Materials assessment and optimisation of aluminium alloys for fatigue resistance

Khor, Kern Hauw (2005) Materials assessment and optimisation of aluminium alloys for fatigue resistance. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

In quantifying the fatigue crack growth behaviour of a material, it is possible to identify intrinsic and extrinsic contributions to failure resistance, with intrinsic resistance representing the inherent mechanical and environmental material response to cyclic loading at the crack tip. The incidence of crack closure has been widely recognised as a major factor extrinsic factor affecting fatigue crack growth rates (via the shielding of cyclic load conditions in the crack tip region), for many engineering alloys (include AI-alloys). Significant origins of crack closure may be identified as the essentially mechanical process of plasticity-induced crack closure (PICC), and the more microstructurally dependent roughness-induced crack closure (RICC). However, significant problems exist in both the experimental determination and micromechanical modelling of closure behaviour. In the present work, several advanced variants of the established aerospace alloy 2024-T351 (with different dispersoid contents and degrees of recrystallisation) are studied for micromechanistic influences on fatigue crack growth behaviour under constant amplitude (CA) and variable amplitude (VA) loading conditions. Degrees of recrystallisation level have been seen to have limited influence on CA crack growth resistance and closure levels in the advanced 2024 alloy variants. Dispersoids are found to be a key factor controlling fracture surface faceting levels and hence, closure processes in these materials. Physical interpretation of RICC effects in terms of residual shear displacements at 3D crack wake asperities has been compared to a variety of experimental results. 3D surface profilometry techniques are applied to obtain crack deflection parameters (deflection angles and length-scales) from fatigue fracture surfaces. For CA loading conditions, where the model is most readily applied, predictions of plane strain crack closure levels from real fracture surface features has been shown to be in reasonable functional accord with experimental data. Crack behaviour has been further investigated via synchrotron X-ray tomography at an enhanced resolution level of 0.7/lm per isotropic voxel. Microstructural displacement gauging and ray casting analysis technique have been successfully developed as novel methods to measure 2D and 3D crack opening displacements. A liquid Gallium (Ga) grain boundary wetting technique has been investigated in conjunction with the microtomography to visualise and analyse simultaneously the correlation between the 3D grain structure and fatigue crack paths. Subsequent electron backscattering diffraction (EBSD) assessment of grain orientation in the computed tomography samples provides detailed crystallographic information on crack propagation mechanisms, where large crack deflections were found to be generated mostly by {Ill} and {I OO} plane oriented fracture. In terms of VA loading (simple peak load transients), closure measurements close to the crack tip appear to describe growth rate transients well. Reasonable correlation between transient crack growth and measured closure variations has been observed when used in conjunction with 'intrinsic' CA crack growth data. Fractographic observations and previous FE modelling results suggest that PICC may be the predominant closure mechanism over RICC in controlling post-overload retardation effects in these materials at intermediate to high baseline stress intensity loading, consistent with a limited effect of microstructure on transient crack growth.

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