13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Surface Crack Initiation Phenomenon in Very High Cycle Fatigue MuhammadKashif Khan1,*, QingYuan Wang1 1 Department of Mechanics and Engineering Science, Sichuan University, Chengdu 610065, China. * Corresponding author: email@address.aa.b.cc Abstract Very High Cycle Fatigue behavior of Stainless Steel AISI 310 has been investigated through ultrasonic fatigue testing. In fatigue cycles, a horizontal asymptote of S-N curve was found without any step at 106 cycles. Surface crack initiation was found irrespective of the stress level. However, the fracture surfaces of the material showed different behavior of crack propagation for different stages of fatigue cycles. It was found that up to 106 cycles, cracks initiated from surface defects and carbide precipitates present on the grain boundaries. The mechanism of crack initiation changed at low stress levels and slip bands and deformation twins were found at the surface of the specimen which acted as fatigue crack initiation sites. At lower stress levels, at or below fatigue limit, PSBs were developed but the cracks were not initiated. The lower slope in S-N curve was attributed to the development of PSBs all over the surface of the specimen before crack initiation. Keywords Very high cycle fatigue, Crack initiation , PSBs, Local plasticity 1. Introduction The fatigue crack initiation and growth in metallic materials is now well-understood. Fatigue cracks initiate at stress concentrating features within a structure. An area of fatigue assessment is still under investigation, however, is how cracks are initiated from surface and subsurface regions at very low stress levels in the very high cycle fatigue (VHCF). It is believed that the fatigue life of numerous aerospace, locomotive, automotive and biomedical structures may go beyond 108 cycles [1]. Hence, better understanding of the long life fatigue behavior is extremely important for efficient design of the components and structures, especially between the 107-1010 cycles. Most engineering designs were based on the assumption that materials exhibit a fatigue limit and any cyclic stress below fatigue limit yield infinite life. Recent studies, however, point out the fact that most materials experience failure up to 1010 cycles or above at low stress levels, and concept of infinite fatigue life is not true [1-4]. The VHCF behaviour of metallic materials is generally divided in two types, based on the crack initiation region [1-4]. In first type, the cracks are initiated from the internal defects of the material and a fish-eye region is obtained at the crack initiation site. In second type, the cracks are initiated from the surface of the material. These surface cracks are attributed to the surface defects, heterogeneities, pores and absence of inclusions and microstructural defects in some other materials. In this type of materials, the S–N curve between 106 to 109 cycles show very little decreasing slope without any transition at 107 cycles and horizontal asymptote is obtained. The VHCF behavior of some materials shows subsurface crack initiation from the internal defects and inclusions [4]. However, surface crack initiation is also obtained in structural alloys which show no inclusions and microstructural defects [5-7]. Plastic deformations at localized regions initiate the fatigue cracks during cyclic loading. During fatigue loading, the stress concentration due
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