ULTRASONIC CHARACTERIZATION OF DEFECT ROOT GEOMETRY

The potential for crack initiation at a surface-breaking defect is strongly dependent on the radius of curvature p at the defect root: a small radius can lead to large stress concentrations which greatly increase the risk of crack initiation. In this project, the objective is to develop a technique for estimating the radius of curvature at the root of a fretting wear defect on the inside surface of a CANDU nuclear reactor pressure tube. One unusual aspect to this problem is that many geometrical features of the defects under investigation are already known. The defects are in the shape of flat-bottomed axial gouges approximately 3 mm wide. Their depths, which range up to a few hundred microns, can be accurately estimated by ultrasonic time-of-flight techniques. [1] The radius of curvature at the bottom comers of the gouge is the only significant feature for which no information is readily obtainable by any conventional nondestructive technique in the hostile environment of a nuclear reactor core. WAVE THEORY Considerable work has been done on the diffraction of ultrasound from geometric discontinuities. Characterization of surface-breaking defects such as those under investigation here has frequently been conducted using Rayleigh waves. Features such as defect depth and surface roughness have been studied, e.g. [2,3], but generally for defects that were very sharp cracks or had idealized shapes such as hemispherical pits. [4] Th