Ultrasonic Propagation Imaging System 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

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.

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.

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.

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.

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.

Composite repair patch evaluation using pulse-echo laser ultrasonic correlation mapping method

Composite patch bonding repair is widely applied to metal and composite aircraft repair owing to its light weight and high resistance to fatigue and corrosion. However, composite patch repairing is done by vacuum bag processing in general, and the process itself is susceptible to manufacturing defects. In addition, the composite repair patches require more frequent inspections owing to the difficulty of the damage prediction. In this paper, we propose pulse-echo laser ultrasonic correlation mapping for a rapid quality evaluation method of composite repair patching. This method uses a pulse-echo ultrasonic propagation imaging system as a tool for in-situ and full-field inspection, and an ultrasonic correlation mapping (UCM) method was developed for fast and straightforward defect visualization. Two composite patch repair specimens were tested, and then UCM results were compared with other results using existing visualization methods. We also tested a composite laminate plate to verify the applicability of UCM to general composite parts. The UCM results showed similar or better performance than current visualization methods in all cases. Test results showed that the proposed system is suitable for both in-process quality evaluation and in-service field inspection.

Application of the ultrasonic propagation imaging system to an immersed metallic structure with a crack under a randomly oscillating water surface

Ultrasound is widely used and studied to satisfy the increased demands of the Non-destructive evaluation (NDE) and testing of underwater structures. However, because of the large size and mass of underwater structures, such as submarines, ship hulls, or nuclear reactor pipe lines, it is difficult to inspect the structures during operation. This underwater NDE technology is challenging but could be highly beneficial because the time and cost of maintenance will be effectively reduced. We propose an NDE method for immersed structures using an ultrasonic propagation imaging system with a piezoelectric sensor. The underwater sensing capability of a piezoelectric sensor is experimentally demonstrated using an aluminum plate specimen. A piezoelectric sensor can compensate for the decreased signal amplitudes due to leaky waves that are generated on interfaces between structures and water, since water transmits signals better than air. Additionally, a piezoelectric sensor can be applied even if the water surface is oscillating. Using these properties, the laser induced guided Ultrasonic propagation imager (UPI) inspected a T-shaped steel structure with a 2-mm crack on the weld zone. The inspection was implemented in three cases: a specimen without water, a specimen immersed in water and a specimen immersed in water with a randomly oscillating surface. The crack was visualized and measured using the ultrasonic wave propagation imaging algorithm, the adjacent wave subtraction algorithm, and the variable time window amplitude mapping algorithm. In the case with a randomly oscillating water surface, the laser pulse was refracted randomly based on Snell’s law. This phenomenon may cause degradation of the inspecting results. However, a repeated scanning process and outlier elimination led to an improved signal-to-noise ratio such that it was able to detect the crack. These results demonstrate the possibility to apply the laser UPI to submerged structures even if the water surface is randomly oscillating.

Dual-energy wave subtraction imaging for evaluation of barely visible impact damage with an ultrasonic propagation imaging system

Barely visible impact damage from low-velocity impacts have been studied as critical design factors of composite structures. In this article, a dual-energy wave subtraction algorithm using an ultrasonic propagation imaging system is proposed to evaluate barely visible impact damage as a strategy of fast in situ nondestructive evaluation or structural health monitoring (SHM). The ultrasonic propagation imaging system is a type of nondestructive evaluation or SHM system and is based on scanning laser-induced guided ultrasound and fixed sensors. The amplitude of ultrasonic signals generated by the ultrasonic propagation imaging system increases with the increasing energy of the laser beam. Two ultrasonic signals generated by different excitation energies of the laser beam can be equalized by multiplying a constant factor to one of them. Therefore, the residuals after subtraction of two signals may be close to zero. However, the two different energy induced signals in the damaged area will be nonzero due to the change in material conditions regarding the laser ultrasonic generation mechanism. The dual-energy wave subtraction algorithm eliminates most of the incident ultrasonic waves and amplifies anomalous waves. A composite wing skin including two barely visible impact damages as well as a composite sandwich panel, including a single barely visible impact damage, were inspected to validate the proposed algorithm.

