Fatigue Crack Initiation by Cyclic Slip Irreversibilities in High-Cycle Fatigue

Abstract

Dislocation mechanisms of fatigue crack initiation in high-cycle fatigue are formulated, with special consideration given to those material properties which determine the cyclic slip mode. The models developed are related to pertinent experimental observations, referring mainly to copper mono- and polycrystals fatigued at room temperature. In particular, the following topics are considered: (1) the origin of cyclic slip irreversibilities in the bulk and near the surface, (2) computer simulations of surface roughening by random irreversible slip processes in planar- and wavy-slip materials, and (3) fatigue crack initiation by cyclic strain localization in persistent slip bands (PSBs) in mono- and polycrystalline wavy-slip materials. The evolution of the surface profile at emerging PSBs is described by a new semiquantitative model which is compared with other models. The model distinguishes between the rather rapid formation of extrusions and the more gradual development of surface roughness. It is shown that specific predictions regarding differences between PSB surface-profiles in mono- and polycrystals are borne out fully by the observations. In the case of copper polycrystals, the experiments show that PSBs are not only responsible for slip-band cracking in PSBs but also induce intergranular cracks. A model is proposed according to which cracks initiate as a result of the piling-up of PSB-matrix interface dislocations against the grain boundaries. Furthermore, the supporting role of environmental interaction and/or deformation-induced diffusion and grain boundary sliding is discussed.

Microstructural mechanisms of cyclic deformation, fatigue crack initiation and early crack growth

“…Emergence of dislocations at the surface, cross slip of screw dislocations, mutual annihilation of dislocations, random to-and-fro glide of dislocations (leading to surface roughening), cutting of shearable precipitates and the so-called slip asymmetry in body-centred cubic (BCC) metals (see §6c) are microscopic mechanisms that occur in the material and can give rise to cyclic slip irreversibilities in the sense that forward and reverse glide of dislocations do not occur exactly along identical paths (cf. [26,[30][31][32]). At the surface, unreversed slip steps (or even microcracks) are left behind which can be conveniently studied by scanning electron microscopy (SEM), replicas or, more recently, by atomic force microscopy (AFM) [27,33].…”

“…PSBs develop only if well-defined thresholds of the plastic strain and the stress amplitude are exceeded. For example, in copper single crystals which deform in single slip, the PSB thresholds are defined by critical values of the shear stress amplitude of τ PSB /2 ≈ 28 MPa and the shear strain amplitude γ pl,PSB /2 ≈ 6 × 10 −5 , respectively [26,40,41]. Similar thresholds apply also to polycrystals.…”

Evolution and microstructural characteristics of deformation bands in fatigued copper single crystals

“…The GB cracking model in polycrystals has been proposed by Mughrabi et al [45] and was further developed by using copper bicrystals with different GBs [46,47]. It was proved that the impingement of PSBs against GBs plays a decisive role in intergranular cracking.…”

Comparison of fatigue cracking possibility along large- and low-angle grain boundaries

“…Kim and Laird [6] have developed a step-mechanism for intergranular fatigue crack nucleation in polycrystalline copper fatigued at high strain amplitudes. Later, for lower strain amplitude, it is recognized that the intergranular cracking is often associated with the interactions of PSBs with GBs and a PSB-GB mechanism for intergranular fatigue cracking was proposed by Mughrabi [7] and developed by Christ [8]. In fact, intergranular cracking is a dominant mode in bicrystals [9][10][11][12][13][14][15][16] and polycrystals [6][7][8][17][18][19][20] fatigued at intermediate or high strain amplitudes.…”

“…Later, for lower strain amplitude, it is recognized that the intergranular cracking is often associated with the interactions of PSBs with GBs and a PSB-GB mechanism for intergranular fatigue cracking was proposed by Mughrabi [7] and developed by Christ [8]. In fact, intergranular cracking is a dominant mode in bicrystals [9][10][11][12][13][14][15][16] and polycrystals [6][7][8][17][18][19][20] fatigued at intermediate or high strain amplitudes. To improve the bulk properties of polycrystalline materials, Watanabe [19], Lim and Watanabe [20] introduced a concept of 'GB design and control (GBDC)', which emphasized that the performance of materials can be improved by increasing the number of some 'special' GBs in polycrystals.…”