Issue 28

B. Ye et alii, Frattura ed Integrità Strutturale, 28 (2014) 32-41 ; DOI: 10.3221/IGF-ESIS.28.04 33 In ECNDT, one of the principal challenges is to determine the size of deep defects in the multi-layered structures based on information contained in the signal representing the change in impedance of the coil as it scans over the specimen. For single-layered structure materials, the high degree of accuracy of some numerical simulation techniques has been demonstrated and several fast computational methods have been presented [9-11]. However, the quantitative estimating size of deep defects inmulti-layered structures still poses amajor challenge and remains tobe dealt with [12]. In this paper, we propose amethod of inversing eddy current NDT signals to quantify deep defect size using an iterative improved ant colony algorithm (IACA). The forward problem is calculated via the finite element method (FEM). This method simplifies the inversion process and can be used more easily for complicated defects owing to the flexibility of finite elements. The accuracy and robustness of two-dimensional (2D) estimation results for a deep defect in the two- layered structures demonstrate the potential of implementing the algorithm. P ROBLEMDESCRIPTION he arrangement for the detection problem of the two-layered structures using the ECNDT technique is shown in Fig. 1. The typical deep defect investigated in this work is assumed to be a regular rectangle defect with different values of length, height and depth, representing themodel of a real inner deep defect increasing in time and size. The ECNDT system taken into account here has been developed in our laboratory. The right-cylindrical air-cored coil probe scans over the surface of the structures from left to right under testing. (a) (b) Figure 1 : Schematic diagramof the detection system. (a) Sectional view; (b) Top view. In order to describe and calculate the defect size conveniently, Cartesian coordinates are used. The global coordinates are O (x, y, z) and the origin lies in the upside of the two-layered structure surface. The local coordinates of the coil are O 1 (x 1 , y 1 , z 1 ) and the origin O 1 is (0, 0, ½( l 2 − l 1 )) in the global coordinates. ( l 2 − l 1 ) is the length of the coil, l 2 , l 1 respectively represent the global coordinates of the upper and lower side of the coil. The local coordinates of the defect areO 2 (x 2 , y 2 , z 2 ) and the originO 2 is assumed the geometry centre of the defect. F ORWARDPROBLEM he EC problem can be described mathematically by the partial differential equations in terms of the magnetic vector potential. The FEM based on variation principles can obtain the numerical solution of magnetic field by transforming the partial differential equations into the liner algebraic equations, which are built by combining the boundary conditions and theminimum energy functional [12]. T