Harvard team claims 'crack resistance' advance in reinforced rubber

Boston, Massachusetts – Researchers at Harvard University have reported new research findings that could significantly enhance the ability of particle-reinforced rubber materials to bear high loads and resist crack-growth over repeated use.

The team, led by Zhigang Suo, professor of mechanics and materials at School of Engineering and Applied Sciences (SEAS), found that a 'multi-scale approach' could surprisingly improved the performance, longevity and sustainability of rubber products such as tires.

Rubber is generally reinforced by rigid particles such as carbon black and silica for a range of applications, including tires, hoses, and dampeners, set out a release issued 15 Dec by the US university.

The particles greatly improve the stiffness of rubbers, but not the resistance to crack growth when the material is cyclically stretched, i.e. fatigue threshold.

Indeed, according to the university researchers, the fatigue-threshold of particle-reinforced rubbers “has not improved much since it was first measured in the 1950s”.

The Harvard team had previously succeeded in "markedly increasing" the fatigue-threshold of rubbers by lengthening polymer chains and densifying entanglements.

This time, they added silica particles to their highly-entangled rubber molecules, assuming – as widely understood – that the particles would increase stiffness but not affect fatigue-threshold.

The actual research findings came as "quite a surprise,” reported Jason Steck, a former graduate student at SEAS and co-first author of the paper.

“We did not expect that adding particles would increase the fatigue-threshold, but we discovered that it increased by a factor of ten.”

In the Harvard team’s material, the polymer chains are long and highly entangled, while the particles are clustered and covalently bonded to the polymer chains.

“As it turns out,” said Junsoo Kim, a former graduate student at SEAS and co-first author of the paper, the material “deconcentrates” stress around a crack over two length scales: the scale of polymer chains, and the scale of particles.

“This combination stops the growth of a crack in the material,” he added.

The team demonstrated their approach by cutting a crack in a piece of their material and then stretching it tens of thousands of times.

In these experiments, the crack never grew, according to the Harvard statement.

“Our approach of multiscale stress deconcentration expands the space of material properties, opening doors to curtailing polymer-pollution and building high-performance soft machines,” said Suo.

Traditional approaches, said co-author Yakov Kutsovsky, “missed these critical insights of using multi-scale stress deconcentration to achieve high performance elastomeric materials for broad industrial uses.”

According to Kutsovsky, the design principles can be applicable to a wide range of industries, including high-volume applications such as tires and industrial rubber goods, as well as emerging applications such as wearable devices.