Understanding fatigue of additively manufactured TPMS metallic metamaterials: Experiments and modeling

Abstract

Triply periodic minimal surfaces (TPMS) are specific types of lattice structures that can only be fabricated using the geometric freedom of additive manufacturing (AM). These structures are gaining traction in fields, such as biomedical and aerospace industries, due to their reduced stress concentrations and increased surface area to volume ratio compared to traditional strut-based lattices. Prior to acceptance into industry design applications, it is vital to understand the fatigue behavior of such porous structures under various loading conditions. The purpose of this study is to characterize and predict the fatigue behavior of Ti-6Al-4V TPMS lattice structures under a variety of loading conditions. Diamond and gyroid unit cell specimens of 50 % and 70 % porosity are fabricated for testing. Finite element analysis (FEA) and X-ray computed tomography (XCT) are also conducted for stress distribution and geometrical accuracy analysis. Various modeling techniques are used to correlate fatigue data of solid specimens and the lattice structures. It is found that popular methods from literature based on monotonic properties are not successful at correlating solid and porous data. This study expands to more robust local stress models, such as fatigue notch factor, that result in significantly improved life predictions but can be computationally expensive. A new model based on nominal applied cyclic loads and stress intensity factor is also proposed that produces comparable results to local stress models with reduced computational expenses.