Evaluation of crack parameters by a nonlinear frequency-mixing laser ultrasonics method

Abstract

Nonlinear acoustic methods are commonly used in crack detection because of their high sensitivity in comparison to linear ones. However, the dependence of the nonlinearities on the crack state can not only localize it but also provides information on its characteristics. We present a laser ultrasonic method, based on nonlinear frequency-mixing, to locally evaluate several crack parameters, including some, like the local crack elasticity, which are assessed uniquely by the present technique. Two laser beams, independently intensity modulated at two cyclic frequencies ωH and ωL(ωH>>ωL), excite the sample. For a large sinusoidal thermo-elastic stress generated by the low-frequency modulated laser beam, the crack oscillates between closed and open states. In the presence of this variation, nonlinear frequency-mixing ultrasonic components at frequencies ωH±nωL (n is an integer) are detected. By modifying the intensity of the laser beam modulated at ωL, we can influence the time spent in opened and closed state over a period (2π/ωL). The developed theoretical model demonstrates the dependence of the nonlinear components at ωH±nωL on the time spent by the crack in each state. Comparison between theoretical and experimental results offers a way to characterize some local crack properties, including its width and effective rigidity.