Local fracture strain measurement in AHSS uniaxial flat tensile tests considering specimen geometry and fracture morphology

The local formability of AHSS sheet steels can be estimated by means of the relative average reduction of cross section area (or thickness) at fracture in flat tensile specimens. A 2D/3D characterization of tensile fracture shapes is performed with an optical stereo microscope for various typical fracture shapes. The initial width to thickness ratio (w₀/t₀) of flat tensile samples quantifies their non-proportionality behavior. A charpy like lower and upper shelf region with a highly scattered transition zone in between at a critical w₀/t₀ ratio is observed when plotting the fracture area or thickness reduction versus w₀/t₀ ratio. The higher the w₀/t₀ ratio of tensile specimens (samples wider, material thinner) the higher the local fracture strain level. The higher the material work hardening ability (n-value), the higher the critical w₀/t₀ ratio below which high scattering in thickness - and area reduction occurs. The local tensile fracture strain is therefore strongly dependent on specimen thickness, width and work hardening behavior. This is a severe drawback for a broader use of such tensile test based fracture strain measurements.