The Measurement Technology Of The Tensile Deformation Of Material Based On Digital Image Correlation

Various new materials appear in succession and have been widely applied in differentareas with the continuous development of material technology. However, the properties ofmaterial will determine the quality of products, so it is necessary to measure the mechanicalproperties of material. At the same time, Traditional contact strain measurement methodwill be limited when it is used in some special areas, such as flexible material, hightemperature and high pressure environment, large deformation measurement and microspecimen. Digital image correlation method has advantages of non-contact, high precisionand low requirements in experiment environment, so in this paper, the practical andeffective measurement method is applied to measure the tensile deformation to analyze themechanical properties of material and obtains good measurement accuracy.In order to improve the solving accuracy and efficiency of integer pixel displacementand sub-pixel displacement, the main theoretical and experiment research work of thisarticle are listed as following:(1) Properties of the common used displacement mappingfunctions and correlation coefficients were analyzed to establish the mathematical model ofdigital image correlation method of this article. Then, the searching accuracy and efficiencyof two practical integer pixel search algorithm were compared using simulated speckleimages. The results showed that with the same solving accuracy, the genetic algorithm wasmore efficient than the coarse-fine search algorithm.(2) Beginning with the mathematicalmodel, four classical sub-pixel displacement measurement algorithms were discussed.Especially, this article compared the choice of the mapping functions and interpolationschemes of the iterative and spatial-gradient algorithm. In order to analyze the performanceof the four algorithms, numerical images were used to simulate the process of objectoccurring translation, tensile deformation or distortion. The measurement results illustratedthat the iterative and gray-gradient algorithm had advantages of easy to implement, high precision and strong robustness, so it was more suitable for actual measurement.(3) Aimingat reducing the effect of the change of gray intensity due to the system itself andenvironment condition on measurement accuracy in actual measurement, the iterative andgray-gradient algorithm based on nonlinear intensity change model was proposed in thispaper and then compared with linear intensity change model and constant model. Theresults showed that the improved algorithm was more robust to noise and more efficientwhen the illumination was unstable, so it was suitable for displacement field and strain fieldmeasurements under complex environment conditions.Finally, the accuracy of the method used in this article was verified by actualmeasurement experiments. Firstly, the accuracy of the iterative and gray-gradient algorithmbased on nonlinear intensity change model was tested by translation experiment.Experiment results showed that the displacement measurement errors were less than0.001mm. Secondly, this method was applied to measure the tensile deformation tocalculate the elastic modules of material under tensile loading and then the results werecompared with extensometer. The experiment results illustrated that the relative errorsbetween the method proposed in this paper and the extensometer were small, and thecalculations of elastic modules were in the range of the true value, so it was able to satisfythe requirement of actual measurement.The research work in this article has certain guided signification for realizing thenon-contact and real time measurement of the mechanical properties of material.