Cracking the Crack Code

The seemingly ordinary phenome-non of cracks spreading has always been something of a mystery to scientists. “People assume everything’s known about it, but it’s actually very complicated,” says Markus Buehler, a researcher in MIT’s Department of Civil and Environmental Engineering.

Theories of how cracks spread through brittle materials have histori-cally looked at the macroscale and ignored what goes on at the atomic level, in part because, until recently, computers weren’t powerful enough to run atomic-level simulations. But cracking is all about breaking atomic bonds. Buehler and Huajian Gao of Brown University and the Max Planck Institute for Metals Research in Stuttgart, Germany, published a paper in the January 19 issue of Nature detailing a model of how cracks happen that they think will be applicable to materials such as glass and polymers.

They found that near the leading edge of a crack, the bonds between individual atoms weaken as force is applied, then break. The greater the force, the faster this happens. At slow cracking speeds, cracks spread in a straight line, breaking a material into two pieces with smooth surfaces. As the force increases, the cracks speed up; when they reach a certain speed, they become unstable, wriggling left and right and leaving a rough fracture.

Older theories of cracking held that cracks could propagate no faster than the speed at which sound could travel through the material – a few kilometers per second in the case of glass – and that instability should set in when a crack reaches about 74 percent of that speed. But that didn’t jibe with what researchers were seeing in experiments; in the lab, cracks started going wild at only 30 to 40 percent of the speed of sound.

Buehler and Gao’s new model matches those experimental findings. The researchers also studied crack speeds in materials like rubber, which stiffen near the leading edge of a crack as molecules stretch apart. The stretching allows the energy from the pressure that drives the crack to spread very rapidly, so that the crack can propagate 10 to 15 percent faster than the speed of sound. Buehler says this discovery was almost like finding out that information can travel faster than the speed of light.

Having a model that more accurately describes how cracks spread could help scientists control cracking, possibly enabling them to design structures, from auto bodies to office towers, that are more durable. Engineers might inscribe nanometer-scale patterns in a material to steer any cracks away from load-bearing areas, for instance. -Buehler and Gao’s theory might also make models of earthquakes, which are fractures on a large scale, more accurate. Or it might lead to more–effective cutting tools for the manufacture of devices that need smooth surfaces, such as certain types of lasers.

Min Zhou, associate professor of mechanical engineering and materials science and engineering at the Georgia Institute of Technology, finds the work exciting. “Just imagine the importance of knowing how – and how fast – earthquake faults, fatigue-initiated cracks in airplane fuselage, and cracks in pressurized space stations propagate,” he says. “The technological and economic impact can be extremely significant.”

Though this model offers some generalizations about crack propagation, it can’t predict precisely how cracks will spread in specific materials, each of which has its own complex pattern of atomic structures. But Buehler is already building on his work; a paper in the March 10 issue of Physical Review Letters, which he coauthored with a Caltech team, details an atomic model of cracking in silicon, where defects can affect computer chip performance.

Ultimately, Buehler hopes that his work will improve materials. “Once we understand how they break, why they break, we can take action to prevent things from breaking,” he says.