Description

T itle of the Invention: System and method of detecting changes in structural health of a composite panel

¾ T echnical F ield:

It is a known fact that composite panels are used in in al l walks of life including the manufacture of anti- ballistic gear which is used by defense systems to protect human beings, vehicles and objects from ballistic damage. It is conventional practice for an organization supplying anti -ballistic gear to provide long term assurances of performance to the agencies

3a procuring such composite panels. The time period for this performance assurance varies from two years to ten years. The performance assurance provided is simply based on the sample testing and ballistic testing methods to test if the manufacturing process parameters such as pressure, temperature, material tolerances, humidity, room temperature have been adhered to. Moreover, manufacturing process parameters such as pressure and temperature can be

¾ measured only on the surface of the composite panel and not at the interfaces. The interfaces of a composite panel constitute the intra layers of the composite structure forming the composite panel. Pursuant to such testing most manufactures have still not been able to identify a material change to composite panels manufactured. It is part of existing practice to f ol I ow the same process steps and parameters i n manufacturi ng the panels and assume that al I , interfaces of the panel are subjected to the same parameters and therefore exhibit the same characteristics. T hese assumptions are not always true for panels which are made out of different types of materials li ke ceramic and Aramid. Moreover, hard materials like ceramic have manufacturing tolerances and surface irregularities which hinder proper i nterface bondi ng.

i¾ Composite panels are subjected during their lifetime to external forces such as varying environmental conditions. Additionally, they may also be subject to external forces such as rough use by the user of the composite panel. This holds especially true for composite panels used by the defense systems as these composite panels could be subjected to external forces such as varying environmental conditions and rough-use conditions arising out of the

†3a professional hazards involved. As a result it is likely that the material properties which contribute to the structural health of these panels can change or deteriorate. This can happen as a result of thei r exposure to changi ng external forces exerted upon them when they are use. During the life time of these composite panels, they are exposed to extreme environmental conditions and other dynamic and static forces such as:

a) Exposure to temperature between the range of -40eC to +90eC

b) Swift change i n humidity from 0 to 100%

¾ c) Immersi on i n sea water and river water

d) E xposure to di rect sun I ight or i nf rared or ultra vi ol et rays

e) H ighest order of vi brati ons duri ng transportati on

f) Swift changes i n altitude pressure

g) Accidental exposure of the composite panel to hard surfaces..

3a h) E I ectri c and magneti c f i el ds

All of the aforementioned environmental effects cause a change to the material properties which contributes to the structural health of composite panels thereby causing a deterioration in its performance.

¾ Additionally, by their very nature, such composite panels are subject to distortion due to bal listic damage and other forms of impact based damage. These forms of damage cause cracking, del ami nation and to a very large extent causes a change in the physical and material characteristics of the composite panel. Once a composite panel becomes damaged, it results in a drastic decrease to its impact resistance properties and anti- ballistic capabilities thereby , affecti ng its performance.

The i nventor has developed a system and method to monitor the changes in material properties and at the interfaces by monitoring changes in intra-layer properties such as the breakages in ceramics or other hard layers and the interlayer bonding properties (del ami nation of layers because of environmental and usage and storage depended i¾ conditions) of composites thereby determining the changes in the structural health of the composites.

The method involves the transmission of wave packets from one end of the composite panel which are subsequently received at the other end of the composite panel at selected frequencies by way of usi ng sensors which are embedded into the composite panel. A wave †3a packet is a short burst of a localized wave action that travels as a unit. Based on the structural health condition of the medium, the following wave characteristics of these wave packets are thereafter measured:

a. Travel time b. Amplitude

c. Phase

d. Shape

A part of each wave packet transmitted gets attenuated. The extent of attenuation of each ¾ wave packet depends on the material properties of the medium such as the strength of the composite panel and its intra- layer and i nter layer bonding. These wave packets take time to reach the receiver and their travel time depends on the material properties such as interlayer bonding (de- 1 ami nation), distortion of individual or multiple panels (distortion) or intra layer strength (breakages) of the composite panels which contribute to the structural health of the

3a medium The wave packets undergo reflections at the boundaries of the composite panel before reaching the receiver because of which the travel time may increase thereby also resulting in a change in the amplitude of the wave packets. Each wave packet may also change its phase and shape depending on the material properties of composite panel and the relative position of the sensors. The final wave packet reaching the receivers is therefore the

¾ sum of each i ndi vi dual wave packet reachi ng the receivers at the same ti me.

