In this paper R-ratio effects on fatigue crack growth near threshold region of a metastable austenitic stainless steel (MASS) in two different conditions, i.e. annealed and cold rolled, is investigated. The authors present two approaches to correlate FCGR data for R = 0.1, 0.3, 0.5, 0.7 and K-max = 23 MPa root m using a two-parameters approach (Delta K, K-max and alpha in Kujawski's model) and crack closure model (using Elber's K-op, and in Donald's ACRn2 approaches). The K-op and ACRn2 were experimentally measured on a single edge tension specimens. The K-op measurements were performed using a modified method and based on ASTM standards. While the two driving force approaches correlate data well in the Paris region, they fail to correlate them in the threshold region. However, this correlation can be improved in the threshold region when a different alpha value from the Paris region is used. The authors indicated that two different mechanisms operate; one in the Paris region and another in the near threshold. Hence, they proposed to combine the two-parameter and crack closure approaches where Delta K is replaced by Delta K-eff (estimated by a new method proposed in this paper), which is shown to correlate the FCGR data for different stress ratios for annealed steel. The correlation for cold rolled condition shows improvement with the new approach but is not as good as for the annealed one. The author further suggests to modify K-max in the two-parameter approach. (C) 2015 Elsevier Ltd. All rights reserved.

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