WO2024112259A1 - Surface defect monitoring with rayleigh wave - Google Patents

Abstract

A system and method to monitor a surface defect of a structure includes a transmitter and a receiver that can be coupled to the surface. The transmitter is configured to generate an ultrasonic Rayleigh wave. The receiver is configured to receive an incident Rayleigh wave from the transmitter and a reflected Rayleigh wave reflected by the surface defect. The receiver includes a polymer piezoelectric film with a plurality of electrodes disposed thereon. A controller may be configured to perform a method including: performing a signal enhancement on the incident Rayleigh wave and the reflected Rayleigh wave to obtain an enhanced incident Rayleigh wave and an enhanced reflected Rayleigh wave respectively, the signal enhancement being based on a delay-and-sum method, and calculating a depth value based on the enhanced incident Rayleigh wave and the enhanced reflected Rayleigh wave, the depth value corresponding to the defect depth of the surface defect.

Description

SURFACE DEFECT MONITORING WITH RAYLEIGH WAVE

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority to the Singapore application no. 10202260156T filed November 22, 2022, the contents of which are hereby incorporated by reference in their entirety for all purposes.

TECHNICAL FIELD

[0002] This application relates to the monitoring of surface defects using Rayleigh waves.

BACKGROUND

[0003] Fatigue cracks in metals are common causes of failure and can result in catastrophic collapse of entire structures. The surface fatigue crack typically develops in two stages of crack initiation at the surface and crack propagation into the material.

SUMMARY

[0004] In one aspect, the present application discloses a system to monitor a surface defect that extends for a defect depth from a surface of a structure. The system includes a transmitter, a receiver, and a controller. The transmitter may be coupled at the surface. The transmitter may be configured to generate an ultrasonic Rayleigh wave. The receiver may be coupled at the surface. The receiver may be configured to receive an incident Rayleigh wave from the transmitter and a reflected Rayleigh wave reflected by the surface defect. The receiver may include a polymer piezoelectric film with a plurality of electrodes disposed on the polymer piezoelectric film. The controller may be in signal communication with the receiver. The controller may be configured to perform a method. The method may include performing a signal enhancement and calculating a depth value. The method may include performing a signal enhancement on the incident Rayleigh wave and the reflected Rayleigh wave to obtain an enhanced incident Rayleigh wave and an enhanced reflected Rayleigh wave respectively. The signal enhancement may be based on a delay-and-sum method. The method may include calculating a depth value based on the enhanced incident Rayleigh wave and the enhanced reflected Rayleigh wave, in which the depth value corresponds to the defect depth of the surface defect.

[0005] In another aspect, the present application discloses a method to monitor a surface defect that extends for a defect depth from a surface of a structure. The method includes generating an ultrasonic Rayleigh wave, detecting Rayleigh waves, performing a signal enhancement, and calculating a depth value. The method includes generating an ultrasonic Rayleigh wave using a transmitter coupled at the surface. The method includes detecting Rayleigh waves using a receiver coupled at the surface, in which the receiver may be a piezoelectric transducer array configured to receive an incident Rayleigh wave from the transmitter and a reflected Rayleigh wave reflected by the surface defect, and in which the receiver includes a polymer piezoelectric film with a plurality of electrodes disposed on the polymer piezoelectric film. The method includes performing a signal enhancement on the incident Rayleigh wave and the reflected Rayleigh wave to obtain an enhanced incident Rayleigh wave and an enhanced reflected Rayleigh wave respectively, in which the signal enhancement is based on a delay-and-sum method. The method includes calculating a depth value based on the enhanced incident Rayleigh wave and the enhanced reflected Rayleigh wave, in which the depth value corresponds to the defect depth of the surface defect. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Various embodiments of the present disclosure are described below with reference to the following drawings:

[0007] FIG. 1 is a schematic diagram of a system for monitoring a surface defect in a structure according to various embodiments of the present disclosure;

[0008] FIG. 2 is a graph showing the defect depth of the surface defect against the Rayleigh wave frequency;

[0009] FIGS.3A to 3C are schematic diagrams of experimental setups to compare the present system with a conventional laser interferometer setup;

[0010] FIGS.4A to 4C are the frequency spectra obtained from the setup of FIGS.3 A to 3C;

[0011] FIG. 5 is the attenuation of the incident Rayleigh waves when traveling across three different locations obtained from the setup of FIGS.3 A to 3C;

[0012] FIG.6 is a schematic diagram of a system for monitoring a surface defect in a structure according to one embodiment of the present disclosure;

[0013] FIG.7 is a schematic diagram of a system for monitoring a surface defect in a structure according to one embodiment of the present disclosure;

[0014] FIG.8 is a schematic diagram of a system for monitoring a surface defect in a structure according to one embodiment of the present disclosure;

[0015] FIGS.9A to 9C are images of spectrograms of the Rayleigh waves generated from the system of FIGS.6 to 8;

[0016] FIG. 10 is a schematic diagram illustrating one example in which multiple sets of the present system can be provided on the same structure;

[0017] FIG. 11 is a schematic diagram of another useful application of the present system and method; [0018] FIG. 12 is a schematic diagram of another useful advantage of the present system and method.

