Evaluation of typical conventional and unconventional plasticity models for prediction of softening behaviour of soils

Conventional elastoplasticity is based on the idealisation that the interior of the yield surface is a purely elastic domain. This idealisation would not lead to unrealistic prediction of hardening behaviour, but would cause unrealistic prediction of softening behaviour, which is observed typically in overconsolidated soils, whereas metals and normally consolidated soils exhibit hardening behaviour. By contrast, unconventional elastoplasticity that does not use this idealisation makes it possible to describe the plastic deformation due to the change of stress inside the yield surface exhibiting a smooth elastic-plastic transition, and is thus expected to describe softening behaviour more accurately. In this paper the capabilities of conventional and unconventional elastoplasticity for describing softening behaviour are examined, comparing the Drucker-Prager model as the conventional one and the subloading surface model as the unconventional one for predicting the deformation behaviour of overconsolidated soils. It is concluded that unconventional elastoplasticity has to be adopted for predicting softening behaviour.