Mean Stress and Environmental Effects on Near Threshold Fatigue Crack Growth

Abstract

The effect of mean stress and environment on the threshold and near threshold fatigue crack propagation behavior has been studied for D6ac steel and for 7050-T73651 aluminum. Fatigue crack propagation experiments were conducted in the range of 10−7 to 10−9 in./cycle (2.5 × 10−9 to 2.5 × 10−11 m/cycle) at 375 Hz. A mean stress effect on the threshold stress intensity factor range, ΔKth, is only observed for low mean stresses, in the regime of the minimum stress intensity factor, Kmin, 0≤Kmin≤ 2 ksi √in. (2.2 MN/m3|2), typically 0≤R ≤0.5. For higher values of Kmin, the fatigue crack propagation threshold is controlled by ΔKth. For the high test frequency used, no effects of humid argon and room air as compared to a dry argon environment were observed on the fatigue crack propagation threshold ΔKth. Environmental effects were noted in the near threshold region. For D6ac steel room air produced the highest crack propagation rate while dry and wet argon produced identical but lower crack propagation rates for the same ΔK values. For 7050-T73651 alumium both wet argon and room air environments produced the highest crack propagation rates, while dry argon produced lower crack propagation rates. Changing the test frequency for 375 to 100 Hz had no noticeable effect on the threshold and near threshold region results for D6ac steel in room air but in dry argon 100 Hz gave slightly higher crack propagation rates with respect to 375 Hz.