Search

Signal instability due to temperature fluctuations, sensor degradation, and debonding introduces additional amplitude loss in the detected signals during acousto-ultrasonic detection, which may be falsely attributed to defects in a structure. First, we determined that the amplitudes of both high-frequency and low-frequency Lamb waves decrease after propagation through a damaged area. Then, we found that the amplitude ratio of such waves not only exhibits a downward trend but is also immune to fluctuations in the input signals. A qualitative numerical expression was proposed to explain this phenomenon, and preliminary experiments were conducted to demonstrate that the amplitude ratio is an effective parameter for mitigating instability in signal detection. Particularly, the number of impacts on a composite laminate was evaluated with respect to changes in the input signal amplitude. Notably, this method can be further simplified by designing a dual-frequency input signal. After conclusively validating the performance of the novel method in a composite subjected to temperature fluctuations, we conclude that the proposed acousto-ultrasonic detection method is robust in mitigating signal instability, and that it yields reliable information for damage evaluation. [ABSTRACT FROM AUTHOR]

We applied a highly sensitive phase-shifted fiber-optic Bragg grating (PS-FBG) sensor with a broad bandwidth to acoustic emission (AE) detection in a composite material. Based on AE detection, this research proposed a novel method to identify damage types in carbon fiber-reinforced plastic (CFRP) laminates. Because the PS-FBG sensor measured pure dynamic strain, the detected AE signals had great physical reliability, which made elastic wave theory suitable for analysis of the signal. The analysis clarified the different characteristics of Lamb wave modes in three types of AEs caused by a transverse crack, delamination and fiber breakage. Additionally, the ratios of the amplitudes of the S 0 mode to the A 0 mode and the peak frequencies quantitatively evaluated the mode characteristics in the AE signals. Simultaneously using the two physical parameters, we identified the three types of damage among the AE signals detected in a three-point bending test. Furthermore, the finite element method-based AE wave propagation simulation agreed well with the identification results. Hence, the proposed identification method with the PS-FBG sensor has great physical reliability for evaluating damage in composite laminates. [ABSTRACT FROM AUTHOR]

In this study, a phase-shifted fiber Bragg grating sensor was employed for acoustic emission detection in cross-ply carbon fiber–reinforced plastic laminates. First, we evaluated the response of the phase-shifted fiber Bragg grating sensor to a simulated acoustic emission source using a pencil lead breakage test. The existing modes of Lamb waves were observed, and their relations to the acoustic emission source orientation were investigated using a modal acoustic emission method for discriminating between acoustic emission signals generated by transverse cracking and delamination. As a result, those two types of damage were identified during a tensile test. In the time–frequency analysis of the waveforms detected by the phase-shifted fiber Bragg grating sensor, in addition to the S0 and A0 modes that are usually used in modal acoustic emissions, we found an A1 mode in the high-frequency range of the acoustic emission generated by a transverse crack. Then, we conducted a finite element analysis to examine the inherent mechanism of the A1 mode excitation. The results of the calculation almost agreed with the experimental results and indicated that a phase-shifted fiber Bragg grating sensor has a high sensitivity response to acoustic emission in the carbon fiber–reinforced plastic laminates over a broad bandwidth. Therefore, this novel fiber-optic sensor has been shown to have high potential for monitoring the ultrasonic structural health of composites. [ABSTRACT FROM AUTHOR]

Highly sensitive ultrasonic fiber-optic Bragg gratings (FBGs) have been used as acoustic emission (AE) sensors in the non-destructive inspection (NDI) for analyzing damage progression during the material tests of carbon fiber reinforced plastic (CFRP) laminates. Localization of the damage-induced AE source is one of the desirable functions for enhancing the efficiency of the AE-based NDI method. Hence, we attempted to establish a linear source localization method in a coupon-shaped CFRP test specimen with a highly sensitive FBG sensor. In particular, to protect the FBG-AE detection from influence caused by the static strain in a test specimen, the FBG sensor was installed by a remote adhesive configuration, i.e., one point of optical fiber was glued on the specimen, and the FBG sensor in the optical fiber was located away from the adhesive point. In the particular bonding way, the FBG sensor detected AE wave through an optical-fiber-based ultrasonic waveguide. In the first half of the paper, an ultrasonic experiment was conducted to show this installation technique can stably and accurately detect the behavior of Lamb wave modes in AE signals. With the help of the sensing characteristics, we clarified that a time difference between the arrival of S 0 and A 0 modes in a one AE signal had a linear relation to the distance between the AE source and the adhesion point when the length of optical fiber-based waveguide was fixed. Based on the examination, the linear AE source localization method was established using only one single FBG sensor. After confirming the ability of the proposed approach through a simulated AE experiment in a cross-ply CFRP laminate, we used the strategy to localize the actual damage-induced AE sources during a three-point bending test of the laminate. [ABSTRACT FROM AUTHOR]