Alternatively, one can use two or more receivers at the same time. The use of additional receivers al I ows one to determi ne whether there i s any difference i n the materi al properti es of the path through which these waves travel. If there is any difference in the material properties , of the path through which these waves travel, then there would be a difference in the wave characteristics of each of these waves received by the sensor. T his would in-turn help in i dentifyi ng that there i s change i n the structural health of the composite panel .

The aforementioned characteristics of the wave packet are influenced by the medium that it i¾ travels through. These material characteristics are reflected in the properties of the received signal. Using several receiver sensors at the same time it is possible to map any changes in the structural health of the composite panel.

The following are the steps involved in determining the appropriate wave packet to be sent †3a through the composite panel in order to detect any change in the structural health of a composite panel.

Step one: Determination of pass frequency of the medium: The method involves the propagation of continuous periodic waves through a composite panel. The composite panel acts as a band pass filter as it allows propagation of waves which are part of the continuous periodic waves of certain frequencies and attenuates or suppresses all other waves which do not belong to that frequency range. This frequency at which the ¾ waves propagate with less difficulty from transmitter to the receiver is called the pass frequency (Pass Frequency).

This Pass Frequency depends on the material properties of the composite panel and the position of the sensors. The Pass Frequency changes if the material properties of the composite panel change. Thus, any change in the Pass Frequency indicates a change i n the

3a material properties as the path through which the waves have travelled has been materially altered. The strength of the wave propagated depends on the relative positions of the transmitter and the receiver. As a number of sensors can be embedded into the composite panel as indicated above under the heading ^" field of the i nvention _, , the signals received by each sensor differ in frequency, amplitude and phase due to the relative position of the sensor

¾ used to transmit the signal and the sensor used to receive the signal in the composite panel.

The Pass Frequency is thereafter determined by plotting the amplitude and phase of these signals received by the sensor receiving the signal as a function of input signal frequency. The result obtained consists of waves of different ampl itude and the wave frequency of the wave with the highest amplitude is selected in order to determine the Pass Frequency. The , Pass Frequency is thereafter selected for further analysis.

Step two: Use of wave packets at selected frequencies for characterization of the composite panel:

Once the Pass Frequency has been determined, wave packets of that particular Pass i¾ Frequency are transmitted into the composite panel. The wave characteristics such as amplitude, shape, phase and travel time of the continuous periodic wave and wave packets of that Pass Frequency are measured in order to find any change in the structural health of the composite panels. Transmitting a wave packet through the transmitter and measuring the time taken for it to reach a receiver, one can calculate the velocity of the wave packet travelled †3a through the medium The speed, amplitude or attenuation of this wave packet is a measure of the quality or state of the medium Similarly, the shape or distortion in the received wave packet is a measure of the number of reflections it has undergone and medium it has travelled. The wave packet samples the properties of the medi um through which it is travelling and delivers the wave characteristics of final wave packet to the receiver in the form of amplitude, shape and phase. These properties of received wave packets are measured at different points in time in the service life of the composite panel.

Any changes in the interlayer bonding (de- 1 ami nation), distortion of individual or multiple ¾ panels (distortion) or intra layer strength (breakages) of composite panels directly affects the strength of the panels. Such delamination, distortion or breakages influence the wave characteristics of such waves passing through the composite panel. The changes in the characteristics of the composite panels resulting from such delamination, distortion or breakages also affect the propagation of continuous periodic waves or wave packets of a

3a particular Pass Frequency. The changes in intra-layer structural integrity, interlayer spacing or interlayer bonding or breakages or micro cracks in the panel also changes its wave characteristics. The developed method makes use of the wave characteristics of wave packets across the panel to enable the detecting of such changes over time. These changes can be measured and recorded as and when required and compared with the original data recorded

¾ immediately after manufacturing and before supplying the composite panel to the customer.

