The Effects of Very Dry and Fully Water Saturated Air Environments on the Fatigue Life of D6AC Steel Under Constant and Variable Amplitude Loading Sequences

Axial load fatigue tests have been carried out on keyhole notched (K_t = 3.15) specimens of D6AC steel (UTS = 1600 MPa) in very dry air and in air fully saturated with water vapor under both constant-stress-amplitude and four-load-range program loading sequences. Estimates were made of the crack initiation lives (corresponding to cracks of 0.1 mm), crack propagation lives, and crack propagation rates under program loading from the “program markings” on the fracture surfaces. Under the program loading sequence used in this investigation, the total fatigue lives in a fully water saturated air environment were about 35% of those in very dry air–the crack initiation life was about half, and the propagation life to failure about one-third of that in dry air. Thus, the environment has a greater effect on the crack propagation phase than on the crack initiation phase of the fatigue process. Fatigue life predictions for program loading were made using the constant-stress-amplitude fatigue data and the simple Palmgren-Miner linear cumulative damage hypothesis. For both environments, the predicted program lives were less than the actual lives, the ratio predicted/actual being about 0.36 under dry air and about 0.33 under wet air conditions. Thus, under the particular sequence of loads and environments used, the predictive method was conservative.