Non-contact visualization of nonlinear ultrasonic modulation for reference-free fatigue crack detection

Abstract

This paper presents a fatigue crack detection technique based on visualization of nonlinear ultrasonic wave modulation produced by a fatigue crack. When distinctive low frequency (LF) and high frequency (HF) inputs are generated and applied to a structure, the presence of a fatigue crack can provide a mechanism for nonlinear ultrasonic modulation and create spectral sidebands around the frequency of the HF signal. In this study, the two input signals are created by two air-coupled transducers (ACT), and the corresponding ultrasonic responses are scanned over a target specimen using a 3D laser Doppler vibrometer (LDV). The crack-induced spectral sidebands are isolated using a combination of linear response subtraction (LRS), and continuous wavelet transform (CWT) filtering. Then, the extracted spectral sideband components are visualized near the fatigue crack. The effectiveness of the proposed non-contact scanning technique is tested using an aluminum plate with a real fatigue crack.

Baseline-free damage visualization using noncontact laser nonlinear ultrasonics and state space geometrical changes

“…As the percentage of false neighbors decreases below 20% (m is equal or larger than 3), the visualization results are improved. Furthermore, it is clearly demonstrated that the strongest nonlinearity is observed near the crack tip [23]. The damage visualization results demonstrate that the proposed technique can successfully localize the fatigue crack and is quite sensitive to nonlinear damage.…”

“…Previous examples include the detection of fatigue crack from the variation of the ultrasonic amplitude profile [21] and the visualization of delamination in composites using a standing wave filter [22]. However, with only a few exceptions [23], to date there have been no laser ultrasonic scanning techniques that visualize damage using nonlinear features.…”

Fatigue crack localization using noncontact laser ultrasonics and state space attractors

“…It is clearly demonstrated that strongest nonlinearity is observed near the crack tip. 27 The visualization results further imply that the proposed technique can not only detect the crack but also localize it. Although it is not demonstrated in this study, the proposed technique is expected to detect even subsurface crack if the energy of the excitation laser beam is high enough to "activate" the opening and closing of the hidden crack.…”