Systems and methods for damage detection in structures using guided wave phased arrays

A method for ultrasonic guided wave defect detection in a structure is disclosed. The method includes driving a plurality of transducers to cause guided waves to be transmitted in the structure in a predetermined direction or focused at a predetermined focal point, receiving at least one reflected guided wave signal, and generating image data of the structure based on the at least one reflected guided wave signal. Processed image data are generated by performing at least one of baseline image subtraction or image suppression on the image data, and a location of at least one possible defect in the structure is identified based on the processed image data.

FIELD OF DISCLOSURE

The disclosed systems and methods relate to structural heath monitoring and non-destructive examination. More particularly, the disclosed systems and methods relate to structural heath monitoring and non-destructive examination of plates and plate-like structures using guided wave phased arrays.

BACKGROUND

Various systems exist for structural heath monitoring (“SHM”) and/or non-destructive examination (“NDE”) of plates or plate-like structures like those used on pressure vessels, aircraft fuselage and wings, ship hulls and storage tanks to identify only a couple possible uses. However, these systems and monitoring/examination techniques are mostly based on point-to-point inspections and are not capable of performing rapid large area monitoring and/or inspection.

SUMMARY

In some embodiments, an ultrasonic guided wave system for defect detection in a structure includes at least two guided wave transducers configured to be disposed on a structure and a controller electrically coupled to the at least two guided wave transducers. The controller includes a machine readable storage medium and a processor in signal communication with the machine readable storage medium. The processor is configured to cause a pulse generator to pulse the at least two guided wave transducers in accordance with at least one of time delays or amplitude controls such that guided wave energy is steered in a predetermined direction in the structure or is focused at a predetermined focal point, generate image data of the structure based on the at least one reflected guided wave signal, generate processed image data by performing at least one of baseline image subtraction or image suppression on the image data of the structure, identify a location of at least one possible defect in the structure based on the processed image data, and have defect detection data of the structure including the location of the at least one possible defect in the structure stored in the machine readable storage medium.

In some embodiments, a method for ultrasonic guided wave defect detection in a structure is disclosed. The method includes driving a plurality of transducers to cause guided waves to be transmitted in a structure in a predetermined direction or focused at a predetermined focal point, receiving at least one reflected guided wave signal, and generating image data of the structure based on the at least one reflected guided wave signal. Processed image data are generated by performing at least one of baseline image subtraction or image suppression on the image data of the structure, and a location of the at least one possible defect in the structure is identified and stored in a machine readable storage medium.

In some embodiments, a computer readable storage medium is encoded with program code. When the program code is executed by a processor, the processor performs a method. The method includes causing a plurality of transducers to be driven such that guided waves are transmitted in a structure in a predetermined direction or focused at a predetermined focal point, generating image data of the structure based on at least one reflected guided wave signal, and generating processed image data by performing at least one of baseline image subtraction or image suppression on the image data of the structure. A location of at least one possible defect in the structure is identified based on the processed image data.

DETAILED DESCRIPTION

This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description.

Ultrasonic guided waves have shown good potential for SHM and/or NDE of plates or plate-like structures due to their capability of interrogating a large area with a small number of transducer locations. The system and methods disclosed herein utilize a real time phased array concept with specially designed guided wave transducers to produce large area SHM and/or NDE of plates or plate-like structures with improvements on guided wave penetration power, signal-to-noise-ratio (SNR), and defect detection sensitivity. As used herein, the term “plate-like structure” includes plates and refers to a structure confined by two planar or curved surfaces including, but not limited to, those used on pressure vessels, aircraft fuselages and wings, ship hulls, and storage tanks, to list only a few examples.

In some embodiments, the system includes a plurality of ultrasonic guided wave transducers, which can be excited individually and/or simultaneously. In some embodiments, the guided wave transducers are placed closely together on the structure to form a compact array. In some embodiments, the guided wave transducers are distributed on the structure at a distance from each other in a random or orderly configuration. The system includes a number of pulser and receiver channels. Time delays and possible amplitude factors can be input into each pulser channel for steering the guided wave energy in a specific direction or to focus the energy at a specific location in the structure. In some embodiments, guided wave phased array techniques are combined with the guided wave computational tomography (“CT”) techniques for damage imaging.

FIGS. 1A-1Gillustrate one example of a non-destructive inspection system100configured to inspect plates and plate-like structures using guided wave phased arrays. As shown inFIG. 1A, inspection system100includes a number, n, of transducers102-1,102-2, . . . ,102-n(collectively “transducers102”) communicatively coupled to a controller130. In some embodiments, as described below, system100is a portable system in which the transducers102are not fixedly connected to a plate or plate-like structure, and in some embodiments, system100is a “fixed” system in which the transducers are secured in some manner to a plate or plate-like structure. These transducers102can be piezoelectric stack transducers, shear piezoelectric transducers, electrical magnetic acoustic transducers (“EMATs”), or other suitable transducer as will be understood by one of ordinary skill in the art. Transducers102can be configured as a transmitter or a receiver in a through-transmission setup. Each of the transducers102can also be used as a dual mode transducer under a pulse-echo test mode.

In some embodiments, such as the embodiment inFIG. 1B, a plurality of transducers102are arranged in circular phased array103disposed in a body or housing104of a probe105such that the array103is portable such that the probe105can be placed in contact with plate or plate-like structure10, be moved around structure10, and be removed from contact with structure10. As shown inFIG. 1B, probe105is tethered to controller130. Each of the sensing elements, e.g., transducers102, can be disposed around body104at an equal distance from the directly adjacent sensing elements. In some embodiments, transducers102are equally spaced about body104.

FIG. 1Cillustrates one example of a housing104prior to transducers102being installed. As shown inFIG. 1C, housing104includes a plurality of holes or internal chambers106arranged in a circle near the peripheral edge108of housing104. In some embodiments, housing104is formed from Noryl; however, housing104can be formed from other materials including, but not limited to, rubber, metal, and plastic to list a few possible alternative materials. Holes and/or internal chambers106can be formed by drilling, milling, injection molding housing104with holes106, or by any other suitable manufacturing method. Although a plurality of holes/internal chambers106are illustrated, a single hole or internal chamber106can be provided and a plurality of transducers can be disposed therein in some embodiments. Housing104also defines a slot108at the approximate center with a central hole110defined within slot108.

