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    Matrix Fatigue Cracking in α2 Titanium Matrix Composites for Hypersonic Applications

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    The objective of this work was to understand matrix cracking mechanisms in a unidirectional α2 titanium matrix composite (TMC) in possible hypersonic applications. A [0]8 SCS-6/Ti-24Al-11Nb (atomic %) TMC was first subjected to a variety of simple isothermal and nonisothermal fatigue cycles to evaluate the damage mechanisms in simple conditions. A modified ascent mission cycle test was then performed to evaluate the combined effects of loading modes. This cycle mixes mechanical cycling at 150 and 483°C, sustained loads, and a slow thermal cycle to 815°C. At low cyclic stresses and strains more common in hypersonic applications, environment-assisted surface cracking limited fatigue resistance. This damage mechanism was most acute for out-of-phase nonisothermal cycles having extended cycle periods and the ascent mission cycle. A simple linear fraction damage model was employed to help understand this damage mechanism. Time-dependent environmental damage was found to strongly influence out-of-phase and mission life, with mechanical cycling damage due to the combination of external loading and CTE mismatch stresses playing a smaller role. The mechanical cycling and sustained loads in the mission cycle also had a smaller role.


    titanium matrix composites, fatigue (materials), thermomechanical fatigue, hypersonic, damage mechanisms, titanium, life prediction, titanium alloys, modeling

    Author Information:

    Gabb, TP
    Research metallurgists, NASA Lewis Research Center, Cleveland, OH

    Gayda, J
    Research metallurgists, NASA Lewis Research Center, Cleveland, OH

    Committee/Subcommittee: D30.04

    DOI: 10.1520/STP18233S