to control a manufacturing process based on dynamic modulus per unit density measures determined using examples disclosed herein.

[0026] The figures are not to scale. Instead, to clarify multiple layers and regions, the thickness of the layers may be enlarged in the drawings. Wherever possible, the same reference numbers will be used throughout the drawing(s) and accompanying written description to refer to the same or like parts. As used in this patent, stating that any part (e.g., a layer, material, area, or plate) is in any way positioned on (e.g., positioned on, located on, disposed on, formed on, bonded on, mounted to, etc.) another part, indicates that the referenced part is either in contact with the other part, or that the referenced part is above the other part with one or more intermediate part(s) located therebetween.

DETAILED DESCRIPTION

[0027] Examples disclosed herein may be used to perform non-destructive dynamic modulus measurements of materials. Among other uses, examples disclosed herein may be used to measure friction materials and brake pads in a non-destructive manner such that measured brake pads can be subsequently used as intended for braking applications. Prior techniques of measuring properties of friction materials of brake pads involve destructive analysis. For example, prior techniques involve removing the friction material from a steel backing member (e.g., a back plate or base plate), and extracting or cutting small rectangular and trapezoidal samples/pieces from each friction material pad such that the brake pad is destroyed and no longer useful for a braking application. Prior techniques for determining dynamic modulus employ ultrasonic measurement methods that use vibrations in the low megahertz (MHz) range, sub- micron strains, and zero net stress. Such prior techniques determine dynamic Young's modulus of a friction material sample based on precise measurements of time-of-flight (ToF) of propagating ultrasonic waves (e.g., the amount of time for an ultrasonic wave to propagate through the friction material) to determine five independent velocity measurements corresponding to the ultrasonic waves propagated along different axes (e.g., two in-plane axes, one out-of-plane axis, two 45-degree axes) through the friction material sample. Such prior ultrasonic measurement methods measure both in-plane and out-of-plane dynamic Young's modulus and in-plane and out-of-plane dynamic shear modulus, employ preload forces applied to the friction material up to 5

megapascals (MPa), and can be operated over a temperature range from ambient temperature to 325°C. Such prior ultrasonic measurement techniques have been used to measure isotropic and orthotropic materials. However, such prior ultrasonic measurement techniques are destructive, require a time consuming process for measuring multiple shear and longitudinal/compression ultrasonic propagation modes (e.g., based on propagating ultrasonic signals through the material in different directions and using a combination of both shear waves and longitudinal/compression waves) to determine a complete set of elastic constants, require significant sample preparation (e.g., removal of the steel backing plate as well as destructive sectioning of the friction material of the brake pad), and require use of a fluid coupling agent between signal transducers and friction material to promote ultrasonic transmission.

[0028] Examples disclosed herein overcome drawbacks of prior time consuming and destructive analysis techniques of friction materials that require significant time for preparation and measurement of five independent velocities along different axes of cut friction material samples, and require discarding measured brake pads. Examples disclosed herein may be used in a manufacturing environment to measure dynamic modulus per unit density based on a single out-of-plane velocity measure, and use the dynamic modulus per unit density to determine a substantially accurate estimate of dynamic Young's modulus for all, a significant number of, or any number of brake pads produced in a manufacturing environment using in-process measurements while the brake pads move along a manufacturing process without the drawbacks of prior measurement techniques such as requiring extensive preparation of items to be measured and needing to discard the measured items as a result of the prior

measurement techniques. Although dynamic modulus per unit density is a useful measure for a number of purposes, including as feedback information for use in a manufacturing environment to modify manufacturing parameters to change (e.g., improve) qualities of manufactured brake pads, dynamic Young's modulus measures are useful for NVH modeling and analysis. Examples disclosed herein measure a dynamic modulus, as opposed to a static modulus, because high frequency oscillations are used (e.g., ultrasonic frequency in the MHz range). Dynamic modulus per unit density (DMPUD) is the ratio of stress to strain under vibratory conditions.

[0029] Since examples disclosed herein estimate dynamic Young's modulus as DMPUD based on a single out-of-plane velocity without needing velocity measures along any other axes of a friction material, examples disclosed herein can be used to measure as-manufactured, unmodified brake pads without needing to cut or extract friction material samples/pieces from the brake pads. As such, because measured brake pads can remain unmodified using examples disclosed herein, examples disclosed herein improve on quality control monitoring of friction materials by not contributing to a decrease in production yield during manufacturing. In addition, because examples disclosed herein may be used as in-process measurements of brake pads, examples disclosed herein are useful to collect analyses data that can be used to modify manufacturing parameters that improve production yield.

[0030] Examples disclosed herein are also useful to measure brake pads before, during, and after performance tests to determine the variation of properties with use. Such performance tests can be done as a quality control operation using either dynamometer testing or vehicle testing. In either case, a brake pad may be measured before such testing to determine the DMPUD of the brake pad before it is put into use, and can be measured after such testing to determine the change in DMPUD of the brake pad attributable to its use. In some examples, ToF measurement configurations of transducers described below may be installed on a dynamometer to measure the

DMPUD of the brake pad during the dynamometer test.

[0031] In addition, because example measurement techniques disclosed herein are nondestructive, examples disclosed herein are scalable to different types of brake pad configurations and form factors. That is, examples disclosed herein facilitate measuring friction material parameters of a brake pad while excluding the effects of other structures (e.g., a backing plate, a shim, etc.) of the brake pad material. As such, differing structures between different types of measured brake pads can be accounted for using measurement techniques disclosed herein such that friction material parameters can be obtained as isolated and unaffected by other structures of the brake pad that are not relevant to the friction material measurements. Examples disclosed herein can be similarly used to measure other as-manufactured multi-layer components to determine the DMPUD and dynamic Young's modulus of a single layer of a multi-layer component by excluding the effects of other layers as disclosed herein.

[0032] Examples disclosed herein enable meeting a need of the automotive industry to control the uniformity and stiffness in friction material used for braking applications. The primary factor controlling the stiffness of a brake pad is the dynamic modulus of the friction material/underlayer in the direction perpendicular to the pad-to-rotor (or pad-to- disc) interface. In brake manufacturing processes, controlling material characteristic uniformity and dynamic stiffness of a brake pad affects noise, vibration, and harshness (NVH) in braking applications.

[0033] FIG. 1 is an example brake pad 100. Brake pads may be manufactured in a number of different shapes and sizes. Examples disclosed herein for performing nondestructive dynamic modulus measurements may be adapted to measure different brake pads of such differing shapes and sizes. In the illustrated example of FIG. 1 , the brake pad 100 includes a friction material 102 bonded to a backing plate 104. In the illustrated example, the friction material 102 includes a braking surface 106 that engages a rotor or disc of a rotary brake assembly to decelerate a vehicle's wheel during a braking operation. The friction material 102 is manufactured with a coefficient of friction conducive to stopping the rotating motion of the rotor or disc to decrease and/or stop the rotating motion of the corresponding wheel within a suitable amount of time.

[0034] The manufacture of the friction material 102 for automotive brake pads involves a complex process of mixing particulate materials and fibers which have a wide range in size and density. The mixture is then loaded into a die with the backing plate 104 forming a base of a mold cavity. The friction material 102 is then formed into a block structure or slab by hot pressing the mixture using a high compression force in a multi-opening press to bond the friction material 102 to the backing plate 104. Post- curing of the brake pad 100 can be done in a separate oven. The resulting friction material 102 is a composite material that is transversely isotropic and has five independent elastic constants that contribute to the dynamic modulus of the friction material 102. Prior measurement techniques for determining the dynamic modulus of friction material involve measuring the five independent elastic constants using five independent velocity measurements. Using such prior measurement techniques, each velocity measurement is a ToF measurement of a separate signal propagated through a friction material sample cut out from a brake pad along a different axis of the friction material sample (e.g., longitudinal/compression and shear along in-plane axes, longitudinal/compression and shear along out-of-plane axis, and shear along 45-degree axes).

[0035] Non-uniformity in dynamic modulus of the friction material 102 can result from poor mixing, poor mold filling, poor temperature gradients, and non-uniform pressure distribution in the press. Measuring the average dynamic modulus of the friction material 102 is related to the formulation and degree of cure. Examples disclosed herein can be used to measure the DMPUD at a single measurement region of the friction material 102, measure the spatial variation of the DMPUD across multiple measurement regions of the friction material 102, or an inter-pad variation (e.g., a pad or pad variation) of dynamic modulus. In addition, examples disclosed herein may be used to measure the variation in DMPUD resulting from one brake pad manufacturing press to another. Such inter-press variation in DMPUD can be used for brake pad manufacturing process development and process control. [0036] The example backing plate 104 is a plate of suitable thickness for supporting the forces exerted on the friction material 102 during braking operations. The example backing plate 104 may be manufactured using steel (e.g., a steel backing plate) or any other suitable metal or metal alloy. The backing plate 104 provides a mounting structure to mount the brake pad 100 to a brake caliper of a rotary brake assembly. The brake caliper enables engaging and disengaging the braking surface 106 of the friction material 102 to and from a surface of the rotor or disc during braking operations. For example, the brake caliper is used to apply a force to the backing plate 104 as the braking surface 106 of the friction material 102 engages the rotor or disc. The amount of force applied by the brake caliper controls the degree of frictional drag between the friction material 102 and the rotor or disc.

[0037] Effects of frictional drag on the brake pad 100 can sometimes lead to vibration and noise, or squeal, in the brake pad 100. Such vibration and noise, or squeal, are attributable to the properties of the friction material 102 and/or to the interface between the brake pad 100 and the brake caliper. In some examples, a brake pad may be provided with an anti-noise shim to reduce or substantially eliminate vibration and noise. In the illustrated example of FIG. 1 , an example anti-noise shim 108 is shown bonded to the brake pad 100 on an opposing surface of the backing material 104 relative to the surface of the backing material 104 that is bonded to the friction material 102. The anti-noise shim 108 may be metal or any other suitable material. The example anti-noise shim 108 is configured to engage a brake caliper when the brake pad 100 is mounted on the brake caliper. In this manner, the anti-noise shim 108 creates a vibration and noise dampening interface between the brake pad 100 and the brake caliper. In some examples, the anti-noise shim 108 is not present on the brake pad 100. Examples disclosed herein may be used in connection with brake pads having anti-noise shims and/or brake pads without anti-noise shims.

[0038] FIG. 2 depicts dimensional properties of the example brake pad 100 of FIG. 1 . The illustrated example shows a friction material thickness dimension (XFM) 202 of the friction material 102, a backing plate thickness dimension (XB) 204 of the backing plate 104, and a shim thickness dimension (Xs) 206 of the anti-noise shim 108. In addition, the example of FIG. 2 shows an out-of-plane axis 208 and an in-plane axis 210 of the brake pad 100. In the illustrated example of FIG. 2, the out-of-plane axis 208 is perpendicular or substantially perpendicular to the braking surface 106 of the friction material 102, and the in-plane axis 210 is parallel or substantially parallel to the braking surface 106.

[0039] Although the example brake pad 100 is described in detail in connection with FIGS. 1 and 2, and although examples disclosed herein are described in connection with friction materials and brake pads, examples disclosed herein may be used to measure other types of materials used in connection with other types of manufactured components.

[0040] FIG. 3 is an example measurement configuration to perform non-destructive dynamic modulus measurements of the example brake pad 100 of FIGS. 1 and 2.

Examples disclosed herein enable performing non-destructive measurements by keeping the brake pad 100 in its intact form, as shown in FIG. 3, without needing to cut or otherwise destroy the brake pad 100. To perform the non-destructive dynamic modulus measurements, the example configuration of FIG. 3 includes an example signal transmitter transducer 302, an example signal receiver transducer 304, and an example load cell 306. The example transducers 302, 304 are acoustic signal transducers to transmit/receive acoustic signals such as ultrasonic signals through a material. In examples disclosed herein, the transducers 302, 304 may be used with longitudinal/compression waves and/or shear waves in the 100 kilohertz (kHz) to 3 MHz frequency range to measure dynamic modulus. The transducers 302, 304 may be implemented using conventional piezoelectric elements. Alternatively, other types of transmitting sources and receiving sensors can be used including, for example, electromagnetic transducers (EMAT), laser transducers (e.g., for laser generation and interferometric reception), etc., or any combination thereof. In the illustrated example, the transducers 302, 304 operate in a through-transmission method such that the signal transmitter transducer 302 emits ultrasonic signals propagating in the direction of the signal receiver transducer 304. In this manner, the example signal receiver transducer 304 detects the ultrasonic signals emitted by the signal transmitter transducer 302 through the brake pad 100. [0041] The example ultrasonic measurement configuration of FIG. 3 is used to measure the DMPUD in the volume of the friction material 102 that is within the propagation path between the transducers 302, 304. This volume is primarily controlled by the diameter of the transducers 302, 304, but is not significantly affected by the center frequency of the broadband pulse (e.g., in the range from 100 kHz to 3 MHz). An example transducer size is 12 mm in diameter which defines a cylindrical volume bounded on opposing surfaces of the brake pad 100 at contact surfaces of the transducers 302, 304. However, smaller or larger diameters may also be used, and may be selected based on suitable sensor construction and operating frequency. By controlling the diameters of the transducers 302, 304, examples disclosed herein may be implemented with different levels of spatial resolution in the brake pad 100. In some examples, multiple positions (e.g., the measurement regions 602a-f of FIG. 6) are measured to obtain the average DMPUD for the brake pad 100. Determining DMPUD variability within a brake pad may be used to vary brake pad manufacturing parameters to control noise performance of manufactured brake pads.

