Structure property relationships in fibre reinforced aluminium laminates and SiC particulate aluminium composites

Hunt, Edward Patrick (1991) Structure property relationships in fibre reinforced aluminium laminates and SiC particulate aluminium composites. University of Southampton, Doctoral Thesis.

Abstract

Aluminium based laminates offer potentially attractive properties for light weight aerospace applications in fatigue critical structures such as fuselage and lower wing skins. Thin sheet aluminium-lithium alloy and aluminium based metal matrix composite (MMC) skins further increase specific stiffness, specific strength and transverse laminate properties. The present work set out to investigate the behaviour of monolithic aluminium-lithium and aluminium-lithium based MMC fibre reinforced laminates. A detailed analysis of fatigue life and fatigue crack growth resistance of laminate materials was carried out. Laminates exhibit improved fatigue crack growth resistance compared to equivalent thickness metallic materials and at low ΔK values crack growth rates decrease for the majority of laminate lay-ups. By contrast fatigue life and crack initiation is only improved for certain laminate lay-ups. These observations are discussed in terms of reinforcing prepreg stiffnesses, loading conditions, delamination and fibre bridging in the wake of the crack. Improvements of the inherent properties of particulate reinforced MMCs are needed to optimise their use in both laminated and unlaminated applications. Work has consequently been carried out on 8090 (Al-Li-Cu-Mg) and 2124 (Al-Cu-Mg) based MMCs reinforced by 20wt% SiC particles (3μm) to gain a better understanding of the ageing kinetics in these materials and to thereby complement efforts towards property optimisation. The MMCs exhibit earlier hardening which cannot be explained solely in terms of dislocation density or enhanced precipitation. An alternative mechanism involving the role of internal tensile hydrostatic stresses on solution strengthening of the metal matrix is proposed. This results in an imbalance of tensile and compressive mechanical properties which may have implications for the use of current MMC systems in stiffness critical structures. A significant step has been made in the current work towards a better understanding of the mechanisms affecting ageing kinetics and mechanical properties in MMC materials. Modifications to reinforcement shape, matrix composition including high lithium and low copper contents, and improved surface treatment may be expected to help improve the fatigue and mechanical properties of MMCs. As a result, the design of future laminate materials to incorporate the high fatigue limit of MMCs and reinforced by a hybrid of carbon and either aramid or glass fibres would be expected to give improved all-round fatigue performance, notch sensitivity and mechanical properties for use in future aerospace applications.

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