Rigid materials break more easily than floppy ones. This simple observation allows to predict and control the width of cracks. Theoretical understanding of how materials break is useful in for example the production of cars or screens. Publication in PNAS.

If you are unlucky enough to have broken a limb at some point in your life, did you wonder why it was the bone that broke, and not the skin? After all, the skin took the first impact. From our intuition we know that rigid materials break more easily than soft ones.

Crack
The research group of theoretical physicist Vincenzo Vitelli at Leiden University and his colleagues from the Nagel Lab have exploited this phenomenon to design materials that resist breaking. A rigid material has many bonds and generates a narrow crack in an approximately straight line (see figure 1a). A material composed of fewer bonds is softer, and produces a diffuse failure region: a crack that can be as wide as the sample size (see figure 1b). When that happens the material can resist catastrophic failure thanks to its softness. To discover this, the physicists simulated and built artificial structures—called metamaterials—with tunable numbers of bonds that break in unusual ways. They publish their findings in the journal PNAS.

Einstein
Earlier, Vitelli’s group published a paper in Nature Materials in collaboration with the Irvine Lab, on the path that a crack follows as it propagates in a curved thin layer. They discovered a remarkable parallel with Einstein’s theory of general relativity, where a ray of light is bent by the curvature of space-time. In the case of fracture, the crack path is bent by the curvature of the underlying surface (see figure 2).
‘When you have a theory on how things break, you can use it to control the properties of real materials,’ says Vitelli. ‘This is potentially useful. For example, you may want to deflect a crack from a portion of a given structure, like the centre of your glasses. Or, to prevent breaking altogether, you can design floppy metamaterials.’

Publication

'The role of rigidity in controlling material failure', Michelle M. Driscoll, Bryan Gin-ge Chen, Thomas H. Beuman, Stephan Ulrich, Sidney R. Nagel, and Vincenzo Vitelli, PNAS

Figure 1. (a)Simulations show a rigid structure with many bonds that generates a straight, narrow crack when broken. (b)In a soft structure with few bonds, the broken bonds (shown in color) are spread over a crack that can be as wide as the system size. (c)An experimental realization of a soft structure, using cellular metamaterial.

Figure 2. (a)Theoretical calculations of the crack path (shown as a black line) on a curved surface on which a crystalline monolayer is deposited. The curved spots are plotted in red and blue. The crack path is bent by these bumpy spots, just like light is bent by the curvature of space-time. (b)Experimental realization of a bent crack in curved space.