OF SOFT-TO-RIGID ASSEMBLIES AND METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63/255,099, filed on October 13, 2021 , entitled “CASSETTE-LIKE PEELING SYSTEM,” the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND

TECHNICAL FIELD

[0002] Embodiments of the subject matter disclosed herein generally relate a system and method for adhesion testing of soft-to-rigid bonded assemblies, and more particularly, to a spooled-based kit to be added to an existing testing machine for improving the measuring bonding quality of soft-to-rigid bonded assemblies.

DISCUSSION OF THE BACKGROUND

[0003] The “peel test” is a common approach to evaluate the adhesion of f lexible-to-f lexible and flexible-to-rigid bonded assemblies, usually by using a dedicated frame mounted on a tensile universal loading machine [1]. To render a self-similar fracture process zone (the fracture process zone, FPZ, is defined as the region ahead of the traction free crack tip and this region contains plural microcracks; a self-similar FPZ is a FPZ with consistent configuration during the peeling process, the peeling arm 1 10 of a bonded assembly 102 (usually two bonded layers), which is generally loaded in a tray 104 of a loading machine 100, is pulled by a head 106 of the machine at a constant angle 9 (i.e. , the peeling angle) with a force F _p , as illustrated in Figure 1 . The tray 104 is linearly moved by a different force F _m so that the peeling angle is maintained constant during the test. The assembly 102 includes a first layer 102A, which is typically solid, and a second layer 102B, which is also solid. The end part 1 10 of the second layer 102B that is free from the first layer 102A is called the peeling arm.

[0004] Different protocols have been established to guide the practice of peeling tests. Representative protocols are ASTM D3167 for measuring the floatingroller peel resistance of adhesives (ASTM, 2017), ASTM D1781 for measuring the climbing drum peeling resistance (ASTM, 2012), and ASTM D6862 for measuring 90° peel resistance (ASTM, 2021 ). The ideal configuration for the peeling tests assumes that the peeling arm 110 of the second layer 102B has infinite tensile stiffness and zero bending stiffness, which gives a test equivalent to peeling away a material behaving as an “infinitely-rigid string.”

[0005] Generally, the peeling tests produce a saturation of the peel force F _p during a steady crack propagation. Under this condition, Kendall’s formula (Kendall, 1975) can be used to determine the fracture toughness. Classical peeling frames include a driving wire directly linking the traveling head of the arm to the peeling bed such that the peeling angle remains constant. However, this configuration assumes infinite stiffness or limited extension of the delaminated coating. In practice, the classical peeling frames are unsuitable for evaluating the bond between the first layer 102A, which is a solid substrate, and the second layer 102B, when this second layer is a highly stretchable coating that experiences considerable amount of extension when it tears.

[0006] The peel test has found its application in multiple disciplines such as electrochemistry coating, biomimetic adhesives, battery electrodes, 2D materials, etc. However, new industrial applications that use stretchable materials pose challenges to the applicability of the existing peel test standards for evaluating interfacial toughness. Note that in the fields of flexible electronics and biomechanics, the soft adherend film (e.g., elastomeric polymer that constitutes the second layer 102B) is highly stretchable. The conventional configuration for the peel test produces a continuously changing peeling angle 9 during crack propagation owing to the elongation of the flexible peeling arm 110. This may induce an inconsistent crack process that can lead to measurement errors. Various authors performed 90° peel tests to measure the interfacial toughness between a hydrogel and solid substrate. To avoid the abovementioned issues, they introduced a stiff backing (layer 102C in Figure 1 ) for the second layer 102B to prevent the elongation of the hydrogel sheet along the peeling direction. Although such reinforcement can be an effective solution, the change in stiffness of the adherend may alter the delamination behavior of the specimen/assembly 102. The authors in [2] used a peel test with a lap-shear configuration (i.e. , peeling force applied in the direction of the tape) to identify the important role of friction during the peeling process, which demonstrated another limitation of the conventional peeling configurations for dealing with the real response of flexible-to-rigid systems.

[0007] New optical measurement techniques such as scanning electron microscopy can efficiently probe the fracture process at the microscale. Using environment scanning electron microscope (ESEM), some authors performed an in situ micromechanical analysis on the delamination of thermoplastic urethane-copper and PDMS-Copper interfaces to explore the mechanisms of the fibrillar microstructure, which is important to a high fracture toughness at these interfaces. The same group of authors examined the delamination behavior of metal-polymer interfaces in stretchable electronic interconnects through in situ microscale experiments and achieved piezoelectric force sensing. Another group observed the formation of similar fibrils and identified the cohesive zone of bonded self-adhesive polymeric films by using ESEM. The abovementioned studies highlight the potential and viability of in situ optical-mechanical experimental techniques to understand the mechanisms that improve the adhesion performance at the microscale. For the application of these optical instruments to the peel test, a stationary crack front is required for high-resolution measurement of the crack propagation.

[0008] Certain efforts have been made toward improving the peel test methods. For example, the authors in [3] developed a horizontal drum peel system that determines the interfacial toughness using a torque cell to measure the torsional moment. This is suitable for in situ testing of an aircraft production line and does not require extracting coupons for a laboratory test. The authors in [4] evaluated a double drum peel (DDP) concept for testing the delamination behavior of cylindrical laminates, which is related to the peel tests used for adhesives or thin films. The system primarily includes two drums, the motion drum and the specimen drum. A coupled load is applied on the motion drum, which drives the rotation of the other drum, which clamps the specimen, and pulls on the peel arm to propagate the delamination. The specimen is pulled by the peeling arm, which propagates the delamination. To ensure a controlled peeling angle and fracture mode, a reverse coupled load is applied to the specimen drum. However, the DDP system requires sophisticated equipment, and the relatively complex loading system is a bottleneck for its wider engineering application.

