Research On Multi-directional Shearography With Large Field Of View For Defect Measurement Based On Spatial Carrier

With the rapid development of modern industry,especially in engineering fields such as aerospace and automotive industries,the requirements for material performance and defect assessment are becoming increasingly high.Shearography is widely used in the field of optical non-destructive testing due to its full field,non-contact,strong anti-interference,high accuracy,and high sensitivity measurement characteristics.The current multi-directional shearography mainly measures in the double or triple shearing directions,with low sensitivity to defects located in the 45 degree and 135 degree directions,and most measurement systems fail to improve the problem of limited field of view.To this end,this thesis proposes a multi-directional shearography system with large field-of-view based on spatial carrier,which achieves simultaneous four-directional shear measurement by polarisation multiplexing to improve the sensitivity for multi-directional defect detection,and in addition,improves the measurement range by embedding a 4f optical system in front of the interferometric structure to transmit object light imaging.The main work of this thesis is as follows:1.This thesis elaborates on the basic measurement principles of shearography,and then provides a theoretical analysis of how the proposed multi-directional shearography system with large field of view achieves four-directional shearography measurements and large field of view measurements,followed by the introduction of the Fourier automatic spatial filtering algorithm to improve the efficiency and accuracy of extracting multi-spectral components,and also analyses the phase processing methods of shearography,including phase shift methods,phase map filtering methods and phase unwrapping methods.2.In order to verify the performance of the proposed four-directional shearography system,an experimental setup was constructed and the measurements showed a doubling of the field of view.This was followed by a multifaceted analysis of the effects of the key experimental parameters and a simple experimental verification.A central force loading experiment with a circular aluminium plate was then carried out to validate the feasibility of simultaneous four-directional shear measurements.3.In order to prove the detection capability of the four-directional shearography system for multi-directional defects,the defects and materials of the sample to be tested were designed in a targeted manner,and the simulation analysis was carried out in combination with finite element analysis software and Matlab.Further improvement of the sample to be tested based on the simulation results of defect detection under different loading conditions and the analysis of the influence of the structural parameters of the defect.Finally,the defect detection experiments were carried out under central force loading and non-destructive thermal loading conditions respectively.The experimental results validate that the proposed system can effectively detect defects in multiple directions simultaneously and has great potential for achieving dynamic measurements for non-destructive testing.