Bottom surface112of housing104is covered, at least partially, with a conductive epoxy114as shown inFIG. 1D, and then is covered with a wear plate116as illustrated inFIG. 1E. In some embodiments, wear plate116is formed from a metal material, such as aluminum, although one of ordinary skill in the art will understand that other materials can be used. A ground lead (not shown) may be placed within central hole110.

Turning now toFIG. 1F, transducers102are inserted into holes106and sealed therein by epoxy114. A lead wire118is connected to each transducer102and, in some embodiments, are tied together in a bundle120. In some embodiments, the transducers102are thin piezoelectric disks, piezoelectric cylinders, cuboid piezoelectric elements, magnetostrictive transducers, such as those disclosed in commonly assigned U.S. patent application Ser. No. 13/298,758, which is incorporated herein by reference in its entirety, EMATs, or other suitable transducer. In some embodiments, the phased array103is made from a piece of piezoelectric composite material with an array of electrode patterns. The piezoelectric composite material is enclosed in a housing enclosure with appropriate wiring, transducer backing, matching, and wear plates as will be understood by one of ordinary skill in the art.

Referring now toFIG. 1G, controller130includes one or more processors, such as processor(s)132. Processor(s)132may be any central processing unit (“CPU”), microprocessor, micro-controller, or computational device or circuit for executing instructions and be connected to a communication infrastructure134(e.g., a communications bus, cross-over bar, or network). Various software embodiments are described in terms of this exemplary controller130. After reading this description, it will be apparent to one of ordinary skill in the art how to implement the method using other computer systems or architectures.

In some embodiments, controller130includes a display interface136that forwards graphics, text, and other data from the communication infrastructure134(or from a frame buffer not shown) for display on a monitor or display unit138that is integrated with or separate from controller130.

Controller130also includes a main memory140, such as a random access memory (“RAM”), and a secondary memory142. In some embodiments, secondary memory142includes a persistent memory such as, for example, a hard disk drive144and/or removable storage drive146, representing an optical disk drive such as, for example, a DVD drive, a Blu-ray disc drive, or the like. In some embodiments, removable storage drive may be an interface for reading data from and writing data to a removable storage unit148. Removable storage drive146reads from and/or writes to a removable storage unit148in a manner that is understood by one of ordinary skill in the art. Removable storage unit148represents an optical disc, a removable memory chip (such as an erasable programmable read only memory (“EPROM”), Flash memory, or the like), or a programmable read only memory (“PROM”)) and associated socket, which may be read by and written to by removable storage drive146. As will be understood by one of ordinary skill in the art, the removable storage unit148may include a non-transient machine readable storage medium having stored therein computer software and/or data.

Controller130may also include one or more communication interface(s)150, which allows software and data to be transferred between controller130and external devices such as, for example, transducers102and optionally to a mainframe, a server, or other device. Examples of the one or more communication interface(s)150may include, but are not limited to, a modem, a network interface (such as an Ethernet card or wireless card), a communications port, a Personal Computer Memory Card International Association (“PCMCIA”) slot and card, one or more Personal Component Interconnect (“PCI”) Express slot and cards, or any combination thereof. Software and data transferred via communications interface150are in the form of signals, which may be electronic, electromagnetic, optical, or other signals capable of being received by communications interface150. These signals are provided to communications interface(s)150via a communications path or channel. The channel may be implemented using wire or cable, fiber optics, a telephone line, a cellular link, a radio frequency (“RF”) link, or other communication channels.

In this document, the terms “computer program medium” and “non-transient machine readable medium” refer to media such as removable storage units148or a hard disk installed in hard disk drive144. These computer program products provide software to controller130. Computer programs (also referred to as “computer control logic”) may be stored in main memory140and/or secondary memory142. Computer programs may also be received via communications interface(s)150. Such computer programs, when executed by a processor(s)132, enable the controller130to perform the features of the method discussed herein.

In an embodiment where the method is implemented using software, the software may be stored in a computer program product and loaded into controller130using removable storage drive146, hard drive144, or communications interface(s)150. The software, when executed by a processor(s)132, causes the processor(s)132to perform the functions of the method described herein. In another embodiment, the method is implemented primarily in hardware using, for example, hardware components such as application specific integrated circuits (“ASICs”). Implementation of the hardware state machine so as to perform the functions described herein will be understood by persons skilled in the art. In yet another embodiment, the method is implemented using a combination of both hardware and software.

Controller130also includes a pulse generator152configured to output a variety of pulses to transducers102. For example, pulse generator152may transmit time-delayed control signals to transducers102, and/or pulse generator152may transmit control signals of varying amplitudes to transducers102.

An amplifier154is configured to amplify signals received from transducers102. Such signals received by transducers102include reflections of waves from structural features and other anomalies, e.g., corrosion in a plate or plate-like structures, in response to signals transmitted by pulse generator152. An analog to digital (“A/D”) converter156is coupled to an output of amplifier154and is configured to convert analog signals received from amplifier154to digital signals. The digital signals output from A/D converter156may be transmitted along communication infrastructure134where they may undergo further signal processing by processor(s)132as will be understood by one of ordinary skill in the art.

Turning now toFIG. 2, which illustrates one example of velocity dispersion curves for a zero degree fiber direction in a 16 layer quasi-isotropic composite plate with the first 8 modes by their respective numbers, i.e., 1, 2, 3, etc. There are infinite numbers of possible guided wave modes in a plate-like structure such as, for example, a composite plate. These wave modes in a plate have different phase and group velocities and energy distributions across the thickness, which may vary with frequency and/or excitation conditions. For guided wave beam steering or beam focusing, guided wave modes with similar velocities can be excited.

