What happens at the moving edge of a crack?

August 25, 2017
What happens at the moving edge of a crack?
The trajectory of a crack tip, showing one cycle of oscillation. The horizontal wavy line shows the trajectory of the tip of the crack. Credit: Weizmann Institute of Science

It is said that a weak link determines the strength of the entire chain. Likewise, defects or small cracks in a solid material may ultimately determine the strength of that material – how well it will withstand various forces. For example, if force is exerted on a material containing a crack, large internal stresses will concentrate on a small region near the crack's edge. When this happens, a failure process is initiated, and the material might begin to fail around the edge of the crack, which could then propagate, leading to the ultimate failure of the material.

What, exactly, happens right around the edge of the crack, in the area in which those large stresses are concentrated? Prof. Eran Bouchbinder of the Weizmann Institute of Science's Department of Chemical Physics, who conducted research into this question with Dr. Chih-Hung Chen and Prof. Alain Karma of Northeastern University, Boston, explains that the processes that take place in this region are universal – they occur in the same way in different materials and under different conditions. "The most outstanding characteristic we discovered," says Prof. Bouchbinder, "is the nonlinear relationship between the strength of the forces and the response taking place in the material adjacent to the crack. This nonlinear region, which most studies overlook, is actually fundamentally important for understanding how cracks propagate. Most notably, it is intimately related to instabilities that can cause cracks to propagate along wavy trajectories or to split, when one would expect them to simply continue in a straight line."

By investigating the forces at play near the crack's edge, Prof. Bouchbinder and his colleagues developed a new theory – published recently in Nature Physics – that will enable researchers to understand, calculate, and predict the dynamics of cracks under various physical conditions. This theory may have significant implications for materials physics research and for understanding the ways in which materials fail.

What happens at the moving edge of a crack?
(l) A sequence of snapshots revealing the onset of the wavy (oscillatory) instability of ultra-fast cracks as obtained from numerical solutions of the new theory, in quantitative agreement with experiments. (r) An experiment in brittle polyacrylamide gel agrees with the theory. The experiment was performed in the laboratory of Prof. Jay Fineberg of the Hebrew University of Jerusalem. Credit: Weizmann Institute of Science

Islands of Softness

Exploring a different topic, in a paper that recently appeared in the Proceedings of the National Academy of Sciences of the United States of America (PNAS), Prof. Bouchbinder and a group of colleagues investigated the fundamental properties of the "glassy state" of matter.

The glassy state can exist in a broad range of materials if their liquid state is cooled quickly enough to prevent them taking on an ordered, crystalline state. Glasses are thus disordered, or amorphous, solids and include, for example, window glass, plastics, rubbery materials, and amorphous metals. Even though these materials are all around us and find an enormous range of applications, understanding their physical properties has been extremely challenging, owing, in large part, to the lack of tools for characterizing their intrinsically disordered structures and characterizing how these structures affect the materials' properties.

Dr. Jacques Zylberg of Prof. Bouchbinder's group, Dr. Edan Lerner of the University of Amsterdam, Dr. Yohai Bar-Sinai of Harvard University (a former PhD student of Prof. Bouchbinder's), and Prof. Bouchbinder found a way to identify particularly soft regions inside glassy materials. These "soft spots," which are identified by measuring the local thermal energy across the material, were shown to be highly susceptible to structural changes when is applied. In other words, these soft spots play a central role when glassy deform and irreversibly flow under the action of external forces. The theory developed by the researchers thus brings us a step closer to understanding the mysteries of the glassy state of matter.

Explore further: Understanding brittle crack behaviors to design stronger materials

More information: Jacques Zylberg et al. Local thermal energy as a structural indicator in glasses, Proceedings of the National Academy of Sciences (2017). DOI: 10.1073/pnas.1704403114

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not rated yet Aug 27, 2017
This theory and analysis doesn't seem to take into account that the materials where cracking is most significant are metals having crystalline structure with intrusions. The cracks tend to be influenced by the places they can most easily spread, namely the boundaries. metal materials are not homogeneous!
not rated yet Aug 28, 2017
Whilst not downplaying the significance of cracking in metals and the relevance of their crystalline nature I would also suggest that cracking in non-crystalline materials such as concrete and glass are also important areas for study and analysis. Not to mention materials such as fibre-reinforced plastics, (fibre-glass, carbon fibre, basalt fibre etc). Beginning with amorphous materials seems like a good place to start.
And as an aside, in my previous life as a bicycle frame builder/repairer I also saw both steel and aluminium alloy tubing which failed with a crack propagating from a stress-riser on the underside where the tube was under tension, growing around the sides of the tube and then branching on each side with one branch per side only growing to about 3 mm. The other branches continued to grow until either the tube failed when they met on the top of the tube, or the crack was detected and the frame repaired or replaced.
Aug 28, 2017
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