[0042] In the illustrated example of FIG. 3, the signal transmitter transducer 302 is placed in contact with a surface of the anti-noise shim 106 of the brake pad 100. If the anti-noise shim 106 is not present on the brake pad 100, the example signal transmitter transducer 302 is placed in contact with a surface of the backing plate 104. Also in the illustrated example of FIG. 3, the signal receiver transducer 304 is placed in contact with the braking surface 106 of the friction material 104 opposite the signal transmitter transducer 304. In this manner, a through-transmission measurement technique can be used to perform out-of-plane time-of-flight (ToF) measurements of the brake pad 100 as ultrasonic signals propagate through the brake pad 100 from the signal transmitter transducer 302 to the signal receiver transducer 304. For example, the signal transmitter transducer 302 emits an ultrasonic signal through the brake pad 100 along the out-of-plane axis 208 (FIG. 2) towards the signal receiver transducer 304. In this manner, a ToF measurement can be measured as the duration of signal propagation that is bounded between the time that the signal transmitter transducer 302 emitted the ultrasonic signal and the time the signal receiver transducer 304 detected the ultrasonic signal. Such ToF measurement can be used to determine the sound velocity (VFM) of the friction material 102 as described below in connection with Equations 2 and 6. In the illustrated example, when an ultrasonic signal propagates through the brake pad 100 along the out-of-plane axis 208, the ultrasonic signal propagates through the anti- noise shim 108, the backing plate 104, and the friction material 102. Alternatively, the ultrasonic signal propagates through the backing plate 104 and the friction material 102 if the anti-noise shim 108 is not present.

[0043] The ToF of the propagated signals can be measured based on any suitable timing measurement techniques which include peak detection, trough detection, cross- correlation methods, etc. In addition, the ToF measurements can be made using analog-to-digital conversion rates that range from 50 MHz to 250 MHz to convert the propagating signals detected by the signal receiver transducer 304 into the digital domain.

[0044] Although the signal transmitter transducer 302 is shown in contact with the anti-noise shim 108 (or the backing plate 104), and the signal receiver transducer 304 is shown in contact with the braking surface 106, in other examples, the locations of the transducers 302, 304 may be interchanged. In such other examples, the signal receiver transducer 304 is placed in contact with the anti-noise shim 108 (or the backing plate 104), and the signal transmitter transducer 302 is placed in contact with the braking surface 106 of the friction material 102. In addition, although the transducers 302, 304 are shown as respective signal transmitter transducer 302 and signal receiver transducer 304, the transducers 302, 304 may instead be implemented using signal transceiver transducers. In such examples, each of the transducers 302, 304 may operate in both receive mode and transmit mode at different times. For example, at a first time, the transducer 302 in contact with the backing plate 104 (or the anti-noise shim 108) may operate in a transmit mode to transmit a signal, and the other transducer 304 in contact with the friction material 102 may operate in a receive mode to detect the propagating signal. At a second time, the transducer 304 in contact with the friction material 102 may operate in a transmit mode to transmit a signal, and the other transducer 302 in contact with the backing plate 104 (or the anti-noise shim 108) may operate in a receive mode to detect the propagating signal. [0045] When the example configuration of FIG. 3 is used to perform a ToF measurement, an example load 308 (e.g., pre-load) is applied to the brake pad 100 along the out-of-plane axis 208, as shown in FIG. 3, at the portion of the brake pad 100 through which the signal transmitter transducer 302 emits an ultrasonic signal toward the signal receiver transducer 304. The example load 308 may be applied using a force actuator (e.g., the force actuator 1310 of FIG. 13) in contact with the signal transmitter transducer 302. The example load 308 is used to create effective interfaces between the signal transmitter transducer 302 and the backing plate 104 (or the anti-noise shim 108), and between the signal receiver transducer 304 and the braking surface 106 that promote strong, or useful, signal transmissions through the brake pad 100. Prior ultrasonic management techniques use fluid coupling agents as signal promoting interfaces between transducers and materials under measure. However, such fluid coupling agents can contaminate the materials under measure and can create undesirable effects (e.g., fluid coupling agent build up) on manufacturing lines when used for in-process measurements. Unlike prior ultrasonic measurement techniques that use such fluid coupling agents, examples disclosed herein use the load 308 to create signal-promoting interfaces between the transducers 302, 304 and

corresponding services of the brake pad 100 by creating signal-tight seals between the transducers 302, 304 and corresponding surfaces of the brake pad 100.

[0046] In the illustrated example of FIG. 3, the load cell 306 is provided to measure the force of the applied load 308. The load cell 306 of the illustrated example is placed in contact with the signal receiver transducer 304 opposite the direction of the applied load 308. In some examples, the non-destructive dynamic modulus measurements can be made based on different measured forces of the applied load 308 on the friction material 102.

[0047] To ensure repeatability and consistency for the ToF measurements, an automatic gain control circuit (AGC) is used to maintain the detected signal at a constant voltage level as the measurements are made, and a fixed, user specified compression load (e.g., in the range from 100 Newton (N) to 900N) is monitored using the load cell 306. However, the technique can also be used in a cyclic loading mode, in which the ToF is measured continuously as the applied load 308 on the brake pads is loaded and unloaded. This is used to measure the non-linear characteristics of the brake pads by quantifying the variation in DMPUD as a function of the applied load 308. As described below in connection with FIG. 4B, modulus measurements can reflect hysteresis in a brake pad where the modulus is dependent upon the loading rate as well as the direction of the loading (e.g., loading the brake pad vs. unloading the brake pad). This provides an additional characterization tool for brake noise studies. In some examples, the same measurement techniques can be applied at temperatures above and below ambient temperature. In such examples, suitable transducers can be selected based on being thermally stable over the temperature range to be measured. Generally, such thermally stable transducers are bonded to thermally insulating buffers such as fused quartz, which has suitable ultrasonic transmission characteristics, with low ToF variation with temperature.

[0048] The example measurement configuration of FIG. 3 may be used to perform non-destructive dynamic modulus measurements of materials by applying ultrasonic techniques to intact, as manufactured brake pads (e.g., the brake pad 100) to estimate modulus or stiffness of those brake pads, their spatial uniformity (e.g., uniformity of material characteristics across the friction material 102 of the brake pad 100), and dependence of the modulus on the magnitude of a static pre-load (e.g., the applied load 308 of FIG. 3). Disclosed techniques are non-destructive, rapid, and use the

measurement of only a single ultrasonic propagation mode. For example, the single ultrasonic propagation mode measurement is a ToF measure of an ultrasonic signal propagation along only the out-of-plane axis 208 (FIG. 2) without needing to measure a signal along the in-plane axis 210 (FIG. 2) or along any other axis. Examples disclosed herein eliminate the influence or effects of the backing plate 104 of the brake pad 100, yielding direct measurement of underlayer properties of the friction material 102.

Because the disclosed measurement techniques are non-destructive, they can be applied as a quality assurance tool used at a point of manufacture. Additional applications include determining NVH performance as well as an aid to developing new, improved friction material formulations during a brake pad manufacturing process and/or in a laboratory environment separate from the manufacturing process. [0049] For highly attenuating materials the through-transmission method illustrated in FIG. 3 is a suitable technique to achieve high detectability of propagated signals. With reference to FIG. 3, a short burst of ultrasound is generated by the signal transmitter transducer 302 and propagates through the backing plate 104 and the friction material 102 (and the anti-noise shim 108 if present) along the out-of-plane axis 208 (FIG. 2) toward the signal receiver transducer 304 in a direction generally indicated by arrows 310. After some propagation delay required for the ultrasonic signal to propagate through the brake pad 100, the signal is detected by the signal receiver transducer 304. In some materials that are less attenuating, a single transceiver transducer may be employed in pulse-echo mode, which involves the single transceiver transducer operating in a transmit mode at a first time to pulse or emit an ultrasonic signal, and switching to a receive mode at a subsequent, second time to detect an echo or reflection of the ultrasonic signal as it reflects off of a material surface and propagates back toward the single transceiver transducer. However, the inhomogeneous structure of composites like automotive friction materials (e.g., the friction material 102 of FIGS. 1 -3) often prevents the use of pulse-echo measurement methods because the high signal attenuation properties of the composites attenuate the signals too much such that the reflected signal is not useful for effective detection.

[0050] Initially, before performing actual measurements, a calibration of timing measurements for corresponding hardware is performed. For example, the calibration of the timing measurement can be accomplished using a calibration standard block (e.g., a "golden part") of which a propagation time is known and pre-established through independent measurement means (e.g., measured using an independent highly accurate signal propagation measurement instrument in, for example, a laboratory environment). The calibration standard block may be a block of steel or any other material that is certified as creating a known propagation delay for signal transmissions. During the calibration phase, the calibration standard block is placed between the transducers 302, 304 in place of the brake pad 100 while the signal transmitter transducer 302 emits a signal, and the signal receiver transducer 304 detects the signal. The calibration process is used to measure trigger delays in the signal digitizer (e.g., an analog-to-digital converter (ADC)) between the time that a trigger feature (e.g., a zero- crossing point, a peak, a trough, etc.) of the signal feature is detected at (e.g., arrives at) the signal receiver transducer 304 and the time it takes the signal digitizer to respond to the trigger feature in the signal. For example, responding to the trigger feature involves the signal digitizer acknowledging the presence of the feature to begin quantization (e.g., analog-to-digital conversion) of the signal. Other factors that could contribute to additional delay in the measured ToF include the analog-to-digital conversion rate (e.g., a digitizer clock) of the signal digitizer and energizing of the signal transmitter transducer 302. By knowing the expected ToF of the calibration standard block and collecting the measured ToF, a calibration process can subtract the expected ToF from the measured ToF to determine the additional delay introduced by the measurement equipment into the ToF measurement. This additional delay can then be subtracted from subsequent ToF measurements acquired using the same measurement equipment to generate more accurate ToF measurements of brake pads without the additional ToF delay of the measurement equipment.

[0051] After calibration, a measurement phase can commence during which actual measurements of brake pads can be performed. Initially, a base measurement is acquired. The base measurement is the ToF required for the ultrasonic signal to propagate through the brake pad 100 along the out-of-plane axis 208 (FIG. 2), which includes the propagation times through the anti-noise shim 108 (if present), through the backing plate 104, and through the friction material 104. The ToF can be measured in any suitable signal measurement technique including, for example, peak detection methods, trough detection methods, zero-crossing methods, or methods of cross- correlation with reference waveforms.

[0052] An advantage of the ultrasonic measurement techniques disclosed herein is that the properties of the friction material 102 can be measured non-destructively, independent of the properties of the backing plate 104 and/or the anti-noise shim 108. Examples disclosed herein enable separating the propagation time through the backing plate 104 from the propagation time through the friction material 102. For example, metal backings (e.g., the backing plate 104) used for manufacturing brake pads have well-controlled propagation velocity, spatial uniformity, and thickness that can be used to subtract the ToF effects of the backing plate 104 from the ToF measurement of the brake pad 100 to determine the ToF measurement of the friction material 102. Equation 1 below is a mathematical representation of the relationship between the ToF of a propagating signal through the brake pad 100 (ToFpad), the ToF of the signal through the backing plate 104 (— ), and the ToF of the signal through the friction material 102 (

^V B

^V FM

Equation 1 ToF _pad =

[0053] In Equation 1 above, the friction material thickness (XFM) 202 (FIG. 2) is divided by a friction material sound velocity { VFM) of the friction material 102 to determine a friction material quotient ( ), which is the ToF of an emitted signal through the friction material 102. Also in Equation 1 , metal backing material thickness {XB) 204 (FIG. 2) is divided by a metal backing sound velocity ( VB) to determine a metal backing quotient (— ), which is the ToF of the emitted signal through the backing

^V B

material 104. As such, in Equation 1 , the ToFpad of an ultrasonic signal through the brake pad 100 without the anti-noise shim 108 (e.g. , through the friction material 102 and the backing plate 104, but not the anti-noise shim 108) is the sum of a friction material ToF ( ) and a metal backing ToF ( - ). In Equation 1 , the sum of the friction material ToF ( ) and the metal backing ToF (— ) is equal to the ToFpad of the

^V FM ^V B

ultrasonic signal through the multiple layers or sub-components of the brake pad 100 (e.g. , through the friction material 102 and the backing plate 104 of the brake pad 100).