The guided wave modes with different velocities are considered as unwanted wave modes and may result in significant wave energy traveling to directions other than the desired beam steering direction or create energy focal points other than at the desired focal point. Furthermore, the velocity differences may introduce coherent noise in guided wave damage detection applications. For instance, if the pulse-echo method is used to detect a single defect, the received signal may have multiple reflected wave packets due to the existence of wave modes with different wave velocities. The redundant wave packets coming from the unwanted wave modes may cause false alarms. To avoid the influence of the unwanted wave modes, transducers with the capability of dominantly exciting guided wave energy with the desired wave velocity while minimizing the energy of the unwanted wave modes can be used. The design of such transducers can be carried out based on theoretical calculations. As described above, examples of such guided wave transducers include, but are not limited to, annular array transducers, time delay annular array transducers, piezoelectric elements on angle wedges, EMATs, and magnetostrictive transducers, to list a few possibilities.

With the energy of unwanted wave modes controlled, time delays can be applied to the transducers102to perform phased array beam steering or focusing. Each transducer102in the array103excites guided wave energy that can propagate in any direction. As described above, pulse generator152can transmit time-delayed control signals to transducers102to physically focus guided waves at a focal point or to form a guided wave beam in a particular direction. The direction of wave propagation can be controlled via a “phasing” approach.FIG. 3illustrates one example of the image obtained using a circular phased array of a portable system100, such as an array103of probe105described above in accordance withFIGS. 1B-1F, inspecting an aluminum plate. As shown inFIG. 3, by applying time delays to the array103, the individual transducer elements102can be “phased” in such a way to allow the guided wave energy to be steered in any direction.

The steering direction can then be controlled to allow 360° scanning. This is different from the guided wave array systems for plate structures that are presented in the articles “Tuned Lamb Wave Excitation and Detection with Piezoelectric Wafer Active Sensors for Structural Health Monitoring,” by V. Giurgiutiu; “Directional Piezoelectric Phased Array Filters for Detecting Damage in Isotropic Plates,” by Purekar et al.; “Omni-Directional Guided Wave Transducer Arrays for the Rapid Inspection of Large Areas of Plate Structures,” by P. D. Wilcox; and “On the Development and Testing of a Guided Ultrasonic Wave Array for Structure Integrity Monitoring,” by Fromme et al., the entireties of which are incorporated by reference herein. In those systems, only one element of an array is pulsed at a time, and, as a result, there are no physically formed guided wave beams. The “beam steering” or “focusing” of those arrays are conducted through post data acquisition signal processing only.

In contrast, the systems and methods disclosed herein generate a physically formed beam of guided wave energy and direct such physically formed beam to different directions by varying the phase delays applied to the different elements of the phased array in a so-called “real-time phased array approach.” Benefits of using the real-time phased array approach for guided wave inspection of plate structures include, but are not limited to, higher penetration power, better signal-to-noise ratio, and the capability of rapidly scan selected directions and/or locations, to list a few examples.

In some embodiments, hardware time delays are applied to a probe to physically form guided wave beams for different beam steering directions, and a back propagation wave number domain signal synthesis approach is utilized for the syntheses of both the pulse-echo signals received by the elements102of the phased array and the through-transmission signals received by the receiving array103. The back propagation wave number domain signal synthesis approach can be used in favor of a delay-and-sum time domain approach. Using plate structures as an example and taking into account the guided wave dispersion and the wave divergence in the plate, the time signal at a point located in the far field of an array element can be approximately expressed as:

S(ω) is the Fourier transform of the time domain guided wave input signal;

x is the distance away from the array element; and

k represents the wave number.

The wave number k is a function of circular frequency co for guided wave modes with dispersion. For the pulse-echo mode, the reflected guided wave signal introduced by a defect located in the far field of the array can then be approximately written as:

where δ is the signal magnification coefficient introduced by the constructive interference of the signals generated by all of the phased elements;

γ is the reflection coefficient;

rdis the distance from the defect to the center of the array;

the subscript n represents that the reflection is received by the nth array element; and

d denotes the propagation distance that needs to be compensated for beam steering to the angle where the defect locates.

The wave number domain signal synthesis of the signals described by Equation (2) can be conducted using the following equation:

N is the number of array elements, and

As shown in Equation (4), the dispersion relation of the guided wave modes is included in the back-propagation process so that the dispersion effects that could decrease defect detection resolution can be removed from the wave number domain synthesized signals. In some embodiments, Equation (3) can be implemented using Fast Fourier Transforms (“FFT”). The wave number domain signal synthesis is therefore also fast. An advanced deconvolution method can be combined with the real-time guided wave phased array and the wave number domain signal synthesis as well to suppress image artifacts caused by the side lobes of the phased array as disclosed in the Ph.D. thesis, “Ultrasonic Guided Wave Phased Array for Isotropic and Anisotropic Plates,” by F. Yan, the entirety of which is herein incorporated by reference.

FIG. 4Aillustrates one example of a phased array scanning image obtained in an experiment on a 4 ft.×4 ft. aluminum plate (1 mm thick) using the wave number domain signal synthesis with a fixed system100where the transducers102were secured to the plate10. A 16-element circular array103was mounted or otherwise secured to the approximate center of the plate using epoxy114as illustrated inFIG. 4B. Lead wires118electrically connected array103to a controller130(not shown). The phased array was operated under a pulse-echo mode, and the locations and shapes of the defects are indicated in the image for comparison. As can be seen, defects12,1416, and18, which are shown inFIGS. 4C, 4D, 4E, and 4F, respectively, were well detected and located. A 5 mm hole20was also detected and is visible in the image.