FPGA-based ultrasonic energy mapping with source removal method for damage visualization in composite structures

This paper presents the implementation of an ultrasonic energy mapping (UEM) method in a field programmable gate array (FPGA)-based processing board. This method is integrated into an ultrasonic propagation imaging (UPI) system for real-field application. UEM method involves two-dimensional Fourier transform and wavenumber filtering to capture the energy scattering waves. Performing multi-dimensional Fourier transform in FPGA is a challenging task because it cannot fit into a single FPGA chip, and therefore, external memory is required to handle a large amount of data. The row–column decomposition method is used to perform 2D Fourier transform using two DDR3 SDRAM memories. Additionally, a source removal function is added into the system to highlight the damage when high energy at the source location occurs. The UPI system was applied for inspection of a composite wing box with impact damages. The UEM results showed that a damage of 20 × 20 mm2 was detected successfully in the presence of the source inside the scan area. Moreover, the UEM result is produced in less than 5 s for 57,600 points, which is practical for a real-world, non-destructive testing application.

FPGA-based design and implementation of data acquisition and real-time processing for laser ultrasound propagation

Ultrasonic propagation imaging (UPI) has shown great potential for detection of impairments in complex structures and can be used in wide range of non-destructive evaluation and structural health monitoring applications. The software implementation of such algorithms showed a tendency in time-consumption with increment in scan area because the processor shares its resources with a number of programs running at the same time. This issue was addressed by using field programmable gate arrays (FPGA) that is a dedicated processing solution and used for high speed signal processing algorithms. For this purpose, we need an independent and flexible block of logic which can be used with continuously evolvable hardware based on FPGA. In this paper, we developed an FPGA-based ultrasonic propagation imaging system, where FPGA functions for both data acquisition system and real-time ultrasonic signal processing. The developed UPI system using FPGA board provides better cost-effectiveness and resolution than digitizers, and much faster signal processing time than CPU which was tested using basic ultrasonic propagation algorithms such as ultrasonic wave propagation imaging and multi-directional adjacent wave subtraction. Finally, a comparison of results for processing time between a CPU-based UPI system and the novel FPGA-based system were presented to justify the objective of this research.

Multi-directional adjacent wave subtraction and shifted time point mapping algorithms and their application to defect visualization in a space tank liner

We propose the multi-directional adjacent wave subtraction (MAWS), shifted time point (STP) mapping and variable time window amplitude (VTWA) mapping algorithms for crack visualization based on our finding that the processing direction of the adjacent wave subtraction with respect to crack orientation can dramatically affect crack visibility. The proposed MAWS, STP and VTWA mapping algorithms were implemented in a laser ultrasonic propagation imaging system and were applied in a nondestructive evaluation of an Al-alloy tank liner for space applications. A crack and void on the fusion zone of a weld were more effectively visualized using the proposed algorithms, and the crack could be visualized regardless of its orientation. The multi-directional processing results of the anomalous wave propagation imaging were visualized as video clips. The STP mapping algorithm, which maps the number of shifted time points to minimize the difference between adjacent waves, was developed to enhance the crack visualization capability of the system. Additionally, the multi-directional VTWA mapping algorithm performed data projection using the residuals of MAWS processing and provided stationary mapping results of crack-induced anomalous waves. The presented results demonstrate that the proposed visualization algorithms are promising techniques that do not overlook unknown crack orientations.

전영역 펄스-에코 초음파전파영상화 시스템의 CN-235의 도색된 샌드위치 조종면 In-situ 비파괴평가 기술

전영역 펄스-에코 초음파전파영상화 시스템의 CN-235의 도색된 샌드위치 조종면 In-situ 비파괴평가 기술

In situ non-destructive evaluation of an aircraft UHF antenna radome based on pulse-echo ultrasonic propagation imaging

Most aircraft antennas are protected by radomes made of composite materials. However, antenna radome structures are easily damaged by external forces and internal manufacturing defects. To detect damage in the composite antenna radomes while in service, a full-field pulse-echo ultrasonic propagation imaging (PE UPI) system is proposed as an in situ non-destructive inspection technique, which is mobile inside a hangar. The radome is generally a double-curvature structure, and thus, a curvature compensation algorithm is also developed to increase damage visibility. The proposed hardware and software have been tested by detecting a resin rich region formed between the E-glass/epoxy plies. To visualize the damage, PE ultrasonic wave propagation imaging has been performed, and ultrasonic energy mapping has also been applied. A curvature compensating algorithm has been newly developed and compared for its performance in damage visibility improvement. To verify the size of the real defects of a UHF antenna radome, destructive testing has been performed by cutting the defect area, polishing the cross section and investigating the section using a 50× microscope. The results have shown the feasibility of the proposed PE UPI system and the inspection strategy as an in situ non-destructive evaluation technique for radome structures.

