Fatigue life calculation of lattice structure by using strain based method

Today, with the advancements of additive manufacturing techniques, to benefit from the great lightweight properties, the use of lattice structures has been focused and many studies have been carried out to reveal the static strength of the lattice structures. Although it is as important as the static strength, the methods that will carry out the fatigue behavior of the lattice structures produced by additive manufacturing methods under dynamic loads are not yet mature. It is seen from the literature that the studies related to this field are in general based on experimental investigations. A small number of studies that use numerical analyses are seen to be based on the stress-based life determination methods. However, it has been observed that the experimental studies are costly, and the stress-based numerical calculations are not efficient particularly in the low cycle fatigue life region. For this reason, in this study, the fatigue life determination of additively manufactured lattice structures from Ti6Al4V material is studied by using the strain-based fatigue life numerical method. In the methodology used in this study, first, the unit cell geometry of the lattice structure, for which the fatigue life is determined, is modeled. To obtain the stress-strain curve of this geometry, a finite element analysis is carried out under quasi-static compression load. The stress values, which are determined by gradually decreasing starting from the yield strength calculated from the stress-strain data obtained from this analysis, are defined to be the maximum stress values for the fatigue life analysis. For each stress value, a repeated loading profile, which will be applied to the lattice structure, is created. For each loading profile, the fatigue life analysis has been carried out by using the strain-based Brown-Miller method. Thus, the fatigue life is calculated for each stress value and the fatigue life curve of the lattice structure is obtained. When compared to the experimental fatigue life curve results presented in the literature for the same lattice structure, higher fatigue life results are observed to be obtained from the numerical fatigue life analysis in this study. The reason for the difference is considered to be the surface defects on the lattice structure, which occurs due to the additive manufacturing process. The stress concentration factor due to these defects is determined using the finite element method. As a result of this study, an analytical equation is presented to calculate the stress concentration factor that will allow estimating the fatigue life of all lattice structures made of Ti6Al4V alloy and selective laser melting directly using the life data of the same size bulk metal. By considering the stress concentration factor value, the life analyses are repeated for different lattice types and compared with the experimental results in the literature. According to these comparisons, it is observed that the life analysis of the lattice structures by using the presented calculation method of stress concentration factor and the strain-based fatigue life method with the unit cell both produces more accurate results when compared with the stress-based methods and can be performed in agreement with the experiments for different lattice types.