[0009] Despite these efforts, an economical and easy-to-use system for testing the adhesion between soft films and rigid substrates is missing. Thus, there is a need for a new system that is capable of measuring the adhesion between a soft film and a rigid substrate that overcomes the above noted limitations of the existing measuring systems.

BRIEF SUMMARY OF THE INVENTION

[0010] According to an embodiment, there is a peeling kit configured to be added to a peeling machine. The peeling kit includes an axle extending along a first horizontal direction X, a test tool supported by the axle and configured to rotate about the axle, wherein a circumferential face of the test spool is flat, and a winding spool supported by the axle and configured to rotate about the axle, wherein the winding spool has a circumferential groove for receiving a cable.

[0011 ] According to another embodiment, there is a peeling machine for testing a peeling of an assembly, the peeling machine including a body, a head configured to grab one end of the assembly and to pull a first flexible layer of the assembly with a constant force along a first direction of a vertical axis Z, and a peeling kit configured to be attached to the body and to hold the assembly coiled around a circumference of a cylindrical test spool. The test spool is configured to freely rotate when the head debonds the first flexible layer from a second solid layer while the second solid layer is fixedly attached to the test spool.

[0012] According to yet another embodiment, there is a method for measuring a peeling parameter of an assembly that includes a first solid layer bonded to a second flexible layer. The method includes fixing the assembly around a cylindrical test spool, connecting an end of the second flexible layer to a movable head of a peeling machine, moving the head to apply a constant tensile force on the end of the second flexible layer, unbonding the second flexibly layer from the first solid layer while the test spool rotates, controlling a balancing force applied on the test spool so that a net torque due to (1 ) the tensile force applied by the head and (2) the balancing force is substantially zero during the step of unbonding, and calculating the fraction energy G _c of the assembly based on the applied tensile force and a peeling angle between the first solid layer and the second flexible layer.

[0013] According to yet another embodiment, there is a peeling kit configured to be added to a peeling machine, and the peeling kit includes an axle extending along a first horizontal direction X, a test spool supported by the axle and configured to rotate about the axle, wherein a circumferential face of the test spool is flat, and a braking mechanism attached to the axle and configured to slow down a rotation of the test spool about the axle. The test spool is configured to hold an assembly formed by a first solid layer and a second flexible layer, and the braking mechanism is configured to generate a zero net torque on the axle when the second flexibly layer is debonded from the first solid layer as the assembly is fixedly attached to the test spool.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0015] Figure 1 is a schematic diagram of a peeling machine that peels a first layer from a second layer of an assembly by linearly moving the assembly;

[0016] Figure 2 is a schematic diagram of a modified peeling machine that peels a first layer from a second layer of an assembly by rotating the assembly;

[0017] Figures 3A to 3E illustrate how a linear peeling machine generates a changing peeling angle when the second layer is stretchable;

[0018] Figure 4 schematically illustrates the forces and torques present in the modified peeling machine;

[0019] Figures 5A to 5C illustrate possible implementations of a peeling kit for the modified peeling machine;

[0020] Figure 6 illustrates the peeling kit with a test spool and a winding spool;

[0021] Figure 7 illustrates the peeling kit attached to a peeling machine;

[0022] Figure 8 illustrates tape test results for a traditional peeling machine and a modified peeling machine;

[0023] Figures 9A and 9B illustrate uniaxial test results for a given assembly that includes an elastomer as the flexible layer; [0024] Figures 10A and 10B illustrate the theoretical peeling behavior for the assembly including the flexible layer, for different thicknesses of the assembly;

[0025] Figures 11 A and 11 B illustrate theoretical predictions of the assembly including the flexible layer for various fracturing energies;

[0026] Figures 12A to 12C are schematics of the variation of the peeling angle due to the misalignment of the crack point for the assembly having the flexible layer when tested in a traditional peeling machine;

[0027] Figure 13A illustrates the peeling force versus displacement and Figure 13B illustrates the peeling angle versus displacement for an assembly having a flexible layer when tested in a traditional peeling machine;

[0028] Figure 14 illustrates the modified peeling machine with the peeling kit that includes a cylindrical test spool;

[0029] Figures 15A and 15B illustrate the force versus the displacement for two different assemblies, each including a flexible layer when tested with the modified peeling machine;

[0030] Figure 16 illustrates the fracture energy versus the peeling angle when the assembly includes the flexible layer and the test is performed with the modified peeling machine;

[0031] Figure 17 illustrates the fracture energy versus delamination speed when the assembly includes the flexible layer and the test is performed with the modified peeling machine; [0032] Figure 18 illustrates the measured fracture energy of the assembly when the substrate layer is steel and the test is performed with the modified peeling machine; and

[0033] Figure 19 is a flow chart of a method for measuring with the peeling kit the fracture energy of an assembly that has a flexible layer.

