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 propa gating at the surface of a sample under cyclic tensile loading is used to m onitor the growth of small surface-breaking cracks. The scattered signals a re used to detect the presence of cracks as small as 80 mum in length, in t he specimens made of AISI 4130 steel, even in the presence of strong curvat ure of the surface in the path of the Rayleigh waves. Ultrasonic data is re corded as a function of the number of cycles and compared with crack length measured optically with a microscope. Experiments indicate that the rms am plitude of the scattered signal is proportional to the square of the crack radius measured optically. On specimens with rougher surfaces, ultrasonic d etection occurs before optical detection of the cracks. Results also indica te that the short-time Fourier transform (STFT) may be useful to determine whether a single crack thigh cycle fatigue) or a distribution of cracks (lo w cycle fatigue) is present. (C) 2001 Elsevier Science Ltd. All rights rese rved.