3D Printed Oyster Shell
In 2012, Leon Dimas and I simulated the crack confusion observed in natural "brick & mortar" structured materials: teeth, nacre and antler, and especially oyster shells. Under a microscope, these materials show parallel layers of strong but brittle material connected by weak but extensible mortar. By combining the two materials' pros and cons in this layered structure, you get an anisotropic bulk material that sacrifices strength for toughness, i.e. the area beneath the stress/strain curve. Instead of energy being expended in crack propagation, energy is diverted to many other smaller deformations and delaminations. Crack propagation is slowed, creating so-called "crack confusion", and the bulk material is able to reach a higher ultimate stress and larger strain than would be expected.
To study this, we compared 2D simulations to planar 3D-printed samples in the same tensile loading conditions. MIT labs are very cooperative in sharing equipment (when the owner isn't using it), and we found a Stratasys Objet 3D printer that can contiguously print a solid white polymer, a black elastomer, or any mix of the two you set. After printing and testing some dogbone samples to get the material properties (strong but brittle, weak but extensible) to match the ratios in 2D simulation, we printed a representative brick & mortar structure.
Without a clear path horizontally, the crack tip is blunted, then switches to shear deformation and failure between the bricks. You can see space open up between the short ends of the bricks, the right side peel open, then the two cracks meet in the middle.
Since the structure was based off of that of oyster shell, I called the Legal Seafood in Kendall Square, and they gave me a bag of shucked oyster shells. My labmates were a little confused by the smell when they walked in and saw my photoshoot, but those photos went on to some real digital journalism:
A comparison of the FEA model (left) to the 3D printed model (right) at similar points of bulk deformation and showing similar failure mechanisms near the crack tip.