Formability analysis of sheet metals by cruciform testing

Cruciform biaxial tests are increasingly becoming popular for testing the formability of sheet metals as they achieve frictionless, in-plane, multi-axial stress states with a single sample geometry. However, premature fracture of the samples during testing prevents large strain deformation necessary for the formability analysis. In this work, we introduce a miniature cruciform sample design (few mm test region) and a test setup to achieve centre fracture and large uniform strains. With its excellent surface finish and optimized geometry, the sample deforms with diagonal strain bands intersecting at the test region. These bands prevent local necking and concentrate the strains at the sample centre. Imaging and strain analysis during testing confirm the uniform strain distributions and the centre fracture are possible for various strain paths ranging from plane-strain to equibiaxial tension. Moreover, the sample deforms without deviating from the predetermined strain ratio at all test conditions, allowing formability analysis under large strains. We demonstrate these features of the cruciform test for three sample materials: Aluminium 6061-T6 alloy, DC-04 steel and Magnesium AZ31 alloy, and investigate their formability at both the millimetre scale and the microstructure scale.