Application Of Digital Image Correlation Method In Study Of Material Deformation

Digital image correlation method is an image measurement method using computer vision technology, it can be used to measure contactlessly the full field shape change, deformation and motion. The objects processed directly by DIC method are digital images or speckle patterns with certain gray level distributions, and the displacement field and strain field of a deformed material are obtains through the correlation analysis of the two digital images acquired before and after the deformation. Comparing with other traditional strain measuring techniques, the advantage of digital image correlation method is remarkable, it can measure full-field strain of samples with different size and different shape. In this paper, the2D-DIC method was used to measure the deformation of DP590and304stainless steel during the uniaxial tensile test both with smooth tensile samples and V-notch tensile samples. Outcomes show the following results:(1) The engineering stress-engineering strain curves obtained using both from2D-DIC and the extensometers results are generally the same, showing that this new method is reliable.The small errors in the measurement caused by asynchronism can be avoided by using tensile force output sigal from test machine as analog signal to trigger picture gathering.(2) The average plastic strain ratios of DP590measured by virtual extensometers is closed to the traditional measurement method, while values obtained by DIC full average method is smaller than traditional measurement method. R values in different positons can be easily obtained by DIC method. Also DIC revealed full-field R values at15%strain levels and at approximately fracture time, the former were uniform while the latter were showing that the closer the distence from the necking centre,the value of R will be smaller.(3) It turned out that the process of plastic instability following load maxima can been divided into two different parts:diffused necking and localized necking. The moment when diffused necking and localized necking emerge varies from tested materials by comparing the stretching process of304stainless steel and DP590steel. The bigger the strain hardening exponent, the true strain at maximum load time and approach to fracture time is larger. In addition, it is demonstrated that plastic flow within the bounds of gauge length is nonuniform during the whole drawing process, the deformation in center is lager than that in the edges.(4) Both materials exhibit the same deformation rule during the V-notch tension test, it is that the whole surface of the sample can be divided into four region which having different deformation characteristcs. The transverse strain along midline right in front of the V-notch is nearly zero, while longitudinal strain keeping the same with edge area. The strain in a local small area above or below gap sideline is almost zero. The transverse strain and axial strain mainly concentrate in the area above or below the midline which forming a quadrilateral region.