Fatigue Crack Growth at High Load Ratios in the Time-Dependent Regime

Experimental crack growth rates were determined in Inconel 718 middle cracked tension [M(T)] specimens at 649°C under conditons of high frequency (10 to 100 Hz) and high load ratio, R. Under these conditions, the materialexperiences cycle-dependent crack growth as well as time-dependent crack growth. At very high R values approaching unity, the observed growth rates are lower than those obtained under sustained load at the same mean load in the absence of the superimposed cyclic loading. Tests on compact tension [C(T)] specimens at lower frequencies were used to demonstrate the existence of three regions of behavior-cycle-dependent, mixed mode, and time-dependent. A linear cumulative damage model was used to predict the growth rates due to combined cycle-dependent and time-dependent mechanisms. The model was developed from 427°C data for the cyclic term and sustained load crack growth data for the time-dependent term. Although the model cannot predict the synergistic effect at high R, it provides a reasonable representation of much of the data. The authors conclude that the use of low-temperature data for the cyclic term is inadequate for representing the threshold values and growth rates at low AK values at the higher temperature.