Mixed-Mode small fatigue crack growth rates and modeling in additively manufactured metals

The fatigue life of metals can be generally divided into three stages: crack nucleation, small crack growth up to about 2 mm, and large crack growth up to fracture. In additively manufactured metals, where defects up to 100 mu m can be present from the time of production, crack nucleation life may be negligible. Therefore, the characterization of small crack growth behavior in these metals is important for making accurate life predictions. This study investigates mixed-mode small crack growth behavior in Laser-Powder Bed Fusion Ti-6Al-4V and 17-4 PH stainless steel. Experiments were conducted on specimens with three different geometries containing artificial defects, which were subjected to various loadings including uniaxial, pure torsion, torsion with static compression, and in-phase axial-torsion. Nominal stress states were corrected for roughness-induced closure, and crack growth rates for all loading conditions were correlated using a critical plane-based stress-intensity factor. The Hartman-Schijve equation was then used to predict mixed-mode crack growth rates by converting mode I material constants from the literature.