Scattering of Elastic Waves by Inclined Subsurface Cracks

Recently analytical techniques using the T-matrix of an isolated flaw were developed to study the problem of ultrasonic wave scattering from a subsurface flaw in an elastic half space that interfaces with a fluid half space.1,2 he scattered far field in the fluid was expressed in orders of multiple scattering between the flaw and the fluid-solid interface. It was concluded from both the theoretical study and the experimental study3 that for incident wave angles oblique to the interface multiple scattering effects are negligible for depths greater than the flaw diameter. At incidence normal to the interface, it was found that first order multiple scattering modulated the scattered field of the isolated flaw. The modulation wavelength could be related directly to the depth of the flaw and its amplitude became smaller as the depth increased. At oblique angles of incidence, mode conversion at the fluid-solid interface is the major cause of the observed interference pattern in the frequency spectrum. Atdepths below one crack diameter, multiple scattering effects are significant and the full scattering series must be retained.