Study On The Application Of Digital Photoelasticity Method In Birefringent Crystal Stress Measurement

The digital photoelasticity method determines the stress distribution within the model by the interference fringes produced by the stressed model.The stress field contains information about the isoclinic of the principal stress direction and the isochromatic of the principal stress difference.It is important to accurately and quickly obtain stress distribution information from isochromatic and isoclinic.Due to monochromatic light and the mismatch error of quarter-wave plate,the isoclinic are easily interfered by the noise of isochromatic in six-step and four-step phase shifting methods.The ten-step phase-shifting method adds four-color photoelastic images,but reduces data acquisition efficiency.High-speed polarization cameras,polarization sensors,and other equipment can improve acquisition efficiency but face the challenge of high prices.Stress calculation requires the phase unwrapping of isochromatic and stress separation to obtain stress distribution results.However,severe noise interference in high-density stripe areas can affect the accuracy of phase unwrapping,and errors can accumulate during stress separation,which ultimately affects the stress calculation results.to address these issues and meet the practical needs of stress measurement using birefringent crystals in engineering applications,the research content of this paper is as follows:（1）A Six-step mixed phase-shifting method has been proposed to address the issues of interference from isochromatic on the calculation of iscolinic using the six-step and four-step phase-shifting methods,as well as the low efficiency of image acquisition in the ten-step phase-shifting method.This method calculates the isoclinic using three photoelastic images under white light in a planar polarized light field,and calculates the isochromatic using three photoelastic images under monochromatic light in a circular polarized light field.Experimental results show that the method has good noise immunity and phase shift error immunity;effectively avoids the iscolinic coupling isochromatic problem compared to the traditional four-step and six-step phase shift methods;and improves the image acquisition efficiency by 40%compared to the ten-step phase shift method,ensuring a balance between accuracy and efficiency.（2）Aiming at the problem of low accuracy of phase unwrapping in high-density striped and severely noisy areas,and noise interference in stress calculation results caused by error propagation during stress separation process.In this paper,a deep convolutional neural network regression method is used,and a stress correlation loss is designed to directly map three stress distribution images from the isochromatic wrapping phase.The experimental results show that the structural similarity（SSIM）of the boundary stresses,,shear stressand principal stresses₁ andare 0.9929,0.9896,0.9951,0.9800 and 0.9915respectively,and the shape and trend of the stress distribution of the model are basically consistent with the true values,indicating that the stress calculation method is feasible.（3）Aiming at the stress measurement application of polycarbonate birefringent crystal,a digital photoelasticity-based birefringent crystal stress measurement system is designed and implemented.The results obtained by the above two methods have an average deviation of0.03 rad for isoclinic and 0.09 rad for isochromatic when compared with the ten-step phase-shifting method.Finally,five stress distribution maps of the birefringent crystal are calculated,which meet the requirements of practical engineering.