A B S T R A C T Fatigue fracture of high-strength steels often occurs from small defect on the surface of a material or from non-metallic inclusion in the subsurface zone of a material. Under rotating bending loading, the S-N curve of high-strength steels consists of two curves corresponding to surface defect-induced fracture and internal inclusion-induced fracture. The surface defect-induced fracture occurs at high stress amplitude levels and low cycles. However, the subsurface inclusion-induced fracture occurs at low stress amplitude levels and high-cycle region of more than 10 6 cycles (giga-cycle fatigue life). There is a definite stress range in the S-N curve obtained from the rotating bending, where the crack initiation site changes from surface to subsurface, giving a stepwise S-N curve or a duplex S-N curve. On the other hand, under cyclic axial loading, the S-N curve of high-strength steels displays a continuous decline and surface defect-induced or internal inclusioninduced fracture occur in the whole range of amplitudes. In this paper, influence factors on S-N curve characteristics of high-strength steels, including size of inclusions and the stress gradient of bending fatigue, were investigated for rotating bending and cyclic axial loading in the giga-cycle fatigue regime. Then, based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and it was clarified that the shape of S-N curve for subsurface inclusion-induced fracture depends on the inclusion size.Keywords fatigue crack growth rate; giga-cycle fatigue; high-strength steels; nonmetallic inclusion; subsurface crack. N O M E N C L A T U R E da/ dN= crack propagation rate d inc = depth of non-metallic inclusion from the specimen surface N f = total fatigue life (number of cycles to failure)A IG = size of internal inclusion with GBF K = stress-intensity factor range K GBF = stress-intensity factor range at GBF K S = stress-intensity factor range at surface inclusion K I = stress-intensity factor range at internal inclusion without GBF K IG = stress-intensity factor range at internal inclusion with GBF K th = threshold stress-intensity factor range for surface crack growth σ a = stress amplitude σ at = true stress amplitude considering the stress gradient

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Nine PP systems were synthesized to investigate the coupling effect of elastic modulus and yield strength on its scratch behavior. Adopting an integrated approach of scratch test and finite element simulation, the tangential force, the residual scratch depth and the groove shoulder area are identified as the evaluation criteria of the scratch resistance of PP. With those identified criteria, the coupling effect of elastic modulus and yield strength on PP scratch is revealed that the PP with large yield strength and small elastic modulus owns a remarkable scratch resistance. The implication of the present work on designing the PP with good scratch resistant is also discussed. POLYM. ENG. SCI., 58:118-122, 2018.

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