Photoelastic Stress Analysis Based On Digital Phase Shifting Technique

Photoelasticity is a non-contact full-field stress measurement method,which can judge the stress distribution in the stress model by the interference fringes generated in the stress model.In 1980s,researchers applied computer technology and digital image processing technology to photoelasticity,and digital photoelasticity appeared.The research focus on digital photoelasticity is to realize the automatic acquisition of photoelastic parameters and the automation of stress separation,obtaining good photoelastic parameters plays a vital role in stress separation.When the photoelastic parameters are obtained by the traditional phase shifting technique,noise will appear in the isoclinic phase map.This is because the isoclinic angle parameter is undefined in this area when monochromatic light is incident.In order to obtain more accurate photoelastic parameters,an improved ten-step phase shifting technique is proposed in this paper.Combining the four-step color phase shifting technique with the six-step phase shifting technique,the four-step color phase shifting technique is used to obtain the isochromatic phase map without noise influence,and the value of the first principal stress direction angle?after unwrapping is used to obtain the isochromatic phase map in the six-step phase shifting technique.After processing,two important photoelastic parameters,the first principal stress direction angle?and the phase difference?,can be obtained.In this paper,the improved ten-step phase shifting technique is simulated experimentally,and the optical stress pattern under experimental conditions is reconstructed by using the theoretical formula of discunder diametral compression.The corresponding unwrapping algorithm is designed to process the isoclinic and isochromatic phase maps,and the value of the first principal stress direction angle?in the range of[-?/2,?/2]and the value of the fringe order N in the whole field are obtained.After stress separation by sheardifference method,the values of normal stress?_x,?_yand shear stress?_xyare obtained,and the simulation results and stress separation results are compared with the theoretical values.Finally,the photoelastic experiment of the disc under radial compression was carried out.According to the improved ten-step phase shifting technique,10 optical stress patterns corresponding to the light field were photographed,and the photoelastic parameters were processed by the designed algorithm and stress separation was carried out.Finally,the stress distribution at any point in the polycarbonate disc was obtained,and the stress separation results were compared with the theoretical values.By comparing the stress separation results of the simulated optical stress pattern with the theoretical values,it is found that when the disc diameter D=30mm,the thickness h=5mm,the radial load P=300N,and the material fringe value is 7.2222N/(mm·bar),except for the stress concentration area,the average error between the normal stress?_xand the theoretical values in the whole field is 6.32%,the normal stress?_yis 3.18%,and the shear stress?_xyis 0.14%.Comparing the stress separation results of photoelastic experiment with the theoretical values,when the polycarbonate disc diameter D=50mm,the thickness h=5mm,the radial load P=300N and the material fringe value is 8.0000N/(mm·bar),except for the stress concentration area,the average error between the normal stress?_xand the theoretical value in the whole field is 7.48%,the normal stress?_yis 4.67%and the shear stress ?_xy is 0.33%.