Constitutive modeling of metastable austenitic
stainless steel
E.S. Perdahcıo ˘glu, H.J.M. Geijselaers, J. Hu ´etink University of Twente, Faculty of Engineering Technology, Section of Applied Mechanics , P.O. Box 217, NL 7500 AE, Enschede phone: +31-(0)53-4894175,
e-mail: e.s.perdahcioglu@utwente.nl
Introduction
Metastable steels such as TRIP steels are beginning to make their way into commercial products. These materials are very attractive since they combine exceptional mechanical properties such as, high formability and high strength, thanks to the strain-induced martensitic transformation phenomenon. However, it is still quite a
challenge to simulate forming operations on these materials because of their complex mechanical behavior.
Objective
Develop a physically based constitutive model for understanding and simulating the behavior of TRIP steels under complex forming operations.
Methods
Mechanical tests To understand the underlying mechanisms and reasons for transformation, mechanical tests, Biaxial and Prestrain, have been performed. 0 0.05 0.1 0.15 0.2 0.25 0 0.2 0.4 0.6 0.8 1 equivalent strain martensite fraction 0 15 30 45 60 75 90
Figure 1 : Biaxial and prestrain test results, respectively.
Crystallography Crystallographic models reveal that transformation is favored among the grains under mechanical loading according to the stress resultant on the martensitic variants. Figure 2 shows the distribution of maximum driving force in a polycrystal with 1000 grains.
Figure 2 : Distribution of driving force in a non-textured polycrystal under two different stress states.
Homogenization The material during transformation is a metal-matrix-composite with austenite and martensite phases. Figure 3 shows the calculated mechanical response of the composite based on an input martensite fraction vs equivalent strain curve.
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0 100 200 300 400 500 600 equivalent strain
shear stress (MPa) martensite − input
τxy − model τxy − exp martensite − exp
Figure 3 : Results of the homogenization model under simple shear deformation.
Discussion
The experiments show the effects of plastic strain and driving force on the rate of transformation. The homogenization model can predict the stress and strain distribution in the phases hence allows physically
based constitutive models to be implemented.
