Development of a novel testing methodology for in-situ microstructural characterisation during continuous strain path change

Abstract

Strain path transition is a common phenomenon during continuous stamping operations of sheet metal and can potentially alter the forming limit of the material. Enabling strain path change in a single experiment is a key challenge faced by several researchers. To understand the effect of continuous strain path change on material forming limits, detailed material characterisation is needed where the material is deformed while the strain path of the material is changed continuously. In this work, a test method was developed, which consisted of a novel mechanical rig and specimen design. The mechanism allowed DP600 steel and AA5182-O aluminium samples to change strain paths continuously without unloading the specimen. Digital image correlation measurements of the strain evolution of the samples during tests showed that the technique was able to alter the strain path of the sample from uniaxial to biaxial strain path. In particular, the measurements showed that the transition from uniaxial to biaxial strain paths was sharper in DP600 than AA5182-O. The test was repeated in a scanning electron microscope (SEM) chamber to observe the behaviour of the microstructure during the strain path transition. The microstructural strain evolution showed rotation of strain bands while the evolution of electron back scattered diffraction (EBSD) maps conveyed grain rotation during continuous strain path change in both the materials. The strain path transition controlled the rotation of grains with preferred crystallographic orientations.