Honeycomb structures can absorb great amounts of mechanical energy, but with one major drawback. Traditional honeycomb materials deform when they absorb their peak energy, making them one-use. Think of the traditional bicycle helmet, which cushions the head with a honeycomb polymer foam structure; but it’s a one-use item. Like the helmet I was wearing when I went down on my bike in April—you have to replace it each time it’s in a crash.
But in a study just published (as a provisional PDF) in Integrating Materials and Manufacturing Innovation, Dixon M. Correa, Carolyn Conner Seepersad, and Michael R. Haberman at the University of Texas at Austin describe a model of honeycomb materials they developed that includes negative stiffness in their construction and design, which allows the cells in the honeycomb to return to their original shape, making these materials reusable.
Curves and negative stiffness
Correa et al. designed honeycomb cells using different beam and wall shapes to combine the strengths of each to create this resilient structure. The figure here shows the geometry of what they designed. As they write:
All of the features in the negative stiffness honeycomb structure shown in [the figure] have a specific purpose. The double concentric beams are utilized to constrain the beams to transition from one first-mode-buckled shape to another via the third buckling mode, rather than the second mode, which is known to significantly reduce the force threshold of the beam and the magnitude of its negative stiffness. The flat, horizontal walls constrain the horizontal expansion of the unit cell upon application of in-plane compression, thereby enabling snap- through-like behavior. Chamfers near the intersection of the horizontal and vertical walls help prevent twisting of the cell walls during loading.
They conclude by noting that this new material offers great opportunities for further investigation, including verifying the computational materials model with experimentation (some of which the authors have already conducted).
You can find the entire study online here.