Ultrasonic Propagation Imaging Research Articles

Development of Wireless Ultrasonic Device for Wireless Guided-Wave Ultrasonic Propagation Imaging System

Development of Wireless Ultrasonic Device for Wireless Guided-Wave Ultrasonic Propagation Imaging System

Automatic defect detection for ultrasonic wave propagation imaging method using spatio-temporal convolution neural networks

Ultrasonic wave propagation imaging enables the detection of anomalies in various structures; hence, it has been applied as one of the promising techniques for damage identification in structural health monitoring (SHM). The interpretation of imaging data is vital to SHM; however, it relies significantly on expert subjective judgment, rendering the results vulnerable to human errors. Recent advances in the field of computer vision arising from the adoption of deep neural networks have resulted in new perspectives for substituting human roles in laborious data interpretation tasks. This paper presents an effective learning architecture that can characterize the ultrasonic wave propagation videos for automatic non-destructive inspection. The main contribution is threefold: 1. To the best of our knowledge, this is the first study to leverage video content analysis techniques to exploit ultrasonic wave propagation image series. Previous approaches that focused on the still wavefield images are likely to lose critical temporal information, thereby resulting in an inferior performance. 2. We devise a model that progressively aggregates both temporal and spatial information encoded in multiple adjacent snapshots of ultrasonic wave propagation motions for efficient data analysis. We presented the details regarding the system implementation and critical parameter settings. 3. The proposed approach is validated through extensive experimental comparisons with other state-of-the-art computer vision techniques on a real dataset which is publicly available. We hope that this study will encourage further investigations into video-based non-destructive data interpretation, not limited to ultrasonic signals.

Improving the Ability of a Laser Ultrasonic Wave-Based Detection of Damage on the Curved Surface of a Pipe Using a Deep Learning Technique.

With the advent of the Fourth Industrial Revolution, the economic, social, and technological demands for pipe maintenance are increasing due to the aging of the infrastructure caused by the increase in industrial development and the expansion of cities. Owing to this, an automatic pipe damage detection system was built using a laser-scanned pipe’s ultrasonic wave propagation imaging (UWPI) data and conventional neural network (CNN)-based object detection algorithms. The algorithm used in this study was EfficientDet-d0, a CNN-based object detection algorithm which uses the transfer learning method. As a result, the mean average precision (mAP) was measured to be 0.39. The result found was higher than COCO EfficientDet-d0 mAP, which is expected to enable the efficient maintenance of piping used in construction and many industries.

Detectable Depth of Copper, Steel, and Aluminum Alloy Plates with Pulse-Echo Laser Ultrasonic Propagation Imaging System

Pulse-echo laser ultrasonic propagation imaging is a nondestructive testing technique developed for composite materials and aluminum alloys used in aerospace. Although this method has been in usage for a considerable time, information of the detectable depth and the relationship between ultrasonic frequencies and the acoustic properties of metals is not readily available. Therefore, we investigate the A-scan and C-scan ultrasonic testing data of aluminum alloy, hot rolled steel, stainless steel, and copper alloy, which are used in aircraft bodies, frameworks, and gas pipelines. Experiments are performed with the pulse-width and excitation laser power fixed at 32 ns and approximately 4 W, respectively. The metal specimens include 24 artificial cylindrical defects with a diameter of 5 mm, located at depths of 1–12 mm on the front surface. The A-scan and C-scan data obtained at room temperature indicate the detectable depth for metals via the pulse-echo laser ultrasonic propagation imaging technique.

Novel distance‐based slicing algorithm for ultrasonic propagation imaging

In this paper, a novel distance-based slicing (DBS) algorithm is proposed for the visualization of inclusion defects within a thick solid structure. The algorithm is based on the propagation distance traveled by the ultrasonic wave signals and was integrated with the generation laser scanning ultrasonic propagation imaging (UPI) system in which the fixed sensor was positioned to detect the induced through-the-thickness ultrasonic signals. The advantages provided by the UPI system allow for an in-situ non-destructive evaluation for real world applications. An investigation of an isotropic solid specimen of large thickness with multiple internal inclusion defects of different shapes and depth location was performed using the proposed algorithm and system. The results in the form of ultrasonic propagation freeze frames show that the algorithm is much more effective in detecting each individual defect and is particularly useful for a thick-walled specimen. Additionally, a relationship was determined between the visualized result of each defect and the source position during data acquisition which enabled us to estimate the depth location of the defect within the specimen.