In some embodiments, computed tomography (“CT”) imaging techniques, such as those disclosed in “Ultrasonic Guided Wave Tomography in Structural Health Monitoring of an Aging Aircraft Wing,” by Gao et al., and “Large Area Corrosion Detection in Complex Aircraft Components using Lamb Wave Tomography,” by Royer et al., the entireties of which are herein incorporated by reference, are used in combination with guided wave activation and reception to accurately detect and locate corrosion and cracking in plate and pipe structures using a small number of sensors to interrogate relatively large areas. Using such a technique, a set of base-line data is acquired and then compared to subsequent data sets, and a CT image is generated by comparing changes in the guided wave signals that occur from damage being introduced into the part.

FIGS. 5A-5Cillustrate one example of the guided wave CT concept. All possible guided wave paths for a 16-element guided wave actuator/sensor network are illustrated inFIG. 5A. The guided wave actuators/sensors can be piezoelectric disk transducers, annular array transducers, magnetostrictive transducers, EMATs, or other suitable actuator/sensor. For structural health monitoring (“SHM”) applications, base-line guided wave signals are collected for all wave paths. Subsequent data sets are acquired in the same manner over time. Guided wave signal variations can be observed when damage occurs in the area covered by the wave paths. Apparently, the signal variations for different sensor pairs will be different. For example, the sensor pair102A-102B inFIG. 5Aproduces consistent signals before and after the corrosion damage occurs as illustrated inFIG. 5B. That is simply due to the fact that the corrosion damage is away from the wave path. In contrast, the sensor pair102A-102C inFIG. 5Aproduces significant signal variations before and after the corrosion because the damage is located in the wave path as illustrated inFIG. 5C. The systems disclosed herein include guided wave CT algorithms that utilize the signal variations for different wave paths to reconstruct CT images that reveal the location, approximate size, and severity of possible damage to the structure under monitoring. The algorithms are applicable to sensor arrays with arbitrary sensor placements and also take into account guided wave beam divergence in plate-like structures. Examples of such algorithms are disclosed in “Ultrasonic Guided Wave Tomography in Structural Health Monitoring of an Aging Aircraft Wing,” by Gao et al., and “Large Area Corrosion Detection in Complex Aircraft Components using Lamb Wave Tomography,” by Royer et al., the entireties of which are herein incorporated by reference.

Different features of the guided wave signal, such as amplitude ratios of different modes and/or time of flight, can be input into the reconstruction algorithm, which is executed by processor(s)132of controller130. Other features could come from a Fourier Transform, a short time Fourier Transform spectrogram, or a wavelet transform as examples. Different features are sensitive to different types of damage or material conditions.

FIG. 6Aillustrates one example of a first side10A of a 4 ft. by 4 ft. aluminum plate10on which 16 packaged piezoceramic sensors102are fixedly mounted (using epoxy or other adhesive) and electrically connected to a controller130(not shown) via leads118.FIG. 6Billustrates the opposite side10B of the aluminum plate20, which includes first and second defects, i.e., Defect 1 and Defect 2, respectively.FIGS. 7A-7Cillustrate sample results of data that were acquired before and after introducing the simulated corrosion defects on the “exposed” surfaceFIG. 6B, i.e., before and after Defect 1 and Defect 2 were formed. In particular,FIG. 7Aillustrates an example of a CT image showing the detection and imaging of Defect 1 inFIG. 6,FIG. 7Billustrates an example of a CT image showing the detection and imaging of Defect 2 inFIG. 6, andFIG. 7Cillustrates an example of a CT image showing the detection and imaging of both Defect 1 and Defect 2 inFIG. 6.

In some embodiments, piezoelectric disc transducers102, and/or guided wave transducers102with guided wave mode and frequency selection capabilities are used as guided wave CT sensors. Examples of guided wave sensors102include, but are not limited to, annular array transducers, time delay annular array transducers, piezoelectric elements on angle wedges, EMATs, and magnetostrictive transducers, to list just a few possibilities.

Ultrasonic guided wave signals taken from a guided wave CT system are generally complicated, and this is especially true when using guided wave CT for large area monitoring of structures with complex geometries, for instance, rivets, and stiffeners. The multiple guided wave scatterings and possible mode conversions at the geometry variations make guided wave signals hard to integrate. This is the main reason why most current guided wave CT systems use only the so-called damage indexes (“DI”) that are defined based on some overall changes in guided wave signals. An example of such a system is described in “Detection and Monitoring of Hidden Fatigue Crack Growth Using a Built-in Piezoelectric Sensor/Actuator Network: II. Validation Using Riveted Joints and Repair Patches,” by Ihn et al., the entirety of which is herein incorporated by reference.

With the controlled guided wave excitations provided by the guided wave transducers, the quality of the guided wave signals can be greatly increased, in the sense that the signals become much easier to integrate based on the knowledge of the guided wave inputs. Physically based guided wave features may then be extracted from the guided wave signals for damage detection and evaluation. Examples of such physically based features include, but are not limited to, amplitude ratios of different modes, mode conversions among different guided wave modes, phase shifts of a specific mode, TOF changes of different modes, and changes in dispersion characteristics, to list a few non-limiting examples.

Guided wave signals obtained with these types of transducers are easier to interpret due to the controlled guided wave input. However, because of possible wave scatterings and mode conversions which are actually quite common for structures with complex geometries such as rivets and stiffeners, advanced signal processing methods are used for accurate feature extractions. Many signal processing tools are available for guided wave signal analysis including, but not limited to, FFT based spectrogram, wavelet based scalogram, and Hilbert-Huang transform. Each of these signal processing techniques can be used to obtain time-frequency representations of guided wave signals for in-depth guided wave mode and frequency analyses.

In some embodiments, the two technologies, guided wave phased array beam steering and guided wave tomography, can be combined together to provide more reliable damage detection and characterization as well as to potentially reduce the sensor density.FIGS. 8A, 8B, and 9illustrate examples of the combination of the two technologies. Referring first toFIG. 8A, a plate-like structure00is provided with a plurality of sensors102being positioned about the periphery of plate10.

