13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- to the induced microscale plasticity around surface defects and pores [5], planar slip bands [6] and grain boundaries [7], results in crack initiation. The crack initiation site for these materials remains at the surface of the specimens, even for higher fatigue cycles from 108 to 109 cycles. The most common plastic deformation in FCC materials is the development of Persistent Slip Bands (PSBs) which yields intrusions and extrusions on the surface of specimens. In this case, the S–N curve decreases linearly with very little slope without transition at 107 cycles [3]. One important structural material which show surface based crack initiation is high temperature grade stainless steel alloy AISI 310. The alloy is very commonly used in moist conditions owing to its excellent characteristic to resist against corrosion. It is used in heat exchangers, pipes, machinery parts etc. The alloy is subjected to pressure cycles, temperature cycles and many start up and shut down cycles during service life. Hence, it is expected to serve for large number of loading cycles. However, there is no such study for this material, to the best of authors’ knowledge, which details the behavior of fatigue crack initiation phenomenon especially up to very high cycle fatigue domain. Hence, it is important to be able to assess the very high cycle fatigue behavior of the material. The surface crack initiation phenomenon even at lower stress levels has been studied previously and different conclusions have been made. Tokaji et al. [8] and Yang et al. [9] in Mg alloys, Miura et al. [10] and Sakai et al. [11] in steels studied the surface based crack initiation. The surface cracks initiation has been attributed to micro-cracks [12], surface roughness [13] and, intrusions and extrusions [14]. In addition, few efforts have been made to characterize the nature of PSBs in range of materials. Man et al. [15] studied the difference in the topography of slip bands in FCC and BCC materials. Polak et al. [16-17] studied the different stainless steels and attributed the local plasticity due to the intrusions and extrusions and their interaction with nearby heterogeneities, for the fatigue crack initiation. The localized plastic deformation and surface roughness developed in fatigue loading, and their interaction with surface defects have been attributed for the surface based crack initiation. It has been concluded that any or combination of these parameters may initiate cracks from the surface of the specimens. However, the crystallographic orientations of the grains play major role in the surface crack initiation. Owing to the difference in basal slip and twining mechanism for FCC and HCP based crystal structure, difference in the topography of PSBs is obtained. Hence, case by case experimental investigation of the crack initiation region and slip and basal planes for different materials is considered inevitable. Ultrasonic fatigue testing has been used in this study to investigate the VHCF behavior of Stainless Steel AISI 310. The S-N curve showed horizontal asymptote 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 the crack propagation for different stages of the fatigue cycles. It was found that up to 106 cycles, the cracks initiated from carbide precipitates present on the grain boundaries. The mechanism of crack initiation changed at low stress levels and PSBs were observed at the surface of the specimen which acted as the fatigue crack initiation sites. The main aim of this study is to enhance the VHCF life of metallic materials though surface modification techniques like Ultrasonic Nanocrystal Surface Modification (UNSM). A precursor to this goal is better understanding of VHCF behaviour of the material. The surface crack initiation for AISI 310 makes it an ideal material for the application of UNSM. The materials which exhibit subsurface crack initiation, the application of compressive residual stresses may enhance the
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