High Cycle Fatigue Behavior of Shot-Peened Steels
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FORGED steel connecting rods have been conventionally produced from wrought steel billets. However, forging powder metals is an innovative process that is now used for the production of connecting rods.[1] Powder forging has the advantage of producing near net-shape components and reducing material waste, although the cost of the process is relatively high as a result of the material and more detailed manufacturing process. Developments in the forging industry have allowed the introduction of innovative one-piece crackable automotive connecting rods to be made and subsequently broken (fracture split) at the big end, thereby facilitating assembly. The advantages of using these crackable steels are that the costs to separate the cap end are lower and that the broken surfaces mate perfectly on reassembly; hence, the tolerances on the cap end internal diameter are extremely close. These advantages are achieved by using crackable AISI 1070 steel forgings that display a cleavage fracture surface similar to that of the powder forged components However, they are cheaper to manufacture and possess the beneficial directional properties typical of forgings. Shot-peening is applied to extend the fatigue life of components and structures under cyclic loading.[2] This treatment, for example, is used in the automotive and aerospace industries for connecting rods and turbine blades. The induced compressive residual stresses improve the high cycle fatigue resistance delaying crack propagation, and surface work hardening retards crack nucleation. However, the initial surface properties are changed and the increase in surface roughness with its accompanying local stress concentration tends to accelerate the crack nucleation stage.[3] M.M. MIRZAZADEH, Graduate Student, and A. PLUMTREE, Professor, are with the Department of Mechanical and Mechatronics Engineering, University of Waterloo, 200 University Ave., Waterloo, Canada N2L 3G1. Contact e-mail: [email protected] Manuscript submitted March 30, 2011. Article published online July 27, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
Induced compressive residual stresses and work hardening are two essential factors that improve the fatigue behavior of shot-peened parts. Several researchers[2,4–7] proposed that the influence of work hardening was greater than the induced compressive residual stresses, especially for smooth steel shot-peened specimens, whereas others suggested that the greater effect was that of the compressive residual stresses.[8,9] Following shot-peening, the initial compressive surface residual stresses are relaxed and redistributed during the fatigue process.[2] Stress relaxation has been investigated by many researchers.[5,6,10–13] Hoffman et al.[5] demonstrated that in both smooth and notched quenched and tempered medium carbon AISI 1045 steel specimens, the compressive residual stresses decreased considerably during fully reversed bending fatigue tests. They concluded that a small increase in the bending fatigue limit could be the result of increased work h
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