A Study of Stress Dependent Magnetostriction on Steel Plate by Analysis of an Electromagnetically Generated S0 Lamb Wave

A ferromagnetic material's magnetostriction coefficient is generally difficult to measure but it does depend on stress, so that measurement of the magnetostriction coefficient can in principle be applied to evaluate stress. A magnetostriction-based electromagnetic acoustic transducer (EMAT) is used to generate the S0 Lamb wave in a 4 mm thick steel plate, and the peak-to-peak value of the EMAT detected S0 Lamb wave signals are used to evaluate the stress. A nonlinear, magnetostrictive finite element model is developed and employed to stimulate the generation of the S0 Lamb wave in 4 mm thick steel plate in the time domain. The results show that the peak-to-peak amplitude of the S0 Lamb wave is inversely proportional to the applied tensile stress, whereas, with increasing compressive stress, the peak-to-peak value of the S0 wave signal rises to a peak at approximately -50 MPa, and then decreases as compressive stress increases. This non-monotonic behaviour with compressive stress is not conducive to the evaluation of compressive stress, but the method can be used to quantify tensile stress. Experiments are conducted on 4 mm thick Q235 steel plates, in which a local stress is artificially applied. The S0 Lamb wave are generated by a magnetostriction-based EMAT generator, and the results are in a good agreement with the simulation. This method is restricted to measurement of local stress underneath the generator. With suitable calibration, this method can be employed to evaluate local tensile stress in ferromagnetic materials.