Developing a multiphysics computational model to support thermosonics for efficient defect visualization

In recent years, thermosonics has gained a tremendous amount of interest as one of the popular Non-Destructive Testing (NDT) techniques in order to localise a possible internal defect.  In general, NDT techniques gather information about the inside of a material, without destroying the material, or potentially enlarging the present damage. In the specific case of thermosonics, the studied sample is first activated by  an ultrasonic excitation causing  elastic waves that  propagate throughout the material. When these waves interact with a damage, either reflections of the wave will occur (when the defect interfaces are separated) or clapping and friction will be triggered when both surfaces of the defect are in close contact (i.e. when the defect is only incipient). When experiencing friction, mechanical energy will be transferred into thermal energy and the defect will become a heat source. This local injection of heat  will diffuse throughout the sample, until it has reached the surface, where the diffused heat then radiates into the air and can be captured by thermal sensors. By analysing the captured information, essential information about the internal structure can be obtained.

In this manuscript, fundamental research is performed to build a multiphysics finite elements computational model in order to support thermosonics for efficient defect visualization. In order to understand better what is happening inside the material, a multiphysics model has been developed, based on (non-linear) mechanical, dynamical and thermal  aspects to describe the clapping, friction and heat generation at the defect surfaces in a contact model.   A set of different  virtual experiments are conducted through numerical simulations and discussed in order to provide proof of a qualitatively working model. The numerical results, in both 2D and 3D, clearly show a response consisting of both local nonlinear frequency generation and thermal heat production induced by an ultrasonic activation. In conclusion, the fundamental research presented in this manuscript indicates that the presented multiphysics computational model is a viable environment to simulate the thermosonics  behaviour of a defected material and that this model seems fit to be used to support the understanding of  the physics occurring at a defect when visualized through thermosonics.