Abstract
Fibre reinforced titanium matrix composites (TMCs) are being considered for use in future aeronautical gas-turbine compressor discs. Low cycle fatigue is thought to be one of the mechanisms most damaging to such a component. Here, the low cycle fatigue behaviour of Ti-6-4, reinforced with SM1140+ fibre, is investigated over the temperature range 22°C to 600°C. SN curves have a characteristic “S” shape and can be split into three regions. Fractography, acoustic emission monitoring and cyclic strain recording have elucidated damage mechanisms in each region. In region I (high cyclic stress) damage is caused by matrix creep, that leads to fibre failure. In region III (low cyclic stress), the predominant damage mechanism is matrix crack growth. Cracks initiate at surface machining damage and grow, bridged by intact fibres, into the bulk. The matrix crack growth transfers stress to fibres, eventually causing them to fail in overload, resulting in specimen failure. In region II (intermediate cyclic stress) damage is by a combination of the mechanisms observed in regions I and III. Comparison of Ti-6-4/SM1140+ with Ti-6-4/SCS-6 shows that fatigue lives are similar in regions II and III. In region I it is possible that Ti-6-4/SM1140+ has inferior lives to Ti-6-4/SCS-6.
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