[0054] An example backing material thickness (XB) 204 for some steel backing brake pads is on the order of 5 millimeters (mm), and an example steel metal backing sound velocity ( VB) is 3.22 millimeters/microsecond (mm/us) for shear waves and 5.92 mm/us for longitudinal/compression waves. In such examples, the ToF of an emitted signal through the backing plate 104 is 5mm / 3.22mm/us = 1 .55 microseconds for shear waves, and is 5mm / 5.92mm/us = 0.85 microseconds for longitudinal/compression waves. An example friction material thickness (XFM) 202 for some friction materials is 14 mm, and an example friction material sound velocity ( VFM) is 1 .0 mm/us for shear waves, and 1 .3 mm/us for longitudinal/compression waves. In such examples, the ToF of an emitted signal through the friction material 102 is 14mm / 1 .0mm/us = 14 microseconds for shear waves, and is 14mm / 1 .3mm/us = 10.8 microseconds for longitudinal/compression waves.

[0055] By knowing the backing material thickness (XB) 204 and the metal backing sound velocity ( VB) for the backing plate 104 in advance of measuring the brake pad 100, the measured ToFpad of the brake pad 100 can be corrected to represent only the ToF of the friction material 102 by removing the ToF contribution by the backing plate 104 from the ToFpad measurement of the brake pad 100. For the above example thickness and sound velocity values of the friction material 102 and the backing plate 104, the correction in the measured ToF for the backing plate 104 is less than or about 10% of the ToF of the friction material 102. In operation, the ToF of the backing plate 104 is consistent and well controlled across brake pads of the same type (e.g. , the metal backing thickness (XB) 204 and the metal backing sound velocity ( VB) of the backing plate 104 typically vary by less than 0.1 % across brake pads of the same type).

[0056] Based on an algebraic rearrangement of Equation 1 above, the sound velocity ( VFM) of the friction material 102 is expressed as shown in Equation 2 below.

Equation 2 V _T FM

[0057] In Equation 2 above, when the backing material thickness (XB) 204 and the metal backing sound velocity ( VB) for the backing plate 104 are known in advance of measuring the brake pad 100, an estimated backing plate ToF (— ) of a propagating

^V B

signal through the backing plate 104 can be used to determine the sound velocity ( VFM) of the friction material 102. In particular, the friction material sound velocity ( VFM) can be determined using Equation 2 above based on the measured ToFpad of a propagating signal through the brake pad 100, an estimated backing plate ToF (— ) of the backing

^V B

plate 104, and the friction material thickness (XFM) 202. [0058] The sound velocity ( VFM) of the friction material 102 can then be used to determine a DMPUD of the friction material 102 based on ToF measurements collected using either longitudinal/compression waves or shear waves. In Equation 3 below, the DMPUD of a material based on measurements using longitudinal/compression waves is represented as the longitudinal/compressional dynamic modulus per unit density (Eiong p) for the material. The longitudinal/compressional dynamic modulus per unit density (Ei _on g p) is the characteristic of a material's response to longitudinal or compression stresses. Equation 3 represents the relationship between material density (p) and the longitudinal/compressional dynamic modulus {Ei _on g) for the measured material (e.g. , the friction material 102).

υ ^M PUD {.v long) ^~ k ^h long) I

Equation 3 long

[0059] In Equation 3 above, Vi _ong is the sound velocity ( VFM) of the friction material 102 determined based on ToF measurements using longitudinal/compression waves, and p is the material density of the friction material 102. Equation 3 shows that DMPUD as a function of longitudinal/compression sound velocity DMPUD( Viong) is equal to the square of the longitudinal/compressional velocity ( Viong).

[0060] In Equation 4 below, the DMPUD of a material based on measurements using shear waves is represented as the shear dynamic modulus per unit density (Gshear/p) for the material. Shear dynamic modulus per unit density (Gshear/p) is a characteristic of a material's response to shear stress. Equation 4 represents the relationship between material density (p) and shear dynamic modulus per unit density (Gshear) for the material (e.g. , the friction material 102).

DMp _UD (Y _shear ) — G _shear i _ _v 2

Equation 4 lp - shear

[0061 ] In Equation 4 above, Vshear is the sound velocity ( VFM) of the friction material 102 determined based on ToF measurements using shear waves, p is the material density of the friction material 1 02. Equation 4 shows that DMPUD as a function of shear sound velocity DMpuD( \ s/?ear) is equal to the square of the shear velocity ( Vshear).

[0062] In both Equations 3 and 4 above, the material density (p) corresponds to the material density of the friction material 1 02. The material density (p) of the friction material 1 02 can be measured using any suitable material density measurement technique based on one or more friction material samples in, for example, a laboratory environment (e.g., a laboratory environment at a manufacturing site). Such friction material samples should be representative of the brake pads to be measured using examples disclosed herein so that the lab-measured material density (p) is substantially or approximately the same as the material density (p) of the subsequently measured pads. In examples disclosed herein, the lab-measured material density (p) is used in connection with Equation 7 below to determine an out-of-plane

longitudinal/compressional dynamic Young's modulus based on the

longitudinal/compressional DM PUD ( Wong) and/or to determine an out-of-plane shear dynamic Young's modulus based on the shear DMpuD( \ snear).

[0063] In some examples, the as-manufactured brake pad 104 will have the anti- noise shim 1 08 as an additional component. The influence of the anti-noise shim 1 08 on the measured ToF of the friction material 1 02 can be eliminated using an analysis similar to that described above for eliminating the ToF effects of the backing plate 1 04. For example, Equations 5 and 6 below may be used to compensate for the ToF effects of the anti-noise shim 1 08.

Equation s To _Fpad = ¾ + ¾ + ¾

Equation 6 V _FM

(™ -¾-

[0064] In Equation 5 above, the ToFpad of a propagating signal through the brake pad 1 00 (e.g., through the friction material 1 02, the backing plate 1 04, and the anti-noise shim 108) is the sum of the friction material ToF ( ), the metal backing ToF ( - ), and an anti-noise shim ToF (— ). Using examples disclosed herein, by obtaining the

^V S

shim thickness dimension (Xs) 206 (FIG. 2) and a sound velocity ( Vs) of the anti-noise shim 108 in advance of measuring the ToF for the friction material 102, the dynamic modulus of the friction material 102 can be determined in as-manufactured brake pads 100 that have anti-noise shims 108 as well as backing plates 104. For example, when the manufacture of anti-noise shims is well controlled (e.g. , anti-noise shims can be cut or stamped from the same sheet material before being bonded to backing plates), the shim thickness dimension (Xs) 206 and the sound velocity ( Vs) of the anti-noise shim 108 remain substantially constant across brake pads of the same type. In such instances, by measuring anti-noise shims only (separated from the rest of the brake pad), ToF measurements can be collected on multiple anti-noise shims to determine a typical/average representative ToF of anti-noise shims. This representative anti-noise shim ToF (— ) value can be used with Equations 5 and 6 above to determine the sound

^V S

velocity ( VFM) of the friction material 102, which can then be used to determine the

DMPUD of the friction material 102 using Equation 3 and/or Equation 4 above.

[0065] An alternate approach is to measure the ToFpad of a brake pad in multiple measurement regions with an anti-noise shim present and then measure the ToFpad of the brake pad in the same measurement regions after the anti-noise shim has been removed. The differences between the ToFpad measurements with and without the anti- noise shim yield an estimated representative anti-noise shim ToF ( ) value, which can be used with Equations 5 and 6 above to determine the sound velocity ( VFM) of the friction material 102. Determining the representative anti-noise shim ToF ( ) in advance eliminates the need to measure the thickness of the shim independently when subsequently measuring intact as-manufactured brake pads having anti-noise shims bonded thereto.

[0066] Measurements without Fluid Coupling Agent

[0067] Prior measurement techniques at ultrasonic frequencies in the megahertz (MHz) range use a fluid coupling agent to promote the transmission of ultrasound across transducer/material interfaces. However, by using through-transmission signal emissions in the 100 kHz to 3 MHz range and the applied load 308 of FIG. 3, example measurement techniques disclosed herein can be used effectively based on direct contact between the transducers 302, 304 and corresponding surfaces of the brake pad 100 without the need for a fluid coupling agent. To enable such non-coupling agent measurements, signals are amplified higher than if a fluid coupling agent were used. However, even though such higher amplifications of the signals are used, there is sufficient signal detectability for high quality ToF measurements. This is true both for longitudinal/compression waves and shear waves. The signal transmission improves with the amount of force used to couple the transducers 302, 304 to the brake pad 100. For example, the applied load 308 of FIG. 3 is used to improve the signal transmission quality of the interfaces between the transducers 302, 304 and corresponding surfaces of the brake pad 100. In some examples, coupling forces as low as 50N may be used for the applied load 308. In other examples, higher coupling forces for the applied load 308 can be used to yield better ToF measurement results. As shown in FIG. 3, the transducers 302, 304 are aligned co-linearly and the applied load 308 generates a coupling force applied directly to the transducers 302, 304. By using high frequencies on the order of a few MHz between the transducers 302, 304 for measuring ToF using examples disclosed herein, the static load does not influence the transducer

performance. In some examples, the contact area (e.g., "footprint") of the transducers 302, 304 is on the order of 15 mm in diameter. In such examples, the coupling pressure for an applied load 308 of 100 N is -0.5 MPa.

[0068] Pre-load Dependence

[0069] Friction materials (e.g., the friction material 102) belong to a class of materials which are deemed to exhibit non-linear mesoscopic elastic behavior. Unlike elastic materials, where the dynamic modulus is relatively independent of load until near failure, the dynamic modulus or stiffness of friction materials can be strongly dependent upon the magnitude of the static load (e.g., the applied load 308) at relatively low levels (e.g., small applied loads). This property of friction material is influenced by material composition as well as the manufacturing process of the friction material. For ultrasonic measurement techniques, the measurement of the variation in the ToF as a function of pre-load (e.g., the applied load 308) provides a unique and convenient way to characterize friction materials. In some measurement processes using examples disclosed herein, the variation of the ToF of signals through the friction material 102 can be measured continuously as a function of changes in the applied load 308.

[0070] FIGS. 4A and 4B are line graphs representative of dynamic modulus data as a function of load (e.g., as a function of the applied load 308 of FIG. 3). For example, FIG. 4A shows an example line graph 402 representative of dynamic modulus

(gigapascals (GPa)) versus load for discrete measurements, and FIG. 4B shows an example line graph 404 representative of percent dynamic modulus change versus load for continuous measurements. The dynamic modulus values plotted in FIG. 4A are based on ToF measures at discrete increases in the applied load 308. The percent change dynamic modulus values plotted in FIG. 4B are based on continuously collected ToF measures as the applied load 308 increases during a loading phase, and as the applied load 308 decreases during an unloading phase.

[0071] The example line graph 404 of FIG. 4B is representative of percent dynamic modulus change versus load for continuous measurements and shows a hysteresis property of non-linear mesoscopic materials used in friction materials such as brake pads. Hysteresis is the result of the friction material 102 being porous and, thus, compliant two the applied load 308. That is, when the applied load 308 is applied (e.g., during a loading phase), the friction material 102 compresses. However, when the applied load 308 is decreased (e.g., during an unloading phase), the friction material 102 tends to resist an immediate return to its uncompressed state. That is, the friction material 102 returns to its uncompressed state over some duration that is longer than the amount of time over which the applied load 308 was increased during the loading phase. In general, during loading and unloading phases, the dynamic modulus is higher when the magnitude of the applied load 308 is increased. However, in the illustrated example, dynamic modulus exhibited in the line graph 404 of FIG. 4B is dependent on the direction of the pre-loading. For example, the hysteresis is apparent from the separation of a first line plot representing the percent change dynamic modulus during loading from a second line plot representing the percent change dynamic modulus during unloading.

