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 NDTE 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.
Highlights
The NDT procedure based on notions of the metal magnetic memory (MMM) or residual lagnetic field (RMF) method has been promoted by its inventor, A
We have formulated the following questions: Could the field gradients measured by Roskosz and Bieniek over S235 and T/P24 steel samples be due to the residual stress alone? Could a reliable inverse function be defined, allowing for deduction of a local residual stress level from the local stray field gradient?
The essential question is: could the field gradients measured over S235 and T/P24 steel samples be due to the residual stress alone, as postulated by Roskosz and Bieniek? If that statement was true, an inverse relationship σ (|d HTAN /d x|) could be defined, and quantitative, Page 6 of 9
Summary
The NDT procedure based on notions of the metal magnetic memory (MMM) or residual lagnetic field (RMF) method has been promoted by its inventor, A. Most of MMM researchers refer to Jiles’ model, only few of them [18] emphasize the complex and multi-factorial relationships which require the use of partial differential equations Roskosz with his co-authors systematically studies the qualitative and putative quantitative potential of MMM. He notes the complexity of B − H (σ ) relationship and presents rather complicated patterns of measured magnetic stray field over a sample with a round hole [19]. An inverse function: d H (σ )/d x is plotted and presented as useful for quantitative determination of local stresses Facing these contradictory statements, we have formulated the following questions: Could the field gradients measured by Roskosz and Bieniek over S235 and T/P24 steel samples be due to the residual stress alone? The second experiment, referred to as ‘E2’ was performed by the authors of this article, in order to reexamine conclusions put forward by the aforementioned researchers
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