When continuous periodic waves are propagated through a composite panel, the resonance and the wave characteristics of these waves being propagated are measured by using sensors embedded within the panel or placed on the surface of the panel and suitable measurement , equi pment. Similarly, the travel time is also measured for wave packets using sensors placed at different locations. The travel time of these wave packets varies as each signal reaches different sensors at a different time. These wave packets are affected by the material properties of the medium through which they are travelling, thus providing information on the strength and properties of the composite panel as they travel. As a result, the amplitude, i¾ phase and shape of these wave packets vary based on the current condition of the composite panel. The results obtained after measuring such wave characteristics and the travel time of these wave packets are recorded before the composite panels are supplied to the customer. All these properties of the wave packet are recorded as ^" Signature Properties _, of that particular composite panel. Signature Properties are therefore defined as those inherent

†3a properties of the wave packet travelling through the composite panel recorded subsequent to the manufacture of the composite panel. Such Signature Properties are an indication of material properties such as the interlayer and the intra-layer bonding or lamination strength of the composite panel. Any change to the Signature Properties would be an indication of distortion or breakage of one or more layers of the composite panel.

The present invention is aimed at detecting any changes in the structural health of these composite panels as a result of various external conditions. Initial baseline tests are conducted ¾ in order to create a standard result for a composite panel, and the wave characteristics of the composite panel at the time of manufacture are recorded by the manufacturer. The composite panel is then tested by the manufacturer after a certain period of time and the wave characteristics of the composite panel are recorded again and compared with the standard results. These subsequently Recorded Properties are referred to herein as ^" Recorded

3a Properties _, . More specifically, Recorded Properties are therefore defined as those properties of the wave packet travelling through the composite panel recorded at any time after the supply of the composite panel by the manufacturer to any person. Any significant difference in the recorded results from the standard results would imply that the composite panels have undergone a structural or material change such as de- 1 ami nation, distortion or breakage or the

¾ material at the molecular level and that they may not be accordingly fit for use.

Background and Problems with the Prior A rt:

The followi ng are the conventional methods of detecting or analyzing the health condition of a composite material:

, 1. E mbeddi ng f i ber opti c sensors for strai n measurement

2. Microelectromechanical system (M E MS) accel erometers for vibration measurement

3. Active ultrasonics

4. Passi ve acousti c emi ssi on moni tori ng, and

5. E I ectromechani cal i mpedance measurements

i¾ F urthermore, the use of ultrasoni c gui ded waves for nondestructive eval uati on of structures i s rapidly expanding due to increased understanding of the underlying wave mechanics and improvements in sensors and signal processing. These methods have been developed mainly with regard to identifying any structural damage to a single layer in the composite. However, none of the above methods provides for an inbuilt system to monitor and analyze the

†3a structural health of the composite materi al as a whol e.

Prior Art

T he f ol I owi ng i nventi ons have been i denti f i ed as possi bl e pri or art to the present i nventi on: a. US Patent Number 6370964 B1 discloses a diagnostic layer and methods for detecting the structural integrity of composite and metallic materials.

The invention relates to a layer with an embedded network of distributed sensors and ¾ actuators that can be surface mounted or embedded in a composite structure for monitoring its structural condition and for detecting anomalies in the hosting metal lic or composite structures.

T he system comprises the fol lowi ng:

1. A diagnostic layer which consists of thin dielectric substrate, a plurality of sensors 3a spatial ly distri buted on the substrate

2. A pi ural ity of conductive el ements i n the substrate for el ectri cal ly connecti ng the sensors to an output lead.

3. The layer can also include an actuator or a plurality of actuators distributed on the substrate.

¾ 4. The system also consists of a signal receiver unit electrically coupled to the output lead for receiving output signals from the sensors. This coupling may be by wireless means

5. The system also consists of signal generating unit electrically connected to the output lead for providing an input signal to the actuators.

Working:

, 1. The first input signal is transmitted to the actuator and a first set of output signal is received

2. After a certain period of time, a second input signal is transmitted to the actuator and a second set of output signals are received.

3. The first set and the second set of output signals are compared to determine a difference ¾ between the two.

4. This difference represents the change in structural condition of the material and may indicate the location, the size of damage and the progression of curing.

The aforementioned i nvention deals with a system consisting of a dielectric substrate or a network of sensors and the method adopted in the same is completely different from that of †3a the present invention wherein sensors, actuators or i mpulse generators are used to detect the resonance properties of the composite panel and not the properties of the dielectric layer as mentioned in the prior art. The disadvantage of the aforementioned i nvention in comparison to the subject invention is that it employs a large number of sensors in combination with conducting material for conducting signals within the composite material. The subject invention can be worked by using not more than two sensors to produce the desired result of detecting a change to the structural health of a composite panel, and without the necessity of addi ng any conducting material for conducting signals within the composite material. b. W09119173 discloses a method for detecting the structural integrity of composite structures.

