Cracked! New insights into how glass breaks

Metallic glasses can be made in bulk form, utilised for structural applications and can also offer the possibility to conduct fundamental studies on how glasses crack, claims a new study.
While most glasses are made with silica base, it is possible to make metallic alloys into glasses as well.
In normal circumstances, metals crystallise, i.E. The atoms in them would organise into nice periodic and repeating structures. But, if they are cooled rapidly, the liquid structure, where the atomic positions are random, can be 'frozen' in.
Professors U Ramamurty and R Narasimhan at the Indian Institute of Science (IISc), Bangalore have been looking into this and identified the mechanisms for both ductile and brittle glasses through detailed experiments, complemented with computer simulations and careful autopsy of fractured surfaces, says a Gubbi Labs release.

The researchers gathered evidence by closely observing the fractured surface of a brittle BMG rod. They found patterns indicative of fracture due to brittleness. The fracture starts with the formation of a cavity, which expands to create cracks; the cracks in turn break out as finer channels as they travel away from the origin. These spread outwards, weave through the material and die out once they lose energy.
Apart from the typical brittle fracture features, they also observed 'Wallner lines' - arcs which form on the fractured surface .

These arcs are created when the crest of expanding cracks interacts with vibrations generated in the material during rapid fracture. Using the geometry of Wallner lines, researchers estimated the maximum velocity of crack propagation to be an "astounding 800 m/s".

When the researchers further zoomed into the fracture surface, they observed regular nano-scale trenches cutting across the material, formed during crack extension.
Using the distance between these periodic trenches and estimates of crack velocity, they could easily calculate the duration of crack propagation to be fraction of a second.
"We have been able to identify the time scale of fracture which was never done before. This is our biggest finding," says Ramasubramanian.