Defect visualization of cylindrical and conical CFRP lattice structures using rotational ultrasonic propagation imager

A composite lattice structure has been used in an aircraft and a space launcher to reduce the structural weight. Defects can occur during fabrication because of structural complexity and they are mostly invisible as occurring inside a structure. Also, they lead to the deterioration of structural performance and expensive failures. Therefore, non-destructive evaluation (NDE) to detect defects should be conducted not to damage an initially intact part. So far, various NDEs methods have been attempted, however, a standard inspection method has not been proposed for the large and curved lattice structures. In this study, two modified rotational through-transmission (TT) ultrasonic propagation imagers (UPIs) are proposed to inspect a skinless cylindrical lattice structure and a conical lattice structure with skin. In the first case of the skinless cylindrical specimen, there is a problem with the breakdown of the laser Doppler vibrometer (LDV) when an ultrasonic generation beam directly entered the LDV through an empty space of the specimen. For this issue, a wavelength domain beam splitting mechanism was included in the system. A proof-of-concept for the proposed inspection setup was performed using unit cells of the lattice structure, and defects could be visualized. The results were reliable compared with the radiation tomographic results. We also inspected the full-scale skinless lattice structure and visualized its internal defects. Conventional radiographic and visual inspection verified the location to be the same as that obtained using the TT UPI. In the second case of the conic specimen with skin, there is a problem with a change in a stand-off distance due to its shape. The change of the stand-off distance causes a steady decrease in signal-to-noise ratio (SNR), a change in the incident angle of the LDV beam and a change in a time-of-flight (ToF) at a received signal. These make defect visibility worse. For this issue, an additional manual rotational stage was incorporated into the rotational TT UPI to ensure that the sensing beam from the LDV would be perpendicularly incident on the skin surface. First, an appropriate scanning width was determined by measuring the variation in the SNR based on the stand-off distance. Furthermore, a ToF compensation algorithm was used to match the arrival times of signals for conic geometry. Then, the full-scale conical lattice structure with skin was inspected, and de-bonding defect was visualized via ultrasonic wave propagation imaging. As a result, it was possible to quickly inspect a large area of composite lattice structures regardless of the shape condition.

Simultaneous external and internal inspection of a cylindrical CFRP lattice-skin structure based on rotational ultrasonic propagation imaging and laser displacement sensing

Composite lattice-skin structures have been widely used in the aerospace industry to reduce structure weight and fabrication cost. The filament winding technique can be used to manufacture them automatically. However, several defects can occur in every manufacturing step, such as outward dimples (in which the skin swells during the curing process) and de-bonding (in which the rib and skin detach). They drastically reduce the buckling strength under compression and should be evaluated before operation. In this study, we simultaneously inspected the outside and inside of a cylindrical composite lattice-skin structure and visualized external and internal defects based on laser displacement sensing and rotational ultrasonic propagation imaging. An external inspection algorithm that used a laser displacement sensor was added to a previously developed internal inspection algorithm. Then, the accuracy of the laser displacement sensor was confirmed by measuring the groove depth of the composite brake disk. Thereafter, external and internal inspection was simultaneously performed on the composite lattice-skin structure. This system can clearly visualize not only external defects (dimples) using laser displacement sensing with 2D median filtering and 2D fast-Fourier transform but also internal defects (de-bonding) using ultrasonic propagation imaging.

