The mechanical behavior of additively manufactured porous biomaterials has recently received increasing attention. While there is a relatively large body of data available on the static mechanical properties of such biomaterials, their fatigue behavior is not yet well-understood. That is partly because systematic study of the fatigue behavior of these porous biomaterials is time-consuming and expensive due to the large number of involved factors. In the current study, we propose a computational approach based on finite element method that could be used to predict the fatigue behavior of porous biomaterials given their type of repeating unit cell, dimensions of the unit cell, and S-N curve of the parent material. We applied the proposed approach to predict the fatigue behavior of porous titanium alloy (Ti-6Al-4V) biomaterials manufactured using selective laser melting based on the rhombic dodecahedron unit cell and compared our computational results with experimental observations from one of our recent studies. The evolution of the displacement, elastic modulus, and number of failed struts vs. the number of loading cycle followed a two-stage pattern. In the first stage, there was a relatively slow rate of change in the above-mentioned variables, while they changed very rapidly in the second stage. That compares to the behavior observed in our experimental study. The computationally predicted S-N curve well matched the experimental observations for stress levels not exceeding 60% of the yield stress of the porous structures. For higher stress levels, the presented approach substantially underestimated the fatigue life of the porous structures. The effects of the irregularities caused by the additive manufacturing process on the fatigue behavior of the porous structures were also studied. It was found that those irregularities substantially decrease the fatigue life particularly for lower stress levels.
Honeycombs resemble the structure of a number of natural and biological materials such as cancellous bone, wood, and cork. Thick honeycomb could be also used for energy absorption applications. Moreover, studying the mechanical behavior of honeycombs under in-plane loading could help understanding the mechanical behavior of more complex 3D tessellated structures such as porous biomaterials. In this paper, we study the mechanical behavior of thick honeycombs made using additive manufacturing techniques that allow for fabrication of honeycombs with arbitrary and precisely controlled thickness. Thick honeycombs with different wall thicknesses were produced from polylactic acid (PLA) using fused deposition modelling, i.e., an additive manufacturing technique. The samples were mechanically tested in-plane under compression to determine their mechanical properties. We also obtained exact analytical solutions for the stiffness matrix of thick hexagonal honeycombs using both Euler-Bernoulli and Timoshenko beam theories. The stiffness matrix was then used to derive analytical relationships that describe the elastic modulus, yield stress, and Poisson’s ratio of thick honeycombs. Finite element models were also built for computational analysis of the mechanical behavior of thick honeycombs under compression. The mechanical properties obtained using our analytical relationships were compared with experimental observations and computational results as well as with analytical solutions available in the literature. It was found that the analytical solutions presented here are in good agreement with experimental and computational results even for very thick honeycombs, whereas the analytical solutions available in the literature show a large deviation from experimental observation, computational results, and our analytical solutions.
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