3D finite element simulations of an air-coupled ultrasonic NDT system

A single-sided, air-coupled ultrasonic non-destructive testing (NDT) system based on the generation and reception of the A0 Lamb mode is used for detecting defects in simple or composite structures. Transmitting and receiving transducers, being oriented at the appropriate coincidence angles for the generation and detection of the mode, are fixed and moved together over the sample surface from one end to the other, to scan the area with the defect. This contact-less NDT system is modelled in three dimensions with a finite element-based method. The air-coupled transmitter is modelled by the normal stress that it locally applies on the sample surface, and the air-coupled receiver by integrating normal displacements over corresponding areas selected on the plate surface according to its positions. In this way, beam spreading of both incident and scattered fields is considered. Numerical predictions have successfully been compared with experimental data for various samples: a simple aluminium plate with a through-thickness hole, as a validation case, a glass–polyester composite sample with an impact damage, and then a high-pressure tank made of a titanium liner and a carbon–epoxy winding, with a local disbond hidden in between.