Steels are heavily used in infrastructure and the transportation industry, and enhancing their fatigue resistance is a major challenge in materials engineering. In this study, by introducing a gradient microstructure into 304 austenitic steel, which is one of the most widely used types of stainless steel, we show that a strength gradient substantially enhances the fatigue life of the material. Pre-notched samples with negative strength gradients in front of the notch's tip endure many more fatigue cycles than do samples with positive strength gradients during the crack initiation stage, and samples with either type of gradient perform better than do gradient-free samples with the same average yield strength. However, as a crack grows, samples with positive strength gradients exhibit better resistance to fatigue crack propagation than do samples with negative gradients or no gradient. This study demonstrates a simple and promising strategy for using gradient structures to enhance the fatigue resistance of materials and complements related studies of strength and ductility.

http://dx.doi.org/10.1038/srep22156

Y.W. acknowledges support from the National Natural Science Foundation of China (NSFC) (Grant no. 11425211), and MOST 973 of China (Grant no. 2012CB937500), the Chinese Academy of Sciences, and. G. W. is grateful for support from the Shanghai Pujiang Program (Grant no. 11PJ1403900) and the Eastern Scholar Program at Shanghai Institutions of Higher Learning. H.W. and J.L. acknowledge support from the NSFC (Grant nos 11002124, 11202183, and 11572281). H.G. acknowledges financial support from the National Science Foundation through grant CMMI-1161749 and from the Center for Mechanics and Materials at Tsinghua University.

Ma ZW,Liu JB,Wang G,et al. Strength gradient enhances fatigue resistance of steels[J]. SCIENTIFIC REPORTS,2016,6:22156.