The invention relates to a method and system for non- destructively evaluating the structural integrity of a mechanical component constructed from fiber composites, specifically for assessing stiffness, strength and damping characteristic of a composite structure.

3a The system compri ses the f ol I owi ng:

1. Sensors and actuators which are placed on the composite material

2. T he sensors are used for sensi ng strai n or moti on and may be I aser vi bro- meters or opti cal sensors.

3. E nvironmental sensors can also be used for the compensation of environmental effects. ¾ 4. Signal conditioning processes such as pre-amplifi cation, powering, signal summation, signal differencing, amplification, filtering or phase control is performed on sensor and actuator signals if necessary.

5. T he system also consists of a signal analyzer.

Working:

, 1. T he signal is transmitted i nto the composite material through the signal i nput unit

2. T he signal is received by an output unit

3. T he frequency of the signal is measured and recorded

4. A fter a certai n peri od of ti me, the frequency of a si mi I ar si gnal i s measured and recorded.

5. Any difference between the signals would indicate stiffness change thereby inferring a i¾ corresponding change in strength or corresponding increase in damage to the said composite structure.

The aforementioned system and method deals with the vibration of the entire composite and also consists of an environmental model which calibrates the results considering the external environmental influence on the composite material. The present invention does not involve †3a any vi bration of the composite panel but deals with the measurement of the wave characteristics of a continuous periodic wave and wave packets of a particular Pass Frequency in order to determine the structural health of the composite panel. The objective of the aforementioned invention is to determine the structural damage to the composite panel by monitoring the extent of structural damping. The disadvantage of the aforementioned invention is that the sensors are placed on the surface of the composite structure which will not enable a user to measure the intra-layer properties of the composite structure.

¾. US Patent Number 4983034 deals with providing a system and method for the measurement of di stri buted strai n wel I sui ted for use i n sensi ng the strai n of a composi te structure.

The invention consists of:

a) A composite structure comprisi ng of composite material

b) A n opti cal f i ber whi ch i s embedded i nto the composi te materi al

3a Working:

1. The optical fiber embedded into the composite material is subjected to strain by varying the optical energy transmitted through an optical source.

2. A reference signal representative of the modulated optical energy is recorded

3. Furthermore, the sensor senses the polarization state of the optical energy backscattered ¾ from the f i ber bel ow a sel ected frequency

4. Increasing the selected frequency to correspond to a selected location and mixing a portion of the optical energy backscattered from the fiber with a portion of the reference signal to produce a beat signal representative of strain at a selected location of the composite.

, 5. The aforementioned invention deals with subjecting the composite to strain by varying the optical energy and involves the production of a beat signal representative of strain at a selected location. T he method would only help in detecting the strain at a particular location of a composite and not the composite as a whole. Moreover the method does not involve the measurement of resonance properties or the travel time of a signal.

0 bj ects of the I nventi on:

1 T he mai n obj ect of the i nventi on i s ai med at detecti ng any change i n the structural heal th of the composite panel.

I A further object of the invention is to detect any continuous periodic wave transmitted †3a through a composite panel .

I A further object of the invention is to detect the Pass Frequency of the continuous periodic wave transmitted through the composite panel I A further object of the invention is to detect any wave packet of a selected Pass

Frequency transmitted though the composite panel.

I A further object of the invention is to measure wave characteristics such as amplitude, phase, and shape of the signal received by the receiver sensors as a result of the ¾ conti nuous peri odi c wave transmitted through the composite panel .

I A further object of the invention is to measure wave characteristics such as amplitude, phase shape and travel time of the signal received by the receiver sensors as a result of the wave packet of a selected Pass Frequency transmitted through the composite panel.

3a Statement and Summary of the Invention:

Accordi ng to the invention there is, therefore, provided a system, method, process and apparatus to detect any change in the structural health of a composite panel

(a) A primary mechanism or means of detecting any change in the structural health of the composite panel.

¾ (b) An auxiliary mechanism of detecting any continuous periodic wave propagated through a composite panel and measured by using suitable measurement equipment

(c) An auxiliary mechanism of detecting any wave packet of a selected Pass Frequency transmitted through a composite panel and measured by usi ng suitable measurement equi pment.

