ANISOTROPIC YIELD FUNCTIONS WITH PLASTIC-STRAIN-INDUCED ANISOTROPY

In most anisotropic yield functions, the stress exponent, M, associate d with the shape of the yield surface is usually independent of plasti c-strain accumulation. This does not allow for different work-hardenin g characteristics under various strain states, as has been observed in aluminum alloys. Assuming coefficients characterizing anisotropy do n ot change with plastic deformation, the M value should vary with plast ic strain, relaxing the isotropic hardening assumption. To verify this , plane-strain tests along with numerical analysis were carried out wi th 2008-T4 aluminium and 70/30 brass. The effective stress and effecti ve plastic-strain curve assuming plane strain and plane stress was fit to the corresponding stress-strain data obtained in uniaxial tension. This was done by allowing M value to vary with effective plastic-stra in. Hill's 1979 (case iv), Hosford's 1979 and Barlat's 1991 (6 compone nt) yield functions were evaluated. Results showed that, with all the yield functions tested, the aluminum exhibited substantial variation o f M value especially at larger strains while the brass showed minor ch ange. Relevant numerical analysis indicated that, even though all the yield functions showed noticeable changes of M as strain increases in order for the plane-strain curve to match with the uniaxial curve, thi s variation of M will not affect much to the prediction with Hosford's and Barlat's yield functions, of which the typically valid M is much higher than that of Hill's. FEM simulation of plane-strain sheet formi ng with 2008-T4 aluminium alloy verified that implementation of varyin g M-value with Hill's yield function led to better agreement with expe rimental measurements, while the variation of M with Barlat's yield fu nction exhibited little influence on the strain prediction.