Reseach On Numberical Modeling Of Pulsed Eddy Current Detection System Containing Defect

Pulsed eddy current(PEC) nondestructive testing technology(NDT) has became the research hotspot of testing aerospace structures due to its wide frequency band and easy access to rich information. However, it has been already pointed out that there are significant differences between real cracks and artificial defects that can be easily modeled in numerical simulations. It has less research on the detection of real crack in China. To expand the research of PEC NDT in artificial crack detection, this thesis will do some further research on the crack generation and propagation, and numerical modeling problem. It can be provide a theoretical basis for practical application. The contents are mainly divided into three parts shown as follows.(1) The 7075 aluminum alloy crack generation and propagation is analyzed by ABAQUS in investigation. The influence of the width, depth, location of the crack and load on stress concentration in three-point bending test artificial are also considered. Based on the conclusion, by using the extended finite element method,some further research and analysis on the case of crack generation and propagation will be done.(2) A scheme the phenomenon will be proposed in the contents which the mesh of different regions in PEC NDT system have different effects on simulation results. That means using quantitative method for the critical area to provide support for the simulation and analysis of PEC system.(3) The finite element model for PEC NDT system is built, then a new method for detecting the artificial defects is proposed and the relationship between surface defect sizes and characteristics of the detection signal is found. This relationship is verified by experiments, and the height of lift-off in simulation is determined at the same time.Through these studies concluded that:(1) The greater the distance between defect and the centerline of specimen, the smaller the maximum stress; the deeper the defect the greater the maximum stress; no significant relationship between the maximum stress and width of the defect; there is a linear relationship between the maximum stress and the loading force. In order to obtain the correct crack expansion location and path, only structured and symmetry of the sub-network is desirable in the process of sub-network of cracks.(2) The mesh size on the tip of defects have samller influence on the simulation results and can be unconsidered in the modeling process for the main reason that the hall sensor is far away from the tip of defects; whether the depth of the defects in measured body is less than or greater than the skin depth, the optimal sub-network size are all obtained based on the size of defects.(3) Within a certain range, the peak value of defect differential signals has a linear relationship with the width of surface defects, and has a quadratic function relationship with the depth of defects, the peak value curve of defect differential signals has a certain relationship with the defect sizes, and this can be an important basis for realizing a quantitative assessment of surface defects.(4) The optimal height of the lift-off is got through experiments, and this can improve the curve fitting accuracy of characteristics of the detection signals...