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.

Structural health monitoring (SHM) technology has attracted more and more attention in aerospace and energy industries as it may detect and monitor early degradation and damage in the structures and thus avoid life and financial loss. Guided wave (GW) based inspection is very useful for the SHM of thin-walled structures due to its capability for long-distance and wide-range inspection with high sensitivity. However, the lack of small and high-performance transducers seriously limits the development of GW-based SHM technology. In this paper, a thin-film magnetostrictive patch transducer (MPT) that can generate and receive single-mode and almost nondispersive GW is proposed. The new transducer is composed of an alloy patch and a double-layer flexible printed circuit (FPC). Firstly, the basic structure and principle of the proposed transducer are presented and analyzed. Compared with current single-mode GW transducers, the main advantages of the proposed transducer are lightweight, flexibility, and high transduction efficiency. Both the numerical simulation and experimental results validate that the new transducer has good performance in generating and receiving single and almost nondispersive S0 mode GW in an aluminum plate. It is very promising to be applied for the SHM in curved thin-walled structures.

Until now, most of the bulk-wave electromagnetic acoustic transducers (EMATs) are developed for shear wave (S-wave) testing. This article presents the development of new longitudinal wave (L-wave) EMATs with high conversion efficiency by using a new magnetic configuration and a meander-line coil. The new magnetic configuration consists of several rectangular magnets arranged face to face to generate a strong horizontal magnetic field in periodical distribution under the magnets. By using the meander-line coil under the magnets, a strong L-wave can be generated and detected. The new EMATs have been characterized and optimized using numerical simulation methods. Simulation results show that the flux density of the horizontal magnetic field of the new EMATs can be increased by a factor of ~4.7, compared with that of the conventional U-type magnet. So, the conversion efficiency for L-wave generation and detection can be significantly improved. Finally, experimental measurement and comparison with the current L-wave EMATs with the U-type magnet have also been carried out. The experimental results have confirmed that the signal amplitude of the new L-wave EMATs is increased by a factor of ~8.3.