Research On Digital Speckle Correlation Measurement Methods

Digital speckle correlation measurement method is a new type of optical measurement technology developed from the 1980s,which depends on comparing the change of speckle images on before and after deformation of the specimen surface to obtain the displacement and strain.After more than 30 years development,this method has been widely accepted for its advantages such as full field,non-contact,high precision,measurement sensitivity,and so on.DSCM has been used in many fields such as aerospace,non-destructive testing,micro-scale deformation field measurement,high temperature deformation measurement and electronic packaging.The primary problem of the digital speckle correlation method is the integer pixel search.In this paper,the cross-search method is used to search the integer pixel displacement and the assignment method of the main peak is been improved.The traditional cross-search method is very complex in the process of determining the main peak area.According to the displacement continuity,the initial value of the main peak is assigned to the calculation result of the adjacent point,which improves the calculation speed.Sub-pixel search is the key problem in digital speckle correlation.In the sub-pixel correlation research,the improved gradient method which based on the gray transform function is combined with the surface fitting method to measure the sub pixel displacement.The introduction of the gray-level variation function weakens the influence of the intensity variation on the measurement accuracy.This new method improves the measurement accuracy and the stability of sub-pixel displacement.We selects the least squares fitting method with high calculation precision and small calculation amount to calculate the strain of the object according to the displacement.In addition,three factors,the calculation area size,interpolation magnification and noise,are analyzed to the accuracy of the digital speckle correlation method.In order to verify the performance of the proposed algorithm in this thesis,three mechanical experiments is designed,include the rigid body displacement test,the tensile test of aluminum sheet and the tensile test of single hole specimen.By comparing the theoretical value and the experimental value,the validity and practicability of the digital speckle correlation measurement method is verified.