Spectrum Analysis For Rayleigh Waves Reflected From Small Fatigue Cracks

Cracks act as the sources of acoustic scattering when they are illuminated by incident ultrasonics. They radiate scattering signals, which carry information on the crack such as location, orientation, shape, and size. In this paper. the amplitude spectrum of the Rayleigh wave reflected at a small fatigue crack is interpreted to provide estimates of crack depth, aspect ratio, and closure stress for characterizing the growth behavior. The crack is initiated at a Vickers indentation and develops by cyclic cantilevered bending, The approach is based on an approximate calculation of the three-dimensional reflection coefficient. This treatment contains the sectioning of the half elliptical crack face into thin vertical strips. The reflection intensity at each strip has been given by the two-dimensional computation for a normal edge crack. The proposed technique is capable of measuring the crack depth larger than 0.13 mm and the crack closure stress from the spectrum response to static bending stresses.