Ultrasonic inspection of tilted defects using the 'corner effect'-modelling and experimental validation

In ultrasonic NDT, the 'corner trap'mechanism is a common means of detecting planar cracks which break the back-wall surface of the component. The mechanism is most efficient when the crack is normal or near-normal to the back wall, so that a specular reflection is obtained. However, a response is still obtained when the crack is more tilted, due to diffracted edge wave signals from the crack tip and from the 'wedge' formed between the crack and the back wall. In order to model the wedge mechanism using the Geometrical Theory of Diffraction, it is necessary to know the diffraction coefficients from the elastic wedge. For ferritic steel, these wedge diffraction coefficients are given by solving the 'canonical problem' of the scattering of a plane elastic wave by a wedge vertex of arbitrary angle in a homogeneous isotropic elastic medium. We have obtained a theoretical solution to this very challenging mathematical problem. The underlying mathematical theory has been implemented in software, which is used to generate lookup tables of diffraction coefficients for use in our ultrasonic NDT models. The theory has recently been validated against experiment, using two sector-shaped steel test specimens having different vertex angles. Pulse-echo measurements of both amplitude and phase of the diffraction coefficients were made, for both compression and shear waves, and compared with theory. Good agreement between theory and experiment was obtained.