Stress analysis and configurational forces for cracks in TRIP-steels

摘要

TRIP-steels are known to possess attractive mechanical properties attributed to the austenite-martensite phase transformation, which provides additional deformability and hardening. In the present work the influence of strain-induced phase transformation on fracture is studied numerically for a casted TRIP-steel utilizing a recently developed material model. Large strain finite element analyses are carried out for a two-dimensional crack under small-scale yielding conditions to determine mechanical fields and fracture characterizing parameters. The results show that the hardening effect of martensite formation causes increased stresses and stress triaxiality ahead of the crack tip, which has implications for failure behavior. In order to generalize the classical J-integral for transformation plasticity, the concept of material forces is applied and numerically implemented. An appropriate path-independent formulation of the J-integral is suggested for TRIP-steels. A considerable amount of material forces is due to plastic deformation and phase transformation. The resultant material force at the crack tip is considered as the relevant energetic driving force for fracture. Furthermore, crack growth resistance curves J - Delta alpha are simulated by means of a cohesive zone model, which allows to simulate the intrinsic fracture toughness. From the analyses, the beneficial impact of strain-induced phase transformation on the fracture resistance R-curves can be concluded. The transformation zone affects an energetic shielding of the very fracture process zone.