Influence of creep-fatigue-environment interactions on fatigue crack growth behaviour of Ti-1100 at elevated temperature

Advanced aerospace as well as safety-critical nuclear and other applications demand high strength, light weight damage-tolerant materials capable of safely withstanding high temperature environment. Some conventional titanium alloys and intermetallics exhibit great potential as replacement for traditional nickel-base superalloys. In this study, fatigue crack growth behaviour of an advanced beta-processed, near- proportional to titanium alloy, Ti-1100 at 866K has been analyzed with a view to understand the influence of high temperature creep-fatiguc-environment interactions. During the experimental investigation, a wide range of loading frequency was employed inorder to differentiate between the cycle- dependent and time-dependent crack growth behaviour of this material. A novel testing technique earlier employed by the author for Titanium aluminide intermetallics was used in the form of superimposed hold times at maximum and minimum loads over 1.0 Hz baseline frequency. Experiments performed over a wide range of frequencies from 0.01 Hz to 200 Hz indicated a strong dependence of the crack growth rate upon cyclic loading frequency. Superposition of hold time at maximum and minimum loads over a baseline 1.0 Hz cyclic frequency produced an insignificant variation in crack growth rate, which might be attributed to the combined effects of enhanced environmental degradation, crack-tip blunting and increased asperity-induced closure level in this material. It is believed that a hold time at maximum load results in an interaction of the environmental effects with a retardation effect due to crack tip blunting as a consequence of creep under maximum applied load, whereas for hold at minimum loads, extensive crack-branching and micro-cracking appear to enhance crack closure loads resulting in lower crack growth rates. A line superposition model has been employed to account for the complex interactions due to fatigue, creep and environmental degradation.