Abstract
Powdered polystyrene was compacted into cylindrical specimens having various void fractions. These specimens were triaxial-compressed and triaxial-extended under various confining pressures σ3. Yield loci were obtained from Mohr's stress circles using the confining pressure σ3 and the axial stress σ1 at which the specimens broke down. These results were expressed by the equation τ2=μi2σ2+4σt(1+μi2)(σ+σt), where τ was the shear stress, σ was the normal stress, μi was the coefficient of effective internal friction and σt was the tensile strength of the powder compact. This equation was preferable to the conventional power law of the yield locus because of the conformity to the test results, and furthermore the constants contained are the most basic ones for powder-like materials.
The yield locus obtained by the triaxial extension test differed from that by the triaxial compression test, but both approached each other with an increase in the initial void fraction of specimens. his fact was attributed to the difference in two kinds of tensile strengths, that is, one was the value in the perpendicular plane to the precompaction plane and the other was the value in the parallel plane. The former determines the yield locus obtained by the triaxial compression test and the latter determines the one by the triaxial extension test.