DETAILED DESCRIPTION

[0019] The following detailed description is made with reference to the accompanying drawings, showing details and embodiments of the present disclosure for the purposes of illustration. Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments, even if not explicitly described in these other embodiments. Additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0020] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0021] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance as generally understood in the relevant technical field, e.g., within 10% of the specified value.

[0022] As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

[0023] As used in the present disclosure, the term "surface defect" refers to a discontinuity in the material present on the external surface of a structure. The present system and method are useful for monitoring surface defects, regardless of the cause of the surface defect. Some surface defects may be fatigue cracks commonly found in metals. Some surface defects result from fatigue damage of the metal materials and initiate as shear cracks on crystallographic slip planes. Some surface defects may be cracks of corrosion. Surface defects may include but are not limited to cracks that appear in welded structures at or near welded joints. A surface defect may be characterized by irregular crack dimensions. Taking reference from the surface where the surface defect can be found, the surface defect may be described as having a defect depth, e.g., the surface defect may extend from the surface into the material for a distance into the material. In various examples, the surface defect includes a surface crack that initiated at a surface of a structure and subsequently developing or increasing in the defect depth, e.g., the surface crack propagating into the material of the structure.

[0024] The term "structure" as used herein refers generally to any physical article to which the present system and method may be applied, including but not limited to one or more parts of buildings, bridges, tunnels, transportation equipment, pipes, etc., for example.

[0025] As used herein, the term "transmitter" refers to a device operable to generate Rayleigh waves. As used herein, the terms "transmitter" and "Rayleigh wave transmitter" may be used interchangeably. The transmitter may be used to provide or direct ultrasonic Rayleigh waves along a propagation direction. An example of the target includes a surface detect to be monitored. As used herein, the term "receiver" refers to a device operable to receive or detect Rayleigh waves. As used herein, the terms "receiver" and "Rayleigh wave receiver" may be used interchangeably. The transmitter and the receiver may be selected to operate within similar frequency ranges. In various examples described below, the transmitter and the receiver are selected to operate in the ultrasonic frequency range.

[0026] Conventional wisdom teaches away from using a piezoelectric sensor that involves physical contact and attachment with the surface because the destructive interference between the various detected Rayleigh waves results in an even weaker signal detected for the Rayleigh wave with the smaller wavelength, e.g., the reflected Rayleigh wave. For this and other reasons, conventionally, the detection of Rayleigh waves is preferably done using non-contact methods such as using a laser interferometer or an electromagnetic acoustic transducer (EMAT). Conventionally, non-contact methods are preferred because Rayleigh waves are known to be sensitive to surface discontinuities and the physical presence of a detector contacting the surface may interfere with the Rayleigh waves.

[0027] In contrast, the present disclosure proposes a receiver that can be physically coupled to the surface in the vicinity of the surface defect to be monitored. The receiver may be physically bonded to the surface using an adhesive such as an epoxy coupler. Alternatively, the receiver may be in-situ fabricated on the surface of the structure. It was further experimentally verified that, using the present system and method, the Rayleigh wave signals acquired can be used to determine the defect depth.

[0028] FIG. 1 is a schematic diagram to illustrate a system 100 and a method 300 for monitoring a surface defect 101 in a structure 200 according to various embodiments of the present disclosure. The system 100 may include a transmitter 110 and a receiver 120 arranged in one of various configurations at a surface 202 of a structure 200, to monitor a surface defect 101 in the structure 200. For example, the transmitter 110 and the receiver 120 may be arranged in alignment with the surface defect. For example, the transmitter 110, the receiver 120, and the surface defect may be aligned along a straight line. The receiver 120 is positioned between the transmitter 110 and the surface defect 101.

[0029] The transmitter 110 may include an ultrasonic transducer 112 installed on a wedge 114 such that the transmitter 110 is angularly displaced relative to the surface 202 at a critical angle 0_R . The wedge 114 may be bonded to the surface 202 with adhesive. A longitudinal wave emitted from the ultrasonic transducer 112 is converted into a Rayleigh wave at an interface of the wedge and the surface. In use, the transmitter 110 generates a Rayleigh wave in the ultrasonic frequency range that propagates along the surface 202 of the structure 200 towards the surface defect 101.

