Research On Multilayer Sheet Electromagnetic Nondestructive Testing Technology

Non-destructive Testing is key technologies to protect vital engineering equipment manufacturing quality and operational safety. Multilayer materials are widely used in key areas such as nuclear industry and aerospace because of their special structure and performance, it is critical to protect quality safety of these vital components. Because multilayer materials generally have complex structure, certain thickness, diversity of materials, and restriction of actual detection environment, it is a challenge to most non-destructive techniques. Electromagnetic nondestructive testing technology is the one which make use of the principle of electromagnetic induction to detect defects or discontinuous in material. Electromagnetic techniques can achieve fast testing without contact and coupling. In this thesis, two advanced electromagnetic non-destructive testing techniques, eddy current thermography(ECT) and remote field eddy current(RFEC) are proposed to study defects detection ability in the multilayer structure, and impact damage in carbon fibre reinforced plastic(CFRP) and internal defects in multilayer steel-lead bonding structure was studied.The main contributions are:1) Multidimensional defects and physical characteristics mining of eddy current pulsed thermography. Eddy current pulsed thermography is a new emerging nondestructive technique, through ECPT system a thermal video which record thermal information of specimen surface was abtained. Currently, this technique is limited by the manual selection of proper contrast frames. In order to select a specific frame from transient thermal image sequences to maximize the contrast of thermal variation and defects pattern from complex structure samples, an energy driven approach to compute the coefficient energy of wavelet transform is proposed which has the potential of automatically selecting both optimal transient frame and spatial scale for defect detection using ECPT technique.2) Micro impact damage detection in carbon fibre reinforced plastic using ECPT. According to the variation of different frequency component and the comparision study of the detection performance of different scale and mother wavelets, the fibre information and impact damage information was separated in the thermal image and impact damage was detected in the low-frequency component.3) The defect detection performance of eddy current thermography and remote field eddy current in multilayer bonding structure was studied. ECT can only detect surface defects in reflection mode due to the limitation of skin depth effect. The heat of single pulse can not effectively penetrate the sample when using transmission mode. Multi-pulse excitation can make heat penetrate the specimen, while the heat diffusion in lateral direction makes it difficult to recognize defects in the thermal image. Remote field eddy current technique can effectively detect defects in two layer steel-lead bonding structure. While it is still difficult to detect four layer steel-lead structure. In the future, magnetic sensors design research for four-layer steel-lead bonding structure will be carried out and computer numerical control machine will be used to achieve more precisely detection.