Defect visualization of aircraft UHF antenna radome using full-field pulse-echo ultrasonic propagation imaging system

Most of aircraft antennas usually have various types of radome made of composite materials for protecting antenna structures. However, these antenna radome structures, which are installed on the outside of airplane, are easy to be damaged by external forces such as drag, foreign object, bird strike and others. In this study, full-field pulse-echo ultrasonic propagation imaging (PE UPI) system is proposed as the non-destructive inspection technique to visualize manufacturing defects in composite antenna radome. Based on the results of the sample case study, it is shown that the ultrasonic wave propagation imaging (UWPI) that is generated by the proposed full-field PE UPI system is able to highlight the intact internal condition of antenna structure and its defect area. Additional damage visualization techniques like ultrasonic energy mapping (UEM), variable time window amplitude map (VTWAM) and also ultrasonic spectral imaging (USI) algorithms are applied to improve the reliability of the damage visualization. It can be concluded that the proposed PE UPI system is an effective non-destructive inspection technique for the composite radome structures.

Development of a long-range multi-area scanning ultrasonic propagation imaging system built into a hangar and its application on an actual aircraft

Materials such as aluminum alloys and carbon composites are widely used in aircraft structures. In the case of the use of Al-alloys in aircraft structures, fatigue cracks occur because of excessive and repeated loading and vibrations experienced during frequent flights. Meanwhile, composite materials are also damaged—for example, impact damage and debonding—and have defects, including voids, prepreg gaps, and overlaps created during the manufacturing process. Ultrasonic propagation imaging is a damage visualization technique that is used in structural inspections employing a laser scanning system and ultrasonic sensors. However, conventional ultrasonic propagation imaging or other scanning systems, such as a scanning laser Doppler vibrometer, only permit a single area to be inspected at one time. It is also difficult to inspect inaccessible areas, such as the upper skins of the aircraft wings. In this work, we describe a multi-area scanning ultrasonic propagation imaging system built in a hangar that is able to rapidly scan at a pulse repetition rate of 20 kHz. After acquiring the generated ultrasonic wave signal induced by laser excitation, ultrasonic propagation imaging videos for the in-plate guided wave are displayed. Finally, internal damage can be identified in a damage visualization platform. The developed multi-area scanning ultrasonic propagation imaging system is demonstrated by performing simultaneous inspections on two areas containing manufacturing defects in a large carbon/epoxy laminate. We also performed a demonstration of the hangar-based multi-area scanning ultrasonic propagation imaging system on an actual aircraft containing back surface cracks. The multi-area scanning ultrasonic propagation imaging system with tilting mirror systems installed in the hangar ceiling permitted a clear visualization of the damage. The damage visualization results confirm that the proposed multi-area scanning ultrasonic propagation imaging system and approach have excellent applicability as a built-in ultrasonic propagation imaging system for a Smart Hangar, which is a future structural health monitoring solution that will be used to realize a full-scale structural inspection of an actual aircraft.

Development of single channeled serial-connected piezoelectric sensor array and damage visualization based on multi-source wave propagation imaging

Piezoelectric sensors have been widely used for structural health monitoring. However, a single piezoelectric sensor has limited coverage, and multi-channel piezoelectric sensors require massive cabling and channeling devices. This article presents a serial-connected piezoelectric sensor array equipped with conductive fabric tape for damage diagnosis on a wide area, structure using an ultrasonic propagation imaging system. The sensor array was developed as a single channel sensing technique with multiple sensors. Using conductive fabric tape for installation provided convenient and compact installation, light weight, and comparatively low attenuation, compared with the lead wire for the easy smartilization of structures. An aluminum plate with various artificial damages was tested with the proposed sensor array combined with the ultrasonic propagation imaging system. The ultrasonic propagation imaging system stimulated the sensitive areas of the serially connected piezoelectrics and generated multi-wave sources covering a wide area. The multi-source wave propagation video visualized the damages in the form of anomalous waves. Consequently, multi-cabling and multi-channeling are not required because multiple piezoelectrics can be deployed by serial connections using conductive fabric tapes because of the multi-source wavefield generating capability of the ultrasonic propagation imaging system. Therefore, light-weight, compact, and easy installation of the sensor network and inspection for full-scale structures become possible.