Study for the Design of an Eddy Current Array Probe for the Imaging of Aeronautical Fastener Holes

Abstract

Abstract:The design of an eddy current imaging array probe dedicated to the inspection of aeronautical fastener holes is presented. The probe aims at enhancing the characterisation of defects such as surface fatigue cracks as well as at simplifying the inspection procedures (limited probe displacements). In this paper, a general probe design featuring separate inducing and sensing functions is proposed and studied thanks to 3D finite element computations. Then, a first experimental validation of the structure is given. Finally alternative pickup sensor technologies are considered. A quantitative study of the influence of both the nature of the used sensors and their arrangement as part of a multisensor array is proposed and discussed.

Experimental validation of an eddy current probe dedicated to the multi-frequency imaging of bore holes

“…This configuration enhances the sensitivity of the probe to the defects [16], and enables the use of high gain amplifiers to provide low noise measurement data. In order to rapidly sample the two-dimensional distribution (C-scan) of the radial magnetic field, the use of a sensing coil array is considered (Figure 1.D).…”

“…The configuration of the proposed probe aims at allowing efficient acquisition of relevant data to be carried out (thanks to a sensing array), as well as computationally efficient defect reconstruction algorithms to be implemented. The design of the probe itself, including the study of the influence of the sensing element technology, was carried out by the means finite element computations and presented in [16]. In paper [17], the authors have proposed an inversion algorithm, developed in a total variation regularized optimization framework, able to carry out the three-dimensional reconstruction of surface breaking defects in the bore-hole, starting from EC data simulated in a large frequency bandwidth.…”

Regularized inversion of a distributed point source model for the reconstruction of defects in eddy current imaging

“…The material under inspection is Al 2024 T3 aluminium alloy the skin depth of which is given in Table I. The considered defect is a half disc notch due to mechanical fatigue (Thomas et al , 2009). We choose six incident field frequencies and six layers of inspection (Table I and Figure 2), to build the discrete observation relation in matrix notation: Equation 4 where: f ∈ C N is the vector of EC data, and contains the electromagnetic flux density at six frequencies equal to 800, 350, 150, 90, 55 and 40 kHZ, see Figure 1 for a representation of the image obtained at one frequency. A ∈ C N , M is the DPSM transfer matrix of the probe. u ∈ R M is the vector to be estimated: u contains a three‐dimensional description of the relative conductivity of the inspected part, the elements of which taking the values 0 or 1, according to the presence or the absence of voids (defect voxel) in the part. n ∈ C N is the vector of additive white noise, distributed according to a zero‐mean complex circular Gaussian random vector, i.e.…”

“…We choose six incident field frequencies and six layers of inspection (Table I and Figure 2), to build the discrete observation relation in matrix notation: Equation 4 where: f ∈ C N is the vector of EC data, and contains the electromagnetic flux density at six frequencies equal to 800, 350, 150, 90, 55 and 40 kHZ, see Figure 1 for a representation of the image obtained at one frequency. A ∈ C N , M is the DPSM transfer matrix of the probe. u ∈ R M is the vector to be estimated: u contains a three‐dimensional description of the relative conductivity of the inspected part, the elements of which taking the values 0 or 1, according to the presence or the absence of voids (defect voxel) in the part. n ∈ C N is the vector of additive white noise, distributed according to a zero‐mean complex circular Gaussian random vector, i.e. n ∼CNC(0, σ 2 I N ), which stands for both the acquisition noise and a possible mispositioning of the probe (Thomas et al , 2009). d ∈ C N is the vector of error taking into account forward model errors due to the discretization approximations (Section II‐A2).At each frequency, the data are recorded on a 44×44 grid, so the length of the vector of EC data is N =6×44 2 .…”

Simulation of Cracks Detection in Tubes by Eddy Current Testing

“…It can be used such as perfect tool to characterize defects in materials [4]. However, the sensitivity of the characterization process is highly dependent on the probe choice and the operation frequency [5].…”