Optimization of coil design for eddy current testing of multi-layer structures

Abstract

The detection of a crack under fastener heads (CUF) in a multi-layered aircraft structure remains a challenge in non-destructive evaluation (NDE). An EC-GMR system using a linear eddy current (EC) coil with giant magnetoresistive (GMR) sensors located on the axis of symmetry is proposed for detecting discontinuities in conducting materials. The signal received from sensors is greatly influenced by the excitation field. An ideal excitation field is purely tangential and uniform, however this is not achievable with coils of finite dimension. This paper proposed a methodology to optimize the coil design in order to approximate the ideal excitation field to the best extent. In the proposed approach, the coil design is cast as a multi-objective optimization problem. The algorithm for solving this optimization problem is the NSGA II (non-dominated sorting genetic algorithm II). The original version of this algorithm is not efficient in finding the solution, i.e. an evenly-distributed Pareto front. Therefore a problem specific modification is proposed. Numerical simulations showed the effectiveness of the method.