On the role of martensitic transformation on damage and cracking resistance in trip-assisted multiphase steels

The damage resistance, fracture toughness and austenite transformation rate in transformation-induced plasticity (TRIP)-assisted multiphase steel shee ts were comparatively characterised on two steel grades differing by the vo lume fractions of the phases (i.e. ferrite, bainite, retained austenite) an d by the mechanical stability of retained austenite. The influence of stres s triaxiality on austenite transformation kinetics and the coupling between martensitic transformation and damage were investigated using double edge notched (or cracked) plate specimens tested in tension. The map of the dist ribution of transformation rates measured locally around the notch (or the crack) was compared with the map of the effective plastic strains and stres s triaxialities computed by finite element simulations of the tests. The me chanically-activated martensitic transformation was found to progress conti nuously with plastic straining and to be strongly influenced by stress tria xiality. Fracture resistance was characterised by means of J(R) curves and crack tip opening displacement (CTOD) measurements using DENT specimens. Th e fracture toughness at cracking initiation was found to be lower for the s teel with higher tensile strength and ductility. The contrasted influence o f the TRIP effect, which improves formability by delaying plastic localisat ion but reduces fracture toughness at cracking initiation, is shown to resu lt from parameters such as the volume fraction of non-intercritical ferrite phases or the mechanical properties of martensite. (C) 2001 Acta Materiali a Inc. Published by Elsevier Science Ltd. All rights reserved.