EDDY CURRENT MODELLING FOR NONDESTRUCTIVE TESTING

Eddy current nondestructive evaluation is widely used to inspect conducting materials during manufacture or in service. In this context, modeling is a powerful tool for inspection improvements : it helps probecoil designers to optimise sensors for each examination requirement, it gives better understanding of the involved physics, it helps operator training and it also increases defect analysis reliability. This contribution presents the recent progresses in developing models which have the capability to predict quickly the signal of an eddy current probe used in nondestructive testing. These codes form a part of the tools available in CIVA which is a multitechnique (ultrasonic, radiographic, electromagnetic) platform for NDT. It is shown how these models, included in the convivial user interface of CIVA, can be used in order to optimize the design of probe-coils and study various configurations of control. It is a powerful tool for implementation of industrial NDT methods (e.g. coil shape, frequencies) and evaluation of perturbation parameters (e.g. lift-off, tilt) by limiting the number of experimental tests. This paper tackles the various analytical and semi-analytical approaches used in these models. In particular, it presents simulations of the response of eddy current probes due to a 3D flaw located in a plate or in a tube, obtained with models based on dyadic Green function calculations in the framework of the volume integral formalism. The case of testing with a ferrite core probes is also detailed. All models developed and integrated in CIVA have been validated using experimental results. Introduction: Eddy current nondestructive testing of conductive materials is of importance in many domains of industry: energy production (nuclear plants), transportation (aeronautic), workpiece manufacturing etc. This technique, based on the analysis of changes in the impedance of one or more coils placed near the workpiece to be tested, is used to detect and characterize possible flaw or anomalies in the workpiece. Typical testing configurations may consist of ferrite or air core bobbin probes which are placed above a planar (or at least locally planar) layered workpiece or inside or outside a tubular workpiece and which are operated in the time-harmonic regime. The probes can as well operate in absolute mode as in differential mode with additive or subtractive flux, or in transmitreceive mode. This contribution presents the recent progresses in developing models mainly based on the volume integral method using the Green’s dyadic formalism [1] which has the capability to predict quickly the signal of an eddy current probe used in nondestructive testing. These codes form a part of the tools available in CIVA which is a powerful multi-technique (ultrasonic, radiographic, electromagnetic) platform for industrial NDT applications including a convivial user interface. All models developed and integrated in CIVA have been validated using experimental results. Tools dedicated to the design of the probes: Before even simulating the response of a probe to the presence of a defect, it is often useful to study the way in which a probe excites the workpiece to be tested in order to optimise its design and to assess the impact of perturbation factors (e.g. lift-off, tilt). Two tools were designed for this purpose. The first of them makes it possible to calculate [2, 3, 4, 5, 6] and represent (figures 1 and 2) the incident field in the workpiece (i.e. the electric field in the absence of flaw). The second is dedicated to the study the probe performance which is usually assessed via the normalized impedance diagram (figure 3).