[0072] Spatial Uniformity [0073] FIG. 5 is an example line graph 500 representing the spatial extent of an ultrasonic beam emitted by the signal transmitter transducer 302 (FIG. 3) for a dynamic modulus measurement. The spatial extent illustrated in the example line graph 500 represents the beam shape of an emitted ultrasonic signal as it propagates through a material across different axial distances between the signal transmitter transducer 302 and the signal receiver transducer 304 (FIG. 3). A lateral distance (mm) axis 502 of the line graph 500 represents the diameter of the ultrasonic beam, and an axial distance (mm) axis 504 of the line graph 500 represents the distance between the signal transmitter transducer 302 and the signal receiver transducer 304. In the illustrated example, the zero (0) mark on the lateral distance (mm) axis 502 is the center of the signal transmitter transducer 302 and, thus, the center of the ultrasonic beam emitted by the signal transmitter transducer 302.

[0074] Some example transducers used to measure the ToF and, thus, the velocity and modulus of friction materials are on the order of 12.5 mm in diameter. Based on such diameter, the transducers 302, 304 of FIG. 3 measure a cylindrical volume having a 12.5 mm diameter which occupies the space between the signal transmitter transducer 302 and the signal receiver transducer 304. In the illustrated example of FIG. 5, the beam diameter data of the line graph 500 is based on a material having a sound velocity of 1 mm/us (V=1 ), a transducer diameter of 12.7 mm (D=12.7), and a frequency of 1 MHz (f=1 ). For these parameters, a near field transition is on the order of 40 mm, as shown along the axial distance axis 504 of FIG. 5. The near field transition is the distance from the signal transmitter transducer 302 at which the transmitted ultrasonic beam begins to diverge or spread outside of the diameter of the signal transmitter transducer 302. This indicates that the physical dimensions of the ultrasonic beam are comparable to the diameter of the signal transmitter transducer 302. Thus, a cylindrical volume directly between the transducers 302, 304 is measured.

[0075] FIG. 6 shows a plurality of measurement regions 602a-f across the example brake pad 100 (FIGS. 1 -3). The well-formed and consistent dimensions of the ultrasonic beam shown in the example line graph 500 of FIG. 5 facilitate measuring the multiple measurement regions 602a-f of the brake pad 100 to determine the spatial uniformity of dynamic modulus across the brake pad 100. Such spatial uniformity is a useful parameter for determining noise performance as well as a useful process control parameter for improving the manufacture of brake pads. By measuring the ToF for the multiple measurement regions 602a-f on multiple brake pads from a given group of brake pads, examples disclosed herein can be used to measure intra-pad dynamic modulus variations and pad-to-pad dynamic modulus variations which are useful measurements of quality. For example, FIG. 7 is an example line graph 700 of intra- pad dynamic modulus variations of a brake pad, and FIG. 8 is an example bar graph 800 of average inter-pad dynamic modulus variations across multiple brake pads (e.g., brake pads having brake IDs A, B, C, D, E).

[0076] In addition to quantitative measurement of the ToF at each measurement regions 602a-f of the brake pad 100, the magnitude of the ultrasonic signal transmitted through each measurement region 602a-f can also be measured. For example, automatic gain control (AGC) circuitry (e.g., the gain controller 1516 of FIG. 15) used with a digitizer (e.g., the signal digitizer 1514 of FIG. 15) connected to the signal receiver transducer 304 (FIG. 3) can be used to control gain so that ToF measurements can take full advantage of the dynamic range of the signal receiver transducer 304 to collect useful signal magnitude measurements. Variations in signal magnitude measurements across the different measurement regions 602a-f of the brake pad 100 (e.g., spatial variation in signal loss by position) can be used to measure the quality of the brake pad 100 in terms of delamination (e.g., between the anti-noise shim 108 and the backing plate 104), poor bonding between the backing plate 104 and the friction material 102, and excessive cracking in the friction material 102, all of which give rise to increased signal loss which can be detected from the variations in the signal magnitude measurements across the different measurement regions 602a-f.

[0077] Use of Dynamic Modulus Per Unit Density for Brake Pad Manufacturing

[0078] Examples disclosed herein enable converting the ToFpad measurement of the brake pad 100 obtained using techniques disclosed above to a friction material sound velocity (VFM) of the friction material 102 and to a measure of DMPUD in a brake pad manufacturing environment. The DMPUD (e.g., determined using Equation 3 and/or Equation 4 above) can be used as a design parameter for improving manufacturing processes of brake pads that have improved noise, vibration, and/or harshness characteristics. Dynamic modulus per unit density (DMPUD) is the ratio of stress to strain under vibratory conditions. As such, DMPUD is representative of the dynamic stiffness of a brake pad. The dynamic stiffness of a brake pad is higher than the static stiffness and is greatly dependent on pre-load (e.g., the applied load 308 of FIG. 3) and strain amplitude. In some examples, dynamic stiffness of the brake pads correlates better with squeal and NVH performance than static stiffness. Using examples disclosed herein, DMPUD can be determined for brake pads to modify parameters in manufacturing processes to improve noise performance of braking systems. Example manufacturing parameters that may be modified include changing proportions of different components of a friction material mixture before hot pressing friction material pads, modifying temperatures of a hot press used to form the friction material pads, modifying the amount of pressure applied by the hot press, modifying the hot press compression time, modifying post-press curing temperatures, modifying post-press curing times, etc.

[0079] Dynamic Modulus Per Unit Density as an Estimate of Young's Modulus Per Unit Density

[0080] Using examples disclosed herein, the DMPUD of the friction material 102 can be determined as an estimate of out-of-plane Young's modulus per unit density of the friction material 102. In examples disclosed herein, the DMPUD of the friction material 102 is derived from an ultrasonic signal ToFpad measurement of a single velocity mode propagating signal along the out-of-plane axes 208 of the brake pad 100 without needing to use ToF measurements for signals along any other axis of the brake pad 100. That is, unlike an actual out-of-plane Young's modulus per unit density

measurement that requires cutting friction material samples from a brake pad to perform multiple propagating signal velocity measures along different axes of the sample, examples disclosed herein use a single out-of-plane velocity measure to determine the DMPUD measure as an estimate of Young's modulus per unit density. For both the

DMPUD and the Young's modulus per unit density, the DM and the Young's modulus are computed by multiplying each of the DMPUD and the Young's modulus per unit density by the density (p) of the measured material (e.g., the friction material 102).

[0081] Deriving DMPUD using a single velocity mode propagating signal as disclosed herein is an unexpected result that is contrary to conventional knowledge because it is well known that, in theory, it is not possible to measure the Young's modulus of transversely isotropic friction material using a ToF measurement of only a single velocity mode signal propagation. The DMPUD of a friction material determined using a single velocity mode signal propagation in accordance with examples disclosed herein is a substantially accurate estimation of Young's modulus, which is determined by prior techniques using multiple propagating signal velocity measures along different axes of the friction material. For example, multiple lab-based measurements of DMPUD of friction material are plotted against corresponding Young's modulus values in FIGS. 9 and 10 to show that examples disclosed herein can be used to determine accurate estimations of Young's modulus by deriving dynamic modulus per unit density using a single velocity mode propagating signal.

[0082] FIG. 9 is an example graph 900 of data plots showing a correlation between longitudinal/compression wave-based dynamic modulus (DM( Vi _on g)) and dynamic Young's modulus for friction material along the out-of-plane axis 208 (FIG. 2). For the data plots of the example graph 900, dynamic Young's modulus values were computed in accordance with Surface Vehicle Standard SAE J2725 defined by the SAE

International. The longitudinal/compression wave-based dynamic modulus (DM( Vi _on g)) values of the example graph 900 were determined by multiplying the material density (p) of the friction material 102 (FIGS. 1 -3) by longitudinal/compression wave-based DMPUD( Viong) values. The longitudinal/compression wave-based DMpuD(Viong) values of the example graph 900 were determined using Equation 3 above based on

longitudinal/compression wave measurements obtained using non-destructive measurements of an intact, as-manufactured brake pad as disclosed herein, and the dynamic Young's modulus measures of the example graph 900 were determined based on destructive measurements of the same brake pad in accordance with SAE J2725 measurement procedures. That is, the longitudinal/compression wave-based dynamic modulus (DM(Viong)) values of the example graph 900 are directly proportional to the square of the longitudinal velocity, Viong, in a brake pad along an out-of-plane axis (e.g., the out-of-plane axis 208).

[0083] FIG. 10 is an example graph 1000 of data plots showing a correlation between shear wave-based dynamic modulus {DM( V _S hear) and dynamic Young's modulus for friction material along the out-of-plan axis 208 (FIG. 2). For the data plots of the example graph 900, dynamic Young's modulus values were computed in accordance with Surface Vehicle Standard SAE J2725 defined by the SAE

International. The shear wave-based dynamic modulus {DM( V _S hear)) values of the example graph 1000 were determined by multiplying the material density (p) of the friction material 102 (FIGS. 1 -3) by shear wave-based DMpuD( \ s/?ear) values. The shear wave-based DMPUD( values of the example graph 1000 were determined using Equation 4 above based on non-destructive shear wave measurements of an intact, as- manufactured brake pad as disclosed herein, and the dynamic Young's modulus measures of the example graph 1000 were determined based on destructive

measurements of the same brake pad in accordance with SAE J2725 measurement procedures. That is, the shear wave-based dynamic modulus {DM( V _S hear)) values of the example graph 1000 are directly proportional to the square of the shear velocity, V shear, in a brake pad along an out-of-plane axis (e.g., the out-of-plane axis 208).

[0084] Given the variability in friction materials and measurements, the correlations of FIGS. 9 and 10 are very good. The example graph 900 of FIG. 9 shows that the correlation factor (R ² ) for longitudinal wave-based dynamic modulus (DM( Viong)) vs. dynamic Young's modulus is 0.88 (R ² =0.88). In the illustrated example, the correlation factor (R ² ) is a factor representative of similarity between compared data (e.g., dynamic modulus (DM( Viong)) vs. dynamic Young's modulus). The correlation factor (R ² ) is indicative of a perfect match between the compared data when it is equal to one. The example graph 1000 of FIG. 10 shows that the correlation factor (R ² ) for shear wave- based dynamic modulus {DM( V _S hear)) vs. dynamic Young's modulus is 0.81 (R ² =0.81 ). The example graphs of FIGS. 9 and 10 show strong correlations between their respective data, especially in view of the Young's modulus values being computed quantities and the DMPUD values being measured.

[0085] The correlations of FIGS. 9 and 10 show that sufficiently accurate Young's modulus of friction materials can be estimated using examples disclosed herein as a DMPUD by measuring a single velocity propagation mode (e.g. , either the shear velocity or the longitudinal velocity) along a single out-of-plane axis (e.g., the out-of-plane axis 208 of FIG. 2). Although DMPUD is a useful measure for a number of purposes, including as feedback information (e.g. , the process feedback 1418 of FIG. 14) for use in a manufacturing environment to modify manufacturing parameters to change (e.g. , improve) the qualities of the manufactured brake pads, as described below in connection with FIG. 14, dynamic Young's modulus measures are useful for NVH modeling and analysis. The correlation graphs of FIGS. 9 and 10 show that suitably accurate estimates of longitudinal/compression-based and shear-based out-of-plane dynamic Young's modulus of the friction material 102 can be determined based on DMPUD measures. Equation 7 below is a linear equation characterizing the linear relationship of DMPUD to actual dynamic Young's modulus of the same material.

Equation 7 can be used to estimate an out-of-plane dynamic Young's modulus (Ez) of the friction material 102 based on DMPUD measures obtained using non-destructive measurement techniques as disclosed herein. The linear equation of FIG. 7 can be derived using the longitudinal/compression wave-based correlation in example graph 900 of FIG. 9 to determine a longitudinal/compression-based out-of-plane dynamic Young's modulus, and/or can be derived using the shear wave-based correlation in example graph 1000 of FIG. 10 to determine the shear-based out-of-plane dynamic Young's modulus.

Equation 7 E _z = mpDM _PUD + b = mpV ² + b

[0086] In Equation 7 above, the correlation slope (m) (e.g. , a correlation slope factor)_and the offset (b) of the linear equation are determined based on a fitted line for the linear correlations represented in the graphs 900, 1000 of FIGS. 9 and 10. For example, to determine the longitudinal/compression-based out-of-plane dynamic Young's modulus (Ez, long), the correlation slope (m) and the offset (b) are determined based on a fitted line 902 of the linear correlations between the

longitudinal/compression wave-based dynamic modulus (DM( Vi _on g)) (e.g. , Vi _ong ) and the dynamic Young's modulus for the friction material 102 along the out-of-plane axis 208 (FIG. 2). The density (p) can be the lab-measured material density (p) of the friction material 102 (e.g. , a friction material that is the same or similar as the friction material used in the manufacture of brake pads being measured). In addition, the velocity of sound ( V) in Equation 7 when determining the longitudinal/compression- based out-of-plane dynamic Young's modulus (Ez, long) is the longitudinal/compression wave-based velocity of sound { Vi _on g) along the out-of-plane axis 208 of the friction material 102.

