An optical method to determine the strain field on micro samples during electrohydraulic forming

Abstract

Abstract The surface integrity of a component usually has a significant influence on its functional properties. This applies in particular to metallic micro components with sub-millimeter dimensions where little to no core material is present. Since the surface regions of these components can also exhibit changed material properties compared to macroscopic components due to size effects, the material properties cannot be adopted directly from conventional material tests such as tensile tests on macro samples of the same material. The electrohydraulic forming process opens up the possibility of directly investigating the elongation behavior of metallic micro samples in the forming process. However, an in situ strain measurement on the deformed micro sample is required in the mold, for which no measurement method is known so far. For this reason, an in situ measurement approach is presented for detecting the 2D deformation field of the micro sample surface during the electrohydraulic forming process. The approach is based on the optical methods digital speckle photography and digital image correlation. They record the movement of locally characteristic surface patterns in the 0.7 mm wide forming channel through an optical access. By calculating the gradients of the local surface motion, the strain fields on the micro samples are determined. The optical access is achieved by a 10 mm thick sapphire pane, which was tight integrated into the forming die. The 2D strain field measurement of hard sample materials (e.g. bronze) can be performed with digital speckle photography that offers a higher image contrast for optically smooth surfaces, while softer materials such as aluminum tend to get smeared at the pane, so that the captured images are evaluated with the more robust digital image correlation. An assessment of the measurement method on the basis of initial measurement results shows its principle applicability for the material characterization of micro samples.