Discussion of Derivability of Local Residual Stress Level from Magnetic Stray Field Measurement

The NDT procedure dubbed ‘metal magnetic memory’ method and the related ISO 24497 standard has found wide industrial acceptance in some countries, mainly in Russia and China. The method has been claimed by some researchers (Roskosz and Bieniek in NDT&E Int 45:55–62, 2012 ; Wilson et al. in Sens Actuators A 135:381–387, 2007 ) as having potential for quantitative determination of local residual stress state in engineering structures, at least for some steel grades. This work presents a critical reexamination of a previous important study by Roskosz and Bieniek, who claimed to have found a direct relationship between local residual equivalent stress levels ranging from 0 to 50 MPa, and the stray field gradients in T/P24 steel sample placed in the Earth’s ambient magnetic field. We reconstruct their experiment in a magnetic finite element simulation, computing stray magnetic field and its tangential gradients along the axis of the sample. Different combinations of remanent induction and relative magnetic permeability levels have been modeled, and the influence of geometrical discontinuity is quantified. In order to validate magnetic finite element methodology, a new experiment is presented, along with its numerical counterpart. The magnetic finite element method allowed to obtain a good quantitative correlation with well-controlled stray field measurements. It is demonstrated, that the residual stress level of order of 50 MPa is not the only factor, on which the stray field measurement depends. The geometrical discontinuity and the remanent induction contribute to a higher extent to the field amplitudes. Consequently we prove, that a bidirectional correlation between the magnetic field gradient and local stress levels cannot be determined because of at least three concurrent inseparable factors on which the measured stray field and its spatial gradient depends.