A Method for Measuring Plastic Strain Ratio Over a Wide Strain Range Using the Digital Image Correlation Technology

Plastic strain ratio, also defined as Lankford coefficient or r-value, is an intrinsic material property in estimating the mechanical anisotropy and drawability of metal sheets. It has the meaning of resistance of materials to thin or thicken when plates are subjected to tension or compression, respectively. The plastic strain ratio is generally evaluated in the gauge region of the specimen at an axial strain range of 10%–20% in the uniform elongation regime under the precondition that there is no volume change during plastic deformation. Most materials can achieve precise plastic strain ratio measurement according to the international standard, but there are limitations for metallic materials with short uniform elongation regimes such as advanced high-strength steels and as-rolled sheets. However, plastic strain ratio is needed in material and shape design in a large deformation regime, even after necking. In this study, we propose a new method to obtain reliable plastic strain ratios even in the post-necking elongation regime using the digital image correlation technique. Graphical Abstract