Abstract
ABSTRACTThis paper examines fatigue crack paths in coarse‐grained magnesium alloys. Visualization of the microstructure by etching and continuous crack monitoring with a microscope provide evidence of the crack‐grain interaction at different fatigue crack growth regimes. Near‐threshold fatigue crack growth promotes the development of rough surfaces by the activation of single slip mechanisms and crack deflection. The R‐ratio effect on near‐threshold results is discussed in the light of the partial crack‐closure model. The role of grain boundaries and grain‐to‐grain crack deflections on fatigue crack growth rates is discussed.
Highlights
Fatigue crack propagation of long cracks is customarily divided in three regimes, [1]
This paper examines fatigue crack paths in coarse-grained magnesium alloys
Evolution of the fatigue crack through the microstructure was monitored during the tests with a microscope and digital images were periodically recorded with a CCD camera
Summary
Fatigue crack propagation of long cracks is customarily divided in three regimes, [1]. Crack deflections due to microstructural heterogeneity can lead to mixed-mode displacements on the microscopic level and a faceted fracture surface. These displacements cause mismatch between upper and lower crack faces which in turn results in a positive closure load. According to [1], RICC is promoted by: i) low stress intensity factor levels where the plastic zone, rc, is smaller than the average grain diameter, dg ; ii) small crack opening displacements (i.e. low ∆K and low R-ratios) of a size comparable to surface asperities; iii) coarse grained microstructures; iv) periodic deflections of the crack due to grain boundaries, second-phase particles and composite reinforcement; v) enhanced slip irreversibility. Near-threshold fatigue crack growth test results are discussed in the light of the partial crack closure model of [2]
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