Gauge length and frame rate dependence of the onset of instability and the fracture limit of DP 980 sheets

Despite the importance of the triaxiality failure diagram (TFD) for damage prediction, the quantitative influences of the gauge length Lo and the frame rate f of the DIC (digital image correlation) system's camera on the necking strain εu and the fracture strain εf from tensile testing have hardly been dealt with. In this study, DP 980 sheets were subjected to uniaxial tensile, shear, and near plane strain conditions, and deformations were recorded by DIC. The investigation of the influence of varying Lo and f on the ability to capture εu and εf accurately reveals the following: the influences on εu is negligible; to avoid overly conservative values for εf, it is essential to limit Lo to the smallest value possible, i.e. the necking band width; the influence of f becomes relevant for low Lo; at a test velocity of 5 mm min−1, f should not be lower than 10 frames per second (fps) (i.e. 120 frames mm−1) when using the minimum Lo. Nonetheless, even with the minimum Lo, εf values are below those obtained on a local level by DIC. The TFDs for different f make clear that the influence of f holds also for other loading modes and is most pronounced for shear. Therefore, application in conjunction with a high-speed camera (≥10 fps) is necessary for the measurement of realistic parameters, which is critical to sheet forming analysis.