Modelling of electromagnetic acoustic transducers (EMAT) for the nondestructive testing (NDT) of ferromagnetic materials

An electromagnetic acoustic transducer (EMAT) induces without contact dynamic sources which radiate ultrasonic waves used for the nondestructive evaluation (NDE) of a mechanical part. Here, a comprehensive model to predict all the transduction sources induced by EMAT in a ferromagnetic material is developed. It takes into account an hysteretic magnetization and magnetostriction curves of the tested material by means of a simplified multi-scale approach, and it formally derives electromagnetic and magnetostrictive sources induced by a given EMAT in a very large range of NDE configurations by means of energy principles. In order to efficiently simulate the ultrasonic field radiated by these sources using the CIVA software, a formal method to transform the body sources into equivalent surface stresses is also developed. Finally, the proposed model formulates a source of surface stresses that embodies all the transduction phenomena involved by EMAT in ferromagnetic materials. The model is used to quantify the weight of each transduction mechanism, to illustrate effects of nonlinear magnetoelastic behaviors of materials, and to enlighten the impact of elastic residual stresses on transduction sources. It is applied in a given EMAT NDE configuration to illustrate how the developed tools can help optimizing EMAT design, and its predictions are successfully compared to experimental measurements. (author)