Accuracy assessment of electromagnetic modelling of a stress measurement instrument

In the design, manufacture and maintenance of many structures, knowledge of stress is of great importance in assuring integrity. AEA Technology, through its National NDT Centre, has developed a stress measurement technique that is nondestructive and capable of use in-situ. The technique, named ´MAPS´, is based on magnetic measurements and is thus restricted to ferromagnetic materials. The processing algorithms are based on analytic models derived by AEA Technology. During the development programme, electromagnetic finite element modelling has made important contributions to improving performance. Recently, such modelling work has been used to gain further insight into the interaction of the MAPS probe with structures, with a view to enhancing the underlying approximate models and consequently extending the capabilities of the MAPS technology. This has involved detailed comparisons of finite element and MAPS instrument data, and a by-product has been the validation of various aspects of finite element modelling. As well as discussing this validation, the paper describes approaches adopted in modelling the MAPS probe and the degree of validation obtained. All the modelling represents the B-H properties nonlinearly and some of the approaches involve anisotropic permeabilities and approximate representation of hysteresis.