Internal Microcrack Detection with Nonlinear Ultrasonics

Abstract

The conventional ultrasonic techniques are ineffective for detection of minute cracks of which surfaces are partially in contact. The detection of the ensemble of such cracks is demonstrated for the steel plates which have minute cracks nucreated by spall. Large amplitude, say 10nm, burst waves of 5MHz were transmitted into the plate and the waves propagated through the plate were received a 10MHz broadband transducer. The fundamental and 2^<nd> harmonic amplitudes were calculated by FFT. For the plates containing minute cracks which can't be imaged by a C-scan acoustic microscope, the amplitude ratio of the 2^<nd> harmonic component to the fundamental one is nearly proportional to the input voltage, which is proportional to the amplitude of the incident wave. The proportional factor increases with an increases in impact stress. Namely severity of the spall damage. To the contrary, the ratio is kept nearly constant for defect-free plates.