Nondestructive residual stress measurement in steel using magnetoacoustic interactions.

Abstract

In ferromagnetic materials, ultrasonic velocity varies with both stress and external magnetic field. This phenomenon occurs due to the magnetoelastic interactions, and it is called magnetically induced velocity change (MIVC). The behavior of MIVC depends on the stress, magnetic field, ultrasonic mode (longitudinal or shear), and their relative orientation including the polarization direction of the shear wave. Its magnitude is of the order of 10-5, being comparable with the acoustoelastic effects. In the present study, the MIVC for longitudinal and polarized shear waves were first measured with mild steel specimens for the changing magnetic fields applied parallel or perpendicular to uniaxial stresses. The MIVC curves thus obtained showed salient dependence on the stress. They were then used to nondestructively evaluate the thermomechanical residual stress introduced by gas cutting. The results agreed with the strain gage measurements and the conventional acoustoelastic methods, both destructive, showing the usefulness of the MIVC method to measure residual stresses nondestructively. The MIVC was found to be insensitive to the micros-tructures.