Ultrasonic surface wave propagation and interaction with surface defects

Electromagnetic acoustic transducers (EMATs) are non-contact ultrasonic transducers capable of generating wideband surface acoustic waves on metallic samples. We describe some lab based ultrasonic measurements using EMATs to generate wideband, low frequency (approximately 50-500kHz) ultrasonic surface waves on a number of samples including aluminum billets and sections of rail track that contain simulated defects. A stabilized Michelson interferometer has. been used to measure accurately the absolute out-of-plane displacement of the ultrasonic waves generated on the sample, which propagate along the sample to interact with a simulated surface breaking defect. Transient finite element analysis has been used to model the ultrasonic wave propagation on the sample and the interaction of these waves with surface breaking defects. These simulations compare very favorably with the experimental results obtained using the Michelson interferometer to measure the out-of-plane displacement of the surface waves. We describe different approaches that can be used to determine the depth and presence of the crack. The non-contact nature of EMATs and the pitch-catch test geometry that we propose to use for testing make them especially suitable for online detection and depth gauging of surface breaking cracks at high inspection speeds