Veryhigh-cyclefatiguebehaviourofshot-peened high-carbon±chromium bearing steel K. SHIOZAWA and L. LU Department of Mechanical and Intellectual Systems Engineering, Toyama University, 3190 Gofuku, Toyama 930-8555, Japan Received in final form 20 June 2002 ABSTRACT Effectofshot-peeningonfatiguebehaviourinthegigacycleregimewasinvestigatedin order to clarify the duplex S±N curve characteristics. Cantilever-type rotary bending fatigue tests were performed in laboratory air at room temperature by using hourglass- shaped specimens of high-carbon±chromium bearing steel, JIS SUJ2. Fatigue crack initiation site changed from the surface of untreated specimen to the subsurface of the specimenbecauseofhardeningandcompressiveresidualstresswithshot-peeninginthe region of high-stress amplitude. On the other hand, no difference in fatigue life con- trolledbythesubsurfacecrackinitiationbetweenuntreatedspecimenandshot-peening onewasobservedinhigh-cycleregion.Itwassuggestedthatthe S±N curvecorrespond- ingtotheinternalfracturemodeisinherentinthematerial,ascomparedwiththe S±N curve of surface fracture mode, which is affected by surface conditions, environmental conditions and so on. Subsurface crack initiation and propagation behaviour were discussed under the detailed measurement of crack initiation area and shape of the fish-eye fracture surface. Keywords fish-eye; high-cycle fatigue; internal crack; shot-peening, surface crack, surface treatment. NOMENCLATURE 2a  Tangentialdiameteroffish-eye 2b  Radialdiameteroffish-eye 2a/2b  Aspectratiooffish-eye c  Distancefromtheouter-sideedgeoffish-eyetothespecimensurface d inc  Depthofnonmetallicinclusionfromthespecimensurface N f  Numberofcyclestofailure DK A  Stress-intensityfactorrangeattheouter-sideedgeoffish-eye DK B  Stress-intensityfactorrangeattheinner-sideedgeoffish-eye DK T,GBF  Stress-intensity factor range at the granular-bright-facet (GBF) DK T,inc  Stress-intensityfactorrangeatthenonmetallicinclusion DK th  Thresholdstress-intensityfactorrangeforsurfacecrackgrowth  area p inc  Sizeofnonmetallicinclusion  area p GBF  Size of granular-bright-facet (GBF) s a  Stress amplitude s at  Truestressamplitudeconsideringthestressgradientandresidualstress INTRODUCTION In recent years, attention has been paid to the fatigue behaviour of materials in the high-cycle region, that is ultra-long life (> 10 7 cycles), because of requirements for high efficiency and reliability of the machines and structures and increases in expected endurance beyond 10 8 cycles. 1±9 Generally speaking, ordinary fatigue crack initiationoccursmostlyonthespecimensurfacebecause of slip deformation of crystals or some surface defect. However, fatigue failure does occur at small internal ß 2002 Blackwell Science Ltd. Fatigue Fract Engng Mater Struct 25, 813±822 813 Correspondence: K. Shiozawa, Department of Mechanical and Intellectual Systems Engineering, Toyama University, 3190 Gofuku, Toyama 930-8555, Japan. E-mail: shiozawa@eng.toyama-u.ac.jp