Ultrasonic Propagation Imaging System Research Articles

A health management technology for multisite cracks in an in-service aircraft fuselage based on multi-time-frame laser ultrasonic energy mapping and serially connected PZTs

The problems of multisite fatigue crack growth and multiple interacting cracks in aircraft fuselage structures may incur catastrophic failure. This article proposes a structural health management (SHM) strategy for multisite cracks in the skin and the vicinity of fastener holes in lap joints and demonstrates that the strategy is effective in an in-service metallic aircraft fuselage. To this end, the present article uses a conductive fabric tape-equipped serially connected piezoelectric sensor array combined with an ultrasonic propagation imaging (UPI) system. The serially connected piezoelectric sensor array is defined as a single sensor that has multiple sensor nodes with one electrical output. It was connected with conductive fabric tape instead of lead wire due to its comparatively low attenuation, light weight, and compact and easy installation on the internal structures of the complex fuselage. This technology does not require multi-cabling and multi-channeling because of its ultrasonic propagation imaging capability from multiple sources of the UPI system. The fuselage side skin of a Cessna-150 with various crack locations and orientations was tested using the proposed SHM technology. The ultrasonic wave propagation imaging video showed multisource wave propagation, and all of the crack-induced scattering wavefields appeared at different times because of the large inspection area. Therefore, we have developed a multi-time-frame ultrasonic energy mapping algorithm, which was processed in the wavenumber domain to visualize the cracks. Consequently, the lightweight, easy, and compact installation of the serially connected PZTs and their combination with the UPI system were shown, demonstrating that the SHM strategy for multisite cracks in an in-service aircraft fuselage is effective.

Investitation of mobile ultrasonic propagation imager as a tool for composite wind blade quality control

According to the statistical reports, almost 50% of wind blade failures are due to manufacturing defects. Therefore, reliable nondestructive testing (NDT) methods are required to create a standard for blade manufacturing quality control that reduces maintenance costs and catastrophic accidents. Applying conventional NDT methods is currently impossible without major modifications; this would also be time-consuming because wind turbine blades are large, thick and heterogeneous composite structures.This study proposes an in-process quality control (IPQC) method based on an ultrasonic propagation imaging (UPI) system. The UPI system is based on laser-induced ultrasound and has the capabilities of noncontact, fast, curved structural and real-time inspection results. These capacities have the potential to be applied to IPQC in blade manufacturing and assembly in a Smart Blade Factory. To provide a proof-of-concept for Smart Blade Factory based on the UPI system as an IPQC tool, a 750-kW actual blade section was tested, and the inspection results were presented based on a multiple-defect visualization platform. Debonding, inclusion and void defects in a wind blade made of glass fiber reinforced plastic and PVC grid panels were studied. The inspection results proved high feasibility for the application of the UPI system to IPQC in blade manufacturing.

Fully Noncontact Wave Propagation Imaging in an Immersed Metallic Plate with a Crack

This study presents a noncontact sensing technique with ultrasonic wave propagation imaging algorithm, for damage visualization of liquid-immersed structures. An aluminum plate specimen (400 mm × 400 mm × 3 mm) with a 12 mm slit was immersed in water and in glycerin. A 532 nm Q-switched continuous wave laser is used at an energy level of 1.2 mJ to scan an area of 100 mm × 100 mm. A laser Doppler vibrometer is used as a noncontact ultrasonic sensor, which measures guided wave displacement at a fixed point. The tests are performed with two different cases of specimen: without water and filled with water and with glycerin. Lamb wave dispersion curves for the respective cases are calculated, to investigate the velocity-frequency relationship of each wave mode. Experimental propagation velocities of Lamb waves for different cases are compared with the theoretical dispersion curves. This study shows that the dispersion and attenuation of the Lamb wave is affected by the surrounding liquid, and the comparative experimental results are presented to verify it. In addition, it is demonstrated that the developed fully noncontact ultrasonic propagation imaging system is capable of damage sizing in submerged structures.

Impact and Manufacturing Defect Visualization of Space Launcher Aluminum Liner/Filament Wound Composite Fuel Tank using Ultrasonic Propagation Imaging System

Impact and Manufacturing Defect Visualization of Space Launcher Aluminum Liner/Filament Wound Composite Fuel Tank using Ultrasonic Propagation Imaging System

