Fatigue and fracture in Ti-6al-4v and 7075-T6 lightweight components

Lightweight alloys are widely used in aircraft engine and structural components due to their high specific strength. However, their durability is affected by defects, notches, surface treatments, impact damage and residual stresses. This contribution summarizes combined experimental and numerical research on fatigue and fracture in Ti-6Al-4V titanium and 7075-T6 aluminum alloys. Axial fatigue tests on smooth and notched Ti-6Al-4V specimens were interpreted using FEM and the Goodman criterion. Notches markedly reduced fatigue strength even at small depths, and fracture surface analysis confirmed the dominant role of stress concentrators in crack nucleation. Quasi-static tests in inert and aggressive environments showed that aggressive media are especially detrimental when combined with high stress gradients. For 7075-T6, rotating bending tests and finite element analyses were used to investigate the effects of DLC coating deposition and impact damage in hourglass specimens. DLC coatings can generally improve wear and corrosion resistance, but their deposition may introduce residual stresses and material modifications that affect fatigue behavior. The coated specimens showed lower fatigue strength in the 2×10^5-10^6 cycle range, while their response became comparable to that of uncoated specimens in high cycle fatigue. Stress superposition analysis revealed a maximum tensile stress beneath the surface, consistent with the observed subsurface crack nucleation. In impact-damaged specimens, fatigue behavior is controlled by the combination of impact-induced residual stresses and cyclic bending stresses, and even a single impact can significantly reduce strength, as also shown in the literature for other impact scenarios. Integrating mechanical testing and FEM enabled the quantification of the effects of stress concentration, residual stresses, environment and surface modification, supporting robust assessment strategies for fatigue- and fracture-critical structures.