Backscattering grain noise modelling in ultrasonic non-destructive testing

Abstract

Many materials present an internal grain microstructure. When these materials are subjected to ultrasonic non-destructive testing, the grains behave like scattering centres producing unwanted backscattered noise that can make the detection of true defects difficult. This paper is devoted to the modelling of the probability density and the spacetime correlation functions of the grain noise complex envelope. Assuming statistical independence between any pair of grains, the authors derive analytical expressions for the above functions. Specifically, the envelope comes to be K-distributed, the parameters of the distribution may be related, under reasonable simplifying assumptions, to the material characteristics (grain density, grain size distribution, propagation velocity). The spacetime correlation function is a separable function. It may be expressed as the product of a spatial factor due to the spatial correlation introduced by the non-zero beamwidth, and a time factor due to the time correlation introduced by the non-zero pulse duration. The analytical expressions are verified by means of real data measured in austenitic stainless steel specimens.