Repeat scanning technology for laser ultrasonic propagation imaging

Laser ultrasonic scanning in combination with contact or non-contact sensors provides new paradigms in structural health management (SHM) and non-destructive in-process quality control (IPQC) for large composite structures. Wave propagation imaging technology based on laser ultrasonic scanning and fixed-point sensing shows remarkable advantages, such as minimal need for embedded sensors in SHM, minimum invasive defect visualization in IPQC and general capabilities of curved and complex target inspection, and temporal reference-free inspection. However, as with other SHM methods and non-destructive evaluation based on ultrasound, the signal-to-noise ratio (SNR) is a prevalent issue in real structural applications, especially with non-contact thin-composite sensing or with thick and heterogeneous composites. This study proposes a high-speed repeat scanning technique for laser ultrasonic propagation imaging (UPI) technology, which is realized with the scanning speed of 1 kHz of a Q-switched continuous wave laser, and precise control of the laser beam pulses for identical point scanning. As a result, the technique enables the achievement of significant improvement in the SNR to inspect real-world composite structures. The proposed technique provides enhanced results for impact damage detection in a 2 mm thick wing box made of carbon-fiber-reinforced plastic, despite the low sensitivity of non-contact laser ultrasonic sensing. A field-applicable pure laser UPI system has been developed using a laser Doppler vibrometer as the non-contact ultrasonic sensor. The proposed technique enables the visualization of the disbond defect in a 15 mm thick wind blade specimen made of glass-fiber-reinforced plastic, despite the high dissipation of ultrasound in the thick composite.

Comparative analysis of laser ultrasonic propagation imaging system with capacitance and piezoelectric air-coupled transducers

Air-coupled nondestructive testing is an attractive technique that allows easy scanning inspection capability due to its noncontact nature with the specimen. A piezoelectric air-coupled transducer has been combined with a laser ultrasonic propagation imaging system as a fixed noncontact sensor. However, the large impedance mismatch between air and a specimen and its narrowband frequency range has prevented it from being efficiently used in real-world applications as a laser-air-coupled transducer ultrasonic propagation imaging system. Therefore, instead of combination with a piezoelectric air-coupled transducer, a combination between a laser and a capacitance air-coupled transducer is proposed through a comparative analysis between the capacitance and piezoelectric air-coupled transducers as the ultrasonic detector of the ultrasonic propagation imaging system. Under the same conditions, a series of experiments was conducted to examine the damage sensitivity in metal and composite specimens that have a hidden crack and barely visible impact damages, respectively. This noncontact system was further improved using repeat-scanning technique to enhance the signal-to-noise ratio and to overcome high attenuation problems, especially in composite specimens.

Composite aircraft debonding visualization by laser ultrasonic scanning excitation and integrated piezoelectric sensing

SUMMARY Two aircraft wings made of carbon fiber-reinforced plastic material were inspected using the laser-based ultrasonic propagation imaging system for the visualization of manufacturing defect and damages of debonding mode. Hypotheses of the detection mechanism of composite debonding were presented and validated through the inspection results for the bonding lines of the composite wing structures. The inspection showed that the location, shape, and size of the poor bonding defect, artificial stringer tip debonding, and real skin–spar debonding can be visualized and/or measured from the imaging results. The performance of the imaging system was found better than that of a manual ultrasonic C-scan in terms of spatial resolution and damage detectability. Another finding from the inspection was the occurrence of skin buckling between two riveted locations. Most importantly, the presence of structural elements or features such as spars, stringers, ribs, lugs, inspection windows, and even embedded PZT elements did not adversely affect the inspection. Copyright © 2012 John Wiley & Sons, Ltd.

초음파전파영상화 시스템 하드웨어 발전

초음파전파영상화 시스템 하드웨어 발전

Laser ultrasonic anomalous wave propagation imaging method with adjacent wave subtraction: Algorithm

The anomalous wave propagation imaging (AWPI) method is proposed for the laser ultrasonic propagation imaging system using a Q-switched laser and a laser mirror scanner to highlight the anomalies in complex structures. The AWPI algorithm was developed based on the observation that the waves from two adjacent scanning points are very similar, and that the propagation direction of the incident wave is different from that of the anomalous wave caused by structural anomalies including damage. The structural anomaly is highlighted by suppressing the incident waves and exaggerating the anomalous wave through adjacent waves subtraction after arrival time and amplitude matching. The variable time window amplitude mapping (VTWAM) method was also developed, based on the difference in arrival time between the residual incident wave and anomalous waves. The VTWAM method enhances anomaly visualization and sizing, notably for composite damages, by mapping the amplitudes of the confining wave within the damage. Our results showed that the AWPI increased the signal-to-noise ratio of a back-surface hole damage in a steel plate by 13.76dB, while in another inspection of a composite wing with two impact damages, the AWPI results enhanced by the VTWAM compared favorably with the results of the immersion ultrasonic C-scan. The AWPI and VTWAM adopt implicit spatial referencing wherein all necessary data can be obtained through a single-time scan, therefore circumvent the disadvantages of conventional temporal baseline referencing.

