Manufacture and evaluation of hoop-wound fibre-reinforced aluminium alloy tube

A composite tube, consisting of aluminium-magnesium-silicon alloy (6061) reinforced with hoop-wound alumina-based fibres, has been manufactured by using the method of liquid metal infiltration. The composite was well-consolidated with a good bond between the fibres and the matrix, as evinced by the close similarity between measured values of modulus and ‘rule of mixtures’ calculations. Although some matrix magnesium was absorbed in the fibre surface regions during processing, sufficient remained in the matrix to effect a significant precipitation-hardening response upon heat treatment. Mechanical measurements were carried out on the tube to determine properties in the principal directions. The tests involved internal pressurisation to produce a tensile stress parallel to the fibre direction, axial tension to create a stress perpendicular to the fibres, and internal pressurisation to give a bi-axial tensile stress state. Stress-quarter diagrams were constructed by using experimental values of yield stress and failure strength to produce failure envelopes. Failure under the different stress conditions and the microstructure of fracture surfaces correlated well with predictions from the stress-quarter diagrams. Heat treatment affected the type of failure mode for a given bi-axial stress ratio and this was reflected in the shape of the failure envelope. The results also indicated a small, but measurable, bi-axial strengthening effect in these materials. In conclusion, the correlation between the failure-envelope constructions and the fracture path observations is considered justification for using a maximum stress-failure approach when designing with this composite material in the bi-axial stress condition.