DETAILED DESCRIPTION OF THE INVENTION

[0034] The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to an existing universal loading machine that is modified with a peeling kit to measure a peeling not only between two solid materials bonded together, but also between a soft material bonded to a solid material. However, the embodiments to be discussed next are not limited to modifying an existing loading machine but may be applied to a new loading machine.

[0035] Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0036] According to an embodiment, a novel kit transforms the translation of a specimen or assembly in a conventional peeling system to rotation via a roller-like spool clamping the specimen. The peeled film is placed under tension to drive the winding of the spool, thus achieving self-similar crack propagation and a stationary peeling front unrelated to the stiffness of the film. These features enable the kit’s compatibility with most universal testing machines and in situ observation of crack tip morphology with optical instruments. Due to its universality and ease-of-use, the proposed peeling kit can be applied to the development of the next generation of peel test standards.

[0037] The peeling kit is configured to achieve a constant peeling angle and a self-similar peeling front during crack propagation. To ensure compatibility with the existing universal testing machines, a vertical tensile load (force F _p ) should be the driving force for the delamination process. Direct visualization of the crack propagation and a stationary crack front are required for in situ optical observation, and thus, the peeling kit should comply with these requirements. Considering the above requirements, a novel roller peeling concept is implemented in a testing kit 200 as shown in Figure 2. More specifically, the testing kit 200 includes a drum or roller or test spool 202 that is configured to rotate relative to a horizontal axle 204, which is attached to a first end of a bracket 206. The bracket 206 extends in a vertical plane ZOY while the axle 204 extends in a horizontal plane XOY. The bracket 206 is attached with a second end, which is opposite to the first end, to a base 208. The base 208 is configured to be attached to the existing testing machine (not shown in this figure). The specimen or assembly 220 to be tested includes in this case a solid first layer 222 and a stretchable second layer 224, and these two layers are bonded to each other with a bonding layer 226, e.g., glue. The first layer 222 is attached to the test spool 202, for example, with a layer 228 of glue. Note that a circumference face 202A of the test spool 202 is flat and smooth and configured to receive the assembly 220. In one application, the assembly 220 is attached to a circular substrate 229 and this substrate is attached to the circumference face 202A of the test spool 202.

[0038] This arrangement illustrated in Figure 2 is advantageous over the exiting testing machines for the following reason. A traditional peeling machine 100 as shown in Figure 1 achieves a synchronized movement of the loading head 106 along the vertical direction and the specimen 102 along a reverse direction of the crack propagation when the peeling arm 110 behaves as a rigid string (i.e., no stretching). For this case, the linear translation of the specimen 102 is equivalent to the vertical distance traveled by the loading head 106 during the peeling process. When the material of the peeling arm 110 is comparable in stiffness to the rigid string, the crack tip 112 is directly below the loading head 106 regardless of the peeling distance PD. However, when the peeling arm 110 is not stiff, i.e., the material forming the peeling arm is stretchable, the crack tip 112 is not aligned with the loading head 106, as illustrated in Figures 3A to 3E. More specifically, Figure 3A shows that the loading head 106 is located above the crack tip 112. However, as the loading heat 106 is moving upwards to apply the load force F _p and as the tray 104 is moving to the right in the figure, the peeling angle 9 increase in size as the peeling arm 110 stretches. Thus, it is noted that by the time the crack tip 112 is visible in Figure 3E, the peeling angle is much higher than the desired value of 60°. The figures show the peeling arm 110 being gripped by the loading head jaw 114 of the head 106 and a length of the peeling arm in this configuration is about 50 mm. In the initial stage shown in Figure 3A, the peeling arm 1 10 experienced elastic elongation without crack propagation. Because of the synchronized movements of the peeling head 106 and the tray 104 holding the specimen 102, the elongation of the peeling arm led to a misalignment between the peeling head 106 and the crack tip 1 12 (see especially Figure 3E). This misalignment generally occurs when peeling any elastic material using the conventional testing machines.

[0039] The peeling kit 200 avoids these problems because of the use of the self-adapting rotation of the test spool 202 to achieve an adaptable crack tip position. In other words, the test spool 202 is not driven by the machine that holds the peeling kit 200, but it is allowed to freely rotate due to the force F _p exerted through the peeling arm 230. The head 232 (only partially shown in Figure 2) of the peeling kit 200 pulls upward the peeling arm 230 with a force F _p , and this force is in turn exerted on the test spool 202 and makes the spool to rotate. For the rotation RO of the test spool 202 to take place, the location of the center C of the head 232 and the axle 204 of the test spool 202 are offset with a distance I along the Y direction, as schematically illustrated in Figure 2. Note that the center C of the head 232 is considered to be the mid-point between the two arms 234A and 234B of the head 232, which grab the peeling arm 230. The test spool 202 has a radius R and the spool is installed with its rotational axis on the fixed axle 204, and the peeling force (load) F _p is applied on the arm of the second layer 224, but not on the first layer 222, of the assembly 220. The first layer 222 is clamped to the spool 220 or to the substrate 229 by the bonding layer 228. [0040] To maintain the self-similar peeling front during the peeling test, the following condition should be satisfied, at any instant of the peeling process: where 8L is the delamination or debonding length caused by the peeling force F _p , 80 is the rotation angle of the test spool 202 due to the debonding 8L, and h is the thickness of the assembly 220. Note that the test spool 202 rotates freely, and its rotation 80 is due exclusively to the debonding 8L and not due to any force applied by the machine or the kit.

