A numerical model for Transformation Induced Plasticity (TRIP)

Abstract
The peculiar behavior of steels exhibiting a TRIP (Transformation Induced Plasticity) effect, especially for the case of non-proportional loading paths, has given rise to a wide variety of different material descriptions ranging from purely phenomenological models to more rigorous approaches based on thermodynamic principles as proposed by this group of authors [1]. While this latter model gives reasonable results for the multigrain compound, its validity has limitations at the integration poit level since it selects only one variant within each time increment. Alternatively, this paper suggests an implicit formulation capable of reflecting the simultaneous evolution of multiple variants during transformation and suitable for an implementation into a finite element program. Particular attention has to be paid to the mechanisms inhibiting transformation, collectively characterized as hardening terms. To this end the well known yield surface concept of classical plasticity is extended by the notion of an additional phase transformation surface in stress space. The algorithm allows to identify the active variants and to monitor their contribution to the transformation kinetics. The overall strain component in loading direction is worked out and discussed in context with the orientation effect as the dominating mechanism for the accommodation of the new phase.