[0087] To determine the shear-based out-of-plane dynamic Young's modulus (Ez, shear), the correlation slope (m) and the offset (b) of Equation 7 above are determined based on a fitted line 1002 of the linear correlations between the shear wave-based dynamic modulus {DM( V _S hear)) (e.g. , pV^ _hear ) and the dynamic Young's modulus for the friction material 102 along the out-of-plane axis 208. The density (p) can be the lab- measured material density (p) of the friction material 102 (e.g. , a friction material that is the same or similar as the friction material used in the manufacture of brake pads being measured). In addition, the velocity of sound ( V) in Equation 7 when determining the shear-based out-of-plane dynamic Young's modulus (Ez, shear) is the shear wave-based velocity of sound ( Vshear) along the out-of-plane axis 208 of the friction material 102.

[0088] Effects of Applied Load on Pulse Shape

[0089] Examples disclosed herein use the applied load 308 (FIG. 3) to increase signal transmissivity between interfaces of the transducers 302, 304 (FIG. 3) and corresponding surfaces of the brake pad 100 as shown in FIG. 3. However, when using longitudinal/compression waves, such applied load 308 influences ToFpad

measurements through the brake pad 100 due to the porosity of the friction material 102 causing the friction material 102 to compress under load, which changes the

longitudinal signal transmissivity of the friction material 102. FIGS. 1 1 A-1 1 C are example waveform plots 1 100, 1 1 10, 1 120 representing time domain measurements of a 1 MHz longitudinal/compression wave ultrasound signal emitted through a brake pad (e.g. , the brake pad 100) along the out-of-plane axis 208 (FIG. 2) under different applied loads (e.g. , the applied load 308). For example, FIG. 1 1 A is a time domain response waveform 1 100 of a 1 MHz longitudinal/compression wave ultrasound signal emitted through a brake pad that is under load at 0.26 MPa, FIG. 1 1 B is a time domain response waveform 1 1 10 of a 1 MHz longitudinal/compression wave ultrasound signal emitted through a brake pad that is under load at 1 .55 MPa, and FIG. 1 1 C is a time domain response waveform 1 120 of a 1 MHz longitudinal/compression wave ultrasound signal emitted through a brake pad that is under load at 2.58 MPa. The large variation in the longitudinal/compressional waveform shape with load requires that the load be monitored to ensure repeatability. In addition, the large changes in the shape of the waveform with load leads to ambiguity in determining the measurement point (e.g., a signal peak, a signal trough, a zero crossing, etc.) in the waveform. The shape of the measured ultrasound waveform during a ToF measurement is an important determinant of the precision and repeatability of the ToF measurement. With increased loads, the measured signal becomes more distorted as shown in FIGS. 1 1 B and 1 1 C relative to FIG. 1 1 A. Such distortion makes it difficult to detect a trigger feature (e.g., a peak, a trough, a zero-crossing, etc.) on the waveform as the reference to measure a correct ToF and, thus, to determine an accurate DMPUD. AS such, when using

longitudinal/compression waves, controlling the load or coupling force is useful to minimize distortion of the measured longitudinal/compression wave ultrasonic signal, while creating a sufficiently transmissive interface between the transducers 302, 304 (FIG. 3) and corresponding surfaces of the brake pad 1 00.

[0090] Unlike longitudinal/compression waves which are affected significantly by increased applied loads as described above, shear waves are affected relatively less by such increased loads. FIG. 12 depicts example waveform plots representing time domain measurements of a 2.25 MHz shear wave ultrasound signal emitted through a brake pad (e.g., the brake pad 1 00) along the out-of-plane axis 208 (FIG. 2) under different applied loads (e.g., the applied load 308), specifically a 0.26 MPa load and a 2.58 MPa load. The 2.25 MHz shear wave ultrasound signal represented in FIG. 12 is transmitted through precisely the same volume as the longitudinal/compression wave ultrasound signal represented in FIGS. 1 1 A-1 1 C. Thus, even though the wavelength of the shear wave is half that of the longitudinal/compression wave, the shear wave pulses of FIG. 1 1 show considerably less distortion with increased load. From the standpoint of reliable ToF measurements, the shear wave pulse waveforms of FIG. 1 1 do not appear to be altered significantly with the applied load 308, which means it is easier to detect a ToF measurement feature (e.g., a peak, a trough, a zero-crossing, etc.) on the waveforms as the reference to measure a correct ToF and, thus, to determine an accurate DMPUD. [0091] FIG. 13 is an example in-process material inspection station 1300 that may be implemented to perform non-destructive dynamic modulus measurements of brake pads during a manufacturing process in accordance with the teachings of this

disclosure. The example in-process material inspection station 1300 includes an inspection table 1302 having a conveying surface 1304 on which the brake pad 100 and another brake pad 1306 are shown as being conveyed along a process flow direction 1308. The example in-process material inspection station 1300 also includes the signal transmitter transducer 302, the signal receiver transducer 304, and the load cell 306 of FIG. 3.

[0092] To place the example transducers 302, 304 in opposing contact with the example brake pad 100 when the brake pad 100 is placed between the transducers 302, 304, the example in-process material inspection station 1300 includes an example force actuator 1310 and an example sensor actuator 1312. A rod end of the example force actuator 1310 is epoxied or otherwise bonded to the single transmitter transducer 302. In the illustrated example, the force actuator 1310 provides the applied load 308 of FIG. 3 to the brake pad 100 by applying force against the signal transmitter transducer 302 along a direction 1314 (e.g., along the out-of-plane axis 208 of FIG. 2) towards the signal receiver transducer 304. The example load cell 306 measures the applied load 308. In the illustrated example, the force actuator 1310 is implemented using a stepper motor. However, any other suitable type of force actuating device may alternatively be used including, for example, a hydraulic piston, a pneumatic piston, a solenoid, etc.

[0093] The example sensor actuator 1312 is provided to raise a material-engaging surface of the signal receiver transducer 304 above the conveying surface 1304 to contact the braking surface 106 (FIGS. 1 -3) of the brake pad 100. In the illustrated example, the sensor actuator 1312 urges the signal receiver transducer 304 to protrude from the conveying surface 1304, creating a material engagement transducer protrusion 1316. The material engagement transducer protrusion 1316 is used to lift the brake pad 100 above the conveying surface 1304 so that the applied load 308 provided by the force actuator 1310 is directly applied to the brake pad 100 between the signal transmitter transducer 302 and the signal receiver transducer 304 without interference by the conveying surface 1304. In this manner, the material engagement transducer protrusion 1316 is used to ensure that the applied load 308 acts solely on the volume of the brake pad 100 between the signal transmitter transducer 302 and the signal receiver transduce 304. The example sensor actuator 1312 may be implemented using a stepper motor or any other suitable type of actuating device including, for example, a hydraulic piston, a pneumatic piston, a solenoid, etc.

[0094] Providing the example sensor actuator 1312 enables retracting the signal receiver transducer 304 into the inspection table 1302 when a brake pad measurement is not being performed so that a measured brake pad (e.g., the brake pad 100 in FIG. 13) may be conveyed away from the transducers 302, 304, and a next brake pad (e.g., the brake pad 1306 in FIG. 13) may be conveyed along the conveying surface 1304 for placement between the single transmitter transducer 302 and the signal receiver transducer 304 without the signal receiver transducer 304 interfering with the movement of the brake pads in the process flow. However, in other examples, the sensor actuator 1312 may be omitted, and the signal receiver transducer 304 and the load cell 306 may be mounted in a fixed position in the inspection table 1302 so that the material engagement transducer protrusion 1316 of the signal receiver transducer 304 is fixed above the conveying surface 1304 as shown in FIG. 13. In such examples, brake pads moved along the conveying surface are elevated above the material engagement transducer protrusion 1316 when the brake pads are placed between the signal transmitter transducer 302 and the signal receiver transducer 304. Such elevation of brake pads may be accomplished using trays, conveyor belts, manual movement, and/or any other suitable means for ensuring that brake pads clear the material engagement transducer protrusion 1316 when moved between the transducers 302, 304.

[0095] In the illustrated example, regardless of whether the sensor actuator 1312 is provided, the distance of the material engagement transducer protrusion 1316 is set or controlled to a known or specified value that is used to determine the thickness of the brake pad 100 after the applied load 308 is provided by the force actuator 1310. For example, when the force actuator 1310 is implemented using a stepper motor or any other suitable type of actuator that enables determining a distance of linear

displacement, the thickness of the brake pad 100 between the signal transmitter transducer 302 and the signal receiver transducer 304 can be determined based on the distance of the material engagement transducer protrusion 1 31 6 and the distance traveled by the force actuator 1 31 0. In this manner, the measured thickness of the brake pad 1 00 between the signal transmitter transducer 302 and the signal receiver transducer 304 can be used to determine the thickness of the friction material 1 02 (XFM) by subtracting a known thickness of the backing plate 1 04 (XB) from the measured thickness of the brake pad 1 00. In turn, the thickness of the friction material 1 02 (XFM) can be used to determine the sound velocity { VFM) of the friction material 1 02 based on Equation 2 and/or Equation 6 above, which is in turn used to determine the DMPUD of the friction material 1 02 using Equation 3 or Equation 4 above.

[0096] FIG. 1 4 is an example brake manufacturing process 1 400 with in-process non-destructive dynamic modulus testing using the in-process material inspection station 1 300 of FIG. 1 3. The brake manufacturing process 1 400 includes an example press 1 402 (e.g. , a multi-opening press) that receives backing plates (e.g. , the backing plate 1 04 of FIGS. 1 -3) from an example backing plate supply 1 404 and that receives a friction material composite from an example mixer 1 406 via an example friction material composite delivery path 1 408. In the illustrated example, the mixer 1 406 mixes multiple ingredients 141 0 in different quantities and/or ratios in accordance with a brake bad recipe formulated to produce brake pad friction material (e.g. , the friction material 1 02 of FIGS. 1 -3) with a target DMPUD.

[0097] The example mixer 1 406 mixes the ingredients 1 41 0 (e.g. , particulate materials and fibers having a wide range in size and density) to form a friction material composite mixture. The example press 1 402 then loads the friction material composite mixture into a die with the backing plate 1 04 forming a base of a mold cavity. The press 1 402 hot presses the mixture using a high compression force to form the friction material 1 02 and bond the friction material 1 02 to the backing plate 1 04. The brake pad 1 00 is then moved to the curing oven 1 412 for post-curing of the brake pad 1 00.

[0098] In the illustrated example, after the curing oven 141 2, brake pads are delivered to the in-process material inspection station 1 300 to perform in-process nondestructive DMPUD measurements of the brake pads in accordance with the teachings of this disclosure. Using such measurements, the in-process material inspection station 1300, or some other station, can determine which brake pads satisfy quality standards to be sent to an example parts packager 1414 and which brake pads do not satisfy quality standards and need to be sent to an example rejected parts station 1416.

[0099] In the illustrated example, the in-process material inspection station 1300 sends process feedback 1418 to one or more of the press 1402, the mixer 1406, and/or the curing oven 1412 based on the DMPUD measurements and/or out-of-plane dynamic Young's modulus measurements of the brake pads to change manufacturing

parameters to change (e.g., improve) the qualities of the manufactured brake pads. Example manufacturing parameters that may be modified based on the process feedback 1418 include changing proportions of different components of the friction material composite mixture at the mixer 1406 before hot pressing the friction material brake pads at the press 1402, modifying temperatures used by the press 1402 to form the friction material brake pads, modifying the amount of pressure applied by the press 1402, modifying the compression time at the press 1402, modifying post-press curing temperatures of the curing oven 1412, modifying post-press curing times at the curing oven 1412, etc.

[00100] In some examples, an in-process material inspection station 1300 can additionally or alternatively be integrated with or adapted to operate in the press 1402 so that in-process non-destructive DMPUD measurements can be performed in

accordance with examples disclosed herein while the press 1402 is forming brake pads. For example, the DMPUD measurements can be performed during the pressing operation to continuously monitor and/or modify the development of the brake pads during the pressing operation. In such examples, ultrasound could be generated using

transducers (e.g., the transducers 302, 304 of FIGS. 2, 3, and 13) mounted to the outside of opposing pressure plates of the press 1402 and transmitted through the brake pads during the forming process.

