Investigation of Creep/Fatigue Interaction on Crack Growth in a Titanium Aluminide Alloy

Crack growth under sustained load and sustained load with superimposed fatigue cycles at elevated temperatures in a titanium-aluminide alloy (Ti₃Al) was investigated. The objectives were to determine the creep crack growth characteristics and to evaluate the applicability of linear cumulative damage modeling to the Ti₃Al alloy. The linear elastic stress intensity factor, K, was used as a correlating parameter for all the tests. Sustained load tests were used to characterize the time-dependent crack growth behavior at elevated temperatures. The test results showed that sustained load crack growth rate and fracture toughness were relatively insensitive to temperature between 700 and 800°C. Crack growth rates were only a factor of five apart between the slowest and fastest growth rates over this range of temperatures. Several sustained load tests with periodic fatigue cycles (hold-time tests) were used to test the applicability of linear cumulative damage modeling. The model was developed using data from the sustained load and a baseline fatigue test at 750°C. Crack growth rates calculated using the model were accurate for a fatigue cycle with a ten minute hold time, but were up to 2 times less than the growth rates for shorter hold-time tests based on summation of the sustained load and fatigue crack growth rates only. A mixed-mode correction factor added to the model produced more accurate results.