As shown inFIGS. 8A and 8B, a number of sensors102are placed close to the boundary of the plate-like structure10for guided wave tomography tests. In some embodiments, tomography sensors102are thin piezoelectric disks, piezoelectric cylinders, cuboid piezoelectric elements, magnetostrictive transducers, and EMATs, to list a few possibilities. Using the phased array concept, different phase delays can be applied to the tomography sensors102by pulse generator152of controller130(not shown) to generate physical guided waves in a particular direction to focus from outside-in or from random locations, i.e., to achieve constructive interferences at different locations. The constructive interferences can increase the guided wave energy for damage interrogation and therefore will yield better penetration distance and more reliable damage detection results.

Phase delays may also be applied two or more of the tomography sensors102to focus guided wave energy to or close to the locations of other tomography sensors102. Higher penetration power can be achieved with the phased array focusing. The phased delays may be applied to any tomography sensor groups. The locations of the focal points may be switched among different sensor locations as well. The received signals can be used for tomographic image reconstructions. In SHM applications, the “phasing” process can also be done with the residual signals that are calculated by subtracting base-line signals from the subsequently acquired signals. These calculations can be performed by processor(s)132of controller130as will be understood by one of ordinary skill in the art.

InFIG. 9, a plate or plate-like structure10is monitored using both tomography sensors102-1placed close to the plate edges and a probe105(not shown) including a phased array103of transducers102-2located near the center of the plate10. Transducers102-2of array103is used to direct guided wave energy30in different directions by steering the energy as described above. At least some of the guided wave energy is reflected or scattered by a Damage/Defect in plate10. This reflected/scattered guided wave energy, which is referenced by reference numeral32, can be detected by tomography sensors102-1. Thus, the combination of tomography sensors102-1and array103increase the probability of detection of scattered guided wave energy32, which is reflected/scattered at different angles. All scattered guided waves can be well recorded. Again, a phasing process may be applied to the signals received by the tomography sensors102-1to further improve the inspection results.

System100can also be used to inspect plate and plate-like structures that are subject to water loading conditions, such as ship hulls, storage tank floors, and the like. In such embodiments, guided wave transducers102are designed such that they will excite and/or receive guided wave energies that do not leak into water. Shear horizontal (“SH”) type guided waves with pure shear particle displacements on the structure surfaces do not leak into water and therefore are one example of a suitable transducer102for this type of application. Longitudinal type waves with dominant in plane displacement on the surface of a structure may also be used.

Referring now toFIGS. 10A and 10B, one example of a sensor102A in accordance with some embodiments. Sensor102illustrated inFIG. 10Ais implemented as a shear transducer and is based on a small shear polarized d15PZT element. The transducers/sensors102A are designed for the excitation and reception of SH type waves. As best seen inFIG. 10A, transducer102A includes a piezoceramic block158sized and configured to be received within an internal chamber defined by housing160. Conductive leads162, such as coaxial cable or other electrical wiring, are coupled to housing160and are disposed within a conduit164for electrical connection to a controller130(not shown). As can be seen inFIG. 10A, the size of sensor102A is less than that of a dime. In some embodiments, piezoceramic block158is glued or fixed in housing160using a conductive epoxy, glue, or soldering. The internal chamber of housing160can be back filled with epoxy or soldering to improve the robustness of sensor102A. An AC voltage166is applied to piezoceramic block158by conductive leads162as illustrated by the circuit diagram inFIG. 10Bto provide shear deformations of the piezoceramic block158.

Shear sensors102A in accordance withFIGS. 10A and 10Bwere designed and tested using a fixed system100where the sensors102A were fixedly coupled to plate or plate-like structure10. These tests demonstrate that shear sensors102A reduce and/or eliminate any negative effects, such as false alarms caused by water loading conditions, when performing guided wave SHM/NDE (nondestructive evaluation).

For example,FIG. 11Aillustrates one example of a plurality of shear sensors102A disposed on a surface of a plate10having a defect or damage in the form of corrosion thereon.FIG. 11Bshows the results of performing SHM/NDE of the setup illustrated inFIG. 11B. As shown inFIG. 11B, a system100configured with shear sensors102A was able to sense the corrosion on plate10as the corrosion is visibly presented inFIG. 11B.

FIGS. 11A and 11Bare in contrast withFIGS. 11C and 11D, which illustrate a piezoelectric sensor setup under a water loading condition where the piezoelectric sensors were not d15PZT elements and the resultant image, respectively. As shown inFIG. 11Dthe water present on plate10inFIG. 11Ctriggered a false detection of corrosion that was not present inFIG. 11B.

The shear polarized d15 PZT elements102A can also be used to form a compact phased array for guided wave beam steering. For example,FIG. 12illustrates an example shear PZT element array103disposed in a circular arrangement that is fixedly attached to a plate or plate-like structure10. Each element102A of the array103included a piezoceramic block158was mounted to the surface of a 0.375″ thick aluminum plate10that simulates a section of a ship hull. The results of the phased array defect detection of the setup illustrated inFIG. 12for monitoring the growth of a corrosion defect are shown inFIGS. 13A-13D. The corrosion defect was simulated by pitting. The density of the pit holes was increased to simulate defect growth. Three defect growth stages were monitored.FIGS. 13A-13Dpresent the phased array images for stages 1, 2, and 3 of the growth of the corrosion defect, respectively. The phased array data forFIGS. 13A-13Cwere collected when the plate was dry.FIG. 13Dshows the phased array image for the corrosion defect at stage 3 with the plate10subject to water loading. Clear defect indications can be seen in all four figures. The locations of the defect indications also very well agree with the actual corrosion defect location.

As described above, the system100can be configured to be portable with a probe105including an array103of guided wave phased array sensors102as illustrated inFIG. 1B. In some embodiments, sensors102are formed from d33PZT elements, d15shear PZT elements, magnetostrictive transducer elements, or EMAT elements, to list just a few possibilities. Such a portable system100can be used for NDE of ship hulls or other structures comprising plates or plate-like structures.

