Research On Detection Of Internal Defects Using Digital Speckle

Non-destructive testing of defects is widely used in the aerospace industry,the automotive industry,the rubber industry,etc.,making the research of defect non-destructive testing methods of great significance.Digital speckle interferometry has the characteristics of full field,non-contact,high precision,etc.,and is widely used in the field of defect detection.In this thesis,the research on the application of digital speckle interferometry in non-destructive testing of defects has been done.The main research contents are as follows:1.A method for full-edge detection of defects based on the phase derivative of the out-of-plane displacement is proposed.The phase derivative of the out-of-plane displacement can be obtained after demodulating the phase of fringe patterns of the out-of-plane displacement using windowed Fourier ridges.Then,the phase derivative are scanned to detect the hidden singularities by the continuous wavelet(CWT)combined with the weighted structural intensity method,using Marr wavelet,so as to determine the full-edge of internal defects.Experiments verify that the detection method is effective and feasible.2.The gradient streamlines scanning method for full-edge detection of defects is proposed.The gradient streamlines scanning method can be used to obtain the paths of the highest signal-to-noise ratio in the out-of-plane displacement field,and,using the third order derivative of the Gaussian function(Guas3)as the basic wavelet,the hidden singularities of the gradient streamlines can be detected by CWT combined with the weighted structural intensity method,so that the full-edge of internal defects can be determined directly in the out-of-plane displacement field by using both of them.Experiment and finite element simulation verify that the detection method is effective and feasible.3.The Moment-Stiffness Method(MSM)for full-edge and depth detection of defects is proposed.The method is based on the inherent relationship between bending moment and bending stiffness to detect the full-edge and depth of the internal defects of structures with known stiffness.Experiments and MATLAB simulations verify that the detection method is effective and feasible.4.Radial numerical differential and circumferential numerical differential algorithms are proposed.Both of them are used to process the out-of-plane displacement field of a flawed plate to gain the distribution of the second order derivative,and then the full-edge and depth of the defects can be obtained using MSM.