, (d) An auxiliary mechanism of measuring the wave characteristics of the continuous periodic wave such as amplitude, shape and phase,

(e) An auxiliary mechanism of measuring the wave characteristics of the wave packet of a selected Pass Frequency such as amplitude, shape, phase and travel time. i¾ Detailed description of the Invention:

The description of the preferred embodiment is meant to demonstrate the broad working principles of the invention without limitation as to possible adaptations, extensions, appl ications etc., which would be obvious to a person skilled i n the art. In the interest of brevity and for the purposes of exemplary explanation, references have been made to a

†3a system, depicted in figures 1, 2, 3, 4 and 5 herein without limitation, to describe the invention which is essentially directed toward catering to the problem of detecting the structural health of composite panels. Brief Description of the Drawings

[Fig. 1] describes a schematic wherein wave packets are propagated through a composite panel in which sensors have been placed on the surface or within the composite panel.

[Fig. 2] describes the placement of sensors within the composite panel wherein these sensors ¾ are used as both signal transmitters and signal receivers.

[Fig. 3] describes the placement of sensors T1, R1, R2, R3, R4, R5 and R6 embedded in the composite panel wherein sensor T1 is used as a signal transmitter and sensors R1, R2, R3, R4, R5 and R6 are used as signal receivers. The sensor pairs ^" R1 and R6, R2 and R5, R3 and R4_ are positioned equidistant to the transmitter sensor and preferably located on the same 3a layer as the transmitter sensor. Therefore, assuming the composite panel is uniform and has uniform material properties all over the panel. Hence the wave packet experiences same medium in reaching each equidistantly placed sensor pair (namely R1:R6 and R2:R5 and R3:R4) resulting in same signals at each pair of sensors.

Any difference in the wave characteristics of the signals received by each sensor pairs R1:R6 ¾ or R2:R5 or R3:,R4 indicates a difference in the change in the material properties of the path through which the waves have travelled to reach each sensor.

[Fig. 4] describes the placement of sensors S1 , S2 and S3 within the composite panel wherein sensor S1 is used as a signal transmitter and sensors S2 and S3 are used as signal receivers. Sensors S2 and S3 are positioned equidsitantly to the Transmitter S1. Therefore, any , difference in the wave characteristics of the signals received by sensors S2 and S3 indicates a difference i n the change in the material properties and the non uniformity of material properties in the panel.

[Fig. 5] describes a concept of signal generation and recovery in the invention wherei n the signal is transmitted by the signal transmitter (Sig in) and enters the composite panel through the signal transmitter. The signal is thereafter received by the signal receiver (Sig out), amplified by the pre-ampl ifier, recovered by the Signal recovery unit and processed by the Signal processing unit

[Fig. 6] describes a composite panel which is subjected to the detection of its structural health sample wherein receiver sensors are placed at positions L and R and the transmitter sensor is til placed at position C. The tone generator (8) represents the control unit which sends the signal through C into the composite panel. The signals received at L and R are then amplified by a pre-amplifier (4). The lock- in amplifier (5) as described thereafter processes, recovers and measures the wave characteristics of the signal. [Fig. 7] shows a composite panel and amplitude-frequency plot based on results obtained after measuring the wave characteristics of the said composite panel sample using the structural health monitoring system. A composite panel sample with the attributes 12 cm width, 60 cm long and 0.6 cm thickness was selected. T he composite panel has been marked ¾ with the letters L, C and R indicati ng the left edge, center portion and right edge of the composite panel respectively. The composite panel has been marked with the letter D between L and C indicating the position of the defect in the composite panel. Three sensors in the form of piezoelectric disks of 0.5 mm thickness each were embedded. Two piezoelectric disks were placed at the edges L and R of the panel and one is positioned at C at the center of

3a the panel. The piezoelectric discs at L and R are positioned equidistantly to the piezoelectric disk at C. The piezoelectric disk at C will act as a transmitter- sensor and the piezoelectric disks at the edges L and R will act as the receiver- sensors. The defect has been marked as D situated i n between the transmitter sensor and the receiver sensor on the I eft edge.

The line in the plot which has been designated as magnitude MILO is an indication of the

¾ signal transmitted by the transmitter sensor i n the centre to the receiver sensor at the left edge.

