13th International Conference on Fracture June 16–21, 2013,Beijing, China -1- Fatigue crack growth in a metastable austenitic stainless steel D.F. Martelo and M.D. Chapetti* Laboratory of Experimental Mechanics (LABMEX) Research Institute for Material Science (INTEMA) CONICET - Faculty of Engineering, National University of Mar del Plata J.B. Justo 4302, 7600 Mar del Plata, Argentina * Corresponding author: mchapetti@fi.mdp.edu.ar Abstract Fatigue crack growth in a metastable austenitic stainless steel was investigated in thin specimen under positive stress ratio. Annealed conditions were used to test the influence of the microstructure. 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 and the empirical Kujawski (∆K•Kmax)α parameter. Results show that load ratio effects are not completely explained by these approaches. It was found that the threshold of the material in the annealed condition depends on the load history, especially when the load ratio is low. It seems that the amount of martensite transformation is responsible for the observed differences in fatigue crack growth resistance. Keywords: Fatigue, crack propagation, thresholdMetastable austenitic stainless steel 1. Introduction In the last years, the competitiveness in the automotive sector and the compliance with higher environmental standards has forced the development of new light-weight materials. One of the materials that fulfill with the exigencies of automotive sector are the metastable stainless steels because of they combine good corrosion resistance with versatile mechanical properties [1,2]. Due to his high ultimate strength, metastable stainless steel allows the use of components of less thickness. One of the more important aspects in the automotive sector is the design again fatigue damage.Traditionally, the approach to fatigue design based on the cyclic stress range ∆Shas been used [3]. However, thin walled light components must be designed using more conservative approach, basedmainly in linear elastic fracture mechanics. AISI 301LN steels belong to the metastable austenitic steels and have an austenitic structure in annealed conditions which confers them an excellent ductility. Besides, they have an extraordinary strain hardening because of the transformation of austenite to martensite during deformation. This particular class of steels are called TRIP steels [4]. Significant researches have been conducted on the fatigue behavior of TRIP steels [4-12], and depending of the testing condition, different behaviors have been reported. In the high cycle (HCF) regime, i.e. test under load control or test under K control, the conclusion of the studies show that exists a relationship between the martensitic transformation around the crack tip and the decrease in the fatigue crack growth rate (FCGR) [4-8,10,12], with the exception of reference [9], where it attributed the decrease in the FCGR to the slip characteristic rather than the martensitic transformation. Until now various mechanisms have been proposed trying to explain how the martensitic transformation can affect the FCGR. However, there is no mechanism that can explain satisfactorily the effect of the martensitic transformation over the FCGR having accounted the entire picture of the stress state. The most recurrent mechanism to explain the crack growth retardations in fatigue is the crack closure, which was introduced by Elber in 1971 [13], after observing that fatigue crack surfaces contact under cyclic tensile loading. Even direct measurements have not been achieved, is felt that the martensitic transformation cause crack closure because of the involved volume expansion of 1-4 % [14]. The first mechanism used to explain the premature contact between the faces of the crack is the plasticity induced crack closure. However, others mechanisms induced crack closure
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