[00101] FIG. 15 is an example apparatus 1500 that may be used to perform nondestructive dynamic modulus measurements of materials in accordance with the teachings of this disclosure. Example apparatus 1500 includes an example

measurement controller 1502, an example transducer interface 1504, an example settings initializer 1506, an example actuator controller 1508, an example load detector 1510, an example thickness detector 1512, an example signal digitizer 1514, an example gain controller 1516, an example signal detector 1518, an example time-of- flight calculator 1520, an example sound velocity calculator 1522, an example modulus calculator 1524, and an example process feedback interface 1526. The example measurement controller 1502 is provided to manage the overall management process to perform the non-destructive dynamic modulus measurements of materials (e.g., the brake pad 100 of FIGS. 1 -3 and 13) as disclosed herein. The example transducer interface 1504 is provided to control acoustic signal transmissions via the signal transmitter transducer 302 and to control receiving acoustic signal transmissions via the signal receiver transducer 304. For example, the transducer interface 1504 may be configured to energize the transducers 302, 304 when an acoustic signal measurement is to be performed.

[00102] The example settings analyzer 1506 is provided to program or set parameter settings (e.g., thickness of backing plate 104, thickness of anti-noise shim 108, user defined applied load, etc.) for use in performing dynamic modulus measures. The example actuator controller 1508 is provided to control actuation of the force actuator 1310 and/or the sensor actuator 1312 of FIG. 13. The example load detector 1510 is provided to measure the load applied (e.g., the applied load 308 of FIG. 3) by the force actuator 1310 of FIG. 3. For example, the load detector 1510 may be connected to the load cell 306 of FIGS. 3 and 13 to measure load. The example thickness detector 1512 is provided to measure the thickness of the brake pad 100 after the transducers 302, 304 are brought into contact with opposing sides of the brake pad 100 by the force actuator 1310 and/or the sensor actuator 1312 (FIG. 13).

[00103] The example signal digitizer 1514 is provided to digitize (e.g., perform analog- to-digital conversions) acoustic signals received by the signal receiver transducer 304 (e.g., signals transmitted by the signal transmitter transducer 302). The example gain controller 1516 is provided to perform automatic gain control (AGC) adjustment of the amplification applied to the signal received by the receiver transmitter transducer 304 and digitized by the signal digitizer 1514. In the illustrated example, the gain controller 1516 amplifies the signal in the analog domain before it is digitized by the signal digitizer 1514. The example signal detector 1518 is provided to detect a trigger feature in the signals digitized by the signal digitizer 1514. For example, the trigger feature may be a signal peak, a signal trough, a zero-crossing, a feature identified based on a measured-reference waveform comparison, etc. that demarcates a position in a received signal waveform that is to serve as a point of reference to measure a ToFpad of the signal waveform through the brake pad 100.

[00104] The example ToF calculator 1520 is provided to calculate or measure times of flight, or propagation times, of signals transmitted through the brake pad 100 along the out-of-plane axis 208 (FIG. 2) by the signal transmitter transducer 302 and received by the signal receiver transducer 304. For example, the ToF calculator 1520 may calculate the ToF of a signal by measuring the duration between when the signal was transmitted by the signal transmitter transducer 302 and when the signal was received by the signal receiver transducer 304. The example sound velocity calculator 1522 is provided to calculate the sound velocity through the friction material 102 of the brake pad 100. The example modulus calculator 1524 is provided to determine dynamic modulus measures such as DMPUD and dynamic Young's modulus of the friction material 102. The example process feedback interface 1526 is configured to provide the example process feedback 1418 (FIG. 14) to one or more process stations (e.g., the press 1402 the mixer 1406 the curing oven 1412, etc. of FIG. 14) in a manufacturing process.

[00105] While an example manner of implementing the apparatus 1500 is illustrated in FIG. 15, one or more of the elements, processes and/or devices illustrated in FIG. 15 may be combined, divided, re-arranged, omitted, eliminated and/or implemented in any other way. Further, the example measurement controller 1502, the example transducer interface 1504, the example settings initializer 1506, the example actuator controller 1508, the example load detector 1510, the example thickness detector 1512, the example signal digitizer 1514, the example gain controller 1516, the example signal detector 1518, the example time-of-flight calculator 1520, the example sound velocity calculator 1522, the example modulus calculator 1524, and the example process feedback interface 1526 and/or, more generally, the example apparatus 1500 of FIG. 15 may be implemented by hardware, software, firmware and/or any combination of hardware, software and/or firmware. Thus, for example, any of the example

measurement controller 1502, the example transducer interface 1504, the example settings initializer 1506, the example actuator controller 1508, the example load detector 1510, the example thickness detector 1512, the example signal digitizer 1514, the example gain controller 1516, the example signal detector 1518, the example time-of- flight calculator 1520, the example sound velocity calculator 1522, the example modulus calculator 1524, and the example process feedback interface 1526 and/or, more generally, the example apparatus 1500 could be implemented by one or more analog or digital circuit(s), logic circuits, programmable processor(s), application specific integrated circuit(s) (ASIC(s)), programmable logic device(s) (PLD(s)) and/or field programmable logic device(s) (FPLD(s)). When reading any of the apparatus or system claims of this patent to cover a purely software and/or firmware implementation, at least one of the example measurement controller 1502, the example transducer interface 1504, the example settings initializer 1506, the example actuator controller 1508, the example load detector 1510, the example thickness detector 1512, the example signal digitizer 1514, the example gain controller 1516, the example signal detector 1518, the example time-of-flight calculator 1520, the example sound velocity calculator 1522, the example modulus calculator 1524, and/or the example process feedback interface 1526 is/are hereby expressly defined to include a non-transitory computer readable storage device or storage disk such as a memory, a digital versatile disk (DVD), a compact disk (CD), a Blu-ray disk, etc. including the software and/or firmware. Further still, the example apparatus 1500 of FIG. 15 may include one or more elements, processes and/or devices in addition to, or instead of, those illustrated in FIG. 15, and/or may include more than one of any or all of the illustrated elements, processes and devices.

[00106] Flow diagrams representative of example machine readable instructions for implementing the example apparatus 1500 of FIG. 15 and/or the example brake manufacturing process 1400 of FIG. 14 are shown in FIGS. 16-19. In these examples, the machine readable instructions implement one or more program(s) for execution by a processor such as the processor 2012 shown in the example processor platform 2000 discussed below in connection with FIG. 20. The program(s) may be embodied in software stored on a non-transitory computer readable storage medium such as a CD- ROM, a floppy disk, a hard drive, a digital versatile disk (DVD), a Blu-ray disk, or a memory associated with the processor 2012, but the entirety of the program(s) and/or parts thereof could alternatively be executed by a device other than the processor 2012 and/or embodied in firmware or dedicated hardware. Further, although the example program(s) is/are described with reference to the flow diagrams illustrated in FIGS. 16- 19, many other methods of implementing the example apparatus 1500 and/or the example brake manufacturing process 1400 may alternatively be used. For example, the order of execution of the blocks may be changed, and/or some of the blocks described may be changed, eliminated, or combined. Additionally or alternatively, any or all of the blocks may be implemented by one or more hardware circuits (e.g., discrete and/or integrated analog and/or digital circuitry, a Field Programmable Gate Array (FPGA), an Application Specific Integrated circuit (ASIC), a comparator, an operational- amplifier (op-amp), a logic circuit, etc.) structured to perform the corresponding operation without executing software or firmware.

[00107] As mentioned above, the example processes of FIGS. 16-19 may be implemented using coded instructions (e.g., computer and/or machine readable instructions) stored on a non-transitory computer and/or machine readable medium such as a hard disk drive, a flash memory, a read-only memory, a compact disk, a digital versatile disk, a cache, a random-access memory and/or any other storage device or storage disk in which information is stored for any duration (e.g., for extended time periods, permanently, for brief instances, for temporarily buffering, and/or for caching of the information). As used herein, the term non-transitory computer readable medium is expressly defined to include any type of computer readable storage device and/or storage disk and to exclude propagating signals and to exclude transmission media. "Including" and "comprising" (and all forms and tenses thereof) are used herein to be open ended terms. Thus, whenever a claim lists anything following any form of "include" or "comprise" (e.g., comprises, includes, comprising, including, etc.), it is to be understood that additional elements, terms, etc. may be present without falling outside the scope of the corresponding claim. As used herein, when the phrase "at least" is used as the transition term in a preamble of a claim, it is open-ended in the same manner as the term "comprising" and "including" are open ended.

[00108] FIG. 16 is a flow diagram representative of example machine readable instructions that may be executed to implement the example apparatus 1500 of FIG. 15 to perform non-destructive dynamic modulus measurements of materials (e.g., the friction material 102 of FIGS. 1 -3). The example process of FIG. 16 begins at block 1602 at which an acoustic signal is transmitted through a measured component along an out-of-plane axis of the measured component. For example, the transducer interface 1504 (FIG. 15) may control the signal transmitter transducer 302 to transmit a shear wave signal through the brake pad 100 along the out-of-plane axis 208 (FIG. 2). This causes the shear wave signal to propagate through the friction material 102 and the backing plate 104 of the brake pad 100. In other examples, a longitudinal/compression wave signal may be used instead.

[00109] The example time-of-f light calculator 1520 (FIG. 15) determines a ToF of the acoustic signal through the measured component (block 1604). For example, the time- of-flight calculator 1520 determines the ToFpad of the shear wave signal transmitted through the brake pad 100 and detected by the signal receiver transducer 304. In other examples in which longitudinal/compression waves are used, the time-of-flight calculator 1520 determines the ToF of the longitudinal/compression wave signal through the brake pad 100.

[001 10] The example sound velocity calculator 1522 (FIG. 15) determines a sound velocity of a sub-component or material layer of the measured component based on the ToF of the signal along the out-of-plane axis 208 (block 1606). For example, the sound velocity calculator 1522 determines the sound velocity { VFM) of the friction material 102 of the brake pad 100 based on Equation 2 above, the ToFpad of the shear wave signal through the brake pad 100, the estimated backing plate ToF (— ) of the shear wave

^V B

signal through the backing plate 104 along the out-of-plane axis 208, and a thickness of the friction material 102, without using any other ToFs along any other axis of the brake pad 100. For examples in which the brake pad 100 includes the anti-noise shim 108 (FIGS. 1 -3), the sound velocity calculator 1522 determines the sound velocity (VFM) of the friction material 102 of the brake pad 100 based on Equation 6 above and the additional parameters corresponding to the anti-noise shim 108.

[001 1 1] The example modulus calculator 1524 determines a DMPUD of the friction material 102 based on the sound velocity { VFM) of the friction material 102. For example, the modulus calculator 1524 uses Equation 4 above to determine the DMPUD ( Vshear) of the friction material 102 based on the sound velocity ( VFM) of the friction material 102 for a shear wave signal. Alternatively, when longitudinal/compression waves are used, the modulus calculator 1524 uses Equation 3 above to determine the DMPUD { Viong) of the friction material 102 based on the sound velocity { VFM) of the friction material 102 for longitudinal/compression waves. The example process FIG. 16 then ends.

[001 12] FIG. 17 is another flow diagram representative of example machine readable instructions that may be executed to implement the example apparatus 1500 of FIG. 15 to perform non-destructive dynamic modulus measurements of materials in accordance with the teachings of this disclosure. The example process of FIG. 17 begins at block 1702 at which the measurement controller 1502 (FIG. 15) performs a calibration of the in-process material inspection station 1300 (FIG. 13). For example, the measurement controller 1502 can set calibration parameters in a corresponding location of a memory (e.g. , one or more of the memories 2014, 2016, 2028 of FIG. 20). In the illustrated example, the calibration parameters correspond to calibration measurements performed with the in-process material inspection station 1300 using a calibration standard block of which a propagation time is known and pre-established through independent

measurement means (e.g. , measured using an independent highly accurate signal propagation measurement instrument). The example settings initializer 1506 (FIG. 15) sets one or more user-defined load value(s) (block 1704). For example, the user- defined load value(s) is/are to be used to generate the applied load 308 of FIG. 3. For examples in which the measurement process of FIG. 17 is to be performed for only a single applied load 308, the settings initializer 1506 sets only one user-defined load value at block 1704. However, for examples in which the measurement process of FIG. 17 is to be repeated multiple times for multiple applied loads 308, the settings initializer 1506 sets a plurality of user-defined load values at block 1704. The settings initializer 1506 may set the user-defined load value(s) by storing the value(s) in a corresponding location in a memory (e.g. , one or more of the memories 2014, 2016, 2028 of FIG. 20). The example settings initializer 1506 sets a backing plate thickness (block 1706). The backing plate thickness (XB) corresponds to a thickness of the backing plate 104 of the brake pad 100. In the illustrated example, the backing plate thickness (XB) may be provided in a specifications sheet identifying structural dimensions of the components of the brake pad 100. The settings initializer 1506 may set the backing plate thickness (Xe) by storing it in a corresponding location of the user-defined load value in a memory (e.g., one or more of the memories 2014, 2016, 2028 of FIG. 20).