[0030] As the Rayleigh wave travels past the receiver, an incident Rayleigh wave will be detected by the receiver 120. When the Rayleigh wave continues to travel further until the Rayleigh wave encounters the surface defect 101, the Rayleigh wave will be scattered. A reflected portion of the scattered wave is detected by the receiver 120 (herein referred to as the reflected Rayleigh wave).

[0031] According to some embodiments of the present disclosure, the receiver 120 includes an array of sensors and a layer of polymer between the sensors and the surface 202.

[0032] In some examples, the receiver 120 may include a plurality of sensors in the form of ceramic-based piezoelectric transducers. The ceramic-based piezoelectric transducers are arranged in an array and spaced apart at regular intervals and oriented to facilitate detection of both the incident Rayleigh wave and the reflected Rayleigh wave. Each of the plurality of sensors is bonded to the surface 202 by an epoxy coupler, e.g., a polymer-based adhesive.

[0033] In some examples, the receiver 120 may include an array of electrodes and a layer of a polymer-based piezoelectric film between the electrodes and the surface 202. In some embodiments, the electrodes may be discrete electrodes. In other embodiments, the electrodes may be interconnected at one end to form a comb-shaped electrode element. The whole receiver 120 has a relatively low profile and is a lightweight device. Compared to a laser interferometer, the present receiver 120 is significantly less bulky and requires no more than a small footprint. The footprint of the present receiver 120 is so small that a few receivers may be provided around one weld line.

[0034] In some examples, the receiver 120 includes at least three electrodes 121 disposed on a polymer-based piezoelectric film. The at least three electrodes 121 are equally spaced apart from one another. The at least three electrodes 121 may be arrayed such that they are parallel to one another, and oriented to be perpendicular to the propagation direction of the incident Rayleigh wave. For the sake of brevity, in the present disclosure, a transverse direction or transverse orientation refers to a direction that is perpendicular or substantially perpendicular to the propagation direction of the incident Rayleigh. When the Rayleigh wave travels across along a propagation direction and cross the width of a transversely oriented electrode, the Rayleigh wave signal with the smaller Rayleigh wavelength detected by the electrode will be weaker owing to a destructive interference of the signal of the smaller Rayleigh wavelength.

[0035] The Rayleigh wave is generated from interaction of longitudinal and transverse waves at a free surface. Rayleigh waves are surface acoustic waves that travel along the surface 202 of a structure 200 with the majority of the wave energy contained within a depth of approximately one wavelength. As the Rayleigh wave propagates along a surface 202, its amplitude decreases exponentially with the defect depth of any surface irregularities. The presence of a surface defect 101, such as a surface crack, can be detected and the location of the surface defect can be determined, e.g., with reference to the transmitter or the receiver 120. A comparison of the incident Rayleigh wave and the reflected Rayleigh wave can yield information about the integrity of the structure in the local region, e.g., in a region proximal to the receiver 120.

[0036] The ratio of the reflected Rayleigh wave to the incident Rayleigh wave is known as a reflection coefficient C_r. A maximum reflection coefficient may be observed when a ratio of the defect depth of the surface defect to the Rayleigh wavelength is 0.45 or about 0.45. The defect depth of the surface defect may be correlated to the frequency of the Rayleigh wave, based on the following Equation (1): a = 0.45 = 0.45 v/ (1) where a is the defect depth of the surface defect; A is the Rayleigh wavelength; v is the Rayleigh wave velocity; and is the Rayleigh wave frequency.

[0037] The Rayleigh wave velocity here is a constant. The defect depth of the surface defect may be expressed as a function of the Rayleigh wave frequency. As shown in FIG. 2, a graph of the defect depth of the surface defect against the Rayleigh wave frequency can be plotted. Using Equation (1), the defect depth of the surface defect can be correlated to the wavelength or to the frequency distribution of the reflected Rayleigh wave. [0038] The Rayleigh wave frequency may be selected on the basis of the minimum defect depth, and the width of each electrode may be determined on the basis of the corresponding Rayleigh wavelength. The minimum defect depth refers to the smallest detectable defect depth. In some examples, the minimum defect depth refers to the defect depth at which the user would like to start monitoring the surface defect. The Rayleigh wave is selected such that surface defects with a defect depth approximately equal to or greater than the minimum defect depth can be detected. The width of each of the electrodes of the receiver 120 is selected to be half of the wavelength of the Rayleigh wave required to detect the minimum defect depth.

