Variation of Localized Nonlinear Acoustic Parameters around a Crack Tip using a Laser Doppler Vibrometry

A nonlinear acoustic effect is a sensitive tool to detect a micro-scale crack or early stage of cracking during the fatigue process. The damage produces a nonlinear stress-strain relationship, and the nonlinearity can be measured by a higher harmonic component in the frequency domain. The greater the damage, the larger the level of a nonlinearity, which can be used for the diagnosis of micro-cracks. The 2nd harmonic component and higher harmonic components are measured using laser Doppler vibrometry. Because a laser beam can be focused to a small spot, localized nonlinear acoustic parameters can be determined. By scanning the laser beam, localized nonlinear acoustic parameters can be mapped around a crack in a CT (compact tension) specimen. Various nonlinear parameters are chosen and tested around the crack tip, and the most sensitive nonlinear parameter for a micro-crack or closed crack are optimized.