FIGS. 14A and 14Billustrate a pulse-echo phased array scanning image of a representation of a ship's hull. As shown inFIG. 14A, guided wave beam steering sends guided waves into different directions to look for defects, andFIG. 14Billustrates the defects identified by the probe. In pulse-echo mode, the transducers102of the phased array probe105detects defect reflections that propagate back to the probe position. The defect locations are determined by the beam steering angle, the time-of-flight (“TOF”) of the defect reflections, and the guided wave velocity. A pulse-echo phased array scanning image of the structure being inspected can be generated by varying phased array time delays to scan the regions of interest as shown inFIG. 14B. Such an image can be presented to a user on display138of controller130.

System100can be used for anisotropic multilayer composite plates or plate-like structures. As guided wave excitations become more complex when material anisotropy is involved, a Green's function based theoretical method can be employed to study the guided wave excitations in composite plate like structures as described in “Ultrasonic Guided Wave Phased Array for Isotropic and Anisotropic Plates,” by Yan. Amplitude and phase variations of the guided wave field excited by a point source applied normally to a composite plate are non-axisymmetric, but the point source itself can be considered as an axisymmetric loading. The angular dependencies of the amplitude for the mode 3 at 600 kHz and the mode 1 at 160 kHz calculated using the Green's function based method are shown inFIGS. 15A and 15B, respectively. As can be seen by comparingFIGS. 15A and 15B, the amplitude of the mode 3 changes much more dramatically as compared to the one of mode 1. The phased array beam steering directivity profile of a circular array for a composite plate can be calculated as:

where αg(φ) represents the angular dependence of the guided wave amplitude;

Φg(φ) is the corresponding angular dependence of phase variations,

R denotes the radius of the array,

ψndenotes the angular locations of the array elements, and

φ0is the beam steering angle.

Sample directivity profiles for the mode 3 at 600 kHz and the mode 1 at 160 kHz are given inFIGS. 16A and 16B, respectively. FromFIG. 16A, it can be seen that although the beam steering direction is 113 degrees, the strongest beam of the phased array output is close to the 150 degree direction, and the beam steering fails in other directions. The beam steering failure is due to the amplitude of the mode 3 reaching its minimum at the 113 degree direction as shown inFIG. 15A. The large amplitudes of the excited wave in other directions form strong side lobes.

In contrast, the mode 1 beam steering directivity profile for the 110 degree direction, which is the minimum amplitude direction for the mode 1, demonstrates a good beam steering capability inFIG. 16B. This is due to the fact that the amplitude variations of the mode 1 are much less severe than the mode 3. Thus, choosing the wave mode with less amplitude changes in different directions ensures good guided wave beam steering for all directions. Such selection can be made by reviewing the directivity profiles when developing signal processing and defect imaging algorithms.

An example guided wave phased array probe105, which includes a plurality of transducers102that are electrically coupled to a controller130(not shown), designed for beam steering in a composite plate is shown inFIG. 17. The composite array was designed for good beam steering directivity profiles for all directions in a 0.24 inch thick carbon composite plate. The mode 1 at 100 kHz was selected for such applications because the mode 1 is not sensitive to fiber orientations at low frequencies. As a result, the amplitudes of the mode 1 for different directions are close to each other.

FIGS. 18A-18Cillustrate comparisons between the measured directivity profiles of the array103and the theoretically calculated profiles. For example,FIG. 18Aillustrates a comparison between experimental results (trace “E”) and a calculated array defectivity profile (trace “C”) for a beam steering angle of zero degrees.FIG. 18Billustrates a comparison between experimental results (trace “E”) and a calculated array defectivity profile (trace “C”) for a beam steering angle of 60 degrees, andFIG. 18Cillustrates a comparison between experimental results (trace “E”) and a calculated array defectivity profile (trace “C”) for a beam steering angle of 120 degrees. As shown in each ofFIGS. 18A-18C, the experimental results agreement well with the calculated array defectivity profile. Thus, an array as shown inFIG. 17can be used to steer guided wave beams into any direction in the composite plate.

For some composite applications, guided wave energy can be focused in specific directions. In such applications, transducers102that excite guided waves with energy naturally focused to the desired directions are used. For composite materials with unknown material properties, multiple polar scans with different modes and frequencies may be applied to reduce effect of beam skewing, sidelobes, and to improve penetration power.

Turning now toFIG. 19, which is a flow diagram of one example of a method200of SHM/NDE of plates and plate-like structures using system100, the operation and use of system100is described. At block202, a focal point or guided wave beam steering direction on a plate or plate-like structure is selected. In some embodiments, for example, the focal point or guided wave beam steering direction is selected by a user. For example, a user can select a focal point or a guided wave beam steering direction to inspect a region of interest in the plate or plate-like structure. By changing the focal point or beam steering direction, method200can be repeated until a region of interest is completely inspected. In some embodiments, the selection of the guided wave beam steering direction is selected by system100, which can be configured to automatically perform an inspection of the an entire region of interest by repeating method200.

At block204, time delays and/or possible amplitude factors are calculated. In some embodiments, system100calculates the time delays and/or amplitude factors and locations of the transducers. For example, time delays are applied to the array elements to achieve constructive interference in the beam steering direction for the purpose of beam steering. For example,FIG. 20Aillustrates a wave path starting from an origin of a coordinate system as a reference. As shown inFIG. 20A, a time delay is used to compensate the phase difference from the wave generated by each element of the array. Letting Endenote the position of the nth transducer, {right arrow over (s)}ndenote the position vector from the origin to the nth transducer, {right arrow over (φ)} be the unit vector pointing to the steering direction, and c represent the wave velocity, the time delay for compensating the phase difference for the nth element can be written as:

Time delays are chosen to make the waves generated by all the transducers be focused at a focal point such that the waves arrive at the focal point at the same time. As illustrated inFIG. 20B, {right arrow over (r)}nis the vector pointing from the nth transducer to the focal point P, {right arrow over (r)} and is the vector from the origin of the coordinate system to the focal point. The time delay for the nth array element can be calculated as:

At block206, the calculated time delays and/or amplitude factors are applied to the array103of transducers102by controller130. Transducers102are either fixedly connected and/or are disposed in a probe105that is placed in contact with a surface of a plate or plate-like structure. As described above, the plate or plate-like structure can be an anisotropic plate including, but not limited to, a multilayer fiber reinforced composite plate. In some embodiments, such as embodiments in accordance with the embodiment depicted inFIG. 9, additional transducers102other than those transducers102disposed in a probe105and/or provided in first array103, are also placed on a surface of the plate or plate-like structure in an orderly arrangement or are placed randomly.

