Fatigue fracture behavior of additively manufactured titanium alloy superstructures under compressive loading

Laser Powder Bed Fusion (LPBF) additively manufactured superstructures are highly promising for aerospace vehicles due to their excellent weight reduction performance and design flexibility, fulfilling the demand for high-performance, lightweight, and integrated components. However, surface roughness and internal porosity introduced by additive manufacturing tend to serve as initiation sites for fatigue cracks, potentially leading to premature failure of the lattice structures. To investigate effects of surface roughness and porosity on fatigue performance, Body Centered Cubic (BCC) lattice specimens were designed and fabricated by additive manufacturing. Fatigue tests under five different loading levels were conducted to obtain the corresponding Stress-Number of cycles (S-N) curves. A semiempirical formula was used to calculate the stress concentration factor introduced by surface roughness. Additionally, the concept of effective load-bearing area was applied to account for the influence of both porosity and surface roughness. The accuracy of this method was verified through finite element simulations based on 3D reconstructed models, and the Neuber-Kuhn formula was used to estimate the fatigue limit. Based on the effective area correction, a fatigue S-N curve for a single strut was established, then a mapping method from the single-strut S-N curve to the lattice S-N curve was developed, and a corrected S-N curve was proposed. It is shown that the model incorporating effective load-bearing area correction more accurately reflects the coupled effect of surface roughness and internal porosity, and thereby it can improve the predictive accuracy of the fatigue limit and fatigue life of the lattice structures. The periodic boundary conditions in one unit lattice can be built and effectively simulate stress concentration in the whole structure. The corrected S-N curve is used to predict the fatigue life of the lattice, and its results fall within a scatter band of factor 3.