[0039] The system 100 may include a controller configured to execute instructions stored in a computer-readable medium as part of a method 300 to determine the defect depth of a surface defect, in which the method 300 involves determining information about the surface defect 101 based on the incident Rayleigh wave 102 and the reflected Rayleigh wave 103 sensed by the receiver 120.

[0040] The method 300 may include a pre-processing step in which the signal-to-noise ratio of the incident Rayleigh wave 102 is enhanced. The method 300 may include a pre-processing step in which the signal-to-noise ratio of the reflected Rayleigh wave 103 is enhanced. In some examples, the controller is configured to apply a delay-and-sum method or algorithm to the incident Rayleigh wave. In some examples, the controller is configured to apply a delay-and- sum method or algorithm to the reflected Rayleigh wave. The method 300 includes comparing the incident Rayleigh wave 102 and the reflected Rayleigh wave 103; and determining a depth value based on the incident Rayleigh wave 102 and the reflected Rayleigh wave 103. The depth value refers to the computed output of the controller. It has been experimentally verified that the depth value corresponds to or is reflective of the actual defect depth of the surface defect

101. [0041] The controller may be configured to execute the delay-and-sum method 300 which includes: (i) determining respective arrival times for the incident Rayleigh wave 102 and the reflected Rayleigh wave 103; (ii) performing a Fast Fourier Transform (FFT) on the incident Rayleigh wave 102 and the reflected Rayleigh wave 103 to obtain their respective frequency spectra; and (iii) summing the frequency spectra to obtain an enhanced incident Rayleigh wave 102 in the frequency domain and an enhanced reflected Rayleigh wave 103 signal in the frequency domain.

[0042] Example 1

[0043] FIGS. 3A to 3C are schematic diagrams of experimental setups to compare the present system with a conventional laser interferometer setup.

[0044] A surface defect was simulated by machining a slot of 1 millimetre (mm) deep in a 40 mm-think aluminum block, i.e., the defect depth in this case was 1 mm. The transmitter used included an ultrasonic transducer 112 attached to a wedge 114 which was in turn fixed to the surface 202 by an adhesive 104, such that the ultrasonic transducer 112 was disposed at a critical angle 0_R relative to the surface 202 of the structure 200. An excitation signal using a -13 Vp pulse with a pulse width of 385 nanosecond (ns) was applied to the transmitter 110 to generate Rayleigh waves that propagate towards the surface defect 101.

[0045] FIG. 3A illustrates a first setup in which the receiver 120 was disposed between the transmitter 110 and the surface defect 101. The receiver 120 included three electrodes 121 on a piezoelectric-polymer layer 122.

[0046] FIG. 3B illustrates a second setup in which the receiver 120 was disposed between the transmitter 110 and the surface defect 101. The receiver 120 included three piezoelectricceramic transducers 123 coupled to the surface by a conductive epoxy 124. [0047] FIG. 3C illustrates a third setup in which a laser interferometer was used to detect the Rayleigh waves 102 at three predetermined detection points 125 between the transmitter 110 and the surface defect 101.

[0048] FIGS. 4 A to 4C present the frequency spectra of the incident Rayleigh wave 102 signal and the reflected Rayleigh 103 wave signal after the pre-processing step, e.g., after applying the delay-and-sum algorithm to the respective detected Rayleigh waves. FIG. 4A shows the frequency spectra obtained from the first setup. FIG. 4B shows the frequency spectra obtained from the second setup. FIG. 4C shows the frequency spectra obtained from the third setup.

[0049] Reflection coefficients (C_r) were calculated by dividing the magnitudes of the reflected Rayleigh wave signal by the incident Rayleigh wave signal in the frequency domain. To determine the defect depth, the reflection coefficient peak was searched within the effective frequency bandwidth (-6 decibel (dB) frequency bandwidth) of the incident Rayleigh wave 102. FIG. 4A shows a reflection coefficient peak at 1.05 megahertz (MHz) using the first setup. FIG. 4C shows a reflection coefficient peak at 1.22 MHz for the third setup. The lack of a peak in the effective frequency bandwidth in FIG. 4B could be due to the high attenuation rate of the transducers and the low signal amplitude of the reflected Rayleigh wave 103 in the particular case.

[0050] The amplitude of the incident Rayleigh wave 102 as detected in each of the three setups of FIGS. 3A to 3C were recorded at three different locations. The attenuation of the incident Rayleigh waves was calculated and presented in FIG. 5 and tabulated below in Table 1.

Table 1. Performance benchmark based on 1 mm-deep surface defect.

