Researchers from Purdue University have 3Dprinted cement paste that becomes stronger under pressure, based on the structure of arthropod shells.
Bouligand structures are tiny microstructures composed of a series of layered fibers, with each layer rotated slightly from the one above it, forming a kind of "spiral staircase" (see above). Materials containing Bouligand structures, like the inner cuticle of arthropod shells, are stronger because of the way they crack. The cracks grow in twisted patterns following the direction of the fibers, and the material’s structure has “sacrificial links” that can be broken without weakening the structure of the overall system. As a result, the layers reorient, allowing the material to achieve greater toughness. Essentially, the “crackability” of the material actually becomes a strength, spreading the damage over more of the material to prevent localized damage.
Civil engineering PhD student Mohamadreza Moini and his team used these structures in their new 3D-printable cement paste, an essential ingredient in concrete and mortar. "3D printing cement-based materials provides control over their structure, which can lead to the creation of more damage and flaw-tolerant structural elements like beams or columns," Moini said, in a Purdue University statement.
The researchers printed their Bouligand structures with a custom-built printer that combined a 3Dprinter more typically used for thermoplastics, a stepper motor capable of the correct extrusion rate, and an aluminum nozzle holder on the gantry. To test the resulting material's toughness, the researchers looked at the structures' flexural strength (how much stress it can take before it yields in a flexure test), and used a stereo microphone device to capture initial cracking noises. The researchers were able to replicate the properties of organic Bouligand structures, preventing one dominant crack by forming multiple smaller ones.
The researchers also printed structures with different pitch angles (the angle at which each layer's fibers are aligned to the layer underneath it) to see what difference it made in crack formation. They found that a delicate balance was needed; large pitch angles meant that the structure was more likely to form one dominant crack through the filaments, while small pitch angles meant that they were less likely to promote local hardening in the material.
The success of the Bouligand architectures made with cement paste is particularly useful because the material is brittle and prone to cracking. Or, as study author Jan Olek said: "Nature has to deal with weaknesses to survive, so we are using the 'built-in' weaknesses of cement-based materials to increase their toughness.”
Currently, the team is planning to research other ways that cement could be 3Dprinted to achieve resilience.