Nominal vs Local Shot-Peening Effects on Fatigue Lifetime in Ti-6Al-2Sn-4Zr-6Mo at Elevated Temperature
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TRODUCTION
COMPRESSIVE-RESIDUAL-STRESS-PRODUCING treatments, including shot peening (SP), have been shown to enhance the resistance of a surface to fatigue crack initiation and propagation.[1–5] Often, the benefit to lifetime is measured in terms of the meanfatigue behavior, which exhibits significant improvement upon SP due to, depending on the loading conditions, a shift in the failure initiation from the surface to the subsurface regions[6–8] or delayed crack initiation or retardation of crack growth from the surface.[9–12] The driving force for crack initiation is thought to be reduced due to the uniform distribution of immobile dislocations in the peened surface layer, which restricts the motion of dislocations and the development of intense slip bands during fatigue loading.[13,14] Along similar lines, others have attributed the delay in crack initiation to a decreased level of plastic strain accumulation in the work-hardened surface material.[4,14] The retardation of crack propagation or crack arrest is related to the significant reduction in the effective stress intensity factor range under the compressive residual stress (RS) profile[9,10,15,16] as well as the decreased crack-tip plasticity due to work hardening.[17] It is also well documented that the degree of influence on the lifetime is strongly dependent on the stress level S.K. JHA, Research Scientist, is with Universal Technology Corporation, Dayton, OH 45432. Contact e-mail: sushant.jha@ wpafb.af.mil R. JOHN, Principal Materials Research Engineer, and J.M. LARSEN, Senior Scientist, are with the United States Air Force Research Laboratory, AFRL/RXLMN, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433. Manuscript submitted September 15, 2008. Article published online August 25, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
and temperature regime,[7,8,11,18,19] as well as the stress gradient.[9] In particular, the subject of RS relaxation has been given widespread attention[7,9,20–22] in this regard. At cyclic stress levels above the material yield strength, under the global-yield condition, or at high temperatures, the benefit of SP could decrease due to relaxation of the RS profile.[7,14,23,24] At lower stress levels, i.e., the long-lifetime regime, the SP effect may be reduced if failure would have predominantly occurred by subsurface crack initiation in an otherwise RS-free condition[18] and due to the dominant contribution of crack initiation to the total lifetime.[4] The latter is typically a more influential factor in notch fatigue[2,23] than smooth fatigue. The SP-induced surface roughness, which has also been given important consideration in several studies,[7,8,26] can have a competing influence on the beneficial effects of the compressive stress and the cold work level, especially under thermal-mechanical conditions where the RS profile may significantly relax. It is clear that the response of an SP-induced (or any other treatment) RS profile to the variables encountered in service and the resulting effect on lifetime must
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