[0051] Of the three setups, the piezoelectric ceramic transducer-based receiver 120 in the second setup had the highest attenuation rate. The receiver 120 in the first setup had an attenuation rate comparable to that of the non-contact setup using the laser interferometer. Without being limited by theory, comparing the results of the first setup and the second setup, the layer of polymer is believed to provide some mechanical damping and a wider frequency bandwidth of the detected Rayleigh wave. In the case of the first setup using the polymer piezoelectric film 122, the frequency bandwidth was comparable to that of the laser interferometer. In terms of the accuracy of the defect depth, it has been successfully demonstrated that the first setup with the polymer-based piezoelectric film receiver 120 has a performance comparable to that of the laser interferometer.

[0052] Unlike the laser interferometer, the polymer-based piezoelectric film receiver 120 does not require line-of-sight and enables a wider range of applications. Unlike the laser interferometer, the polymer-based piezoelectric film receiver 120 can be coupled to the surface 202 for structural health monitoring over a long period of time. In terms of cost, weight, and footprint, the polymer-based piezoelectric film receiver 120 opens up the possibility of having multiple receivers installed at one or more surface defects of a structure 200 for simultaneous and/or long-term monitoring, even unmanned or remote monitoring. It is also possible now to monitor the development of surface defects 101 at previously difficult to access places, e.g., high-rise structures, suspended structures, etc.

[0053] Example 2

[0054] Experiments were performed to compare various types of transmitters 110 as illustrated schematically in FIGS. 6 to 8.

[0055] FIG. 6 shows a fourth setup in which the receiver 120 is positioned between the transmitter 110 and the surface defect 101. The transmitter 110 includes a comb-shaped electrode element 113 on a piezoelectric polymer layer 115. The receiver 120 includes a combshaped electrode element 121 on a piezoelectric polymer layer 122. In this experiment, both the transmitter 110 and the receiver 120 were coupled to the surface 202 by an adhesive 104. Optionally, either or both of the transmitter 110 and the receiver 120 may be in-situ fabricated on the surface 202 of the structure 200.

[0056] FIG. 7 shows a fifth setup in which the receiver 120 is positioned between the transmitter 110 and the surface defect 101. The transmitter 110 is a thickness-shear transducer 112, e.g., a thickness-shear (d₁₅) lead zirconate titanate (PZT) ultrasonic transducer, that can generate Rayleigh waves along its polarization direction. The receiver 120 includes a combshaped electrode element 121 on a piezoelectric polymer layer 122. In this experiment, both the transmitter 110 and the receiver 120 were coupled to the surface 202 by an adhesive 104. Optionally, the receiver 120 may be in-situ fabricated on the surface 202 of the structure 200.

[0057] FIG. 8 shows a sixth setup in which the receiver 120 is positioned between the transmitter 110 and the surface defect 101. The receiver 120 includes a comb-shaped electrode element 121 on a piezoelectric polymer layer 122. The transmitter 110 includes an ultrasonic transducer 112 mounted on a wedge 114 at a critical angle (0_R). In this experiment, both the transmitter 110 and the receiver 120 were coupled to the surface 202 by an adhesive 104. Optionally, the receiver 120 may be in-situ fabricated on the surface 202 of the structure 200.

[0058] The critical angle can be calculated using Snell's law as shown in Equation (2) below:

where 0_R is the critical angle to generate a Rayleigh wave; V_L is the longitudinal wave speed in the wedge material; V_R is the Rayleigh wave speed travelling on the structure 200.

[0059] The Rayleigh waves generated were processed using a Short Time Fourier Transform

(STFT) algorithm for further analysis, and the corresponding images of spectrograms are shown in FIGS. 9 A to 9C, with the arrival times for the incident Rayleigh wave 102 and the reflected Rayleigh wave 103 indicated as

and t_r , respectively.

[0060] FIG. 9A is an image of the spectrogram from ultrasonic signals detected using the sixth setup of FIG. 8 in which the transmitter 110 includes an ultrasonic transducer 112 with a wedge 114. Both the incident Rayleigh wave 102 and the reflected Rayleigh wave 103 can be clearly observed in the spectrogram.

[0061] FIG. 9B is an image of the spectrogram from ultrasonic signals detected using the fifth setup of FIG. 7 in which the transmitter 110 is a thickness-shear transducer 112. FIG. 9C is an image of the spectrogram from ultrasonic signals detected using the fourth setup of FIG. 6 in which the transmitter 110 includes a piezoelectric polymer film 115 with a comb-shaped electrode element 113 disposed thereon. In both FIG. 9B and FIG. 9C, although various other ultrasonic wave modes may be observed in the spectrograms, the Rayleigh wave of interest is still distinguishable. This suggests the possibility of selecting a transmitter of a lower profile (smaller device size footprint) while balancing the Rayleigh wave mode purity, e.g., in a situation where there is insufficient space to accommodate the wedge.

