An Investigation of the Relationship between Subsurface and Head Waves by Finite Element Modeling

In ultrasonic testing, the energy of subsurface waves is concentrated under the surface of the workpiece and because of that, these waves are not affected by surface roughness and can be used for detection of subsurface defects. Subsurface wave is in direct interaction with its corresponding head wave. A head wave is a plane wave that is generated in a medium because of the free-surface boundary conditions. In order to properly interpret ultrasonic testing signals, it is important to know how different waves propagate inside the material and how they interact with each other. In this study, we concentrate on how the subsurface and head waves interact with each other. The finite element method is used for modeling the interaction of these waves. The modeling is two-dimensional and in plane strain state. A number of specimens with curved surfaces or internal defects are modelled. The results show a close relationship between the subsurface and head waves. When the specimen has a crack-like defect, subsurface and head waves are also generated on the surface of the crack. The results provide a better understanding on how waves propagate in a test specimen and can help in improving the interpretation of ultrasonic testing signals.