[0041] Because a slow and constant peeling rate is expected, the system should be in a quasi-static balance, i.e. , all the forces and torques are balanced and the test spool 202 rotates with a non-zero angular speed and zero angular acceleration. Figure 4 shows the forces and moments (torques) acting on the test spool 202 and the peeling kit. The peeling force is P and is applied by the head 232 of the machine to the peeling arm 230. Because neither the test spool 202 nor any part of the kit 200 moves along a vertical or horizontal direction, the net vertical force should be zero. This means that a reaction force - P is applied by the axle 204 on the test spool 202. To achieve a constant peeling angle 9 that is smaller than 90°, the peeling force P should not be loaded across the center of the test spool 202, but an offset I should be present between the loading line due to the peeling force and the axle of the spool, as shown in Figure 4. Because of this offset, the peeling force P generates a moment M on the spool, as shown in the figure. Therefore, a reverse torque is required to balance the moment M because of the eccentric load P and the moment M is given by:

[0042] With this condition, the peeling arm 230 stays vertical throughout the peeling process as the peeling arm drives the rotation of the test spool 202, and not an external force as in the traditional peeling machine 100. Therefore, the peeling angle 9 remains constant as long as the moment M is set to a value that is not too high. This value depends on the material of the second layer 224 and its properties. [0043] The reverse moment M should be applied to the test spool 202 to ensure the vertical alignment of the peeled arm 230. There are various ways to apply this reverse moment to the test spool 202. Figure 5A illustrates a first embodiment that achieves the reverse torque by applying a constant tensile load T on the edge of a coaxial winding spool 240. The constant tensile load T may be obtained by suspending a given mass 244 having a selected weight Wfrom a cable 242 that is wound around a groove 246 formed on the circumference of the coaxial winding spool 240. The weight Wof the mass 244 may be selected during an initial phase of the peeling test, for example, by trial and error. During this first phase, that may consume up to 20 mm of the specimen 202, various weights may be tried until the test spool 202 rotates with zero angular acceleration. During a second phase, the net torque is zero as the selected weight l/V creates the opposite moment Mio balance the moment created Mby the peeling force P. [0044] In a second embodiment, which is illustrated in Figure 5B, the negative torque is directly applied to the axle 204 by a dedicated device 250, for example, a magnetic brake. A magnetic brake is connected to a power source 252 that generates an electrical current for generating the braking magnetic field. Those skilled in the art would understand that any device that is capable to generate a reverse moment M may be used to counterbalance the original moment M produced by the head 232. In a third embodiment, which is illustrated in Figure 5C, the negative torque is applied by a force cell 260, which is attached to a cable 262 that connects the coaxial winding spool 240 to an auxiliary spool 264. The force cell 260 may be adapted to apply a desired force.

[0045] The first embodiment is more economical as no power source is used. However, the second embodiment is more straightforward because the torque generated by the magnetic brake 250 can be adjusted by changing a setting (e.g., the supplied electrical current to the brake mechanism). Note that the test spool 200 in both of the embodiments shown in Figures 5A and 5B is illustrated without the corresponding bracket that holds it in the desired position while the test spool 200 is shown with the holding bracket 206 in the embodiment of Figure 5C.

[0046] The proposed spool-based peeling kit requires an appropriate design that facilitates compatibility with the universal testing machines 100. Figure 6 illustrates the full kit 200 configured to fit into the universal testing machines 100. The kit 200 includes the base 208, which can be fixedly attached to a body 310 of an existing testing machine 300 (see Figure 7). Note that the sliding tray 104 of the testing machine 300 has been removed in this embodiment and it has been replaced by the kit 200. A positioning device 210 is slidable attached to the base 208 so that it can slide along the horizontal axis Y, as shown in Figures 6 and 7. A measuring tape 212 may be attached to the base 208, along the Y axis, so that the horizontal axle 204 may be placed at the desired length I relative to the midpoint of the head 232. The positioning device 210 may be a mechanical device, for which the operator of the machine needs to manually move it relative to the base 208, or may be an actuated device, where an electrical current is provided to a motor for moving the device 210 relative to the base 208. In one application, the positioning device 210 includes a rail 213 fixedly attached to the base 208 and a sliding part 211 that slides along the rail 213. The bracket 206 is in this case fixedly attached to the sliding part 211. In general, the bracket 206 is fixedly attached to the positioning device 210. [0047] The machine 300 also includes the head 232. The end of the peeling arm 230 is grabbed between the two arms 234A and 234B of the head 232 as these two arms can move along opposite directions when a knob 236 is rotated. The head 232 is configured to move along the vertical direction Z, with the desired force F _p , that is setup by the operator of the machine 300. Figure 7 shows a processor-based interface 320 that has plural buttons that control the movement of the head up or down. An optional controller (processor) 330 may be attached to the machine 300, to control the head 232 and the positioning device 210. For example, the controller 330 may be configured to select a position of the platform 211 along the rail 213 so that a distance between (1) a longitudinal axis of the axle 204 and (2) a tangent to the test spool 202, which is perpendicular to the first and second horizontal lines X and Y, is larger than zero and smaller than the radius R of the test spool 202. The controller 330 may also include a screen and an input/output interface for allowing the operator to visualize various parameters and also to modify these parameters if so desired. [0048] Figure 6 also shows that a second bracket 206’ is attached to the positioning device 210 and together with the first bracket 206, they support the horizontal axle 204. To reduce the rotation friction during the peeling test, each bracket has a ball bearing 207, 207’ that holds the axle 204. The test spool 202 in this embodiment was made of light Al alloy to reduce the effect of inertia on the test. [0049] The peeling angle 9 for the kit 200 can be adjusted by changing the length / of the relative distance between the axle 204 and the midpoint of the head 232 along the Y direction as the length / is given by:

[0050] As long as a suitable reverse torque is applied to ensure the vertical alignment of the peeling arm, the peeling angle 9 only depends on the length I. A positioning line laser 340 may be installed to assist with assessment of the vertical alignment of the peeling arm. The radius of the spool can be customized as per the sample size. In other words, the test spool 202 may be removed and changed with a larger or smaller spool. This configuration is compact and economical.

