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

Abstract

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 B Marek Augustyniak maugustyniak@mif.pg.gda.pl 1 Gdansk University of Technology, 80233 Gdansk, Poland 2 DES ART Ltd, 81366 Gdynia, Poland 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.

Finite Element Method Applied in Electromagnetic NDTE: A Review

“…The only published applications of numerical magnetic analyses in this context are either non-conclusive [112], or bring forth arguments against the concept [113][114][115].…”

“…In Augustyniak and Usarek [113] the magnetostatic 3D FEA was applied to study the influence of concurrent factors (geometrical discontinuity, magnetic permeability and remnant magnetisation) on the field strength gradient, claimed by some MMM proponents as correlated with elevated residual stress. The sensitivity study revealed, that the field gradient did not carry enough information to allow solving of an inverse quantitative problem, so determination of stress levels with a magnetic passive stray field alone is impossible.…”

“…-simulation not referred to any experiment [56,104] -qualitative correlation with an experiment [74,75] -relative quantitative correlation [106,117] -absolute quantitative correlation [97,113] Relative quantitative correlation is defined here as demonstrating the same per cent variation of modelled and measured signals, with differing baseline values, typical for 2D modelling. Often it is not possible to make a sharp distinction between relative and absolute correlation, as in [106], where the measurement is presented in terms of voltage, which naturally is influenced by tunable amplification factor.…”

“…The break-through presented in [116] (a function allowing to quantitatively deduce a residual stress from a stray field level) was later shown to be an overinterpretation [113].…”

Current deflection NDE for the inspection and monitoring of pipes

“…A manifestation of MMM known as the Magnetic Tomography Method (MTM) claims to be able to detect defects at extreme lift-off of over a metre [26,27,28]. The relationship between stress and magnetic field is complex, and Augustyniak and Usarek [29] have shown that it is not possible to retrieve a bidirectional relationship between the magnetic field gradient and the local stress level in the pipe because the magnetic field is a function of several inseparable variables, making a quantitative NDE technique based on the MMM method impossible. The "NoPig" method is an above ground inspection method aimed at identifying corrosion on difficult-toinspect buried pipelines carrying an injected electric current.…”