Fatigue life and mechanistic modeling of interior micro-defect induced cracking in high cycle and very high cycle regimes

• Published in: Acta materialia Vol. 157; pp. 259 - 275
• Main Authors: Zhu, Ming-Liang, Jin, Long, Xuan, Fu-Zhen
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.09.2018
• Subjects: Dislocation structures; Fatigue crack initiation; Focused ion beam (FIB); Microstructure-inclusion interaction; Very high cycle fatigue; Dislocation structures; Very high cycle fatigue; Focused ion beam (FIB); Microstructure-inclusion interaction; Fatigue crack initiation
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2018.07.036

Axially loaded push-pull cyclic tests of a precipitation-hardened stainless steel with different sampling orientations were conducted in high cycle and very high cycle fatigue regimes. Results showed apparent fatigue anisotropy with non-metallic inclusions dominating crack initiation behavior. A fatigue lifing model was developed by combining size, location and shape of inclusions into a new form of Z parameter to rationalize the orientation effect. Using a multi-scale and full-field approach, the inclusion-induced interior cracking mechanisms were found to be associated with inclusion-microstructure interaction resulted plasticity. Micro-hardness at the cracking site was the lowest on the fracture surface, and surrounding microstructures showed formation of small grains with clear interfaces. The fine granular area was characteristic of several nano-scale fine grains formed in terms of dislocation cell structures by martensitic laths breakdown. The coalescence of interfaces or micro-crackings finally became interior early fatigue cracks. The mechanistic modeling of “fragmentation of martensitic laths and formation of dislocation cells” revealed a microstructure-dependent crack initiation and stage I growth for interior fatigue cracking. All these inform the significance of combining metallurgical and processing factors in designing against fatigue of engineering materials. [Display omitted]