13th International Conference on Fracture June 16–21, 2013,Beijing, China -8- Figure 8.Fatigue crack growth rate as a function of the Kujawski`s parameter with (a) α equal to 0.6 and (b) α equal to 0.5 The same parameter α that was found to correlate the fatigue crack growth rate curves in Fig. 7 has been used to analyze the near threshold region, and results are shown in Fig 8(a). It can be observed that in this case the approach cannot success as in Fig. 7. Fig.8(b) shows results obtained by using an α value equal to 0.5. Even though results seem to be somehow better, the approach does not success. It is interesting to note that the value of α parameter calculated for the AISI 301LN decrease with the decrease of the stress intensity factor range and/or with the decrease of the martensite content. This result agree with those of the work of S.Kalnaus et al [11], were they estimated an α parameter equals to 0.36 for an austenitic steel without transformation. According to our measurements, the martensite content decreases in the near threshold region of crack propagation with respect to measurement made in the relative high K fatigue crack propagation region. It is clear that further and more detailed analyses are needed in order to explain the influence of the load ratio R in this region. 5. Concluding remarks Fatigue crack growth of aannealed 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 and the empirical Kujawski (∆K•Kmax)α parameter. Results show that load ratio effects are not completely explained by these approaches. The crack closure effect approaches (both, the traditional Elber approach and its modification by Donald and Paris) cannot explain the influence of the stress ratio R on the fatigue crack propagation rate on the analyzed thin specimens of a TRIP material. The two parameter crack driving force (∆K•Kmax)α, with α = 0.6, seem to be a proper parameter to uniquely explain the fatigue behavior of the analyzed TRIP steel for positive R ratios far from the threshold region. However, it seems to fail in the threshold region, where the crack closure levels are important, and a unique α value cannot be found. So, further investigation should be carried out in order to explain these results. Acknowledgements Authors wish to express their gratitude to the funding provided by CONICET (National Research Council), and by Agencia Nacional de Promoción Científica y Tecnológica (ANPCyT), Argentina (PICT2010 Nro.0379). References [1] S. Lamb, Handbook of Stainless Steels and Nickels Alloy, CASTI Publishing Inc, Edmonton, 2001.
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