EBSD analysis of cyclic load effect on final misorientation distribution of post-mortem low alloy steel: A new method for fatigue crack tip driving force prediction

The effect of applied cyclic load, including the maximum plastic strain epsilon(p)(max) and the plastic strain amplitude epsilon(p)(a) on final misorientation level of post-mortem low alloy steel after low cycle fatigue (LCF) test, was analyzed by Electron Back-Scattered Diffraction (EBSD) in this research at first. Two misorientation parameters widely used in EBSD analysis, Kernel Average Misorientation (KAM) and Grain Reference Orientation Deviation (GROD), are compared in terms of accumulated plasticity characterization after LCF failure, where a bilinear function is found between final KAM and applied cyclic load epsilon(p)(max) & epsilon(p)(a). Then the cyclic load effect on final misorientation distribution of post-mortem low alloy steel after fatigue crack propagation (FCP) test was further analyzed by EBSD based on the above bilinear function between final KAM and epsilon(p)(max) & epsilon(p)(a), as well as the equivalent (epsilon) over bar (p)(max) & Delta(epsilon) over bar (p) distribution functions given by HRR field. - 1/2 power law is established to describe final KAM distribution near crack path. The area S-KAM between final KAM distribution curve and undeformed KAM(0) base line within a certain range, rather than S-GROD, is proposed as a better measurement of fatigue crack tip driving force Delta K in constant load ratio R condition. Meanwhile, the influence of load ratio R on final misorientation distribution is also discussed.