An incrementally continuous deformation theory of plasticity with unloading

For predictions of plastic buckling, and especially postbuckling behaviour and imperfection sensitivity it is desirable to have a plasticity theory th at combines some of the desired characteristics of both the flow and deform ation theories of plasticity. For this purpose a way to include unloading w ithin a deformation theory of plasticity is given that preserves the increm ental continuity of the resulting constitutive equations. This can only be achieved by allowing plastic deformations to occur within the yield surface . Such plastic deformations are controlled by a parameter m, which describe s how rapidly the possibility of such plastic deformations disappears as th e stress state moves away from the yield surface. A finite strain version o f the formulation is given. The approach can be implemented with minimal ch anges to an elastic predictor - radial return algorithm for the flow theory of plasticity, by changing the elastic predictor phase only. For tests inv olving thick-walled (D/t approximate to 10) cylinders with known axisymmetr ic imperfections under axial compression, this new deformation theory overp redicted the concertina wrinkling type deformations for a given amount of a pplied axial shortening, whereas the flow theory underpredicted these wrink ling deformations in some cases. (C) 2000 Elsevier Science Ltd. All rights reserved.