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

Abstract

Crack growth under sustained load and sustained load with superimposed fatigue cycles at elevated temperatures in a titanium-aluminide alloy (Ti3 Al) was investigated. The objectives were to determine the creep crack growth characteristics and to evaluate the applicability of linear cumulative damage modeling to the Ti3 Al 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.