Anyone who’s ever stuck a pin into a balloon knows well what happens next: The tiny puncture swiftly grows into one or more fractures that propagate across the balloon’s surface. Captured by high-speed photography, as shown in figure 1, the phenomenon is spectacular.
Inflated balloons store so much energy in their stretched latex that they invariably burst when pierced. But elastic material failure isn’t always so certain. Car tires, soft medical implants, and O-ring seals all routinely experience stresses and strains that may or may not be enough to cause them to rupture. The safe use of elastic materials depends on understanding just how much deformation they can withstand before they break. In other words, how much energy does it take to propagate a crack in a stretchy substance?
That complicated problem involves physics on multiple size scales, from the macroscopic bulk down to individual molecules. For decades, models...