As described above, processor(s)132communicate with pulse generator152via communication infrastructure134causing pulse generator152to output control signals to transducers102in accordance with the time delays and/or amplitude factors. Transducers102cause one or more guided wave beams to propagate way from the array103.

At block208, reflections of the guided wave signals are received at one or more transducers102of array103. In some embodiments, such as embodiments in accordance with the embodiment depicted inFIG. 9, additional transducers102other than those transducers102that generated the guided wave receive the reflected guide wave energy alone or in combination with the transmitting transducers102. These additional transducers102can be disposed on the plate or plate-like structure in an orderly configuration or in a random configuration. The reflected signals received at transducers102are amplified by amplifier154and converted from an analog signal to a digital signal by A/D converter156. The digital signal can be forwarded to processor(s)132via communication infrastructure134as will be understood by one of ordinary skill in the art.

At block210, the received guided wave signals (e.g., reflected guided wave signals) are combined together. In some embodiments, the combination of the received signals is performed by processor(s)132, which combine together the digital representation of the signals received from A/D converter156from communication infrastructure134.

At block212, the combined signals are used to perform defect detection by processor(s)132. Possible defect reflections can be identified in the combined signals.

At block214, an image of the plate or plate-like structure including an identification of a location of one or more defects is generated by processor(s)132. In some embodiments, the generated image is displayed to a user on graphical interface/display138, which receives signals from processor(s)132via display interface136.

At block216, the inspection data (e.g., defect location data and/or graphical representation data) are stored in a non-transient computer readable storage medium. For example, the data can be stored in main memory140and/or secondary memory142in response to processor(s)132transmitting the data via communication infrastructure134.

FIG. 21Adiscloses one example of a circumferentially shear polarized d15piezoelectric ring element in accordance with some embodiments. When pulsed with one or more AC voltages from an AC voltage source252, circumferentially poled ring-type actuator251excites shear horizontal (SH)-type guided waves in plate-like structures using the d15piezoelectric mode. SH waves are not sensitive to liquid loading conditions that are frequently encountered in tank floor monitoring applications. Additionally, the fundamental SH wave mode, the SH0mode, is non-dispersive in isotropic structures such as steel plates. These circumferentially polarized shear elements oscillate in a torsional mode, which yields omnidirectional SH wave generation and reception. Analytical calculations, numerical finite element simulations, and/or experimental tests can be used to determine the dimensions of the piezoelectric ring transducers251for different applications, and are within the abilities of one of ordinary skill in the art. One example of a circular 16-element phased array253comprising a plurality of poled ring-type actuators251is illustrated inFIG. 21B.

In some embodiments, a structural health monitoring approach is adopted, in which guided wave imaging results and guided wave signals can be compared from data sets collected at different times. This approach improves defect sensitivity by removing pre-existing features such as welds, stiffeners, rivets, access ports, and other discontinuities from the image as only changes in the imaging results are considered. This approach is particularly advantageous for monitoring the growth of defects over time. In some embodiments, a fixture is applied to a structure to achieve consistent probe positioning and coupling.

In some embodiments, the SHM approach is realized by subtracting the image associated with a first state from the image associated with a second state. One example of this baseline subtraction approach is illustrated inFIGS. 22A-1-22A-4. For example,FIG. 22A-1shows a 16-element phased array shear ring probe253disposed on a steel plate structure254with welded steel stiffeners257before and after corrosion simulation defects255and256were introduced. During the SHM processing of this data, the image inFIG. 22A-2, which is associated with a defect-free state, is subtracted, point-by-point, from the image inFIG. 22A-3, which is associated with a state in which corrosion defects have been introduced to the structure. The resulting image shown inFIG. 22A-4includes a reduced amplitude of many of the static features, such as plate edges304and stiffeners305, with features330and331inFIGS. 22A-3 and 22A-4corresponding to defects255and256inFIG. 22A-1. The reduction in amplitudes of these static features advantageously improves the image quality and ease of interpretation.

In some embodiments, additional SHM processing can be applied to further reduce the amplitude of static features by applying a suppression algorithm to the two data sets. Due to natural variations in the amplitude of the reflections from various features in a structure, the static features are often not completely removed from the SHM image by the baseline subtraction algorithm. For example, edge reflections304and stiffener reflections305inFIGS. 22A-2 and 22A-3, respectively, are not fully removed in the image shown inFIG. 22A-4after baseline subtraction and appear as304-1and305-1, respectively, inFIG. 22A-4. These variations in reflection amplitudes can occur due to temperature variations, boundary condition changes, coupling variations, probe inconsistencies, and other variations that cannot be eliminated.

In some embodiments, the suppression algorithm divides a baseline subtraction SHM image point-by-point by an amplified version of a baseline image to produce a suppressed SHM image which greatly reduces or eliminates the static reflectors to yield a clearer image of defects. For example,FIG. 22B-1provides an image, identical to that ofFIG. 22A-3of the reflections generated by the discontinuities in the plate shown inFIG. 22A-1.FIG. 22B-2illustrates the amplified version of the baseline image shown inFIG. 22A-2, which is subtracted from the image inFIG. 22B-1to produce a clearer image of defects330,331as shown inFIG. 22B-3as compared to the baseline-subtraction image shown inFIG. 22A-4. The amplification of the baseline divisor,FIG. 22B-2, can be adjusted to suit the needs of the structure and the data.

