The role of surface treatments on fatigue design of aircraft components

High fatigue properties and excellent corrosion resistance are generally required in aircraft applications to ensure ideal operating conditions in harsh environments and under various and severe loading scenarios. These characteristics can be provided through an appropriate combination of specifically designed surface treatments, such as Tartaric-Sulfuric Acid Anodizing (TSA) and Laser Shock Peening (LSP). The reduction in fatigue life caused by the presence of the TSA anodic layer can be effectively compensated by pre-treating the component with LSP, in order to introduce a specific Compressive Residual Stress (CRS) field and thus delay the nucleation and propagation of fatigue cracks. Previous studies have already demonstrated the validity of this strategy in the case of Shot Peening (SP) combined with anodizing treatment, but very few applications have specifically concerned the combination of LSP and TSA. In this research activity, the effects of the RCS field induced by an optimized LSP process on the fatigue performance of TSA anodized plain (Kt=1) and single edge notched (Kt>1) components (AA 7050-T7451) were investigated to verify and evaluate the fatigue enhancement determined by LSP when used as pre-treatment to anodizing. Four point bending and uniaxial fatigue tests at stress ratio R of 0.1 were performed on four different groups of specimens: as-received, not peened components to obtain preliminary baseline information on fatigue performance of base material; TSA anodized components to assess the individual effect of anodizing; LSP+TSA anodized components to evaluate the enhancement in fatigue life determined by the laser peening process and SP+TSA anodized components as a reference technology in the aviation industry. X-ray Diffractometry (XRD) was employed to analyze the CRS field induced both by SP, LSP and anodizing treatments. Surface morphology and cross-section of the anodized and peened samples were analyzed by Scanning Electron Microscope (SEM) before fatigue tests. A reduction in the fatigue life of the anodized components compared to the base material, especially in the high cycle fatigue region, is expected as a result of the intrinsic properties of the anodic layer. Based on the process parameters adopted, the LSP-induced CRS field should result in a significant increase in the component’s fatigue life when compared to the base material and, even more, to anodized one.