Fatigue Fracture of AM Inconel 718: Surface Effects on Tension-Torsion Capability

摘要

Aerospace structural systems are employing additively manufactured (AM) parts in their designs with the objective of optimizing weight, reducing production costs, and decreasing lead times. While the surface finish of AM parts affects the fatigue life of AM parts, it is not clear if this effect is important for parts with pre-existing flaws. Some launch vehicle and space vehicle parts specifically need to meet damage tolerance requirements. A test methodology is presented to determine whether measures should be taken to characterize the fatigue performance of all as-built finishes when damage tolerance requirements are met. The methodology is then illustrated for cylinders under tension and torsion fatigue loads. Test results of AM Inconel 718 cylinders suggest that the fatigue life of coupons with defects envelopes the fatigue life of coupons with as-built surface finish. For the examples presented, fully characterizing the fatigue life with the as-built surface finishes is not required since assuming an initial flaw causes a stress concentration far greater than the stress concentration from the surface roughness.