The line in the plot which has been designated as magnitude MIRO is an indication of the signal transmitted by the transmitter sensor in the centre to the receiver sensor at the right edge.

C omponents of the structural health monitoring system for detecting the changes in the material properties of a composite panel:

T he Structural H ealth monitori ng system comprises the f ol I owi ng:

1. A Composite Panel

¾ 2. Transmitter sensor(s)

3. Receiver sensor(s)

4. Pre-Amplifier

5. Signal recovery unit

6. Signal processing unit

†3a 7. Measurement Equipment

8. Control unit for the exciter Wherein the sensors are placed as desi red within or on the surface of the composite panel. The sensors can be surface mounted or placed internally at any position within the same layer or different layers in the composite panel.

Wherein the sensors can act as a Transmitter sensor

¾ W herei n the sensors can act as a R ecei ver sensor

Wherein the pre-amplifier pre-amplifies the signals received by the receiver sensor

Wherein the signal recovery unit recovers the signal amplified by the pre-amplifier.

Wherein the signal processing unit processes the signal recovered by the pre-amplifier.

Wherein the measurement equi pment measures wave characteristics such as amplitude, 3a phase, shape and travel ti me of the signal processed by the signal processi ng unite.

Wherein the control unit is connected to the Receiver sensor through an electronic interface

Working of the structural health monitori ng system

The method by which the invention detects the structural health of the composite panel is as ¾ follows:

I A conti nuous peri odic wave is transmitted i nto the composite panel usi ng the T ransmitter sensor

I The signals from the continuous periodic waves transmitted through the composite panel are received by the receiver sensor.

, i The signals received by the receiver sensor from the continuous periodic wave is pre- amplified by a pre-amplifier and wave characteristics such as Pass Frequency, amplitude, phase and shape of the received signal is measured using suitable measurement equi pment.

I The pre-amplifier amplifies the signal received by the receiver sensor

i¾ i The amplified signal is then recovered by a suitable signal recovery unit

I The signal recovered by the signal recovery unit is then processed by a signal processing unit.

I The signal processed by the signal processing unit is then analyzed and its characteristics such as Pass Frequency, amplitude, phase and shape are measured using suitable †3a measurement equi pment and recorded.

I Another aspect of the invention, a wave packet at the selected Pass Frequency is transmitted into the composite panel using the transmitter sensor. T he receiver sensor receives the signal from the wave packet at a selected Pass Frequency transmitted i nto the composite panel through the transmitter sensor.

I T he signals received by the receiver sensor from the wave packet at a selected Pass

Frequency are pre-ampl ified by a pre-ampl ifier and wave characteristics such as ¾ amplitude, shape, phase and travel time of the received signal are measured usi ng suitable measurement equipment

I T he pre-ampl ifier amplifies the signal received by the receiver sensor

I T he ampl ified signal is then recovered by a suitable signal recovery unit

I T he signal recovered by the signal recovery unit is then processed by a signal processing 3a unit.

I T he signal processed by the signal processi ng unit is then analyzed and its wave characteristics such as amplitude, phase, shape and travel ti me are measured usi ng suitable measurement equi pment and recorded to constitute Signature Properties of the composite panel.

¾ i T he Signature Properties are an indication of material properties such as the i nterlayer and the i ntra-layer bonding or lami nation strength of the composite panel, and an indicator of distortion or breakage in one or more layers of the composite panel.

I In another aspect of the i nvention, the composite panels are thereafter subjected to the aforementioned system and method after a period of ti me pursuant to the exposure of the , composite panel to an external envi ronment and wave characteristics such as amplitude, phase, shape and travel ti me of the signals are measured usi ng suitable measurement equi pment and recorded to constitute Recorded Properties of the composite panel wherein the measurement equi pment

I A ny variations that may be detected between the Signature Properties and the Recorded i¾ Properties such as the travel ti me, ampl itude, phase and shape of the wave packet indicates a deterioration in the current structural health of the composite panel. T his i n turn would determi ne the current level of interlayer and the i ntra-layer bondi ng or lamination strength of the composite panel or detect distortion or breakage in one or more layers of the composite panel thereby providi ng an insight i nto the health condition of the

†3a composite panel.

I A ny change i n the i nterlayer bondi ng (de- 1 ami nation), distortion of i ndividual or multi ple panels (distortion) or intra layer strength (breakages) of composite panels directly indicates the change i n the material properties of the panels, thereby identifying its structural health.