A Fully Non-Contact Ultrasonic Propagation Imaging System for Closed Surface Crack Evaluation

Cracks are one of the most common types of damage occurring in engineering structures. A reliable nondestructive evaluation technique is essential to detect any possible damage at the initiation phase. Real fatigue closed-surface cracks are difficult to detect through visual inspection. Ultrasound has been widely used, but conventional contact ultrasonic inspection techniques are not suitable for couplant-sensitive structures. In addition, these techniques are generally laborious for large field structures and the inspection speed is relatively slow. We present a novel fully non-contact hybrid ultrasonic propagation imaging (UPI) system that uses laser ultrasonic scanning excitation and piezoelectric air-coupled sensing. Ultrasonic frequency tomography and wavelet-transformed ultrasonic propagation imaging algorithms are used to extract damage features. These features are used to perform a thorough diagnosis of damage. The system enables remote and fully non-contact automatic one-sided inspection for temporal reference-free damage evaluation, and is also applicable to in-field structures. Optimization provides improved performance of air-coupled transducers (ACTs) used as receivers for the hybrid UPI system, as shown by our experimental results. Surface crack evaluation results were analyzed on the basis of ease of damage visualization, accuracy of crack size estimation, and sensitivity. The proposed hybrid UPI system is sensitive enough to detect a real fatigue closed-surface micro-crack with size detection accuracy as high as 96%. We also show that the relation between the scanning interval and crack width affects damage visualization performance, and the accuracy and sensitivity of damage size estimation.

Laser ultrasonic anomalous wave propagation imaging method with adjacent wave subtraction: Application to actual damages in composite wing

Laser ultrasonic wave propagation imaging methods have great potential for integrated structural health management and non-destructive evaluation. However, application of these techniques to complex structures in the field is difficult because they give rise to complicated wave propagation patterns. We developed an anomalous wave propagation imaging method with adjacent wave subtraction using laser ultrasonic scanning to solve this problem. The proposed method is suitable for non-destructive evaluation of complex structures because it highlights the propagation of anomalous waves related to structural discontinuities, and suppresses complex incident waves without the need of pre-stored reference data. In this study, the method was applied to a real composite wing subjected to bending and impact tests. The method enhanced the visibility of the anomalous waves related to damages such as stringer tip debonding, skin-spar debonding, and invisible impact damage. Based on these anomalous waves, variable time window amplitude mapping was performed to show the damage location, size, and shape resemble to the actual damage. Comparisons showed that the methods performed better than the ultrasonic A-scan in terms of damage detection and sizing accuracy. The presence of structural elements such as spars, stringers, ribs, and surface-mounted PZT elements did not adversely affect the inspection. The proposed wing test setup with a built-in ultrasonic propagation imaging system for automatic NDE could be easily expanded throughout a hanger for maintenance inspection.

Long distance laser ultrasonic propagation imaging system for damage visualization

Wind turbine blade failure is the most prominent and common type of damage occurring in operating wind turbine systems. Conventional nondestructive testing systems are not available for in situ wind turbine blades. We propose a portable long distance ultrasonic propagation imaging (LUPI) system that uses a laser beam targeting and scanning system to excite, from a long distance, acoustic emission sensors installed in the blade. An examination of the beam collimation effect using geometric parameters of a commercial 2 MW wind turbine provided Lamb wave amplitude increases of 41.5 and 23.1 dB at a distance of 40 m for symmetrical and asymmetrical modes, respectively, in a 2 mm-thick stainless steel plate. With this improvement in signal-to-noise ratio, a feasibility study of damage detection was conducted with a 5 mm-thick composite leading edge specimen. To develop a reliable damage evaluation system, the excitation/sensing technology and the associated damage visualization algorithm are equally important. Hence, our results provide a new platform based on anomalous wave propagation imaging (AWPI) methods with adjacent wave subtraction, reference wave subtraction, reference image subtraction, and the variable time window amplitude mapping method. The advantages and disadvantages of AWPI algorithms are reported in terms of reference data requirements, signal-to-noise ratios, and damage evaluation accuracy. The compactness and portability of the proposed UPI system are also important for in-field applications at wind farms.