[0051] An energy conservation analysis of the peeling test kit 200 in particular, and the testing machine 300 in general, has been performed as now discussed. A gradual increase of the applied peeling force during the crack initiation phase is typically observed in typical responses of peeling tests. Afterward, the system establishes a steady self-similar peeling process, and the peeling force exhibits a plateau. According to Griffith theory, the dissipation related to the creation of new cracked surfaces can be characterized by a material parameter G _c , defined as the fracture energy or the work of adhesion (WOA) in adhesion systems, so that in the case that the film has zero bending stiffness and infinite tensile stiffness, the equation of the fracture energy can be expressed as: in which P is the plateau peeling force, b is the width of the peeling arm 230, and Q is the peeling angle. This is the classical equation to derive WOA in peeling tests. In reality, other sources of energy dissipation are generally lumped in the above equation due to the unrealistic assumptions about the properties of the peeling arms. The most significant contribution might be the dissipation due to the bending deformation of the peeling film when the material of the peeling arm is elasto-plastic, such as metal. Considering all the possible energy dissipating sources, the intrinsic fracture energy when peeling a film can be expressed based on the Griffith theory by the following relationship: in which II is the total potential of the peeling system, a is the crack length, U _ex t is the external work (WOS), U _e is the stored tensile strain energy, Un and U are the dissipation due to film tension and film bending, respectively, and Umss is the energy loss because of friction, inertia, etc. Equation (5) applies to any peeling system. Considering the force balance of the system 300, the differential work of the external load can be expressed by: in which p is the rotation angle of the test spool 202. Knowing that the momentum M is given by equation (6) can be rewritten as

[0052] The differential of the stored tensile elastic energy and the dissipation due to tension can be calculated by the following equation: where Fis the deformation gradient and P is the first Piola-Kirchhoff stress tensor.

Under uniaxial tension, the term J _o P. dF can be calculated by the expression

J a _e ■ dA where a _e is the nominal stress in the tensile direction. This term represents the strain energy density with respect to the reference configuration if the material is not damaged (i.e. , no tensile energy dissipation). By substituting equations (8) and (9) into equation (5), the G _c of the modified peeling system 300 is given by [0053] Kinloch et al. (Kinloch, A.J., Lau, C.C., Williams, J.G., 1994. The peeling of flexible laminates. Inf. J. Fract. 66 (1 ), 45-70) theoretically identified the energy dissipation caused by elasto-plastic deformation from local bending of the film near the crack tip in peeling tests. Li and Lubineau (International Journal of Solids and Structures, 2022 - Elsevier) used Sobol’s approach to examine the effect of the adhesion properties on the energy dissipation mechanisms when peeling an elasto-plastic film, and they demonstrated that the energy dissipation associated with film bending was significant. If h is negligible compared to R, it is possible to derive Gc by ignoring the energy loss and assuming a zero-bending stiffness of the film, which is generally true for flexible films, which results in:

[0054] Equation (11 ) is identical to the Kendall’s classical peeling equation if the term (A - 1) - ■ dA, i.e., the complementary strain energy per area of the peeled film, is negligible compared to G _c . For a soft film with high stretchability, such as the elastomeric polymeric film 224, the large deformation experienced by the peeling arm indicates that the complementary energy density is significant and cannot be neglected. In this case, using the classical analysis to derive G _c may lead to an underestimation of the intrinsic fracture energy. Thus, to accurately determine the Gc of the soft-to-rigid bonded assemblies 220, the energy consumption caused by the tensile deformation of the soft film 224 during a peel test needs to be considered. [0055] To validate the developed spool-based peeling kit 200, peeling tests were performed using both the system 300 with the peeling kit 200 and the conventional peeling machine 100. All the tests were performed on the universal tester ZwickRoell TN0.5. The peeling force and the displacement of the peeling head were recorded throughout the loading process.

[0056] Before using the machine 300 for configurations that cannot be tested with the classical machine 100, a benchmark was established with an almost inextensible Kapton® tape with a width of 20 mm. The test kit TH50+SW1 (Grip Engineering, Germany) was adopted to test the conventional peeling method. Since the almost inextensible tape experiences small tensile strain during peeling, similar responses are expected from both systems in this context. A small amount of pretension was maintained in the peeling arm to ensure its straightness without jeopardizing the measured peeling response. Figure 8 shows the responses to the 90° and 60° peeling tests for the traditional machine 100 and the modified machine 300. Good agreements were obtained between the corresponding peeling curves. [0057] Thus, the spool peeling machine 300 was validated for measuring the Gc of flexible-to-rigid assemblies where the peeled adherend is subjected to a small strain. It should be noted that, for the response of the 60° peel test by the spool peeling system, the first plateau was followed by a climbing force, which corresponds to the practice of reverse torque adjustment for achieving the target angle of peeling. For both testing scenarios, the spool peeling machine produces a more stable response. Using the classical derivation of fracture energy (i.e. , equation (4)), the G _c was slightly greater in the 90° peel test than in the 60° peel test. This is consistent with the results known in the art. The tape tests results confirmed that the peeling machine 300 having the peeling kit 200 can handle the scenarios where the film 224 experiences small deformations.

