Fatigue as a process of cyclic brittle microfracture

Abstract

A B S T R A C T While fatigue crack growth in vacuum may occur by slip alone, environmental fatigue including crack growth in air is strongly influenced by crack-tip surface chemistry that adversely affects ductility. Cumulative diffusion, combined with adsorption and chemisorption in the loading half-cycle may promote instantaneous crack extension by brittle microfracture (BMF). Unlike slip, the BMF component will be sensitive to parameters that affect near-tip stresses, such as load history and constraint. BMF dominates near-threshold environmental fatigue. Being a surface phenomenon, it loses its significance with increasing growth rate, as slip-driven crack extension gains momentum and growth becomes less sensitive to environment. The BMF model provides for the first time, a scientific rationale for the residual stress effect as well as the related connection between stress-strain hysteresis and load-sequence sensitivity of metal fatigue including notch response. Experimental evidence obtained on a variety of materials under different loading conditions in air and vacuum appears to support the proposed model and its implications.

Crystallography of Fatigue Crack Propagation in Precipitation-Hardened Al-Cu-Mg/Li

“…Either of these mechanisms could produce a path without a preferred low-index crystallographic plane, consistent with the results of the present study. Sunder speculates that near-threshold fatigue crack extension in Al alloys is dominated by brittle microfracture of an unspecified environmental surface reaction film, [81] paralleling this discussion. However, the environmental exposure conditions for the present research likely involve rapid surface reaction saturation and H-diffusion limitation, justifying the volumetric H-embrittlement perspective.…”

Driving forces for localized corrosion-to-fatigue crack transition in Al-Zn-Mg-Cu

“…A programmed loading sequence was periodically interspersed to create microscopic marks on the fracture surface, as shown in Fig. 2 21,22,72–75 . Marker loads were applied with the baseline σ max , but R = 0.1 and f = 10 Hz.…”

Environmental Fatigue-Crack Surface Crystallography for Al-Zn-Cu-Mg-Mn/Zr

“…This mechanism is consistent with recent focus on both DK and K MAX control of intrinsic fatigue crack-tip damage. [45,65] Preliminary TEM analysis of a thin foil, prepared from the environmental-fatigue crack wake by focused ionbeam machining, demonstrated that an approximately 200-nm-diameter dislocation-cell structure formed in naturally aged Al-Cu-Mg, presumably due to recovery of high strain from crack-tip deformation. The fatigue crack appeared to travel both along and through this dislocation structure.…”

“…Moist-environment enhancement of fatigue-crack propagation in the low growth-rate regime is reasonably attributed to hydrogen embrittlement in precipitationhardened aluminum alloys, [1][2][3]28,29,45] rather than a per cycle geometrically based plasticity mechanism. [25][26][27]46,47] (For the present work, cyclic plastic zone size is on the order of 3 lm, environment enhanced da/dN is on the order of 0.003 lm/cycle (Figure 1), and fatigue-crack advance is likely discontinuous over a distance of a fraction of a micrometer and requiring several hundred load cycles.)…”