Influence of Corrosive NaCl solution on Life times of 7075 Aluminum alloy under Combined Fatigue loading in the VHCF Regime

Abstract

The loading histories of aircrafts and rockets are complex and the components are exposed to combined loading conditions with high numbers of high-frequency cycles superimposed to lower-frequency carrier vibrations. The amplitudes of both frequency ranges usually are variable. The small-amplitude cycles arise from several discrete sources such as structural vibrations which are especially critical at special components and places of an aircraft. The environment plays another important role and ambient air and sea-water have to be considered. Almost no knowledge exists of the material response in the VHCF regime, especially concerning the environmental influence. In this study, experiments were performed on aluminum alloy 7075-T651 in a 3.5%- sodium-chloride solution and in laboratory air of 22 °C of 50% relative humidity at high (105 – 108) and very high numbers of cycles (5 × 108 – 1010). The loading sequences consisted of a low-frequency square wave (0.4 to 1 Hz), being superimposed with a high frequency 20 kHz random vibration. The random vibrations were simulated by a Gauss distribution allowing stress ranges of 10 to 70 MPa or alternatively 50 to 90 MPa. Stress/strain vs. life time curves were measured. The results were correlated with in-situ microscopy observations of the specimen surfaces and with post-experimental fracture-surface images. This technology allowed identifying fatigue crack initiation and propagation stages being needed for an interpretation of the relevant fatigue-life mechanisms under environmental influences.