Two dimensional nonlinear frequency-mixing photo-acoustic imaging of a crack and observation of crack phantoms

Abstract

A two-dimensional imaging of a crack by nonlinear frequency-mixing photo-acoustic method is reported. The imaging contrast is due to nonlinear photo-thermo-acoustic processes taking place in case of simultaneous excitation by lasers of thermo-elastic and acoustic waves in the vicinity of the cracks. The images are obtained by scanning of two co-focused laser beams in region of crack location. The first cw laser beam, modulated in intensity at low frequency f L , generates a thermoelastic wave, which is able to strongly periodically modulate the local crack rigidity up to complete closing/opening of the crack. The second cw laser beam, intensity modulated at much higher frequency f H , generates an acoustic wave incident on the breathing crack. The crack rigidity is also influenced by the stationary non-modulated inhomogeneous thermal stresses caused by the stationary heating of the sample by both lasers. The main contribution to imaging contrast comes from the strong dependence of the reflectivity of acoustic waves on the crack rigidity. The modulation of crack rigidity by thermo-elastic wave leads to the parametric modulation of the reflected acoustic waves and the generation in the spectrum of the acoustic field of the side lobes, which are separated from f H by the integer number of f L . Scan images of a crack with an amplitude dynamics up to 40 dB and a spatial resolution better than 100 lm, are obtained through the mapping of nonlinear side lobe amplitudes. The observed dependences of the images on the power of the lasers are discussed and the physical explanation of the appearance of crack phantom images at high level of optical excitation is proposed. For comparison and elucidation of spatial resolution issues related to this imaging method, the one-dimensional images, obtained with one laser beam focus position fixed and the second beam focus position scanning, are also presented and discussed.

Laser ultrasonic monitoring of reversible/irreversible modification of a real crack under photothermal loading

“…We hypothesize that the irreversible processes that cause the modification of the quasi-equilibrium state of the crack revealed by the measurements of ''tR(cool)'' and ''tL-R(cool)'' signal amplitudes presented in Figure 4(a) are the processes of asperities destruction on the crack surfaces. Several studies have demonstrated that a crack can be closed with the thermoelastic stress induced by the absorption of intensity-modulated [61][62][63][64] or quasicontinuous laser irradiation. 66,67 Moreover, the AFM topography of a similar crack surface (see Figure 7 in the previous publication) 67 reported earlier, which was obtained by the destruction of a similar sample, revealed asperities of different heights on the crack faces.…”

“…This approach was later advanced by applying the laser radiation not only for crack modulation but also for generation of ultrasound. The frequency-mixed components of the spectrum are detected by in-contact accelerometers [60][61][62][63][64] and contactless optical techniques, such as interferometry and beam deflection. 61 Furthermore, the contrast of photoacoustic imaging of cracks is improved because of the strong dependence of optoacoustic conversion efficiency on the state of the crack.…”

“…For example, it has been frequently reported that modulated photothermal loading can be used in crack detection using acoustoelasticity. [61][62][63][64] In addition, it can be applied to distinguish surfacebreaking cracks from other surface structures or attachments. 76 Furthermore, in this study, the reported non-monotonic evolution of peak-to-peak amplitude of the mode-converted signal at the end of cooling time intervals after heating at various heating powers can be potentially used to evaluate the width of the crack in SHM.…”

Multi-mode laser-ultrasound imaging using Time-domain Synthetic Aperture Focusing Technique (T-SAFT)

“…Second, the LUT provides not only possibilities for both wide-band and narrow-band generation, but also for multi-wave-mode generation. In addition, different modes of waves can carry physical information with special directivity patterns [14] , [15] , [16] , relating to the location of surface cracks, thickness of the specimen, and internal defects [17] , [18] , [19] . The LUT also has a high spatial resolution [20] , which is important for detecting and imaging tiny defects (i.e.…”

Surface Crack Detection with Low-cost Photoacoustic Imaging System

“…However, the invention of semiconductor and laser technology in the early 1960s has allowed it to become one of the most advanced imaging techniques available today (Ganguly & Rao, 1981;Luscher et al, 1984;Ni, 2014). Several researchers have reported that applying photoacoustic measurement to detect material cracking yielded satisfactory results (Chigarev et al, 2009;Yan et al, 2012;Mezil et al, 2013;Setiawan et al, 2017). However, they encountered problems due to the high cost and lack of compactness of this technique (Maslov & Wang, 2008;LeBoulluec et al, 2013;Wang et al, 2014).…”