Abstract:The nominal stress-strain relationship is used widely in engineering. The stress and strain is no longer uniform distribution when the large deformation, so it can not reflect the true constitutive relation with the stage of plastic. Tensile testing of stainless steel is produced and series of digital images of specimen speckle surface were recorded, and processed by digital image correlation technique to obtain the deformation field of each recording step, so the deformation process of tensile test is revealed; the optical extensometer technique is also used to measure the real time horizontal and vertical strain of the specimen, and the real stress-strain curve is obtained with combination of the volume invariant theory, then the Johnson-Cook model is employed to match the real stress-strain curve to extract the plastic relationship; finally based on the above experimental results and the real stress-strain curve, the mechanical properties could be obtained. The experimental results shows that the true stress-strain relationship obtaining by digital image correlation is effective. Through the post processing of true stress-strain relationship the elastic modules and yield strength are calculated as 169 GPa and 236.8 MPa respectively, and moreover, all the three parameters of the simplified Johnson-Cook model A, B and n are obtained together.