Combined cycle fatigue of 7075 aluminum alloy – Fracture surface characterization and short crack propagation

• High-frequent ultrasonic VA-blocks are superimposed to low-frequent square loads.
• Life-times under specified combined loading conditions are shorter than predicted.
• Predictions used completely reversed S–N data, FCG-rates and a modified Miner rule.
• Correlation of life-times with fractographic results allows interpretation of results.
• Small crack arrest and micro-plasticity seem to be main life-limiting mechanisms.

Aim of this study is an interpretation of the influence of variable-amplitude (VA) cycles superimposed to low-frequency loads on fatigue life of 7075-T651 Al-alloys. Constant-amplitude (CA) 20 kHz stress/strain-life (S–N) and (ε–N)-curves with and without superimposed mean loads serve as basis. For combined fatigue loading, life-time measurements were performed. Life-time estimations based on the S–N results reveal a damaging effect of the superimposed ultrasonic vibrations in the high cycle fatigue (HCF) and the very high cycle fatigue (VHCF) regimes. The CA and VA-life time results are correlated with fractographic observations. An interpretation of fatigue lives under combined low and high-frequency VA-loading is proposed considering small/short-crack propagation and arrest mechanisms.