Detection of small surface-breaking fatigue cracks in steel using scattering of Rayleigh waves

The long-wavelength (Rayleigh) scattering of a 5 MHz ultrasonic pulse propagating at the surface of a sample under cyclic tensile loading is used to monitor the growth of small surface-breaking cracks. The scattered signals are used to detect the presence of cracks as small as 80 mum in length, in the specimens made of AISI 4130 steel, even in the presence of strong curvature of the surface in the path of the Rayleigh waves. Ultrasonic data is recorded as a function of the number of cycles and compared with crack length measured optically with a microscope. Experiments indicate that the rms amplitude of the scattered signal is proportional to the square of the crack radius measured optically. On specimens with rougher surfaces, ultrasonic detection occurs before optical detection of the cracks. Results also indicate that the short-time Fourier transform (STFT) may be useful to determine whether a single crack thigh cycle fatigue) or a distribution of cracks (low cycle fatigue) is present. (C) 2001 Elsevier Science Ltd. All rights reserved.