Analysis of experimental scattering sources in local ductility measurement from flat tensile test sample fracture surfaces

Local formability measurement from fracture surfaces out of flat sheet tensile test samples is increasingly gaining momentum for AHSS automotive sheet steel grades characterization. A newly released VDA 238-110 test standard specifies the experimental methods and addresses the particular testing challenges with focus on reduction of area and thickness reduction at minimum fracture thickness from postmortem microscope measurements on tensile testing sample fracture surfaces. The scattering of testing results depends on the one hand on the measuring challenges when dealing with a broad variety of fracture shapes. On the other hand, systematic geometric effects such as sample width to thickness w/t ratio typically between ISO, ASTM or JIS tensile sample geometries also play a key role. The w/t ratio influences largely the fracture angle in width direction and fracture morphology in thickness direction and subsequently the overall local ductility level. This issue is discussed with two common cold rolled AHSS steel grades DP800 and CP1000 with large w/t ratio variations. Local formability material testing can be interpreted satisfyingly by sorting out stochastic test result variations within a stable fracture angle region separately from the systematic low vs. high fracture angle regions offset.