A limit surface formulation for plastically compressible polymers

The tightening of industrial safety standards for structures generates a need for refined computational methods, which, among other things, must be able to describe the yield surface and the deformation behaviour of non-reinforced thermoplastics. To describe the plastic behaviour of materials, a potential formulation is suggested. This formulation contains a number of known potentials as special cases. The parameters of the model, which are obtained from test data, are restricted by the convexity condition for the potential. The new model allows one to take into account effects of the second order, for instance, the unequal behaviour under tension and compression, the plastic compressibility, and the Poynting-Swift effect. For each particular choice of the parameters, the Poisson ratio in tension is computed. If the restrictions imposed on the Poisson ratio do not hold, the application of a non-associated flow rule is necessary. A simple non-associated flow rule with different values of Poisson ratio intension and compression is proposed.