I In another aspect of the invention a wave packet at the selected Pass Frequency is transmitted into the composite panel using the transmitter sensor and received ¾ individually by each receiver sensor.

I The signals from the wave packet at a selected Pass Frequency and received by each receiver sensor equi distantly placed is pre-amplified by a preamplifier and wave characteristics such as amplitude, shape, phase and travel time of the received signal is measured using suitable measurement equipment

3a I The pre-amplifier amplifies the signal received by each receiver sensor

I The amplified signal is then recovered by a suitable signal recovery unit

I The signal recovered by the signal recovery unit is then processed by a signal processing unit.

I The signal processed by the signal processing unit is then analyzed and its wave ¾ characteristics such as amplitude, phase, shape and travel time are measured using suitable measurement equipment and recorded.

I Any change in the wave characteristics of the signals received by each receiver sensor is compared and any deviation in the wave characteristics obtained for each sensor indicates a change i n the interlayer bonding (de- 1 ami nation), or intra layer strength (breakages) to , the path through whi ch the wave packet has passed to reach the receiver sensor.

I Therefore any change in the interlayer bonding (de- 1 ami nation) or intra layer strength

( breakages) of composi te panel s di rectly i ndi cates the change i n the materi al properti es of the panels, thereby indicating a change in its structural health. ¾ W or k i ng embod i ment:

A working embodiment of the invention for the purposes of detecting the structural health of a composite panel is disclosed below. T he working embodi ment is illustrated with the assistance of Figures 6 and 7.

1. A composite panel prepared using High Density Poly Ethylene ( ^" H DPE J of s suitable †3a grade i s sel ected and marked as Sampl e i n F igure 6. T he attri butes of the composite panel prepared usi ng H D PE are as fol lows:

a) T hi ckness 6 mi 11 i meters

b) Width ^" 12 centimeters c) Length ^" 60 centimeters

2. Piezoelectric discs having sensory properties are selected and placed in three suitable positions of the composite panel as described in Figure 6. Two of the piezoelectric discs are positioned at the left and right corners of the composite panel prepared using H DPE

¾ and marked as L and R respectively. The third piezoelectric disc is placed in the center of the composite panel and indicated as C as described in Figure 6.

3. The piezoelectric discs at L and R are placed in the same layer as and equidistant to the piezoelectric disc at C.

4. The piezoelectric disc at C will act as the transmitter sensor and the piezoelectric discs at 3a L and R will act as receiver sensors

5. For the purposes of describing the working embodiment, a defect is positioned between C and L and indicated as D in Figure 6.

6. The control unit(8) more specifically described as a tone generator in Figure 6 is connected to the piezoelectric disc at C. A sinusoidal signal which is a continuous

¾ periodic wave of 0.5 V olt is then appl ied to the transmitter sensor and suitable vi brations of the signal are transmitted into the composite panel prepared using H DPE. The frequency of the sinusoidal signal applied at C is varied using the tone generator.

7. These vibrations which are transmitted into the composite panel prepared using HDPE through the piezoelectric disc at C is then received by the piezoelectric discs at L and R. , 8. The received signal at L and R is then amplified using a pre-amplifier (4), processed using a signal processing unit recovered using a signal recovery unit and measured.

9. As described in Figure 6, a commercially available Lock-in Amplifier has been used to carry out the aforementioned processes of signal processi ng, recovery and measurement

10. Based on the frequency and amplitude characteristics of the signal recovered, a frequency i¾ amplitude plot is generated, and described in Figure 7. The plot in Figure 7 shows that the highest peaks for both the signals transmitted from C to L and C to R and marked as MILO and MIRO respectively are obtained at 130000 Hertz indicati ng the suitable pass frequency at which the signals should be transmitted into the composite panel to detect the structural health of the subj ect composite panel prepared usi ng H D PE .

t¾a 11. The different plots obtained and marked as MILO and MIRO in Figure 7 is recorded and compared. Any deviation of the plot obtained and marked as MILO from MIRO is an indication of a difference to the material properties of the path between C to L due to the presence of defect D. The process is further repeated by sending wave packets such as sinusoidal wave packets instead of continuous periodic waves such as sinusoidal waves at a pass frequency of 130000 Hertz through the piezoelectric disc at C and receiving them at L and R as described in Figure 7.