A thermal barrier coating (TBC), consisting of the top and bond coatings, is normally deployed over the alloying body of an aeroengine blade in order to protect the blade from hostile environments. Thickness loss of TBC severely undermines the structural integrity of blades, and thus leads to the failure of aeroengines. Before catastrophic accidents occur, it is important to nonintrusively assess the TBC thickness via effective nondestructive evaluation (NDE) techniques. In this letter, the dual-frequency eddy current technique (DFEC) in conjunction with an efficient inverse model is intensively investigated for quantitative evaluation of TBC thickness in micrometers. The thickness of both top and bond coatings are assessed simultaneously. Comparison between the estimated TBC thickness of blade specimens and true values acquired using scanning electron microscope (SEM) indicates that DFEC is applicable for quantitative evaluation of TBC thickness without much loss of accuracy.

Inverse analysis is a promising tool for quantitative evaluation offering informative model-based prediction and providing accurate reconstruction results without pre-inspections for characterization criteria. For traditional defect inverse reconstruction, a large number of parameters are required to reconstruct a complex defect, and the corresponding forward modelling simulation is very time-consuming. Such issues result in ill-posed and complex inverse reconstruction results, which further reduces its practical applicability. In this paper, we propose and experimentally validate an inversion technique for the reconstruction of complexly-shaped delamination defects in a multilayered metallic structure using signals derived from infrared thermography (IRT) testing. First, we employ a novel defect parameterization strategy based on Fourier series fitting to represent the profile of a complicated delamination defect with relatively few coefficients. Secondly, the multi-medium element modelling method is applied to enhance a FEM fast forward simulator, in order to solve the mismatching mesh issue for mesh updating during inversion. Thirdly, a deterministic inverse algorithm based on a penalty conjugate gradient algorithm is employed to realize a robust and efficient inverse analysis. By reconstructing delamination profiles with both numerically-simulated IRT signals and those obtained through laser IRT experiments, the validity, efficiency and robustness of the proposed inversion method are demonstrated for delamination defects in a double-layered plate. Based on this strategy, not only is the feasibility of the proposed method in Infrared thermography NDT validated, but the practical applicability of inversion reconstruction analysis is significantly improved.

Metallic structures of non-ferromagnetic materials such as aluminum alloy, titanium alloy, etc., are widely used in the engineering field of aviation. Owing to harsh and hostile environments, subsurface corrosion manifests itself in the in-service Metallic Structures of Aviation (MSA), and is usually hidden within the structure body, leaving it barely detected. Such defect has been found to be one of the most critical defects posing a severe threat to structural integrity and safety of MSA. Therefore, it is imperative to conduct imaging and quantitative assessment of the subsurface corrosion with effective Non-destructive Evaluation (NDE) techniques. Pulse-modulation Eddy Current technique (PMEC) has been identified to be superior to other electromagnetic NDE techniques in evaluation of corrosion in conductors. In light of this, in this paper PMEC for imaging and 3D profile reconstruction of subsurface corrosion in MSA is investigated. Novel features extracted from testing signals are proposed in a bid to enhance the sensitivity of the PMEC response. An image processing technique based on the modified Theta Map is proposed and utilized for recognition of corrosion opening profile. An efficient inversion method, which is focused on the corrosion-profile reconstruction right within the corrosion region and based on a sectional scanning strategy, is investigated along with the establishment of the database conceiving PMEC responses to regular-shaped corrosion. It has been found from the experimental results that the proposed inversion scheme is feasible and applicable for 3D profiling of subsurface corrosion in MSA with the averaged evaluation accuracy of 95.4%.

A corrosive environment leaves in-service conductive structures prone to subsurface corrosion which poses a severe threat to the structural integrity. It is indispensable to detect and quantitatively evaluate subsurface corrosion via non-destructive evaluation techniques. Although the gradient-field pulsed eddy current technique (GPEC) has been found to be superior in the evaluation of corrosion in conductors, it suffers from a technical drawback resulting from the non-uniform field excited by the conventional pancake coil. In light of this, a new GPEC probe with uniform field excitation for the imaging of subsurface corrosion is proposed in this paper. The excited uniform field makes the GPEC signal correspond only to the field perturbation due to the presence of subsurface corrosion, which benefits the corrosion profiling and sizing. A 3D analytical model of GPEC is established to analyze the characteristics of the uniform field induced within a conductor. Following this, experiments regarding the imaging of subsurface corrosion via GPEC have been carried out. It has been found from the results that the proposed GPEC probe with uniform field excitation not only applies to the imaging of subsurface corrosion in conductive structures, but provides high-sensitivity imaging results regarding the corrosion profile and opening size.

Multilayered Tubular Structures (MTS) such as pipeline are widely employed in such industrial fields as nuclear energy, petroleum, etc. It is vital to non-destructively evaluate MTS periodically before catastrophic accidents take place. As one of the advanced Electromagnetic Nondestructive Evaluation (ENDE) techniques, Pulsed Eddy Current Testing (PECT) has been found advantageous over the other ENDE methods regarding evaluation of integrity of MTS. In this paper, a fast forward model of PECT inspection of MTS is proposed based on the analytical modeling, namely the Extended Truncated Region Eigenfunction Expansion (ETREE). The closed-form expressions of PECT signals of 3D magnetic field and coil Electromotive Force (EMF) have been formulated. The proposed model has been verified by Finite Element Modeling (FEM) and experiments. The advantages of the model in terms of high computational speed and accuracy have been identified.

Coated conductive structures are widely adopted in such engineering fields as aerospace, nuclear energy, etc. The hostile and corrosive environment leaves in-service coated conductive structures vulnerable to Hidden Material Degradation (HMD) occurring under the protection coating. It is highly demanded that HMD can be non-intrusively assessed using non-destructive evaluation techniques. In light of the advantages of Gradient-field Pulsed Eddy Current technique (GPEC) over other non-destructive evaluation methods in corrosion evaluation, in this paper the GPEC probe for quantitative evaluation of HMD is intensively investigated. Closed-form expressions of GPEC responses to HMD are formulated via analytical modeling. The Lift-off Invariance (LOI) in GPEC signals, which makes the HMD evaluation immune to the variation in thickness of the protection coating, is introduced and analyzed through simulations involving HMD with variable depths and conductivities. A fast inverse method employing magnitude and time of the LOI point in GPEC signals for simultaneously evaluating the conductivity and thickness of HMD region is proposed, and subsequently verified by finite element modeling and experiments. It has been found from the results that along with the proposed inverse method the GPEC probe is applicable to evaluation of HMD in coated conductive structures without much loss in accuracy.