13th International Conference on Fracture June 16–21, 2013,Beijing, China -2- have been identified, as oxide induced crack closure or roughness induce crack closure. The vast majority of mechanisms to explain crack closure are explained, for instance, in the book “Fatigue of Material” written by S.Suresh [3]. Traditionally, in some expression obtained from experimental investigations have been indicated that the crack opening stress intensity factor Kop is only function of the stress ratio R [13,15]. However, some authors recognized that Kop is also influenced by the specimen geometry, the stress state, the stress intensity factor range and the environment. Some models have been theoretically developed in order to explain how the crack opening stress is influenced by the stress ratio R, the maximum applied stress or load level Pmax, and the crack front constraint (plane stress or plane strain behavior). In this work the fatigue crack growth behavior of an annealed metastable austenitic stainless steel was investigated in thin specimen under positive stress ratio. The influence of load ratio on propagation threshold and propagation behavior was analyzed using the Elber`s closure approach, the Donald and Paris partial crack closure approach and the empirical Kujawski (∆K•Kmax)α parameter. Results are analyzed in order to look for answers about the influence of the martensitic transformation on the fatigue behavior of this kind of steel and for a unique relationship between fatigue crack propagation rate and the applied driving force. 2. Experiments The material utilized in the current study was an annealed austenitic stainless steel AISI 301LN provided by OCAS NV, Arcelor-Mittal R&D Industry Gent (Belgium) The chemical composition of this material is shown in Table 1. The microstructure of the material is shown in figure 1. The α‘martensite content was calculated by mean of X-ray diffraction and results showed that the martensite content was less than 2%. Table 1.Chemical Composition AISI 301LN FE Cr Ni Mo C Si P S Mn Cu Annealed bal 17.94 6.30 0.18 0.016 0.513 0.032 0.005 1.481 0.135 Figure 1.Microstructure of annealed AISI 301LN stainless steel Table 2.Mechanical and thermo-mechanical properties σys σUTS Elongation (Pct) Ms ( oC) Md30 ( oC) Md ( oC) 343 MPa 973 MPa 39,39 -66.015 49.042 100* *For AISI 301 stainless steel. Single edge notch test specimen (SENT) of 1 mm thickness and a width of 35 mm and 40 mm, were
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