[0058] Next, the peeling of a highly stretchable film 224 from a rigid substrate 222 has been tested to demonstrate the advantage of the peeling machine 300 when measuring the fracture energy G _c . A theoretical response is discussed first followed by a discussion of the results obtained with the traditional peeling machine 100 and the results obtained with the modified peeling machine 300.

[0059] Polystyrene-block-polyisoprene-block-polystyrene (SIS) is a commercially available triblock copolymer that has been applied to mechanical sensors, electronic products, soft robotics and medical devices. This material was used for the following tests to fabricate the soft films 224 to be peeled. To accurately determine the Gc of soft-to-rigid assemblies 220, the contribution of the complementary energy density from the tensile deformation of the flexible film 224 required to be quantified. Therefore, before the peeling tests, a uniaxial test was performed on the SIS dog-bone specimens with a thickness of 2 mm, which were cut from a larger sheet using a die. The dog-bone specimens, in addition to the films used for the peeling tests, were unconditioned, implying the involvement of tensile dissipation due to the Mullins’ effect. The nominal stress a _e in uniaxial tests was used to present the experimental data because it is the most easily accessible stress metric in peeling tests. Figure 9A shows the nominal stress oe versus the stretch A response of the uniaxial tests. During the unloading process, the observed stress softening and residual strain from the virgin state could be attributed to the combined effect of damage and time-dependent characteristics. For the uniaxial tension, the stretch A can be written as a function of the nominal stress o _e in the loading direction, i.e., A = f(o _e ). Therefore, it is possible to express the complementary energy density as a function of the nominal stress a _e , as shown in Figure 9B.

[0060] Making use of the complementary energy data, Figure 10A plots the theoretical value of the peeling force P corresponding to the stable crack propagation as a function of the peeling angle for different film thicknesses h, resulting from equation (1 1 ). Generally, the fracture toughness has a dependence on the fracture mode. Here, to simplify the discussion, it was assumed a bonding system whose fracture toughness Gc is independent of the fracture mode. Thicker films require greater peeling forces, which indicates that peeling is easier for thinner films in terms of the force magnitude. To peel the same delamination length, the displacement of the peeling head should be greater for a system with a thinner film than with a thicker film. Larger peeling angles require smaller loads and the difference in required loads decreases with increasing peeling angle.

[0061 ] Figure 10B shows the plot of (1 ) the ratio between Gc and the total external work, denoted by y versus (2) the peeling angle. Accordingly, the term (1 - y) quantifies the energy consumption caused by the large deformation of the soft films. To summarize, the fraction of the external work contributing to the formation of crack surfaces increases with the peeling angle 9. The effect of h on y varies as the peeling angle increases. A positive correlation between h and y can be observed in the range of 9 e [0°, 30°]. However, a negative correlation can be identified at larger peeling angles 9. This variation strongly depends on the concavity and convexity of the film material’s constitutive responses. It is known in the art the effect of y on the relative importance of the membrane strain energy stored in the peeling arm for determining the WOA. The theoretical results presented here support those conclusions.

[0062] Figure 11 A plots the theoretical value of P as a function of the peeling angle 9 for a constant h and a varying G _c (from 0.1 to 0.9 N/mm). While still maintaining the assumption that G _c is independent of the mode, P shows a positive correlation with the fracture toughness, and P decreases with an increasing of the peeling angle 9. This agrees with the results shown in Figure 10A. The discrepancy between solutions with different G _c shows no significant change with an increase in the peeling angle 9. Figure 11 B plots the same energy ratio as in Figure 10B for various Gc values. Similarly, different features are observed for the effect of Gc on y with the change of the peeling angle 9. For smaller values of 9, the effect is not significant, although a positive correlation can be identified. As the peeling angle 9 increases, the correlation becomes negative. For larger values of 9, the external work makes a smaller contribution to the formation of new fracture surfaces because the deformation may be greater for a system with a larger Gc. The significant effect of Gc manifests with the larger values of y as the peeling angle 9 approaches 90°. For the system with the weakest interface, i.e., G _c =0.1 N/mm, y can be as high as 0.91 , which indicates that 9% of the external work is stored/dissipated because of the tensile deformation of the film. This value is still significant, which demonstrates the necessity of considering the strain energy when deriving G _c of such bonding assemblies.