[0062] Example 3

[0063] FIG. 10 schematically illustrates one example in which multiple sets of the present system 100 can be provided on the same structure 200. Each set or each transmitter-receiver pair may include a receiver 120 of FIG. 3 A, FIG. 3B, or FIG. 6. Each set or each transmitterreceiver pair may include a transmitter 110 of FIG. 6, FIG. 7, or FIG. 8.

[0064] In this example, the structure 200 is a long hollow structure 200, such as a tubular structure, and the plurality of sets or transmitter-receiver pairs are distributed about the circumference of the tubular structure. Rayleigh waves are generated by transmitters to travel along a longitudinal direction or along the length of the tubular structure. In this manner, the structural health of long hollow structures can be monitored remotely for kilometers. The transmitters 110 and the receivers 120 may be bonded or in-situ fabricated on the surface 202 of the structure 200.

[0065] Example 4

[0066] FIG. 11 schematic illustrates another useful application of the present system 100 and method 300. In this example, the surface defect 101 is not found along a long and straight length of the structure 200, but at a concave part of a bent structure. To monitor the defect depth of such a surface defect 101, the receiver 120 may be coupled at a concave, bent, or corner 210 region of the structure. The transmitter 110 may be coupled to a relatively accessible part of the structure 200 such that the receiver 120 is located between the surface defect 101 and the transmitter 110. For example, the surface defect 101 may be at a horizontal region, the transmitter 110 may be located at a vertical region, and the receiver 120 may be located at a bent region between the vertical region and the horizontal region. For example, the transmitter 110 and the surface defect 101 may be located at differently oriented regions of the surface 202, with the receiver 120 located at a comer 210 defined by the differently oriented regions.

[0067] Such a configuration of the system 100 is possible because the receiver 120 is thin and flexible, enabling conformance of the receiver 120 with the curvature of the surface 202 where the receiver 120 is coupled. For example, the receiver 120 may be bonded, or in-situ fabricated in various complex structures, including structures with curves and/or comers 210. [0068] Example 5

[0069] FIG. 12 illustrates another useful advantage of the present system 100 and method 300. In this example, the transmitter 110 and the receiver 120 are bonded on the surface 202 of a welded structure 200. The present system 100 and method 300 can be used to detect defects and/or discontinuities in the weld line 220 or in the weld. The defect depth of a surface defect 101 in a region proximal to the weld line 220 can be monitored. Welding may subject the material around the weld to undergo significant temperature changes, and be prone to the formation of surface defects 101, such as cracks. The surface defects 101 may initially be too small to be visible to visual inspection. After a welding process has been completed, the present system 100 may be provided near the weld line 220 so that the region may be monitored for any surface defects.

[0070] According to various embodiments of the present disclosure, the present application discloses a system to monitor a surface defect that extends for a defect depth from a surface of a structure. The system includes a transmitter, a receiver, and a controller. The transmitter may be coupled at the surface. The transmitter may be configured to generate an ultrasonic Rayleigh wave. The receiver may be coupled at the surface. The receiver may be configured to receive an incident Rayleigh wave from the transmitter and a reflected Rayleigh wave reflected by the surface defect. The receiver may include a polymer piezoelectric film with a plurality of electrodes disposed on the polymer piezoelectric film. The controller may be in signal communication with the receiver. The controller may be configured to perform a method. The method may include performing a signal enhancement and calculating a depth value. The method may include performing a signal enhancement on the incident Rayleigh wave and the reflected Rayleigh wave to obtain an enhanced incident Rayleigh wave and an enhanced reflected Rayleigh wave respectively. The signal enhancement may be based on a delay-and- sum method. The method may include calculating a depth value based on the enhanced incident Rayleigh wave and the enhanced reflected Rayleigh wave, in which the depth value corresponds to the defect depth of the surface defect.

[0071] The receiver may be coupled to the surface by one of the following: an epoxy bonding and an in-situ fabrication.

[0072] The system may include at least three electrodes arrayed on the polymer-based piezoelectric film. [0073] The at least three electrodes may be interconnected to form a comb-shaped electrode element.

[0074] The at least three electrodes may include discrete electrodes equally spaced apart from one another.

[0075] Each of the at least three electrodes may be characterized by a length-to-width aspect ratio greater than five. Each of the at least three electrodes may be characterized by an electrode width determined by a minimum defect depth to be monitored.