In this study, we propose a straightforward and easy-to-use method (chirp guided wave mixing, CGWM) for detecting delaminations in CFRP laminates. In this method, two long-duration chirp signals with reversely varying frequency bands are applied to two piezoelectric actuators to generate two broadband guided wave bursts. These waves interact within the specimen, producing sum-frequency harmonics around a target frequency at the defect site. The damage can be directly visualized in the amplitude map at the target frequency without needing complicated post-processing procedures. The feasibility of the CGWM method is demonstrated in experiments and FEM simulations. We compare our proposed method with two other approaches, namely root mean square (RMS) method and higher harmonic (HH) method. The results indicate that the CGWM method exhibits superior sensitivity compared to the RMS method and offers greater simplicity and ease of use than the HH method. Overall, this study presents an innovative approach to frequency manipulation in the field of nonlinear acoustics, providing a promising avenue for delamination detection in composites. • This study offers a novel frequency manipulation approach of guided wave mixing using chirp excitation. • A chirp guided wave mixing technique is utilized for detecting delamination defect in CFRP laminates. • The proposed approach has high sensitivity to both shallow and deep delaminations. • Delaminations with varying sizes and depths are directly visualized without complicated post-processing procedures. [ABSTRACT FROM AUTHOR]

Visualizing the propagation of Lamb waves generated by scanning laser injection is a promising and effective method for the nondestructive inspection of carbon fiber reinforced plastic (CFRP) structures. We evaluate the use of two-dimensional time-slowness maps of the acquired data for defect detection in CFRP laminates. The slowness maps can be constructed from the differences in wave propagation characteristics between two adjacent laser injection points. Using a CFRP plate, in which an artificial delamination is located at the center, we show that the delamination can be clearly observed as a hotspot in the two-dimensional slowness map. The visible hotspot is due to the dispersion of the lowest antisymmetric (A 0) mode of the Lamb wave: the A 0 mode slows down at the delaminated region. We also demonstrate that our method can be applied to an L-shaped structure. Two-dimensional slowness images can be useful for developing and applying practical inspection methods based on laser ultrasonic waves. • ・Defects in CFRP laminates are inspected using laser ultrasonic guided waves. • ・A two-dimensional slowness map is made from the data obtained by scanning laser. • ・The slowness map is simple and useful for defect detection. [ABSTRACT FROM AUTHOR]

Detection of transverse crack (TC) is important in establishing reliable structural health monitoring (SHM) and nondestructive inspection (NDI) for warning the potential occurrence of large-scale damage in carbon fiber reinforced plastic (CFRP) structures at an earlier stage. In this research, a diagnostic method that is sensitive to a single TC was established by visualizing laser-induced guided ultrasonic waves in a cross-ply CFRP laminate. First, the phenomenon of TC-induced mode conversion in the laminate was clarified using finite element analysis (FEA) of harmonic oscillations. This FEA method is explicit and precise for investigating the conversion behavior of individual modes in a cracked plate. Then, the conversion phenomenon was also validated by another FEA-based pulse laser ultrasonic simulation. Finally, to observe the mode conversion phenomenon in practice, we conducted an experiment with a laser ultrasonic visualizing inspector (LUVI). The LUVI system could visualize the wave propagation in a two-dimensional (2D) wavefield that was formed on the surface of the CFRP cross-ply laminate with a single TC. By observing the image results of wave propagation, we verified that the wave component of the A 0 mode was converted from the S 0 mode at a TC even though the converted A 0 mode had a much smaller amplitude than the incident S 0 mode in the cross-ply CFRP laminate. Thus, those visualization results could be used to detect a single TC in the CFRP cross-ply laminate reliably. [ABSTRACT FROM AUTHOR]

Lamb waves can be used in carbon fiber reinforced plastics (CFRP) for establishing both structural health monitoring systems and nondestructive inspection techniques. In this study, we investigated the dispersion relation of Lamb waves propagating in a cross-ply CFRP laminate. Using a formalism of the multi-layer Lamb wave model, we compared a homogeneous single-layer model and multi-layer models. The results suggest that the velocity of the S 0 mode was the same for all the models in the low-frequency region. Thus, the use of homogeneous parameters is valid for the S 0 mode. However, it was found that in high-frequency regions, there was a discrepancy between the velocities of Lamb wave modes for the homogeneous model and multi-layer models. This is because Lamb waves can be localized in a particular layer, and the velocity can be affected by the stiffness constants of the layer. This work elucidates the characteristics of Lamb wave propagation by showing that the stacking sequence of the laminate can have an effect on the wave velocities and wave structures, enabling researchers to improve Lamb wave-based defect detection. [ABSTRACT FROM AUTHOR]

This paper presents a local defect resonance (LDR)-based approach to detect barely visible impact damages (BVIDs) in woven CFRP laminates using laser-ultrasonic guided waves. In the experiments, broadband guided waves were excited by an Nd:YAG pulsed laser to induce resonant vibrations of BVIDs. The laser was scanned on the designed area of the woven CFRP laminates by using a galvano mirror system. A fixed broadband PZT sensor received the waves propagated from all the laser irradiation points and the datasets of the recorded waveforms were further processed to visualize the wavefield in the scanned area. A robust post-processing method was proposed to extract automatically the LDR frequencies and to obtain the image of the BVID. The feasibility of the proposed method was well validated on a woven CFRP laminate with a single BVID. In order to evaluate the detection performance, we compared our proposed method with other traditional methods, i.e., root mean square (RMS) and weighted RMS. As a result, it was found that our proposed post-processing procedure could visualize the BVID with higher contrast. Furthermore, the detection of multiple BVIDs using the developed method was demonstrated successfully. [ABSTRACT FROM AUTHOR]