Plane strain compression of aluminium alloy sheets

The theory of plane strain compression is applied to rolled aluminium alloy sheet. Two contrasting grades of the alloy are tested: naturally aged AC 120 and half-hard HE 30. While AC 120 displays a smooth stress–strain curve under homogenous straining, HE30 shows a serrated stress–strain curve due to its banded plastic strain behaviour. It is shown that, provided the r-values can be established reliably to characterise each sheet’s orthotropy, a flow curve to large strain (≃2) is provided by the plane strain test. Certain modifications to the original test procedure are made to achieve this. Equivalence in flow curves, as required of orthotropic plasticity theory, is examined from plane strain, bulge forming and tension tests conducted at various orientations to the roll. Despite the contrasting limiting strains between the three tests (tension ≃ 0.1, bulge forming ≃ 0.8) an acceptable correlation has been found between their equivalent flow curves across three decades of strain. The dependence of equivalent plastic strain upon equivalent stress for each material conforms to the Hollomon law. The Ramberg–Osgood law allows for the addition of elastic strain.

► Theory of orthotropic plasticity applied to rolled aluminium alloy sheets. ► Contrasting material behaviour with smooth and serrated flow curves. ► Usefully large strains obtained from plane strain compression and bulge forming. ► Holomon and Ramberg–Osgood laws apply to an equivalent strain range 0.1–1.