Reverberation-based high-speed guided-wave ultrasonic propagation imager for structural inspection of thick composites

Increasing use of thick composite structures in many fields, including the aerospace and shipbuilding industries, necessitates appropriate inspection techniques to ensure structural integrity. Of many nondestructive testing techniques, laser ultrasonic testing is a non-contact method that yields accurate inspection results with high spatial resolution; this method was adopted in this study for the inspection of thick composite structures. We propose a novel guided-wave ultrasonic propagation imaging (UPI) method, called the reverberation wave UPI (R-wave UPI), which is capable of faster target scanning and clearer highlighting of confining waves. To prove the merits of the R-wave UPI, a relatively thin carbon/epoxy composite (thickness, 12 mm) was first tested using the proposed method and the conventional through-transmission UPI. The R-wave UPI modifies the guided-wave UPI with a faster pulse repetition frequency (PRF) laser and ultrasonic energy mapping with straightforward parameter determination. The through-transmission UPI consisting of a raster mechanical scanner, a Q-switched laser, and a laser Doppler vibrometer, was used as a reference, whereas the R-wave UPI consisting of an angular laser mirror scanner, a Q-switched laser, and a PZT sensor was operated at a high PRF in the kilohertz regime to induce reverberation. The reverberation phenomenon was used in the post-image processing, called ultrasonic energy mapping (UEM). UEM with a high PRF was then compared to the through-transmission inspection reference to verify its accuracy and reliability for mapping of structural anomalies. Finally, a thick and curved composite blade with a rough and curved surface that could not be inspected using the conventional through-transmission UPI was tested using 7 kHz PRF and its delamination was successfully visualized using the UEM.

Development of rotational pulse-echo ultrasonic propagation imaging system capable of inspecting cylindrical specimens

A rotational pulse-echo ultrasonic propagation imager that can inspect cylindrical specimens for material nondestructive evaluations is proposed herein. In this system, a laser-generated ultrasonic bulk wave is used for inspection, which enables a clear visualization of subsurface defects with a precise reproduction of the damage shape and size. The ultrasonic waves are generated by a Q-switched laser that impinges on the outer surface of the specimen walls. The generated waves travel through the walls and their echo is detected by a Laser Doppler Vibrometer (LDV) at the same point. To obtain the optimal Signal-to-Noise Ratio (SNR) of the measured signal, the LDV requires the sensed surface to be at a right angle to the laser beam and at a predefined constant standoff distance from the laser head. For flat specimens, these constraints can be easily satisfied by performing a raster scan using a dual-axis linear stage. However, this arrangement cannot be used for cylindrical specimens owing to their curved nature. To inspect the cylindrical specimens, a circular scan technology is newly proposed for pulse-echo laser ultrasound. A rotational stage is coupled with a single-axis linear stage to inspect the desired area of the specimen. This system arrangement ensures that the standoff distance and beam incidence angle are maintained while the cylindrical specimen is being inspected. This enables the inspection of a curved specimen while maintaining the optimal SNR. The measurement result is displayed in parallel with the on-going inspection. The inspection data used in scanning are mapped from rotational coordinates to linear coordinates for visualization and post-processing of results. A graphical user interface software is implemented in C++ using a QT framework and controls all the individual blocks of the system and implements the necessary image processing, scan calculations, data acquisition, signal processing and result visualization.

Artificial Intelligence-Based Bolt Loosening Diagnosis Using Deep Learning Algorithms for Laser Ultrasonic Wave Propagation Data

The application of deep learning (DL) algorithms to non-destructive evaluation (NDE) is now becoming one of the most attractive topics in this field. As a contribution to such research, this study aims to investigate the application of DL algorithms for detecting and estimating the looseness in bolted joints using a laser ultrasonic technique. This research was conducted based on a hypothesis regarding the relationship between the true contact area of the bolt head-plate and the guided wave energy lost while the ultrasonic waves pass through it. First, a Q-switched Nd:YAG pulsed laser and an acoustic emission sensor were used as exciting and sensing ultrasonic signals, respectively. Then, a 3D full-field ultrasonic data set was created using an ultrasonic wave propagation imaging (UWPI) process, after which several signal processing techniques were applied to generate the processed data. By using a deep convolutional neural network (DCNN) with a VGG-like architecture based regression model, the estimated error was calculated to compare the performance of a DCNN on different processed data set. The proposed approach was also compared with a K-nearest neighbor, support vector regression, and deep artificial neural network for regression to demonstrate its robustness. Consequently, it was found that the proposed approach shows potential for the incorporation of laser-generated ultrasound and DL algorithms. In addition, the signal processing technique has been shown to have an important impact on the DL performance for automatic looseness estimation.

