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

The local evaluation of several parameters of a crack is realized with a nonlinear laser ultrasonic method. The method is based on the sample excitation by two laser beams, independently intensity modulated at two cyclic frequencies omega(H) and omega(L) (omega(H) >> omega(L)) and on the detection of nonlinear frequency-mixing ultrasonic components at frequencies omega(H) +/- n omega(L) (n an integer). Frequency-mixing is a nonlinear process originating from the modulation of the crack state at low frequency omega(L) by laser-induced thermo-elastic stresses, which causes in turn the modulation of the acoustic waves at frequency xH reflected/transmitted by the crack. We carry experiments with increasing laser power and observe a non-monotonous variation in the amplitude of up to 6 nonlinear sidelobes. We also improve a previously introduced theoretical model which leads to interpreting the experimental observations by the combined action on the crack of the thermo-elastic waves at low frequency omega(L) and of the stationary thermo-elastic stresses at omega = 0. The latter are induced by the average laser power absorbed by the sample. While thermo-elastic wave can periodically modulate the parameters of the crack up to its periodic opening/closing, the stationary heating could cause complete local closure of the crack. By fitting the experimental amplitude evolutions for all monitored sidelobes with the theoretically predicted ones, various local parameters of the crack are extracted, including its local width and effective rigidity. (C) 2016 Elsevier B.V. All rights reserved.