Towards a general damage law for interior micro-defect induced fatigue cracking in martensitic steels

Abstract

Axially loaded cyclic tests of a precipitation-hardened martensitic steel at different stress ratios in 3% NaCl solution and steam environment were conducted up to very high cycle fatigue regime. In-depth fracture surface observation, quantitative characterization of microstructural damage, and theoretical modeling were carried out to illustrate the physics and mechanics of micro-defect induced interior cracking. Results showed apparent environmental effect on fatigue strength and crack initiation morphology. The fine granular area was observed for the first time in environmental media, heterogeneously distributed around micro-defect, and was found dependent on local fracture mode with a less probability in the case of faceted failure. The formation of fine granular area was confirmed as a result of microstructural damage with significant contribution from cyclic stress amplitude, and could be assisted by hydrogen speeding up lath martensites breakdown. A chemo-mechanical model of interior cracking was finally established based on the concept of interaction of inclusion, matrix, and environment induced plasticity. All these underpin a general fatigue damage law in micro-defects assessment for long-life structural integrity.