Robotic scanning technology for laser pulse‐echo inspection

The full-field pulse-echo ultrasonic propagation imaging (PE UPI) system using a two-axis linear stage has been successfully used to perform a non-destructive evaluation of flat specimens. This study used a six-axis robot arm to develop a robotic scanning algorithm that can be applied to the laser pulse-echo inspection of non-flat objects. The robotic PE UPI system was constructed and verified for both flat and curved specimens. The internal structures and defects of a honeycomb sandwich plate and a glass-fibre-reinforced polymer specimen were visualised by the robotic PE UPI system. The quality of the evaluation of the flat specimen was equivalent to that of a two-axis linear system, and the result of the curved specimen showed improvement compared to that of the linear system. The ability of the robotic scanning technique to evaluate the structural health performance of arbitrarily shaped surfaces was validated.

Bolt looseness detection based on ultrasonic wavefield energy analysis using an Nd:YAG pulsed laser scanning system

This paper proposes a bolt looseness detection method based on the ultrasonic wavefield energy in the Lamb wave reflected from a target bolt. The full wavefield inside the area of the specimen containing the target bolt was first visualized. The ultrasonic wave propagation imaging (UWPI) approach was used for visualizing the interaction of the wavefield with the elements on the specimen such as the bolts and the boundaries of the specimen. The interaction between the ultrasonic wave and the airgap between the bolt head and the plate was interpreted based on the microcontact theory. This theory, when applied to the interface of two contacting surfaces such as those joined by a bolt, generally states that by increasing the fastening torque, the true contact area will increase and the wave will propagate across the interface with less energy loss. To apply the theory, the reflected wave from the target bolt was isolated from the incident wave and was further assessed to extract a feature indicating the looseness or tightness. The variation of the energy in the wave reflected from the target bolt with varying magnitudes of torque was determined by performing a continuous wavelet transform-based energy computation. The reflected wave energies of the different torque values were first compared by considering single time instants starting with the moment the wave was first observed in the UWPI reflecting from the target bolt. Then, energy accumulation was used to obtain the overall energy differences that resulted solely from the torque differences.

Determination of Plate Corrosion Dimension Using Nd:YAG Pulsed Laser-generated Wavefield and Experimental Dispersion Curves.

Corrosion detection using a pulsed laser scanning system can be performed via ultrasonic wave propagation imaging. This method outputs illustrations of the wave field within the host structure; thus, it can depict wave–corrosion area interactions. Additionally, post-processing can be performed to enhance the visualization of corroded areas. The wavefield energy computed using RMS (Root Mean Square) is a validated post-processing tool capable of displaying the location and area of corrosion-damaged regions. Nonetheless, to characterize corrosion, it is necessary to determine its depth. The measurement of depth in conjunction with that of the corroded area via the RMS distribution enables the determination of all dimensions of corrosion damage. Thereafter, the flaw severity can be evaluated. This study employed a wavefield within a plate on which corrosion was developed artificially to generate frequency–wavenumber dispersion curves. The curves were compared with their counterparts from a corrosion-free plate. Alternatively, they could be compared with dispersion curves drawn using the depth and material properties of a pristine plate via a computer program. Frequency–wavenumber pairs were extracted from the dispersion curves produced using the portion of the wavefield within the corroded area. These were inserted into the Rayleigh–Lamb equation, from which depths were calculated and averaged.

