13th International Conference on Fracture June 16–21, 2013,Beijing, China -1- Deformations and Damage Evolution of Austenitic Steel AISI304 with Martensite Phase Transformation Gang Han1, HuangYuan1,2,* 1School of Mechanical Engineering, Beijing Institute of Technology, 100081, China 2Department of Mech. Engineering,University of Wuppertal,42097, Germany * Corresponding author: yuanhuang@bit.edu.cn Abstract In the present paper martensite transformation in stainless steel 304 and its effects to material damage is discussed. The experiments confirm that martensite phase transformation in SS304 can be described by the Santacreu model and shows dependence on the plastic strain and stress triaxiality. The plasticity model with the martensite transformation is established based on Santacreu model and applied to describe plastic behavior of SS304 with severe plastic deformations. It is shown that the plasticity model predicts strain hardening under both compression and tension uniformly and agrees with experimental results reasonably. Although the fracture strain of SS304 can be characterized by the equivalent plastic strain precisely, fatigue tests display strong influence of the pre-strains to the fatigue life. Whereas the strain-fatigue life curve shows acceleration of fatigue damage in strain-controlled fatigue tests, the stress versus fatigue life curve reveals significantly higher bearing capacity due to pre-strains. This result implies that application of the pre-strain should only be used if the mechanical loading is applied in stress-controlled cases. Keywords：deformation induced phase transformation, plasticity mode, fatigue damage, fatigue life 1. Introduction It is known that severe plastic deformations influence material behavior. For instance, surface treatment of critical mechanical parts is an important step in manufacturing. The improvement of fatigue behavior of the mechanical part is realized just due to compressive residual stresses and distortions of the surface material [1-3]. Quantifying effects of surface treatment in fatigue life improvement requires detailed understanding of the mechanical behavior of the material with strong distortion and variation of the residual stresses in the surface layer material. AISI304 is a popular stainless steel and can be found in many industrial branches. On the other side, the stainless steel SS304 is meta stable austenite material, its crystallographic structures can transform to martensite phase under plastic deformations [4-6]. The deformation-induced martensite transformations have been studied for many years, especially in material science communities on kinetics of transformation [7-10]. Due to martensite phase the stainless steel behaviors significantly differently from the austenitic steel, both in plastic deformation and failure [11-15]. Using known models to establish a continuum mechanics model for quantifying effects of surface treatment needs detailed understanding of evolution of microstructure and meso-mechanical behavior of the distorted material [7-10]. Especially, phase transformation under severe compressive deformation is less investigated in the past. The present work dedicates to identify a plasticity model for SS304 under severe plastic deformations, especially under compressive deformations, and to clarify effects of the pre-strains to material failure under monotonic and cyclic loading. Based on extensive experiments, the plasticity model under consideration of the martensite transformation should be applied to predict fatigue life of the compressive deformed specimens and to quantify effects of the compressive strains to material failure evolution.
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