Nonlinear Ultrasonic Characterization of Thermally Damaged Westerly Granite

摘要

Thermally damaged Westerly granite samples are probed by means of linear and nonlinear ultrasonic methods. With the increase of material damage, linear longitudinal wave velocity decreased down to nearly 50% of its initial value. Nonlinear ultrasonic measurements are performed by emitting 1 MHz tonebursts at increasing excitation amplitude. For increasing damage, the obtained waveform appeared to be the second time derivative of the envelope of the emitted toneburst, revealing a self-demodulation effect. This effect, occuring in a nonlinear manner for increasing excitation amplitude, allows to characterize the material damage with high sensitivity. A simple theoretical model, taking into account a nonlinear stress/strain relationship of the medium, described qualitatively the observed experimental results.