Fatigue limit of Ni-based superalloy 718 relative to the shear-mode crack-growth threshold: A quantitative evaluation considering the influence of crack-opening and -closing stresses

Abstract

Abstract Fatigue strength of Ni-based superalloy 718 was investigated relative to shear-mode crack-growth thresholds, to evaluate the influence of crack-opening/-closing stresses perpendicular to shear-mode crack-planes. The impact of precipitation-hardening was also examined. Using solution-treated and precipitation-hardened materials, three different types of fatigue tests were performed: (i) push-pull; (ii) pure-torsion; (iii) torsion with superposed static tension. All tests revealed non-propagation of small, shear-mode cracks, while confirming the negligible impact of precipitation-hardening on shear-mode crack-growth threshold. Additionally, crack-opening/-closing stresses were determined to only imperceptibly alter shear-mode resistance. Based on these findings, a novel strategy was proposed to quantify the alloy’s fatigue strength.