Fatigue Crack Localization Using Laser Nonlinear Wave Modulation Spectroscopy (LNWMS)

Abstract

Nonlinear features of ultrasonic waves are more sensitive to the presence of a fatigue crack than their linear counterparts are. For this reason, the use of nonlinear ultrasonic techniques to detect a fatigue crack at its early stage has been widely investigated. Of the different proposed techniques, laser nonlinear wave modulation spectroscopy (LNWMS) is unique because a pulse laser is used to exert a single broadband input and a noncontact measurement can be performed. Broadband excitation causes a nonlinear source to exhibit modulation at multiple spectral peaks owing to interactions among various input frequency components. A feature called maximum sideband peak count difference (MSPCD), which is extracted from the spectral plot, measures the degree of crackinduced material nonlinearity. First, the ratios of spectral peaks whose amplitudes are above a moving threshold to the total number of peaks are computed for spectral signals obtained from the pristine and the current state of a target structure. Then, the difference of these ratios are computed as a function of the moving threshold. Finally, the MSPCD is defined as the maximum difference between these ratios. The basic premise is that the MSPCD will increase as the nonlinearity of the material increases. This technique has been used successfully for localizing fatigue cracks in metallic plates.

Application of nonlinear ultrasonic analysis for in situ monitoring of metal additive manufacturing

“…Although the frequency window W ω has been investigated and selected so as to increase the sensitivity of the SPC technique for defect detection, 14,31 the time information of the measured signals is lost in Equation (2). Transient ultrasonic signals, such as pulse-laser-induced guided waves, are often nonstationary signals with time-evolving frequency components.…”

“…Although the frequency window W v has been investigated and selected so as to increase the sensitivity of the SPC technique for defect detection, 14,31…”

“…9 The SPC technique has been successfully applied in analyzing the aging of glass fiber-reinforced cement, 10 monitoring concrete cracks, 11 inspecting impact damage in composites, 12 and detecting and locating fatigue cracks in metallic structures. 13,14 In one of the aforementioned studies, a pulse laser was used to excite the broadband ultrasonic waves into the target structure, and the corresponding response signals were measured using a laser Doppler vibrometer (LDV) in a fully noncontact manner. 13 The incorporation of a laser ultrasonic system leverages the SPC technique for potential in situ applications, such as quality inspection in metal additive manufacturing (AM).…”

Nondestructive Evaluation of Thermal Aging in Al6061 Alloy by Measuring Acoustic Nonlinearity of Laser-Generated Surface Acoustic Waves

“…The most extensively used method in NDE is ultrasonic testing, which is effective because it has high mobility and high feasibility with a relatively simple instrumentation and uncomplicated configuration [6,7,10,11]. Particularly, the measurement of acoustic nonlinearity has shown its effectiveness in the evaluation of microstructural changes in materials, including stress corrosion cracks [12], plastic deformation [13][14][15][16], residual stress [17], fatigue [5,6,14,16,[18][19][20][21], thermal aging [4,8,9,[22][23][24][25][26], creep [10], and so on [18,27,28]. The microstructural changes, such as the evolution of dislocation structure, precipitate growth, and the phase transitions of precipitates, affect the acoustic nonlinearity of any ultrasonic waves propagating through the region where the microstructural change has taken place.…”