[0063] Results obtained with the peeling machine 100 are now discussed. For these tests, a SIS film was used. Before loading, the specimen was tilted to obtain the required peeling angle. The peeling arm was vertically gripped by the loading head jaw. The length of the free arm was 50 mm. Figures 3A to 3E show the peeling process starting from an elastic elongation of the peeling arm 110 to the final detachment of the thin film. In the initial stage (Figures 3A and 3B), the peeling arm 110 experienced elastic elongation without crack propagation. Because of the synchronized movements of the peeling head 106 and the specimen 102 due to the tray 104, the elongation of the peeling arm 110 led to a misalignment between the peeling head 106 and the crack tip 112 as best visible in Figures 3D and 3E. This misalignment generally occurs when peeling any materials using the conventional approach. Figures 12A to 12C schematically illustrate this process, where points O and A correspond to the crack tip and loading head, respectively. If the tensile stiffness of the peeling arm 110 is comparable to that of a rigid string linking the peeling head and the specimen 102, this misalignment is negligible. However, if the peeling arm 110 has a high elasticity as in this case, the vertical misalignment is considerable and gradually accumulates; therefore, the real-time peeling angle 9' accordingly increases as shown in Figure 12C.

[0064] Assuming that the specimen is responding quasi-statically throughout the peeling process shown in Figures 12A to 12C, the following equations describe the system’s status prior to the crack propagation,

[0065] To describe the crack propagation, the following equations may be used: where L _o is the initial length of the free arm, d is the displacement of the loading head, AL is the crack length, and 9 ₀ and 9' are the peeling angles before and during the loading process, respectively. Using the specifications of the SIS elastomer, it is possible to solve the above equations to obtain the response of peeling a SIS film with high stretchability, when the conventional test frame is adopted.

[0066] As indicated by the analytical force response shown in Figure 13A, during the loading stage, prior to crack propagation, the tensile force P increases with the displacement. This process corresponds to pulling the free film 102B uniaxially. Once the crack starts to propagate, the tensile force P starts decreasing with increasing AL. The reason is that the misalignment between the string for driving the specimen’s translation and the soft peeling arm 110 is significant, which induces a misalignment between the crack tip 112 and the loading point (which corresponds to the head of the peeling machine). Figure 13B shows that the peeling angle keeps changing throughout the peeling process, implying that the exact loading conditions seen by the tip of the crack are varying. The real-time peeling angle has changed in this figure from its initial value, i.e., 60°, to around 72.5° at the instant of the crack initiation. During the crack propagation, the change of the peeling angle can still be observed with a relatively slow rate, approaching 90°. Similar results with other values for the peeling angle 9 were observed for the peeling machine 100. These analytical predictions are consistent with the experimental observations shown in Figures 12A to 12C.

[0067] Next, the results of the tests performed with the modified peeling machine 300 are discussed. The tests involved peeling the SIS elastomer bonded to metal substrates for different values of h and 9. The sample preparation process involved directly depositing the dissolved SIS 224 onto a metal sheet 222. After curing, this flexible laminate 220 was bonded onto a plastic rim 229 using strong adhesives 228. The adhesion should be good enough to prevent any failure between the metal sheet and plastic rim. Ideally, the van der Vaals force and chemical bonds between SIS and the metal should be the same for all the samples. Because the SIS was directly deposited onto the metal substrate, fracture occurred at the SIS-metal interface in all the tests, and the peeled films behaved exactly as per the structural data obtained from the uniaxial test shown in Figures 9A and 9B.

[0068] Before loading, the test spool 202 was adjusted along the sliding rail

213 to a position predetermined according to equation (3) and then it was fixed at that position. A positioning laser line 342 generated by a laser 340 (not shown in this figure but illustrated in Figure 7) was adopted to assist with alignment, as shown in Figure 14. To establish a steady crack propagation, the reverse torque was adjusted by changing the counterweight 244 to ensure that the peeling arm 230 was perpendicular to the base platform 208. Once the peeling film was vertical as per the positioning laser line 342, the expected peeling angle was achieved.

[0069] Two batches of SIS/Stainless steel bonding samples 220 were fabricated. The SIS films in batches 1 and 2 had an average thicknesses h of 575 pm and 635 pm, respectively. At least five coupons were tested for all the following testing scenarios. In all peeling tests, after stable crack propagation was established, the peeling angle 9 remained at the expected value. This indicates a consistent crack tip 112 and self-similar crack propagation. Figures 15A and 15B show the representative peeling force versus displacement responses for batches 1 and 2, respectively. The delamination speed for all the tests was controlled to be ~ 4 mm/min to eliminate the effect of the delamination speed on the measured G _c . The climbing regime of the curves indicates the establishment of a steady crack propagation. The random jumps and oscillations of the force responses correspond to the adjustment of the reverse torque to maintain the alignment of the peeled film. After this initial adjustment (described earlier as the first phase of the test), the peeling machine 300 produced a steady force-displacement response. A stable peeling force was observed for all the tests. The tensile force P decreased with an increase of the peeling angle 9 and exhibited a positive correlation with h at all values of the peeling angle 9. The consistent peeling angle 9 and steady response with a stable tensile force P are unlike the response with the conventional peeling machine 100 shown in Figures 13A and 13B. Because Gc depends purely on the material and adhesion, it should be obtainable by substituting the average tensile force P during propagation into equation (11).

[0070] In this regard, Figure 16 shows the derived G _c with error bar for each testing scenario. The measured G _c decreased linearly with the increase in the peeling angle 9. This contradicts the assumption of a constant G _c that was used to derive the theoretical predictions presented in Figures 10A to 11 B. The dependency on the peeling angle 9 also contradicts the observed results for peeling the Kapton® tape (see Figure 8) and the literature. Thus, the inventors believe to be the first to discover the dependency of Gc on the peeling angle 9 for the elastomer-metal interface. The inventors expected that all samples had the same Van der Waals force and chemical bonding properties. Therefore, this difference must be from the energy dissipation from the SIS film, which may be attributed to the failure of polymer fibrils. The scale of fibrillation within the crack propagation zone may vary with the peeling angle 9 (essentially the change of fracture mode). As shown in Figure 16, the measured G _c was greater when a thicker film was being peeled than when a thinner film was being peeled. This observation can possibly also be attributed to different scales of polymer fibrillation formed in the two batches of samples.

