The interaction of eddy-currents with defects is quite a complex phenomenon, but if a firm understanding can be developed then one has taken a large step towards quantitative eddy-current testing. Lord and Palanisamy describe this very well:
Although many eddy-current tests are carried out to determine composition, hardness, dimensions, and other properties of metal parts, the major barrier to further development of eddy-current and, indeed, all electromagnetic testing methods at this time, is the lack of a viable theoretical model capable of predicting the complex field/defect interactions which are the very essence of any sound defect characterization scheme.
The theoretical model is often referred to as the forward problem or forward model and is the heart of any quantitative scheme.
The forward problem is defined to be the predicting of the response of some system when all the inputs into the system are known. The forward problem is well-posed if the system response to a fixed set of inputs is always the same, that is to say the system response is a function of its inputs. This function is represented diagrammatically in figure above. The system response can be multidimensional, but must be completely determined by the input parameters.
The EddyCentre basic model and the EddyCentre advanced model can predict a probe's response due to a work-piece. In the framework of Figure, the inputs consists of all the dimensional and material parameters to describe the probe, work-piece and flaw with the driving force being the alternating current in the probe. The response is the change in impedance, delZ, in the coil due to the work-piece. The system, in this example, governs the relationship between the probe and the flaw in the work-piece. Understanding the interactions of induced eddy-currents in the work-piece is based on an appropriate physical model, mathematical model, and numerical model.
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