Anders Rosell

Model Based Capability Assessment of an Automated Eddy Current Inspection Procedure on Flat Surfaces

The probability of detection (POD) of a well-controlled, automated eddy current (EC) procedure is evaluated in a numerical model and compared with experiments. The procedure is applied in laboratory environment with a single absolute probe which is positioned for raster scan over flat surfaces containing fatigue cracks. The variability of the signal amplitude, due to small fatigue cracks in the Titanium alloy Ti-6Al-4 V, is expected to mainly originate from crack characteristics and the index distance of the raster scan. The POD model is based on the signal versus crack size ( versus a) result. The presented procedure provides a well-defined basis for a comparison between a simulated and an experimentally based POD assessment. Finite element analysis is used to model the EC method. A simplified fatigue crack model is first introduced and evaluated experimentally. Numerical computations are then used to build the corresponding model-based POD curve which shows good agreement with the experimental result. The model-based POD curve is generated both by means of a parametric and a nonparametric approach. Differences between model-based and experimental POD are discussed as well as the delta POD approach using transfer functions.

The transition matrix method for a 2D eddy current interaction problem

A 2D model of the eddy current interaction problem that consists of an inhomogeneity in a conductive half space is presented. The applied analytical method of solution is the transition (T) matrix method. This involves use of the free space Greens function to generate a system of boundary integral relations. In this way, it is easy to identify the contributions to the total solution from each different scattering surface. The different parts are separated also in the computation of the impedance. This leads to low cost simulations in terms of computation time and qualify the method to be used to obtain probability of detection (POD) curves. The T matrix method is a building block method and the possibility to extend the geometry with several inhomogeneities and extra layers will be discussed. The model is compared with a Finite Element (FE) model and numerical examples for the case with a cylindrical inhomogeneity are given.