SH0 interaction with surface-breaking tilted cracks in plates

J Combaniere and P Cawley 

Interactions between SH0 guided waves and simple defects such as vertical notches or cracks have already been thoroughly investigated. However, many defects have more complex morphologies and cannot be modelled under the assumption that the defect is vertical. Among many other industries, the oil & gas industry requires a practical characterisation method, as tilted cracks are a common occurrence in high pH environments. A finite element model was developed for simulating the ultrasonic response from tilted surface-breaking cracks. Waves trapped between the defect and the surface of the plate were observed as the incident wave interacts with the defect to generate transmitted and reflected waves. This can produce destructive interference between reverberating signals, potentially leading to transmission nulls at specific frequencies defined by the geometry of the defect. The initial studies were in 2D and looked  only at the effect of tilt and depth for an incident SH0 wave at 1; this was followed by a 3D study investigating the effect of size. It was shown that the transmission null disappears for defects and sensors smaller than 100 mm (around 3 wavelengths) with the transmission minimum frequency being removed being a weak function of size, supporting the idea that edge effects play an important part in the total transmitted field. The implications of this for defect detection and characterisation will be discussed.