Scientists at Northeastern University have discovered the mechanism that causes cracks to behave strangely when they spread very rapidly in brittle materials. The results of this study will help researchers better understand how fragile materials, such as glass, ceramic, polymers and bone, break – often catastrophically – and how to design materials to avoid failure.

The team was led by Alain Karma, professor in the Department of Physics, Arts and Sciences, and included his postdoctoral research associate Chih-Hung Chen and Eran Bouchbinder, a professor at the Weizmann Institute of Science in Israel. The scientists report their findings in a paper in Nature Physics.

Karma's goal was to understand how things break, since a primary way materials fail is through crack propagation, which has long been an issue in materials science, construction and product development. More specifically, the collaborative research team wanted to understand how the mechanical properties of regions of high stress concentration around the edges of a crack affect the crack dynamics.

"While straight cracks can, in principle, race through a material as fast as the speed of sound, they never reach that speed for reasons that have remained elusive," said Karma. "We have shown that this is because cracks become inherently unstable when their speed is sufficiently high. Instability causes the crack tip to wobble from side to side and trace out a wavy path through the material.

"We believe that the non-linear relationship between force and deformation is at the root of micro-branching instabilities, and we think we can crack that problem."Alain Karma, Northeastern University

"This instability has been completely missed by conventional theories of fracture, which all assume that the relationship between stretch and force inside a material is linear, meaning that doubling the force doubles the amount of stretch. Our work shows that this assumption breaks down near the crack tip and explains how the nonlinear relationship between stretch and force produces oscillations with a well-defined period that can be related to material properties."

Through this research, Karma and his colleagues developed a novel theory to help researchers predict, through large-scale computer simulations, the dynamics of a crack under varying conditions, which has the potential to help explain why and how certain materials fail.

With success in this study, Karma hopes to move on to related work. "This study used very thin sheets of quasi-2D materials. We plan to extend this study to 3D bulk materials. In bulk, the instability that prevents cracks from breaking at the speed of sound happens at a lower crack velocity than in 2D but the mechanism is not understood," he said.

To elucidate this mechanism, the team plans to investigate the 3D phenomenon of micro-branching, where the main crack splits into many micro-cracks, to understand its origins in bulk samples of brittle materials. "We believe that the non-linear relationship between force and deformation is at the root of micro-branching instabilities, and we think we can crack that problem," Karma said.

This story is adapted from material from Northeastern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.