Nondestructive evaluation of microcracks by laser-induced focused ultrasound

摘要

Photoacoustic transformation of laser pulses at a concave stainless steel surface immersed in water delivered short, focused, ultrasonic pressure pulses to the free surface of a sample placed in the focal region. The probe-beam-deflection method was employed to detect the resulting transient surface slope of the surface distortions in the frequency range ≤ 100 MHz. Absolute calibration of the setup indicates strains of ～10~(-3). Clear evidence for characteristic changes in the transient waveforms was observed near single cracks at the surface. Therefore, this novel method is suitable for nondestructive evaluation and characterization of bulk and surface defects.