Biaxial tension under bending and compression - development of a new formability test for incremental sheet forming

A new testing method, Biaxial Tension under Bending and Compression (BTBC), is developed to investigate the effect of different deformation modes on material formability in Incremental Sheet Forming (ISF). A cruciform specimen is designed by simulating the material deformation under biaxial tension using Finite Element (FE) method. In the BTBC experimental testing, the cruciform specimen can be stretched in biaxial directions and the strain ratio of the two perpendicular directions can be varied. Furthermore, the superimposed effect of compression, bending and cyclic loading can be investigated. Material formability of aluminium alloy AA5251-H22 under plane strain path is tested. True strains of the specimen under different deformation modes are obtained by measuring distortions of circular grids inscribed onto the surface of the specimen. The experimental results show that the introduction of bending and compression contributes to localised material deformation. Material formability is improved by the introduction of bending, which is further enhanced by applying compression and cyclic loading. The BTBC test overcomes the limitation of commonly used testing methods in ISF formability studies, providing a fundamental explanation of the effect of strain path and loading conditions on the material deformation and fracture behaviour in ISF.