Filament-wound composite pressure vessel inspection based on rotational through-transmission laser ultrasonic propagation imaging

In this paper, we introduce a rotational through-transmission ultrasonic propagation imaging system to inspect the full thickness of the entire cylindrical section of filament-wound composite overwrapped pressure vessels. This system can achieve high signal-to-noise ratio throughout the cylindrical section of pressure vessels by using a rotational scanning method. Based on the advantages of the laser ultrasonic system that can generate and detect ultrasound waves by using a small-diameter laser beam and making an inspection path in a narrow environment, this system directs the laser beam inside the pressure vessel via a mirror to enable the through-transmission mode for vessel inspection. A type-3 pressure vessel was inspected using the system developed, which successfully detected two impact damage spots and showed the filament winding direction clearly. The test results show that the system developed is highly reliable and can replace the current hydrostatic sample test for pressure vessel safety certification.

Investigation of the damage effect on electromagnetic performance evaluation of a radar absorbing structure

Stealth technology encompasses various techniques to minimize the detection of acoustic, visual, infrared, and electromagnetic (EM) wave signals. Specifically, to minimize the radar cross section (RCS) of the radar system, radar-absorbing structures (RAS) capable of absorbing EM waves are essential. RAS composed of composites can be damaged, and both their structural and EM performances become important issues in RAS studies. Therefore, in this study, EM wave interactions in damaged specimens were evaluated and visualized using a scanning free-space measurement (SFM) system. First, the change in the EM performance due to damage was analyzed using Al-alloy 6061 and Teflon specimens. When the damage size was less than one-half wavelength (30 mm and 20 mm) in both the X (8.2∼12.4 GHz) and Ku (12.4∼18 GHz) bands, the return loss of Al-alloy 6061 was close to 0 dB and the real part of the permittivity was between 2 and 2.1. On the other hand, when the damage size was more than one-half wavelength, the EM performance in terms of the return and insertion loss and the EM properties of the material (such as the permittivity) were changed in both Al-alloy 6061 and Teflon specimens. Second, a nickel fiber-based RAS with lightning strike-induced damage was also tested by the SFM system, and a behavior similar to that of non-stealth specimens was observed. The RAS was also tested by a pulse-echo ultrasonic propagation imager (PE-UPI) system to investigate the internal damage induced by the lightning strike. Much larger delaminations than the penetration occurred at different plies of the RAS. Eventually, it was found that not only the penetration damage but also the delaminations predominantly affected the EM performance, which caused the overall stealth degradation.

Development of autonomous target recognition and scanning technology for pulse-echo ultrasonic propagation imager

This article proposes a pulse-echo ultrasonic propagation imaging system that is capable of autonomous target recognition and scanning an area of a customizable shape for the non-destructive evaluation of structural defects. The proposed system employs through-the-thickness bulk waves for ultrasonic inspection, which is achieved by joining two laser beams: one each for the ultrasonic wave generation and sensing. Moreover, the system is capable of autonomously suggesting a suitable inspection area for a specimen placed in front of the scanning head. The scan area delimitation algorithm uses the specimen image and ascertains the specimen border by means of edge and contour detection operations, following which a scan area closely conforming to the specimen boundary is suggested. The system can scan an area of any arbitrary shape, thereby preventing any wasteful operations that may result from fixed shape (rectangular or square) scanning. A Q-switched laser is used for generating the aforementioned ultrasonic waves, while a laser Doppler vibrometer is used for sensing these signals. A dual-axis automated translation stage is applied for raster scanning of the specimen, and a data acquisition card is employed for taking measurements. A camera mounted on the scan head is used for imaging the specimen for the scan area detection. Graphical user interface software controls all the individual blocks of the system, while implementing the required image processing, scan area detection, signal acquisition, signal processing, and result display. The graphical user interface is created in C++ using the Qt framework. Moreover, Qt Widgets for Technical Applications is used for the result display, and the Open Source Computer Vision Library is employed for the implementation of basic image processing algorithms. Multi-threading is used for real-time updating of the scan results while the scanning is ongoing.