[00113] The in-process material inspection station 1300 receives the brake pad 100 (block 1708). For example, the in-process material inspection station 1300 receives the brake pad 100 between the signal transmitter transducer 302 and the signal receiver transducer 304 as described above in connection FIG. 13. The example actuator controller 1508 (FIG. 15) applies a load to the brake pad 100 (block 1710). For example, if the signal receiver transducer 304 is configured to be actuated by the sensor actuator 1312 (FIG. 13), the actuator controller 1508 controls both the force actuator 1310 (FIG. 13) and the sensor actuator 1312 to move the transducers 302, 304 into contact with the brake pad 100. Otherwise, if the signal receiver transducer 304 is configured to be in a fixed position, the actuator controller 1508 controls only the force actuator 1310 to move the signal transmitter transducer 302 into contact with the brake pad 100. In either case, when the example actuator controller 1508 controls movement of the transducer(s) 302, 304 into contact with opposing surfaces of the brake pad 100, the actuator controller 1508 controls the applied load 308 (FIG. 3) between transducers 302, 304 until the example load cell 306 (FIGS. 3 and 13) detects that the applied load 308 equals the user-defined load set in block 1704.

[00114] The example thickness detector 1512 (FIG. 15) measures the total brake pad thickness (block 1712). For example, the thickness detector 1512 measures the linear actuation of the force actuator 1310 relative to a position of the signal receiver transducer 304 to determine the total thickness of the brake pad 100. The example signal transmitter transducer 302 emits a signal (block 1714). For example, the example transducer interface 1504 (FIG. 15) causes the signal transmitter transducer 302 to emit a shear wave signal through the brake pad 100 towards the signal receiver transducer 304 along the out-of-plane axis 208 (FIG. 2). In other examples, a

longitudinal/compression wave may be used instead. The example signal detector 1518 searches for a signal detected by the signal receiver transducer 304 (block 1716). For example, after the signal receiver transducer 304 receives the signal transmitted by the signal transmitter transducer 302, the example signal digitizer 1514 (FIG. 15) performs an analog-to-digital conversion on the received signal for analysis by the signal detector 1518. In the illustrated example, the gain controller 1516 (FIG. 15) amplifies the received analog signal before the example signal digitizer 1514 digitizes the received signal.

[001 15] The example signal detector 1518 determines whether it has detected the signal (block 1718). For example, the signal detector 1518 may search for a feature in a signal waveform (e.g., a signal peak, a signal trough, a zero-crossing, a signal feature matching a reference waveform, etc.) to confirm that it has detected the signal transmitted by the signal transmitter transducer 302. If the signal detector 1518 determines that it has not detected the signal (block 1718), the gain controller 1516 adjusts the gain applied to the analog signal received by the signal receiver transducer 304 (block 1720) and control returns to block 1716. For example, the gain controller 1516 may perform automatic gain control (AGC) to increase the applied signal gain. If the signal detector 1518 determines that it has detected the signal (block 1718), control advances to block 1722 at which the time-of-flight calculator 1520 (FIG. 15) determines the ToF of the signal (block 1722). For example, the time-of-flight calculator 1520 determines the ToFpad of the signal transmitted through the brake pad 100 along the out-of-plane axis 208 (FIG. 2) and detected by the signal receiver transducer 304.

[001 16] The example measurement controller 1502 determines whether the process is a single load measurement (block 1724). For example, a single load measurement parameter may be set by the settings initializer 1506 based on user input before performing the measurement process of FIG. 17 to cause the measurement process to measure the ToFpad and determine a DMPUD based on only one applied load 308.

Otherwise, if the single load measure parameter is not set, the example process is to perform measurements based on a plurality of applied loads 308. If the measurement controller 1502 determines that the measurement process is not to be performed for only a single applied load 308 (block 1724), the measurement controller 1502 determines whether the multi-load cycle is complete (block 1726). For example, the measurement controller 1502 determines whether the brake pad 100 has been measured for all the specified loads. If the measurement controller 1502 determines that the multi-load cycle is not complete, control returns to block 1710 at which the actuator controller 1508 applies a different load to the brake pad 100. Otherwise, if the measurement controller 1502 determines that the multi-load cycle is complete (block 1726), or if the measurement controller determines at block 1724 that the measurement process is to be performed for only a single applied load 308, control advances to block 1728 at which the actuator controller 1508 retracts the transducer(s) 302, 304 (block 1728). For example, if the signal receiver transducer 304 is configured to be actuated by the sensor actuator 1312, the actuator controller 1508 controls both the force actuator 1310 and the sensor actuator 1312 to retract the transducers 302, 304 away from the brake pad 100. Otherwise, if the signal receiver transducer 304 is configured to be in a fixed position, the actuator controller 1508 controls only the force actuator 1310 to retract the signal transmitter transducer 302 away from the brake pad 100.

[001 17] The example modulus calculator 1524 (FIG. 15) determines a DMPUD of the friction material 102 (block 1730). For example, the modulus calculator 1524 may determine the DMPUD of the friction material 102 based on Equation 4 above for shear waves. Alternatively, the modulus calculator 1524 may determine the DMPUD of the friction material 102 based on Equation 3 above for longitudinal/compression waves. The example operation of block 1730 may be implemented using, for example, the example process described below in connection with FIG. 18. The example modulus calculator 1524 determines an out-of-plane dynamic Young's modulus of the friction material 102 (block 1732). For example, out-of-plane dynamic Young's modulus measures are useful for NVH modeling and analysis and/or for process feedback in a manufacturing environment. In the illustrated example, the modulus calculator 1524 determines the out-of-plane dynamic Young's modulus (Ez) of the friction material 102 by multiplying the DMPUD of the friction material 102 by a material density (p) of the friction material 102 and based on a correlation slope factor (m) (e.g. , the correlation slope (m) of Equation 7 above) corresponding to a linear correlation between a dynamic modulus of a second friction material and an actual Young's dynamic modulus of the second friction material measured using destructive testing of the second friction material. For example, the modulus calculator 1524 may determine a

longitudinal/compression-based out-of-plane dynamic Young's modulus based on the longitudinal/compression wave-based dynamic modulus (DM(Vi _on g)) (e.g., pVi _ong ) as described above in connection with Equation 7. Additionally or alternatively, the modulus calculator 1524 may determine a shear-based out-of-plane dynamic Young's modulus based on the shear wave-based dynamic modulus (DM(V _S hear)) (e.g., pV^ _hear ) as described above in connection with Equation 7. The example process feedback interface 1526 (FIG.15) provides the process feedback 1418 (FIG.14) to one or more processes of the brake manufacturing process 1400 of FIG.14 (block 1734). The example process of FIG 17 then ends.

[00118] FIG.18 is a flow diagram representative of example machine readable instructions that may be executed to determine a DMPUD of a measured material (e.g., the friction material 102 of FIGS.1-3). The example process of FIG.18 begins at block 1802 at which the thickness detector 1512 (FIG.15) determines a thickness of the friction material 102. For example, the thickness detector 1512 can subtract the thickness of the backing plate 104 set at block 1706 of FIG.17 from the total brake pad thickness measured at block 1712 of FIG.17. The example sound velocity calculator 1522 (FIG.15) determines the sound velocity {VFM) of the friction material 102 (block 1804). For example, the sound velocity calculator 1524 determines the sound velocity {VFM) of the friction material 102 based on Equation 2 above using the ToFpad of the brake pad 100 measured at block 1722 of FIG.7, based on the friction material thickness (XFM) determined at block 1802, based on the backing plate thickness (XB) set at block 1706 of FIG.17, and based on a sound velocity (VB) of the backing plate 104 obtained as a user input or programmed value for the particular material of the backing plate 104. For example, the sound velocity (VB) of the backing plate 104 may be obtained from a specification sheet or catalog including a laboratory-derived sound velocity of the material used for the backing plate 104. For examples in which the brake pad 100 includes the anti-noise shim 108 (FIGS.1-3), the sound velocity calculator 1522 determines the sound velocity (VFM) of the friction material 102 of the brake pad 100 based on Equation 6 above and the additional parameters corresponding to the anti-noise shim 108.

[00119] The example modulus calculator 1524 (FIG.15) determines the DMPUD of the friction material 102 (block 1806). For example, the modulus calculator 1524 uses Equation 4 above to determine the DMPUD ( Vshear) of the friction material 102 based on the sound velocity { VFM) of the friction material 102 for a shear wave signal.

Alternatively, when longitudinal/compression waves are used, the modulus calculator 1524 uses Equation 3 above to determine the DMPUD { Viong) of the friction material 102 based on the sound velocity { VFM) of the friction material 102 for

longitudinal/compression waves. The example process FIG. 18 ends. In the illustrated example, control returns to a calling function or process such as the example process of FIG. 17.

[00120] FIG. 19 is a flow diagram representative of example machine readable instructions that may be executed to control a manufacturing process (e.g., the brake pad manufacturing process 1400 of FIG. 14) based on DMPUD and/or out-of-plane dynamic Young's modulus measures determined using examples disclosed herein. For example, the process of FIG. 19 may be based on DMPUD measures and/or out-of-plane dynamic Young's modulus measures provided by the apparatus 1500 in the process feedback at block 1734 of FIG. 17. The example process of FIG. 19 begins at block 1902 when a processor (e.g., the processor 2012 of FIG. 20) of a process station (e.g., the press 1402, the mixer 1406, the curing oven 1412, etc.) of the brake manufacturing process 1400 receives DM-based process feedback (e.g., the process feedback 1418). For example, the DM-based process feedback may be based on or include DMPUD values (e.g., determined at block 1730 of FIG. 17) and/or may be based on or include out-of-plane dynamic Young's modulus values (e.g., determined at block 1732 of FIG. 17). The processor of the process station determines whether the DM needs to be increased (block 1904). For example, the process station may compare the DM to a target value, or may identify an instruction or other information in the process feedback 1418 indicating whether the DM needs to be increased. If, at block 1904, the processor of the process station determines that the DM needs to be increased, the processor of the process station modifies one or more of its process/manufacturing parameters to increase the DM (block 1906). If, at block 1904, the processor of the process station determines that the DM does not need to be increased, control advances to block 1908, at which the processor of the process station determines whether the DM needs to be decreased. For example, the process station may compare the DM to a target value, or may identify an instruction or other information in the process feedback 1418 indicating whether the DM needs to be decreased. If, at block 1908, the processor of the process station determines that the DM does need to be decreased, the processor of the process station modifies one or more of its process/manufacturing parameters to decrease the DM (block 1910). After the process parameter(s) are modified at block 1906 or block 1910, or if, at block 1908, the processor of the process station determines that the DM is not to be decreased, the example process of FIG. 19 ends.

[00121] The example processes of FIGS. 16-19 may be repeated multiple times for any number of brake pads measured in the in-process material inspection station 1300 of FIG. 13.

[00122] FIG. 20 is an example processor platform 2000 capable of executing the example machine readable instructions represented by FIG. 16, FIG. 17, FIG. 18, and/or FIG. 19 to implement the apparatus 1500 of FIG. 15 and/or the brake

manufacturing process 1400 of FIG. 14 to perform non-destructive dynamic modulus measurements of materials (e.g., brake pads) and/or to control a manufacturing process (e.g., a brake pad manufacturing process) based on DMPUD and/or out-of-plane dynamic Young's modulus measures determined using examples disclosed herein. The processor platform 2000 can be, for example, a server, a personal computer, or any other type of computing device.

