3D-printed polymers for lightweight structures

“Our main finding is that structural elements that aren’t directly resisting loading are actually very important for fatigue failure”Christopher Hernandez

Researchers at Cornell University, Purdue University and Case Western Reserve University have shown how a “beam” in human bone is able to endure wear and tear over its lifetime, an insight that could help the development of durable 3D-printed lightweight architected materials in the construction and aerospace industries, as well as improve how we treat patients suffering from osteoporosis.

Our bones contain internal columns and beams that determine how long they last – with columns carrying most of the load and the beams connecting the columns, properties that 3D-printed buildings also possess. However, as reported in the Proceedings of the National Academy of Sciences [Torres et al. Proc. Natl. Acad. Sci. U.S.A. (2019) DOI: 10.1073/pnas.1905814116], this study demonstrated that mimicking the beams and making them around one-third thicker could produce an artificial material that lasts up to 100 times longer.

Despite being quicker and less expensive to produce, increased usage of 3D-printed houses and offices requires them to be sufficiently robust to survive natural disasters. As this can be resolved by redesigning the internal structure, or “architecture”, of the cement used, the team have been researching architected materials inspired by nature to improve on their functionality and key properties.

Bones are so long-lasting as they are made from trabeculae, a spongy structure that is a network of interconnected struts that act as columns and beams, with the denser the trabeculae the more durable the bone. However, disease and aging have an effect on their density, with the horizontal struts being lost first, increasing the chances of breakage due to continual wear and tear. The researchers discovered that while the vertical struts contribute to the stiffness and strength of bone, it is really the horizontal struts that work to increase its fatigue life and are crucial for durability.

Simulations of the bone microstructure were carried out under cyclic loading to identify if the strains would be concentrated in the horizontal struts, and by increasing the struts’ thickness they were able to mitigate some of these strains. On applying loads to 3D-printed polymers, the findings were confirmed as the thicker the horizontal struts the longer the polymer would endure the load.

As thickening the struts did not significantly increase the mass of the polymer, the design could lead to more resilient lightweight materials. As senior author Christopher Hernandez said, “Our main finding is that structural elements that aren’t directly resisting loading are actually very important for fatigue failure”. The team now hope to assess other loading modalities as well as test the application of the fatigue estimates in real-world structures.