Design And Research Of Small-scale Tensile Testing Machine For Material True Stress And Strain Based On Digital Image Correlation Method

The mechanical properties of materials are very important technical indicators in the development and application of materials,and they are usually measured by corresponding testing machines.The measurement of tensile stress and strain is an important content in the measurement of mechanical properties of materials.At present,most material tensile testing machines can only measure the engineering stress and strain of materials.The measurement of true stress and strain of materials mainly adopts non-contact measurement methods,such as photometry,image-based edge detection technology and digital image correlation method.The latter two have high detection accuracy and the detection system based on digital image correlation method is relatively stable.However,the current measurement systems using the above technologies are designed independently of the tensile testing machine,and the measurement accuracy and real-time performance need to be further improved.Therefore,this paper designs and develops a small non-contact true stress-strain tensile testing machine based on digital image correlation methods,and establishes a Gaussian surface fitting algorithm based on a weight function.Based on the review and analysis of the development status of domestic and foreign materials real stress-strain tensile testing technology and testing machine,this paper proposes the overall design plan of a small non-contact true stress-strain tensile testing machine based on digital image correlation method,and confirms The main parameters of the testing machine and the parameters and models of the drive motor,reducer,ball screw and other components;the transmission part of the testing machine,the camera bracket and other structures are designed;the application of CATIA has established a small non-contact real stress strain tensile testing machine Whole machine model;use ANSYS software to carry out finite element analysis on the whole machine transmission module.The testing machine should output accurate measurement results in real time during the process of stretching the sample.Therefore,the calculation speed and accuracy of the digital image correlation method have higher requirements.This paper proposes a Gaussian surface fitting algorithm based on the weight function for the above problems.The algorithm uses the cross-correlation function established by the weight function to obtain the correlation coefficients of 9 points to be fitted around the displacement point of the whole pixel,and then the extreme points of the curved surface are fitted by the Gaussian function to obtain the sub-pixel displacement.The feasibility and effectiveness of the algorithm are verified by simulation experiments of real speckles.The data acquisition,image acquisition,analysis and calculation and motion control programs of the testing machine are developed using LABVIEW software.The whole machine program adopts modular design,and the main program realizes the function of controlling the hardware by calling the sub VIs of each functional module.Finally,the real stress-strain tensile testing machine developed in this paper was manufactured,and the tensile experiment was carried out with 7020 aluminum alloy material as the test piece.The experimental results show that the measurement accuracy of the tensile testing machine meets the design requirements,and the control system is relatively stable..Through the analysis of the error source and principle of the experimental measurement results,the improvement direction of the testing machine is proposed.