Insect exoskeletons inspire better building material

A design based on insect exoskeletons has helped a team of civil engineering researchers crack the balance between strength and damage tolerance in cement.

The project was led by Wenhui Duan, a professor in structural engineering at Monash University, and described in an article published in Nature Communications last month.

The tradeoff between a material’s strength – its ability to bear weight – and its ability to tolerate damage is a classic engineering dilemma.

High-strength materials are usually stiff and don’t change shape when loaded with weight, explains Wei Wang from Monash University, who is co-first author on the study.

“However, damage tolerance requires the material to deform under loading in order to dissipate energy.”

So-called ‘brittle materials’, like concrete, are typically strong but easily broken. If only a small area of concrete becomes cracked, the whole structure can quickly fail.

As in so many things, though, nature has already found a successful way to balance these competing factors.  

“If we regard evolution as a process of optimisation, that optimisation has been happening for millions of years,” says Wang’s co-first author Shujian Chen, a lecturer in structural engineering at the University of Queensland.

An insect’s exoskeleton – in particular, its segmented legs – is both strong and capable of absorbing a lot of energy, making it damage-tolerant.

“Fleas have an amazing skill allowing them to jump up to 150 times their own length – that’s like a human jumping over 300 metres,” says Wang. “It requires the exoskeleton to not only sustain a significant impact but also to absorb or release substantial energy.”

“We found that the exoskeleton of insects has an asymmetrical rotation mechanism that can achieve good strength and damage tolerance,” explains Chen.

Inspired by nature, the team got to work on developing a material design that would use this asymmetrical rotation to create a strong yet damage-tolerant construction material.

Their invention combines a 3D-printed polymer scaffold with cement to form a segmented honeycomb structure.  

Mechanical tests showed that the material had a high compressive strength – about 200% higher than cellular foam concrete.

“The amazing idea behind [this] breakthrough is actually to make [the material] weaker at some points,” Chen explains.

The creation of such controlled weak spots allows the material to undergo the same asymmetrical rotation as the insect exoskeleton.

The new design also means that if the material does get damaged, it will fail layer by layer rather than all at once as conventional concrete does.

“We can contain the damage within a particular region of material, while the rest of the structure can still maintain [its] integrity and most (around 80%) of [its] load-bearing capacity,” explains Duan.

With cement production currently contributing an estimated 8% of global carbon dioxide emissions, the new design highlights a promising pathway to creating safer and more durable building materials that will also help the environment.

Chen explains that because cement is brittle, engineers typically use 30% more material than is technically required in order to make the structure safer.

“So, if we can significantly reduce the amount of cement used, we can of course significantly reduce carbon dioxide emissions globally,” he says.

The concepts behind the design can be applied to other brittle materials, such as glass and ceramics.  

Duan also hopes the research will inspire more interest in civil engineering, which is typically seen as a little low-tech and perhaps less exciting than other engineering disciplines.

“This paper demonstrates the application of 3D printing, robotics, artificial intelligence – how these emerging technologies can transform civil [engineering] so we can prepare our next generation of engineers,” he says.