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.

Cracked! New insights into how glass breaks

New Delhi: 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. 

This discovery led them to critically look at various existing theories of crack propagation, each of which was disqualified owing to inconsistencies in duration estimates or travel velocity.

The team has proposed a new theory in light of their findings to explain the nano-trenches. They believe that rupture of atomic bonds during fracture produces substantial heat, resulting in local melting and formation of ripples. Their next step is to test whether this hypothesis is true.

"Understanding fracture is very difficult. It occurs at time scales we cannot fathom. Understanding the mechanisms is key, it is a chapter that has not been written," Ramasubramanian says.

Until the understanding of BMG fracture is furthered and improvements sought, metallic glass will only be limited to cosmetic applications.

"Scientists had a lot of plans for metallic glass -- they thought it would replace all structural materials," says Lakshmi Narayan Ramasubramanian, a doctoral student with Prof. Upadrasta Ramamurty from the Department of Materials Engineering, IISc and an author on the study. However, the road-block to its wider use has been its brittleness; cracks rapidly zip through the BMG material when force is applied.

This resembles cracking of china, or shattering of glass.

There is a huge dispute between various theories proposed to explain how cracks propagate in BMGs, Ramasubramanian says. "We are trying to find a suitable compromise for this dispute," he says.