[0071] The inventors also measured the Gc of the SIS-Stainless steel interface at different delamination speeds to investigate the rate dependency. Based on the measured nominal stress, the stretch ratios of the peeled film were all less than 1 .5, which indicates relatively little stretching. Thus, the inventors ignored the rate dependence of the constitutive model of SIS when deriving the interface fracture energy. In other words, it is possible to use the constitutive response for SIS as shown in Figures 9A and 9B to derive G _c for all the peeling tests. As shown by the fitted curve in Figure 17, the G _c scaled exponentially with the delamination speed. [0072] To further confirm the reliability of the measurements, the inventors performed peeling tests on a bonding assembly whose substrate is made from metals other than stainless steel. Here, SIS-Steel bonding samples were tested. The results are summarized in Figure 18. Overall, the G _c was greater for the SIS-Steel interface than for the SIS-Stainless steel interface. The possible reason is multifold, can be the difference of either the surface characteristics or the chemical properties of substrate materials, e.g., the surface energy of stainless steel is relatively higher than steel, which may result in a higher molecular attraction to polymer. The effects of h and 9 on the G _c measurements of SIS-steel samples agreed well with the observations for the SIS-stainless steel samples.

[0073] The above results demonstrate the advantage of the spool-based peeling machine 300 over the conventional peeling machine 100 when peeling a highly stretchable film. Note that there is no contact between the peeled film and the substrate with the modified peeling machine 300, which eliminates the effect of friction on the debonding behaviors in the case of 0° peeling.

[0074] The large deformation of the peeling film poses challenges to the conventional peeling machines when the film adherend has a high stretchability. The above embodiments clarified the limitation of the conventional peeling machines, by experiment and theoretical analysis, accounting for the complex deformation behavior of the peeling film. The results obtained for the conventional peeling machines 100 showed a continuous change in the peeling angle during the peeling process, which was associated with a decrease in the peeling force. By virtue of the transformation from translation to rotation, the modified peeling machine 300 overcomes the above noted problems. The delamination for the peeling machine 300 is driven by a tensile load, which makes the peeling kit compatible with most universal testers. In comparison to the conventional method, the simple yet versatile spoon-based peeling machine has one or more of the following advantages: [0075] The peeling angle/crack tip configuration is independent on the tensile stiffness of the peeling arm; it is generally applicable for peeling any flexible films, and it is especially suitable for measuring the adhesion of systems with ultra-elastic adherends.

[0076] The stationary crack front during peeling favors in situ visual observation of the crack with high-resolution optical instruments.

[0077] The configuration of the peeling kit 200 is naturally applicable to evaluating the delamination resistance of curved structures such as composite pipelines and curved panels.

[0078] A method for measuring a peeling parameter of an assembly 220 that includes a first solid layer 222 bonded to a second flexible layer 224 is now discussed with regard to Figure 19. The method includes a step 1900 of fixing the assembly around a cylindrical test spool, a step 1902 of connecting an end of the second flexible layer to a movable head of a peeling machine, a step 1904 of moving the head to apply a constant tensile force on the end of the second flexible layer, a step 1906 of unbonding the second flexibly layer from the first solid layer while the test spool rotates, a step 1908 of controlling a balancing force applied on the test spool so that a net torque due to (1) the moving of the head and (2) the balancing force is substantially zero during the step of unbonding, and a step 1910 of calculating the fraction energy G _c of the assembly based on the applied tensile force and a peeling angle between the first solid layer and the second flexible layer. In one application, the first solid layer is steel, and the second flexible layer is an elastomer. [0079] The disclosed embodiments provide a peeling kit that can be added to an existing peeling machine to change a translation of the specimen to a rotation to stabilize the peeling angle. It should be understood that this description is not intended to limit the invention. On the contrary, the embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

[0080] Although the features and elements of the present embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. [0081] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

References

The entire content of all the publications listed herein is incorporated by reference in this patent application.

[1 ] Kinloch, A., Williams, J., 2002. Chapter 8 - the mechanics of peel tests. In: Dillard, D., Pocius, A., Chaudhury, M. (Eds.), Adhesion Science and Engineering. Elsevier Science B.V., Amsterdam, pp. 273-301. http://dx.doi.org/10.1016/B978-0-444-51140- 9.50035-4.

[2] Ponce, S., Bico, J., & Roman, B. (2015). Effect of friction on the peeling test at zerodegrees. Soft matter, 11 , 9281-9290.

[3] Canas, J., Tavara, L., Blazquez, A., Estefani, A., and Santacruz, G. (2018). A new in situ peeling test for the characterisation of composite bonded joints. Composites Part A: Applied Science and Manufacturing, 113, 298-310.

[4] Daghia, F., Cluzel, C., Hebrard, L., Churlaud, F., & Courtemanche, B. (2018). The double drum peel (ddp) test: a new concept to evaluate the delamination fracture toughness of cylindrical laminates. Composites Part A: Applied Science and Manufacturing, 113 , 83-94.