Frequency effects on fatigue crack growth behavior in a near-α titanium alloy

Abstract

The loading frequency effects on elevated temperature fatigue crack growth behavior in a new near-α, β-processed titanium alloy were examined in both air and vacuum environments. The individual influences of environment and creep are identified in terms of both the macroscopic crack growth rates and detailed fractographic observations as a function of the applied ΔK. Results indicate the existence of two regions of transgranular crack growth: microstructurally sensitive and insensitive. The transition between these two regions was found to depend on the testing temperature. Lower frequencies promoted an increase in cleavage fracture while an addition of hold time at maximum load resulted in prior β grain boundary fracture attributed to creep damage. In addition, the transition to intergranular fracture at high ΔK was found to take place at higher frequencies in vacuum, yielding a higher crack growth rate than in air which is indicative of a faster, more widespread viscoplastic flow without the presence of oxidation. Closure measurements indicated a dominant influence of asperity-induced closure in non-oxidizing environments and at low values of applied ΔK.