[0076] The at least three electrodes may be aligned with respective lengths perpendicular to a propagation direction of the Rayleigh waves.

[0077] The transmitter may include an ultrasonic transducer coupled on a wedge at an angle to the relative to the surface.

[0078] The transmitter may include one of a thickness-shear transducer coupled to the surface.

[0079] The transmitter may include a comb-shaped electrode, the comb-shaped electrode including at least three interconnected electrodes on a piezoelectric polymer layer, and wherein the piezoelectric polymer layer is coupled to the surface.

[0080] The signal enhancement may be based on a delay-and-sum method. The delay-and- sum method may include determining an arrival time, performing a Fast Fourier Transform (FFT), and summing the frequency spectra. The delay-and-sum method may include determining an arrival time for the incident Rayleigh wave and the reflected Rayleigh wave. The delay-and-sum method may include performing a FFT on the incident Rayleigh wave and the reflected Rayleigh waves to obtain respective frequency spectra. The delay-and-sum method may include summing the frequency spectra to obtain the enhanced incident Rayleigh wave and the reflected Rayleigh wave in a frequency domain. [0081] The transmitter and the receiver may be arranged in a straight line with the surface defect, in which the receiver is positioned between the transmitter and the surface defect.

[0082] The receiver may be disposed in proximity of a weld line in the structure.

[0083] In another aspect, the present application discloses a method to monitor a surface defect that extends for a defect depth from a surface of a structure. The method includes generating an ultrasonic Rayleigh wave, detecting Rayleigh waves, performing a signal enhancement, and calculating a depth value. The method includes generating an ultrasonic Rayleigh wave using a transmitter coupled at the surface. The method includes detecting Rayleigh waves using a receiver coupled at the surface, in which the receiver may be a piezoelectric transducer array configured to receive an incident Rayleigh wave from the transmitter and a reflected Rayleigh wave reflected by the surface defect, and in which the receiver includes a polymer piezoelectric film with a plurality of electrodes disposed on the polymer piezoelectric film. The method includes performing a signal enhancement on the incident Rayleigh wave and the reflected Rayleigh wave to obtain an enhanced incident Rayleigh wave and an enhanced reflected Rayleigh wave respectively, in which the signal enhancement is based on a delay-and-sum method. The method includes calculating a depth value based on the enhanced incident Rayleigh wave and the enhanced reflected Rayleigh wave, in which the depth value corresponds to the defect depth of the surface defect.

[0084] In the method, the receiver may be coupled to the surface by one of the following: an epoxy bonding and an in-situ fabrication.

[0085] In the method, the receiver may include at least three discrete electrodes arrayed on a polymer-based piezoelectric film, in which the at least three electrodes may be equally spaced apart from one another.

[0086] In the method, the at least three electrodes may be interconnected to form a combshaped electrode element. [0087] In the method, the at least three electrodes may include discrete electrodes equally spaced apart from one another.

[0088] In the method, each of the at least three electrodes may be characterized by a length- to-width aspect ratio greater than five, in which each of the at least three electrodes may be characterized by a width determined by a minimum defect depth to be monitored.

[0089] In the method, the at least three electrodes may be aligned with respective lengths perpendicular to a propagation direction of the Rayleigh waves.

[0090] In the method, the signal enhancement may be based on a delay-and-sum method, in which the delay-and-sum method includes determining an arrival time, performing a FFT, and summing the frequency spectra. The method may include determining an arrival time for the incident Rayleigh wave and the reflected Rayleigh wave. The method may include performing a FFT on the incident Rayleigh wave and the reflected Rayleigh waves to obtain respective frequency spectra. The method may include summing the frequency spectra to obtain the enhanced incident Rayleigh wave and the reflected Rayleigh wave in a frequency domain.

[0091] All examples described herein, whether of apparatus, methods, materials, or products, are presented for the purpose of illustration and to aid understanding, and are not intended to be limiting or exhaustive. Modifications may be made by one of ordinary skill in the art without departing from the scope of the invention as claimed.

Claims

1. A system to monitor a surface defect that extends for a defect depth from a surface of a structure, comprising: a transmitter coupled at the surface, the transmitter being configured to generate an ultrasonic Rayleigh wave; a receiver coupled at the surface, the receiver being configured to receive an incident Rayleigh wave from the transmitter and a reflected Rayleigh wave reflected by the surface defect, the receiver including a polymer piezoelectric film with a plurality of electrodes disposed on the polymer piezoelectric film; a controller, the controller in signal communication with the receiver, the controller being configured to perform a method including: performing a signal enhancement on the incident Rayleigh wave and the reflected Rayleigh wave to obtain an enhanced incident Rayleigh wave and an enhanced reflected Rayleigh wave respectively, the signal enhancement being based on a delay-and-sum method; and calculating a depth value based on the enhanced incident Rayleigh wave and the enhanced reflected Rayleigh wave, wherein the depth value corresponds to the defect depth of the surface defect.

