Fatigue Limit Improvement by Needle-Peening for Stainless Steel Welded Joint Containing a Crack-Like Defect

Abstract

The effects of needle-peening on the bending fatigue limit of an austenitic stainless steel JIS-SUS316 welded joint containing an artificial semi-circular slit on the weld toe were investigated. Peening was applied to specimens with a semi-circular slit at depths of a = 1.0 mm and 1.5 mm. Then, plane bending fatigue tests were carried out at a stress ratio of R = 0. The fatigue limits of welded specimens containing a semi-circular slit were increased for the peened specimens. Peened specimens with slit sizes of a = 1.0 mm had high fatigue limits that were nearly equal to those of non-slit, peened specimens. We observed that a semicircular slit with a depth of less than a = 1.0 mm could be rendered harmless by peening. Additionally, the values of fatigue limit and the maximum depth of a semi-circular slit that can be rendered harmless by peening were predicted based on fracture mechanics, where we assume that a semi-circular slit is equivalent to a semi-circular crack. The prediction results were consistent with experimental results. NOMENCLATURE a: depth of crack alim: upper limit of depth of non-propagating crack amax: maximum depth of crack that does not decrease fatigue limit FEM: finite element method Kmax: maximum stress intensity factor Kmin: minimum stress intensity factor Kr: stress intensity factor due to residual stress ΔK: stress intensity factor range ΔKT: effective stress intensity factor range for fatigue crack growth ΔKT,A: stress intensity factor at the deepest point of the crack ΔKT,C: stress intensity factor at the surface of the crack ΔKth: threshold stress intensity factor range ΔK(L)th: threshold stress intensity factor range ΔKth for a large crack PPP: portable pneumatic needle-peening TIG: tungsten inert gas UNP: ultrasonic needle-peening x: longitudinal direction R: stress ratio α: shape parameter σnom: nominal stress σx: normal stress for longitudinal direction σr: residual stress σa: stress amplitude σmax: maximum stress Δσw0: stress range at the fatigue limit for non-slit, non-peened specimens