Sustained Load Cracking in Titanium Alloys

摘要

Recent failures of titanium alloy components have drawn attention to sustained load failure phenomena, be they 'cold creep', sustained load cracking (SLC), dwell fatigue or ripple fatigue. These phenomena occur at approximately -50 to 200℃. 'Cold creep' has been used to describe slow extension in titanium under high, sustained loads but a better term is 'sustained load strain' (SLS), since deformation cannot be ascribed to a conventional creep mechanism. SLC is sub-critical crack growth, occurring at stress intensity factors below the fracture toughness. Early service and experimental observations of SLC in titanium were noted in the 1950's but have been largely ignored in non-aerospace fabrications. Dwell fatigue is typified by a square waveform stress cycle incorporating a hold at the high stress. At elevated temperatures this can be considered as creep-fatigue, but dwell fatigue also occurs at lower temperatures (through a combination of SLS/SLC and low cycle fatigue). Ripple fatigue is the synergy of low amplitude, high cycle fatigue crack growth and SLC. In summary, time dependent failures in titanium alloys can occur at stresses below tensile strength (UTS), stress intensity factors below K_(IC) and when dwell or ripple fatigue loads are applied.