2. The system as recited in claim 1, wherein the receiver is coupled to the surface by one of the following: an epoxy bonding and an in-situ fabrication.

3. The system as recited in claim 2, wherein the receiver comprises at least three electrodes arrayed on the polymer-based piezoelectric film.

4. The system as recited in claim 3, wherein the at least three electrodes are interconnected to form a comb-shaped electrode element.

5. The system as recited in claim 3, wherein the at least three electrodes are discrete electrodes equally spaced apart from one another.

6. The system as recited in claim 4 or claim 5, wherein each of the at least three electrodes is characterized by a length-to-width aspect ratio greater than five, and each of the at least three electrodes is characterized by an electrode width determined by a minimum defect depth to be monitored.

7. The system as recited in claim 6, wherein the at least three electrodes are aligned with respective lengths perpendicular to a propagation direction of the Rayleigh waves.

8. The system as recited in claim 1, wherein the transmitter comprises an ultrasonic transducer coupled on a wedge at an angle relative to the surface.

9. The system as recited in claim 1, wherein the transmitter comprises one of a thicknessshear transducer coupled to the surface.

10. The system as recited in claim 1, wherein the transmitter comprises a comb-shaped electrode, the comb-shaped electrode including at least three interconnected electrodes on a piezoelectric polymer layer, and wherein the piezoelectric polymer layer is coupled to the surface.

11. The system as recited in claim 1, wherein the signal enhancement is based on a delay - and-sum method comprising: determining an arrival time for the incident Rayleigh wave and the reflected Rayleigh wave; performing a Fast Fourier Transform (FFT) on the incident Rayleigh wave and the reflected Rayleigh waves to obtain respective frequency spectra; summing the frequency spectra to obtain the enhanced incident Rayleigh wave and the reflected Rayleigh wave in a frequency domain.

12. The system as recited in claim 1, wherein the transmitter and the receiver are arranged in a straight line with the surface defect, and wherein the receiver is positioned between the transmitter and the surface defect.

13. The system as recited in claim 1, wherein the receiver is disposed in proximity of a weld line in the structure.

14. A method to monitor a surface defect that extends for a defect depth from a surface of a structure, comprising: generating an ultrasonic Rayleigh wave using a transmitter coupled at the surface; detecting Rayleigh waves using a receiver coupled at the surface, the receiver being a piezoelectric transducer array configured to receive an incident Rayleigh wave from the transmitter and a reflected Rayleigh wave reflected by the surface defect, the receiver including a polymer piezoelectric film with a plurality of electrodes disposed on the polymer piezoelectric film; performing a signal enhancement on the incident Rayleigh wave and the reflected Rayleigh wave to obtain an enhanced incident Rayleigh wave and an enhanced reflected Rayleigh wave respectively, the signal enhancement being based on a delay-and-sum method; and calculating a depth value based on the enhanced incident Rayleigh wave and the enhanced reflected Rayleigh wave, wherein the depth value corresponds to the defect depth of the surface defect.

15. The method as recited in claim 14, wherein the receiver is coupled to the surface by one of the following: an epoxy bonding and an in-situ fabrication.

16. The method as recited in claim 14, wherein the receiver comprises at least three discrete electrodes arrayed on a polymer-based piezoelectric film, the at least three electrodes being equally spaced apart from one another.

17. The method as recited in claim 16, wherein the at least three electrodes are interconnected to form a comb-shaped electrode element.

18. The system as recited in claim 16, wherein the at least three electrodes are discrete electrodes equally spaced apart from one another.

19. The method as recited in claim 16, wherein each of the at least three electrodes is characterized by a length-to-width aspect ratio greater than five, and wherein each of the at least three electrodes is characterized by a width determined by a minimum defect depth to be monitored.

20. The method as recited in claim 19, wherein the at least three electrodes are aligned with respective lengths perpendicular to a propagation direction of the Rayleigh waves.

21. The method as recited in claim 14, wherein the signal enhancement is based on a delay - and-sum method comprising: determining an arrival time for the incident Rayleigh wave and the reflected Rayleigh wave; performing a Fast Fourier Transform (FFT) on the incident Rayleigh wave and the reflected Rayleigh waves to obtain respective frequency spectra; summing the frequency spectra to obtain the enhanced incident Rayleigh wave and the reflected Rayleigh wave in a frequency domain.