Broadband Laser Ultrasonic Excitation and Multi-band Sensing for Hierarchical Automatic Damage Visualization

It is well known that the selection of a frequency for signal generation and data acquisition significantly affects the quality of ultrasonic or laser ultrasonic nondestructive testing (NDT) results, and the analog filter is essential to prevent the aliasing effect. This paper presents a systematic approach using a hierarchical inspection scheme for automatic inspection processes. A frequency band divider (FBD) with a single-input–multiple-output (SIMO) system was newly created to examine quickly various frequency ranges at single scanning using laser-based broadband excitation. The FBD was implemented into a pulse-echo ultrasonic propagation imaging (PE UPI) system capable of fully noncontact laser ultrasonic inspection. Only two scans are required to determine the optimal frequency range for unknown specimens. This tremendously reduces the number of scanning trials for the investigation of complete frequency ranges.

Damage visualization of a cylindrical CFRP lattice-skin structure based on a pulse-echo ultrasonic propagation imager

Recently, composite lattice structures are attracting significant attention owing to their high specific stiffness and specific strength and are mainly used in aerospace applications. However, the composite lattice structure also exhibits damage occurrence probability during operation as well as defects in the filament winding manufacturing process. Thus a non-destructive evaluation (NDE) technique for defect and damage evaluation is essential although its complexity is not desirable, especially with respect to in-situ NDE. In this study, we propose a pulse-echo ultrasonic propagation imaging (PE UPI) system for in-situ NDE of cylindrical composite lattice-skin specimen and develop a VTWAM (Variable time window amplitude mapping)-based curvature-compensating algorithm that eliminates the curved surface effect of the PE UPI system. As a result of the actual application to a damaged lattice cylindrical specimen, the damage position, size, and shape are clearly visualized by effectively suppressing the curved surface effect in the inspection region via the VTWAM-based curvature-compensating algorithm. We demonstrate the capability of the linear scan PE UPI to inspect a curved and complex lattice-skin structure with a curved surface compensation algorithm and propose the same as a tool for in-situ health management of composite lattice structures for manufacturing to in-service stages.

Nondestructive and electromagnetic evaluations of stealth structures damaged by lightning strike

Stealth technology is very important for the survival of military aircraft. A stealth aircraft structure has both electromagnetic and mechanical functions. Lightning can cause failure on both the points. In this study, we claim that the stealth structure should be evaluated nondestructively and electromagnetically, and we propose a method for full-field evaluations of both the functions. First, a radar absorbing structure was designed and fabricated with stealth capability in the X-band. The radar absorbing structure consisted of a carbon nanotube layer (glass/epoxy dispersed with multiwalled carbon nanotubes), a spacer layer (glass/epoxy) and a perfect electrical conductor layer. A lightning test was performed using an impulse current generator according to standard regulations. Then, nondestructive damage and electromagnetic performance evaluations were performed using a pulse-echo laser ultrasonic propagation imager and a scanning free-space measurement system, respectively. The results showed that neither structural damage nor changes in the electromagnetic properties were observed during the two evaluations. In general, the composites were severely damaged by lightning. However, it turned out that the radar absorbing structure with the carbon nanotube layer could prevent serious damage to stealth function as well as material damage owing to the high conductivity of the carbon nanotubes dispersed in its surface layer.

Nondestructive detection of incipient thermal damage in glass fiber reinforced epoxy composite using the ultrasonic propagation imaging

Incipient thermal damage (ITD) in polymer-matrix composites could cause substantial loss of material properties and it is notoriously difficult to be detected using nondestructive evaluation (NDE) techniques. The emerging ultrasonic propagation imaging (UPI) could be the solution for the detection problem and its detection probability for ITD was studied in this work. Glass fiber reinforced epoxy plate specimens insulted at temperatures ranging between 1.1 and 1.7 times of the glass transition temperature (Tg) for duration ranging between 5 and 30 min were inspected. The results in the form of statistically thresholded anomaly maps showed clear detection of ITD for specimens insulted at 1.4 and 1.7 times of Tg for 30 and 5 min, respectively, and partial detection for specimens insulted at 1.3 and 1.5 times of Tg for 30 and 10 min, respectively. Finally, an insulting duration-temperature plot with distinguished ITD undetectable, partially detectable, and fully detectable zones was estimated for references of future works.