[00123] The processor platform 2000 of the illustrated example includes a processor 2012. The processor 2012 of the illustrated example is hardware. For example, the processor 2012 can be implemented by one or more integrated circuits, logic circuits, microprocessors or controllers from any desired family or manufacturer. The hardware processor may be a semiconductor based (e.g., silicon based) device. In this example, the processor 2012 implements the example measurement controller 1502, the example transducer interface 1504, the example settings initializer 1506, the example actuator controller 1508, the example load detector 1510, the example thickness detector 1512, the example signal digitizer 1514, the example gain controller 1516, the example signal detector 1518, the example time-of-flight calculator 1520, the example sound velocity calculator 1522, the example modulus calculator 1524, and the example process feedback interface 1526. [00124] The processor 2012 of the illustrated example includes a local memory 2013 (e.g., a cache). The processor 2012 of the illustrated example is in communication with a main memory including a volatile memory 2014 and a non-volatile memory 2016 via a bus 2018. The volatile memory 2014 may be implemented by Synchronous Dynamic Random Access Memory (SDRAM), Dynamic Random Access Memory (DRAM), RAMBUS Dynamic Random Access Memory (RDRAM) and/or any other type of random access memory device. The non-volatile memory 2016 may be implemented by flash memory and/or any other desired type of memory device. Access to the main memory 2014, 2016 is controlled by a memory controller.

[00125] The processor platform 2000 of the illustrated example also includes an interface circuit 2020. The interface circuit 2020 may be implemented by any type of interface standard, such as an Ethernet interface, a universal serial bus (USB), and/or a PCI express interface.

[00126] In the illustrated example, one or more input devices 2022 are connected to the interface circuit 2020. The input device(s) 2022 permit(s) a user to enter data and/or commands into the processor 2012. The input device(s) can be implemented by, for example, an audio sensor, a microphone, a camera (still or video), a keyboard, a button, a mouse, a touchscreen, a track-pad, a trackball, isopoint and/or a voice recognition system.

[00127] One or more output devices 2024 are also connected to the interface circuit 2020 of the illustrated example. The output devices 2024 can be implemented, for example, by display devices (e.g., a light emitting diode (LED), an organic light emitting diode (OLED), a liquid crystal display, a cathode ray tube display (CRT), a touchscreen, a tactile output device, a printer and/or speakers). The interface circuit 2020 of the illustrated example, thus, typically includes a graphics driver card, a graphics driver chip and/or a graphics driver processor.

[00128] The interface circuit 2020 of the illustrated example also includes a

communication device such as a transmitter, a receiver, a transceiver, a modem and/or network interface card to facilitate exchange of data with external machines (e.g., computing devices of any kind) via a network 2026 (e.g., an Ethernet connection, a digital subscriber line (DSL), a telephone line, coaxial cable, a cellular telephone system, etc.).

[00129] The processor platform 2000 of the illustrated example also includes one or more mass storage devices 2028 for storing software and/or data. Examples of such mass storage devices 2028 include floppy disk drives, hard drive disks, compact disk drives, Blu-ray disk drives, RAID systems, and digital versatile disk (DVD) drives.

[00130] Coded instructions 2032 representative of the example machine readable instructions of FIGS. 16-19 may be stored in the mass storage device 2028, in the volatile memory 2014, in the non-volatile memory 2016, and/or on a removable tangible computer readable storage medium such as a CD or DVD.

[00131] The following correspond to examples disclosed herein.

[00132] Example methods disclosed herein may be used to determine dynamic modulus per unit density of a friction material. Some such example methods include: transmitting a shear wave signal through a brake pad along an out-of-plane axis of the brake pad, the brake pad including a friction material bonded to a backing plate;

determining a first time-of-flight of the shear wave signal through the brake pad;

determining a sound velocity of the friction material of the brake pad based on the first time-of-flight of the shear wave signal along the out-of-plane axis, a second time-of- flight of the shear wave signal through the backing plate of the brake pad along the out- of-plane axis, and a thickness of the friction material, without using a third time-of-flight along any other axis of the brake pad; and determining a dynamic modulus per unit density of the friction material based on the sound velocity of the friction material.

[00133] In some disclosed example methods, the transmitting of the shear wave signal through the brake pad is performed using a transducer in contact with the brake pad without using a fluid coupling agent between the transducer and the brake pad. Some disclosed example methods further include applying a load to the transducer before transmitting the shear wave signal to increase a signal transmission

effectiveness between the transducer and the brake pad.

[00134] Some disclosed example methods further include determining the thickness of the friction material by: determining a thickness of the brake pad as a distance between opposing acoustic transducers in contact with the brake pad, the shear wave signal emitted by a first one of the acoustic transducers towards a second one of the acoustic transducers; and subtracting a thickness of the backing plate from the thickness of the brake pad.

[00135] In some disclosed example methods, the determining of the dynamic modulus per unit density of the friction material is further based on a material density of the friction material.

[00136] In some disclosed example methods, the transmitting of the shear wave signal through the brake pad and the determining of the dynamic modulus per unit density of the friction material are performed without removing the friction material from the brake pad.

[00137] Some disclosed example methods further include communicating process feedback based on the dynamic modulus per unit density to at least one process of a brake manufacturing process of the brake pad to modify at least one manufacturing parameter of the brake manufacturing process.

[00138] Some disclosed example methods further include determining an out-of-plane dynamic Young's modulus of the friction material by multiplying the dynamic modulus per unit density of the friction material by a material density of the friction material and based on a correlation slope factor corresponding to a linear correlation between a dynamic modulus of a second friction material and a Young's dynamic modulus of the second friction material measured using destructive testing of the second friction material.

[00139] Example apparatus disclosed herein may be used to determine dynamic modulus per unit density of a friction material. Some such example apparatus include: a transducer interface to cause a transducer to transmit a shear wave signal through a brake pad along an out-of-plane axis of the brake pad, the brake pad including a friction material bonded to a backing plate; a time-of-f light calculator to determine a first time-of- flight of the shear wave signal through the brake pad; a sound velocity calculator to determine a sound velocity of the friction material of the brake pad based on the first time-of-flight of the shear wave signal along the out-of-plane axis, a second time-of- flight of the shear wave signal through the backing plate of the brake pad along the out- of-plane axis, and a thickness of the friction material, without using a third time-of-flight along any other axis of the brake pad; and a modulus calculator to determine a dynamic modulus per unit density of the friction material based on the sound velocity of the friction material.

[00140] Some disclosed example apparatus further include the transducer to be in contact with the brake pad without using a fluid coupling agent between the transducer and the brake pad to transmit the shear wave signal through the brake pad along the out-of-plane axis of the brake pad. Some such example apparatus further include an actuator controller to apply a load to the transducer before transmitting the shear wave signal to increase a signal transmission effectiveness between the transducer and the brake pad.

[00141] Some disclosed example apparatus further include a thickness detector to determine the thickness of the friction material by: determining a thickness of the brake pad as a distance between opposing acoustic transducers in contact with the brake pad, the shear wave signal emitted by a first one of the acoustic transducers towards a second one of the acoustic transducers; and subtracting a thickness of the backing plate from the thickness of the brake pad.

[00142] In some disclosed example apparatus, the modulus calculator is to determine the dynamic modulus per unit density of the friction material based on a material density of the friction material.

[00143] In some disclosed example apparatus, the transducer interface is to cause the transducer to transmit the shear wave signal through the brake pad, and the modulus calculator is to determine the dynamic modulus per unit density of the friction material without the friction material being removed from the brake pad.

[00144] Some disclosed example apparatus further include a process feedback interface to communicate process feedback based on the dynamic modulus per unit density to at least one process of a brake manufacturing process of the brake pad to modify at least one manufacturing parameter of the brake manufacturing process.

[00145] In some disclosed example apparatus, the modulus calculator is further to determine an out-of-plane dynamic Young's modulus of the friction material by multiplying the dynamic modulus per unit density of the friction material by a material density of the friction material and based on a correlation slope factor corresponding to a linear correlation between a dynamic modulus of a second friction material and Young's dynamic modulus of the second friction material measured using destructive testing of the second friction material.

[00146] Example non-transitory computer readable storage media comprising instructions disclosed herein may be used to determine dynamic modulus per unit density of a friction material. In some such examples, the instructions, when executed, cause a processor to: transmit a shear wave signal through a brake pad along an out- of-plane axis of the brake pad, the brake pad including a friction material bonded to a backing plate; determine a first time-of -flight of the shear wave signal through the brake pad; determine a sound velocity of the friction material of the brake pad based on the first time-of-flight of the shear wave signal along the out-of-plane axis, a second time-of- flight of the shear wave signal through the backing plate of the brake pad along the out- of-plane axis, and a thickness of the friction material, without using a third time-of-flight along any other axis of the brake pad; and determine a dynamic modulus per unit density of the friction material based on the sound velocity of the friction material.

[00147] In some disclosed example non-transitory computer readable storage media, the transmitting of the shear wave signal through the brake pad is performed using a transducer in contact with the brake pad without using a fluid coupling agent between the transducer and the brake pad. In some disclosed example non-transitory computer readable storage media, the instructions, when executed, are further to cause the processor to apply a load to the transducer before transmitting the shear wave signal to increase a signal transmission effectiveness between the transducer and the brake pad.

[00148] In some disclosed example non-transitory computer readable storage media, the instructions, when executed, are further to cause the processor to determine the thickness of the friction material by: determining a thickness of the brake pad as a distance between opposing acoustic transducers in contact with the brake pad, the shear wave signal emitted by a first one of the acoustic transducers towards a second one of the acoustic transducers; and subtracting a thickness of the backing plate from the thickness of the brake pad.

[00149] In some disclosed example non-transitory computer readable storage media, the instructions, when executed, are to cause the processor to determine the dynamic modulus per unit density of the friction material based on a material density of the friction material.

[00150] In some disclosed example non-transitory computer readable storage media, the transmitting of the shear wave signal through the brake pad and the determining of the dynamic modulus per unit density of the friction material are performed without removing the friction material from the brake pad.

[00151] In some disclosed example non-transitory computer readable storage media, the instructions, when executed, are further to cause the processor to communicate process feedback based on the dynamic modulus per unit density to at least one process of a brake manufacturing process of the brake pad to modify at least one manufacturing parameter of the brake manufacturing process.

[00152] In some disclosed example non-transitory computer readable storage media, the instructions, when executed, are further to cause the processor to determine an out- of-plane dynamic Young's modulus of the friction material by multiplying the dynamic modulus per unit density of the friction material by a material density of the friction material and based on a correlation slope factor corresponding to a linear correlation between a dynamic modulus of a second friction material and a dynamic modulus of the second friction material measured using destructive testing of the second friction material.

[00153] Example apparatus disclosed herein may be used to determine dynamic modulus per unit density of a friction material. Some such disclosed examples include: transducer interface means for causing a transducer to transmit a shear wave signal through a brake pad along an out-of-plane axis of the brake pad, the brake pad including a friction material bonded to a backing plate; time-of-flight calculating means for determining a first time-of-flight of the shear wave signal through the brake pad; sound velocity calculating means for determining a sound velocity of the friction material of the brake pad based on the first time-of-flight of the shear wave signal along the out- of-plane axis, a second time-of-flight of the shear wave signal through the backing plate of the brake pad along the out-of-plane axis, and a thickness of the friction material, without using a third time-of-flight along any other axis of the brake pad; and modulus calculating means for determining a dynamic modulus per unit density of the friction material based on the sound velocity of the friction material.

[00154] Some disclosed example apparatus further include the transducer to be in contact with the brake pad without using a fluid coupling agent between the transducer and the brake pad to transmit the shear wave signal through the brake pad along the out-of-plane axis of the brake pad.

[00155] Some disclosed examples further include actuator controller means for applying a load to the transducer before transmitting the shear wave signal to increase a signal transmission effectiveness between the transducer and the brake pad.

[00156] Some disclosed examples further include thickness detecting means for determining the thickness of the friction material by: determining a thickness of the brake pad as a distance between opposing acoustic transducers in contact with the brake pad, the shear wave signal emitted by a first one of the acoustic transducers towards a second one of the acoustic transducers; and subtracting a thickness of the backing plate from the thickness of the brake pad.

[00157] In some disclosed examples, the modulus calculating means is to determine the dynamic modulus per unit density of the friction material based on a material density of the friction material.

[00158] In some disclosed examples, the transducer interface means is to cause the transducer to transmit the shear wave signal through the brake pad, and the modulus calculating means is to determine the dynamic modulus per unit density of the friction material without the friction material being removed from the brake pad.

[00159] Some disclosed examples further include process feedback interface means for communicating process feedback based on the dynamic modulus per unit density to at least one process of a brake manufacturing process of the brake pad to modify at least one manufacturing parameter of the brake manufacturing process.

[00160] In some disclosed examples, the modulus calculating means is further for determining an out-of-plane dynamic Young's modulus of the friction material by multiplying the dynamic modulus per unit density of the friction material by a material density of the friction material and based on a correlation slope factor corresponding to a linear correlation between a dynamic modulus of a second friction material and a Young's dynamic modulus of the second friction material measured using destructive testing of the second friction material.

[00161] Although certain example methods, apparatus and articles of manufacture have been disclosed herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the claims of this patent.