Effective NDE inspection systems and methods are highly desired in a broad range of engineering applications including metal structure thickness evaluation. Laser-generated ultrasound has been studied to excite wideband Lamb waves for NDE. By using the ultrasound in conjunction with multidimensional wavefield measurements, obtained by high spatial resolution noncontact laser scanning vibrometer, thickness evaluations for metal components can be achieved. This paper applies a fully noncontact/remote ultrasonic Lamb wave NDE system and explores its application for thickness characterization and evaluation of metal components. This paper first demonstrates the actuation and sensing of Lamb waves in metal components. The non-contact system consists of a Pulsed Laser (PL) working in the thermoelastic regime to excite Lamb waves and Scanning Laser Doppler vibrometer (SLDV) sensing the waves and providing high-resolution multidimensional wavefield signals for evaluation. Enhancements through sensing, actuation parameters, and surface enhancement were attempted to excite very high-frequency Lamb waves. The results show that excited Lamb waves can have a frequency beyond 1 MHz in certain components thinner than 1 mm. The paper continues to show how the acquired Lamb waves can be used for measuring the thickness of the metal components. The method adopted multidimensional Fourier analysis to convert time-space wavefield measurements to frequency-wavenumber representation. The resulting spectrum is then compared to theoretical dispersion curves in frequency-wavenumber for thickness matching to derive the thickness parameter. Proof-of-concept tests were performed with aluminum components of various thicknesses first and then the method was applied to much thinner foil-type material and components made of different materials.
Structural health monitoring (SHM)/nondestructive evaluation (NDE) is an emerging multi-disciplinary field that aims at detecting/characterizing structural damage and providing diagnosis/prognosis of structural health status in a real-time or on-demand manner. It can reduce maintenance costs, shorten the machine service downtime, and improve the safety and reliability of the engineering structures. Acoustic emission (AE) is one of the SHM/NDE methods by means of detecting elastic waves due to dynamic motions at AE sources, such as cracking, delamination, cleavage, and fretting in a material. The acoustic emission inspection technique relies on the AE sensors to collect the AE signals from the structure to monitor the structural health. Conventionally, these AE sensors need to be permanently attached to the structure through the bonding adhesive layer which may introduce contamination to the structure. In this work, the research is focused on investigating non-contact passive sensing of acoustic emission (AE) signals using an air-coupled transducer (ACT). The well-acknowledged pencil-lead-break method has been used to simulate the AE source. A resonant type ACT is used to passively sense the AE signals, which leaves the testing object intact and provides a non-intrusive sensing method. The non-contact AE test on a thin aluminum structure as well as a thick steel structure is first conducted. Next, the investigation is extended to composite structures. Both single-layer composite structure and bonded composite structure are investigated. The results successfully demonstrate the capability of non-contact passive sensing of the AE signals using the ACT method.
Since 1986, the above-ground dry cask for spent fuel rods was employed to the Surry Nuclear plant as a temporary solution for high-level waste storage until a more permanent solution could be found. During the service of the dry cask storage, when the crack occurs, it will grow progressively as time goes on. This greatly threatens the safety of the multilayer dry cask structures. Ultrasonic Lamb waves have been shown as an effective nondestructive evaluation (NDE) method due to their ability to propagate a long distance with less energy loss as well as their sensitivity to various defects on the surface or inside the structure. In this study, the research was focused on the laboratory investigation of the nondestructive inspection of the multilayer structures using a fully non-contact Lamb wave method. The non-contact system was constructed using an air-coupled transducer (ACT) for actuation and scanning laser Doppler vibrometer (SLDV) for sensing. The ACT provided a narrowband wave actuation, while SLDV provided high-quality wavefield signals for damage detection and evaluation. To systematically develop the method for dry cask structure inspection, crack inspection in a simple 1-mm aluminum plate was first conducted; then crack inspections in more complicated multilayer structures were further carried out. Besides, to evaluate the crack inspection, two imaging techniques were further developed using a full-wave energy method and a scattered wave energy method for crack detection. The cracks in both simple aluminum plate and multilayer structures were successfully inspected.
Adhesively bonded composites have been widely used in aerospace engineering, renewable energies, and automotive industries. However, the potential weak bonding of the composite structures, which exhibits a low cohesion interface between the adhesive and the composite substrate, greatly threatens the reliability of these structures. The occurrence of weak bonding has yet to be well understood and has posed new challenges for the evaluation of weak bonding in composite structures. Ultrasonic Lamb waves have been shown useful for nondestructive evaluation (NDE) due to their ability to propagate a long distance with less energy loss and their sensitivity to small defects. Among various ultrasonic transducers, air-coupled transducers (ACT) eliminate the need for couplant/adhesive and provide a noncontact actuation method. In this study, a hybrid noncontact system was constructed using an ACT for actuation and a scanning laser Doppler vibrometer (SLDV) for sensing. The ACT provided a narrowband wave actuation, while SLDV provided high-quality wavefield signals for damage detection and evaluation. An adhesively bonded composite structure containing good and weak bond quality areas was manufactured using simulated contamination. Then the composite bond quality inspection was conducted using the ACT-SLDV system. To validate the ACT Lamb wave inspection, a second noncontact inspection by a pulsed laser-SLDV method was carried out.
Ultrasonic Lamb waves have been proved useful for nondestructive evaluation (NDE) due to their abilities to propagate a long distance with less energy loss as well as their high sensitivity to small defects on the surface or inside the structure. However, there are still many challenging tasks for Lamb wave based NDE due to the complexity involved with Lamb waves propagation and the complexity caused by coupling layer used in traditional contact-type transducers. This paper established a fully non-contact Lamb wave NDE system by using a non-contact air-coupled transducer (ACT) which can eliminate the need for couplant/adhesive and actuate pure fundamental A0 mode Lamb wave; and a non-contact scanning laser Doppler vibrometer (SLDV) for sensing which can provide high spatial resolution wavefield data. Through ACT, pure A0 mode was actuated at selected ACT incident angle based on Snell’s law. By SLDV sensing, multi-dimensional wavefields for one-dimensional or two-dimensional wave propagation were obtained and further used for Lamb waves’ characterization analysis. A specimen with a through-thickness crack was manufactured and adopted to evaluate the inspection capability of the ACT-SLDV system. Waves with normal incident and aligned incident w.r.t the longitudinal dimension of the crack were investigated, and our results showed that cracks were detected successfully in both cases. Moreover, crack lengths can be quantitatively evaluated for both situations with 10% error.
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