Effect of residual fatigue quality on fatigue damage accumulated by VHCF loads in variable-amplitude loads

Current fatigue damage analysis of various components (e.g. aircraft parts) focuses on effects of High-Cycle-Fatigue (HCF) loads while overlooking effects of Very-High-Cycle-Fatigue (VHCF) loads, thereby introducing a substantial bias. The crux of decreasing this bias lies in how to reasonably consider the threshold effect and nonlinear effect of VHCF loads’ fatigue damage evolution. This problem is addressed in this paper from the perspective of Residual Fatigue Quality (RFQ, represent residual S-N* curve and residual fatigue limit σ₋₁*). Fatigue tests were conducted on AA2024-T4 under various constant/variable-amplitude loads to reveal the evolution characteristics of RFQ and measure the equivalent fatigue damage of VHCF loads block loaded with various number of pre-loading HCF loads. Corresponding mechanisms were analysed in view of evolution of extrusions/intrusions along persistent slip bands. Theoretical analysis was conducted to reveal the relationship between RFQ and fatigue damage of VHCF loads block. Based on the above results, an isodamage curve-based fatigue damage analysis method was proposed, where bilinear-isodamage curves (consist of S-N* curves intersecting at a point and corresponding σ₋₁*) were adopted to consider the RFQ degeneration and its effect. This method reduces analysis bias to 1/3 of previous methods for typical variable-amplitude loads in HCF and HCF-VHCF regime.