In additional embodiments, a stretch suppression algorithm is applied in which the spatial region of influence of each reflector is extended in at least one dimension to account for minor misalignments of the static reflectors between the baseline and second images.FIGS. 22C-1-22C-4illustrate one example of this concept. For example, the original baseline image shown inFIG. 22A-2is stretched and amplified to generate the suppression image illustrated inFIG. 22C-2. The defect image shown inFIG. 22C-1is then divided by the stretched and amplified baseline image shown inFIG. 22C-2to obtain the stretch-suppressed SHM image shown inFIG. 22C-4. The stretch-suppressed SHM image shown inFIG. 22C-4exhibits a further reduction of static reflections314and315compared to SHM image shown inFIG. 22C-3, which was developed with an un-stretched suppression algorithm and contains artifacts313. Corrosion defect reflections330and331become more apparent after stretch suppression. The degree to which each reflector is extended during stretch suppression can be adjusted to suit the needs of the structure and the data.

An image realignment algorithm may also be applied which compensates for minor misalignment of the probe by adjusting the baseline and second images before applying the SHM routines. This alignment can, in some embodiments, be accomplished by calculating the two-dimensional cross-correlation of the baseline and second images and subsequently offsetting the second image in accordance with the offset required to maximize the cross-correlation value. This process aligns the static features that are common between the two images.

Amplitude scaling factors may be applied to the excitation signals applied across the terminals of the one or more transducer elements in order to reduce the amplitude of the sidelobes of beam directivity profiles during phased array focusing. For example,FIG. 23A-1is the beam directivity profile of a particular array without apodization, andFIG. 23A-2is the beam directivity profile of the same array with Hamming apodization. This method, known as apodization, can be applied using various windowing functions including, but not limited to, a Hann window, a Hamming window, a Blackman-Harris window, a flat-top window, and customized versions of one of these windows. As shown inFIGS. 23A-1 and 23A-2, the side lobes320-2inFIG. 23A-2are smaller than the side lobes320-1inFIG. 23A-1. However, the main beam lobe319-2inFIG. 23A-2is wider than the main beam lobe319-1inFIG. 23A-1.

FIG. 23B-1illustrates a Hamming window function in general terms, andFIG. 23B-2illustrates a Hamming window function as applied to a circular phased array for steering direction340. Similar apodization can be applied to the guided wave signals during post-processing. For example,FIG. 23C-1illustrates a SHM phased array image of the steel plate structure254with welded steel stiffeners257illustrated inFIG. 22A-1without apodization, andFIG. 23C-2illustrates the same image with apodization. Note that the amplitude of the sidelobes326is reduced with respect to the amplitude of the main beam reflection330from the corrosion defect. For the case of a Hamming window apodization function A(n,φ), the relative amplitudes of the various elements of a circular array can be calculated by applying the following equation:
A(n,φ)=α−(1−α)cos [π+2(ψn−φ)]
where

n is the array element number,

φ is the beam steering or focusing angle,

ψnis the angular position of array element n, and

α is a factor between 0.5 and 1.

One or more calibration targets may be affixed to the plate-like structure being inspected to act as references for NDE and SHM embodiments of the system. In some embodiment, for NDE, the one or more calibration targets can be used to measure guided wave velocity in the material, to perform a transducer and system self-check, and to achieve defect sizing. Since the dimensions and reflection characteristics of the one or more calibration targets are known, the guided wave reflections from the one or more targets can be compared to the reflections from defects in the structure to calculate the size of the defects. Several non-limiting embodiments of the calibration target include a metallic or polymer rod or block.

Additional calibration of the guided wave probe can be conducted separately from the structure by coupling the probe to a calibration plate or rod, such as the example of a Plexiglas cylinder327illustrated inFIG. 24for the purposes of calibrating probe328. Pulse-echo and pitch-catch measurements may be collected by each transducer element and compared to evaluate the health of each element and to calibrate the relative sensitivity of each element prior to data collection. Imbalances in the sensitivity of various elements in the guided wave phased array can lead to undesirable image artifacts and incorrect focusing. In some embodiments, the element calibration plate or rod is constructed of a material such as Plexiglas, which has a low wave velocity, in order to reduce the dimensions of the calibration specimen.

The disclosed systems and methods described above advantageously enable SHM/NDE of plates and plate-like structures using guided wave phased arrays. The plates or plate-like structures can be anisotropic materials, including multilayer fiber reinforced composite materials, and can be dry or under water/liquid loading conditions. The transducers of the disclosed systems can be individually or simultaneously excited and can be placed closely together on the structure to form a compact array and/or distributed on the structure at some distance away from each other in a random or orderly configuration. In some embodiments, the transducers include shear d15PZT type transducers for generating and receiving SH-type guided waves for applications on structures subject to water loading conditions. The disclosed systems use a number of pulser and receiver channels into which time delays can be input.

Additionally the disclosed systems can be used to perform real-time phased array beam steering and/or focusing utilizing guided wave transducers with mode and frequency selection capability for guided wave phased array and/or CT testing. Physically based guided wave features can be extracted from guided wave signals for damage detection and evaluation.

In some embodiments, the systems are configured to perform guided wave phased array tests or guided wave CT tests individually. In some embodiments, the systems also are configured to combine the guided wave phased array approach with the guided wave CT approach.

The disclosed systems and methods can be at least partially embodied in the form of program code embodied in tangible media, such as floppy diskettes, CD-ROMs, DVD-ROMs, Blu-ray disks, hard drives, or any other tangible and non-transient machine-readable storage medium, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the method. The disclosed systems and methods can also be embodied, at least partially, in the form of program code, for example, whether stored in a storage medium, loaded into and/or executed by a machine, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the program code is loaded into and executed by a machine, such as a computer, the machine becomes an apparatus for practicing the methods. When implemented on a general-purpose processor, the program code segments combine with the processor to provide a unique device that operates analogously to specific logic circuits.

Although the disclosed systems and methods have been described in terms of exemplary embodiments they are not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments of the disclosed systems and methods, which may be made by those skilled in the art without departing from the scope and range of equivalents of the systems and methods.