Experimental Investigation And Constitutive Modeling Of The Strain-induced Martensitic Phase Transformation In A Metastable Austenitic Stainless Steel

In this paper, the martensitic phase transformations in AISI348 and its influence to the mechanical behavior of the material under monotonic and cyclic loading conditions are investigated in detail. The influence of stress triaxiality to martensitic phase transformation is verified by experiment results. It is also confirmed that for strain rate less than 10-2s-1, temperature is account for the influence of strain rate to martensitic phase transformation. On the basis of Santacreu model, a stress triaxiality dependent transformation kinetics is obtained. A new constitutive model based on this kinetics is developed, in which the transformation hardening is included in the isotropic hardening term. It is shown that for small strain, this constitutive model could describe the mechanical behavior of the material with accuracy.Based on the constitutive model developed above, its implementation in computational mechanics is derived briefly. A user defined material subroutine(UMAT) also coded with FORTRAN, which is implemented into ABAQUS finite element(FE) computation successfully. The computation results of smooth round bar and notched round bar tension agree well with experiment results. The computation model also predicted the evolution of martensitic phase in the material properly.By defining a new variable, a transformation kinetics with the ability to describe martensitic transformation under both monotonic and cyclic loading conditions is developed. This kinetics is built on the basis of the kinetics developed above, and will degrade to monotonic transformation kinetics if no reverse loading exists. The prediction of this kinetics fits well with experiment data in this paper. Due to transformation hardening, AISI348 exhibited substantial cyclic hardening. We developed a cyclic constitutive model by introducing a backstress and by including transformation hardening into the isotropic hardening term. The cyclic model developed captures the cyclic stress-strain response of the material in general. However, as the backstress is assumed to be linear, difference could be seen between model and experiment results.