Owing to difficult economical conditions, machines and structures often have to be used beyond the design lifetime. In this study, fatigue properties of a bearing steel in the long‐life region were experimentally examined under cyclic axial loading. The complicated S–N behaviour was well explained as a combination of S–N curves for surface‐induced fracture and interior inclusion‐induced fracture. Fish‐eye marks were always observed on the fracture surfaces of specimens, which failed in the latter fracture mode, and an inclusion was found at the center of the fish‐eye. Finally, it was found that the fatigue fracture of this steel in the long‐life region occurred through the following three processes: (i) formation of the characteristic area as a fine granular area (FGA), (ii) crack propagation to form the fish‐eye and (iii) rapid crack propagation to cause the catastrophic fracture.

In fatigue tests of high strength steels and surface hardened steels, a characteristic fatigue behavior such that S-N curve tends to come down again in the long life region of N>107 was often observed and reported by many researchers. When the mechanical design is based on the fatigue limit of the material, the above aspect introduces a typical difficulty to provide the reliability of the mechanical structures. In order to clarify such S-N characteristics in wide life region, a series of fatigue tests were performed by means of same type fatigue testing machines and same type of fatigue specimens in a definite high carbon chromium steel for the use of bearing as a collaborative study by the authors. Thus the complicated S-N property of this steel was tentatively interpreted as duplex S-N characteristics given by superposition of S-N curves for the respective fracture modes of the surface-originated fracture and the inclusion-originated fish-eye fracture.

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A B S T R A C T In some high-strength steels, a fatigue crack tends to occur at the interior inclusion after a long-term sequence of the cyclic loadings at low stress levels, although the crack takes place at the surface in the usual life region at high stress levels. Thus, we have the duplex S-N curves consisting of the respective S-N curves for usual life region and very highcycle regime. It is well known that a significant fracture surface having the fine granular morphology is formed around the interior inclusion at the crack initiation site. This surface area is sometimes called as 'fine granular area'. In this work, metallurgical structures around the interior inclusion at the fatigue crack initiation site were carefully observed by combining several special techniques such as focused ion beam technique and highresolution scanning electronic microscopes. Based on the current observation results, it was found that the microstructure around the interior inclusion was changed into the penny-shape fine granular layer from the usual martensitic structure during long-term cyclic loadings. Then, debondings along with the boundaries of the matrix and the fine granular layer have produced the small cracks inside the metallic material, and these interior cracks caused the final fatigue fracture after definite loading cycles of the crack propagation.Keywords bearing steel; fine acicular area (FAA); fine granular area (FGA); interior crack initiation; rotating bending; very high-cycle fatigue. N O M E N C L A T U R Eα = stress concentration factor D = distance from definite section ΔK th = threshold value of stress intensity factor range in crack propagation N = number of cycles N f = number of cycles to failure R = stress ratio (minimum stress/maximum stress) σ = stress σ a = stress amplitude σ B = tensile strength σ w = fatigue limit I N T R O D U C T I O NAs a common understanding in the Metal Fatigue, it is well known that ferrous metals such as structural steels reveal the clear fatigue limit at the number of stress cycles less than 10 7 cycles. 1 However, non-ferrous metals such as aluminium alloys have no fatigue limit such that the S-N curve tends to decrease continuously in the very long life region, longer than 10 7 cycles. Usually, the fatigue limit for the structural steels is proportional to the tensile strength, and we have σ w = σ B /2, approximately, in the wide variety of the strength levels. 2,3 In recent years, practical structures such as railway wheels and axles, offshore structures, energy conversion and transportation systems have been used in a long term, sometimes, beyond their original design life

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