13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Subsurface non defect fatigue crack origin and local plasticity exhaustion Guocai Chai1,2* 1 Strategy Research, Sandvik Materials Technology, 811 81 Sandviken, Sweden 2 Engneering Materials, Linköping University, 581 83 Linköping, Sweden * Corresponding author: guocai.chai@sandvik.com Abstract Besides “fish eye”, subsurface non-defect fatigue crack origin (SNDFCO) in the matrix is another fatigue crack origin observed during very high cycle fatigue (VHCF). This paper provides some discussion on the phenomena and damage mechanisms from the recent investigations using four metallic materials with different microstructures. The results show that the strains in these materials in the VHCF regime were highly localized, especially in the multi-phase materials, where the local maximum strain can be eight times higher than the average strain value in the specimen. This high strain localization can lead to a fatigue damage or fatigue crack initiation at grain boundaries or twin boundaries by impingement cracking. High strain localization causes dislocation accumulation of very small strain during each cyclic loading and consequently the formation of local “fine grain area” and also increases the local hardness of the material. This can cause quasi-cleavage crack origin, and finally the formation of SNDFCO. The results in this paper indicate that fatigue damage and crack initiation mechanisms in the VHCF regime can be different in different metals due to the mechanisms for local plasticity exhaustion. Keywords Very high cycle fatigue, Local plasticity exhaustion, Fatigue crack initiation, stainless steels 1. Introduction Fatigue behaviors of metals in very high cycle fatigue (VHCF) regime have been widely investigated during the last decade [1-5]. It has been found that fatigue crack initiation in metals can shift from surface defects, subsurface defects and subsurface matrix with decreasing applied stress or increasing fatigue life [1, 4, 6]. Subsurface fatigue crack initiation has been mostly reported to start at subsurface defects such as inclusions, pores and microstructure inhomogeneity [4]. The surface treatments like shot peening, case hardening and surface nitriding can prevent the surface fatigue crack initiation, and cause a shift to a subsurface fatigue crack initiation at relatively higher stress amplitudes, and therefore improve the fatigue life or fatigue strength of the material [7]. Recently, another type of subsurface crack initiation, namely subsurface non-defect fatigue crack origin (SNDFCO), has been reported [6-8]. These crack origins were observed in the material fatigue tested for a very high fatigue life, and start in some phase or matrix of the material and are not associated with pre-existing defects. In very high cycle fatigue regime, the applied stresses or strains are sometime well below the bulk yield strength or in the elastic deformation regime. How a cyclic plastic deformation or damage can occur in such a situation is still not well known. Hydrogen has been considered as a source for the formation of fish eye of subsurface inclusion [9], but this cannot explain a subsurface matrix crack initiation or formation of SNDFCO. Other mechanism such as localized deformation by dislocation pileup or grain boundary imcompatibility has been proposed [6, 10]. These observations are however not enough to verify why SNDFCO has only been observed in some alloy systems. On the other hand, the mechanism for the shift of fatigue crack initiation from surface to subsurface is still not fully understood. Another phenomenon correlated to fatigue crack initiation in the VHCF regime is the formation of fine grains in the Fine Grain Area, FGA [11]. The reasons of the formation and the role of this FGA are not well explained either. In this investigation, fatigue crack initiation behavior and fatigue damage mechanism at a stress well below the bulk yield strength have been investigated in three metal materials with different microstructures using scanning electron microscopy, SEM, electron back scatter diffraction, EBSD and electron channel contrast
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