Wave propagation visualization in an experimental model for a control rod drive mechanism assembly

Nondestructive inspection techniques such as ultrasonic testing, eddy current testing, and visual testing are being developed to detect primary water stress corrosion cracks in control rod drive mechanism (CRDM) assemblies of nuclear power plants. A unit CRDM assembly consists of a reactor upper head including cladding, a penetration nozzle, and J-groove dissimilar metal welds with buttering. In this study, we fabricated a full-scale CRDM assembly mock-up. An ultrasonic propagation imaging (UPI) method using a scanning laser ultrasonic generator is proposed to visualize and simulate ultrasonic wave propagation around the thick and complex CRDM assembly. First, the proposed laser UPI system was validated for a simple aluminium plate by comparing the ultrasonic wave propagation movie (UWPM) obtained using the system with numerical simulation results reported in the literature. Lamb wave mode identification and damage detectability, depending on the ultrasonic frequency, were also included in the UWPM analysis. A CRDM assembly mock-up was fabricated in full-size and its vertical cross section was scanned using the laser UPI system to investigate the propagation characteristics of the longitudinal and Rayleigh waves in the complex structure. The ultrasonic source location and frequency were easily simulated by changing the sensor location and the band pass filtering zone, respectively. The ultrasonic propagation patterns before and after cracks in the weld and nozzle of the CRDM assembly were also analyzed. Since this visualization method is not limited in the flat cross section, it will be useful in developing ultrasound-based structural health monitoring technologies, advanced nondestructive methods, and numerical models. In addition, the proposed laser UPI system could be a useful tool in optimizing the receiver and transmitter locations, the ultrasonic path, and the ultrasonic frequency.

Ultrasonic Propagation Imaging for Wind Turbine Blade Quality Evaluation

Soundness of wind turbine blade plays paramount role in supplying cheap and reliable wind energy. To ensure that blades are sound and free from inherent manufacturing defects, easy and reliable quality evaluation before delivering blades for installation is crucial. We propose an ultrasonic amplitude imaging method based on Ultrasonic Propagation Imaging (UPI) system for this purpose. The system utilizes a scanning Q-switched continuous wave laser as ultrasonic wave generator. Various acousto-ultrasonic sensors, either contact or non-contact type, could be adopted as ultrasonic receiver. The system was tested on a blade section made of glass fiber reinforced plastic sandwiched with balsa wood. The leading edge of the specimen with a kissing disbond was inspected. A PZT-based sensor attached temporarily on the outer surface of a blade was used for ultrasonic reception to demonstrate that this imaging method is applicable even for the blades without integrated sensor. The amplitude map clearly showed the disbond defect with the kissing features. The location, shape, and size of the defect evaluated from the result agree excellently with the actual defect. The result proved that the proposed method works well for the quality evaluation of wind turbine blades. Inheriting the advantages of UPI technology, i.e. focusing-free characteristics, acceptance of large laser incident angle up to 60°, and rapid scanning without needing an expensive scan gantry, this system is suitable for automatic quality evaluation of blades with different geometry. On top of that, this system does not require any reference data, making it a flexible quality evaluation system suitable for different methods of blade production flow.

Hot target inspection using a welded fibre acoustic wave piezoelectric sensor and a laser-ultrasonic mirror scanner

The direct attachment of piezoelectric transducers onto hot targets raises formidable challenges as piezoelectric transducers lose their piezoelectric characteristics at elevated temperatures or debond due to thermal expansion coefficient mismatches. We developed a welded fibre acoustic-wave PZT (FAWPZT) sensor to alleviate these temperature limitations. One end of the FAWPZT sensor, made from a stainless steel fibre, was welded onto a stainless steel target plate and the other end was bonded to a PZT sensor. An ultrasonic wave propagation imaging (UWPI) system consists of a laser mirror scanner and a Q-switched pulsed laser (QPL) acting as a non-contact ultrasonic generator was then used to scan a hot target surface with an artificial 2 mm-sized open crack. The result was presented in the form of an ultrasonic wave propagation movie. The damage was detected as a wavefield scattering from the damaged location and its size was evaluated from the plot of amplitude distribution along the propagating wavefront. Sensor performance was briefly discussed and the results confirmed that a FAWPZT sensor combined with a UWPI system has good potential for implementation in hot target integrated structural health management.

Health monitoring of complex curved structures using an ultrasonic wavefield propagation imaging system

An ultrasonic wavefield propagation imaging system is introduced and then applied for ultrasonic wavefield imaging of complex curved surfaces. A Q-switched pulsed laser is utilized as a moving ultrasonic generator, and a PZT ultrasonic sensor is fixed during the laser beam scanning and detects the ultrasonic waves propagated from the points excited by the laser beam. The waveforms are allocated in the spatial domain of the scanned points and then manipulated in the form of a time versus wavefield movie. The visualized wavefields enable easy detection and interpretation of structural defects because anomalies during wavefield propagation can be visualized. Furthermore, this ultrasonic wavefield propagation imaging system enables reference-free inspection, complex curved surface scanning because it does not require control of focal length and incidence angle of the laser beam, and excellent adaptability with built-in structural health monitoring sensors, such as piezoelectric and fiber optic sensors. The system is demonstrated in the applications of wavefield visualization on a drill surface, detection of mass loss parts inside an elbow pipe joint, and detection and characterization of impact damage and stringer disbond in a composite skin–stringer structure.