Effect of Hierarchical Microstructures of Lath Martensite on the Transitional Behavior of Fatigue Crack Growth Rate
In this study, the fatigue-crack growth (FCG) behavior of 20CrMTiH steel with different substructure sizes was investigated. The results showed that coarsen microstructures exhibit excellent growth resistance. Moreover, two transitional behaviors were observed in the FCG curves of all specimens. The first transition point occurs in the near-threshold regime, whereas the second transition point occurs in the Paris regime. A comparison of substructure size to cyclic plastic size showed that the block size is almost equal to cyclic plastic size at ∆KT1, indicating that block size is an effective grain size to control the first transitional behavior of fatigue-crack propagation, whereas the second transitional behavior is related to the packet width or grain size. According to the fracture morphology, the fracture mechanism above and below the transition point responsible for the above phenomenon were distinguished. In addition, two prediction models based on microstructure size were established for lath martensite to evaluate the threshold and stress intensity factor range at the transition point.
KeywordsFatigue Crack growth Lath martensite Microstructure Transitional behavior
The research documented in this work was financially supported by the Joint Foundation of Guizhou province, China (Grant No.  7244 and 5788), the National Natural Science Foundation of China (Grant No. 51461006), and the Natural Science Foundation of Guizhou province, China (Grant No.  2003).
- 6.J. Zheng, B.E. Powell, A method to determine the transition point of fatigue crack growth rate from near-ΔKth to paris regions. J. Test. Eval. 27(4), 2 (1999)Google Scholar
- 16.M. Yingjie, J. Liu, J. Lei et al., The turning point in Paris region of fatigue crack growth rate in titanium alloy. Acta Metall. Sin. 44(8), 973–978 (2008)Google Scholar
- 21.G.R. Yoder, L.A. Cooley, T.W. Crooker, in A critical analysis of grain size and yield strength dependence on near-threshold fatigue crack growth in steels. Fracture mechanics: Fourteenth symposium—Volume I: Theory and Analysis (ASTM International, 1983)Google Scholar
- 25.C. Du, J.P.M. Hoefnagels, R. Vaes et al., Block and sub-block boundary strengthening in lath martensite. ScriptaMaterialia 116, 117–121 (2016)Google Scholar