Diabolical ironclad beetle adds inspiration to structural strength

This is the claim of a team of engineers at the University of California, Irvine (UCI) and Purdue University who believe the beetle – which can survive a load 39,000 times its body weight - could inspire new materials. Their findings are published in Nature.

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The study found that the diabolical ironclad beetle's toughness lies in two armour-like elytron that meet at a suture running the length of the abdomen.

In flying beetles, the elytra protect wings and facilitate flight. The diabolical ironclad beetle does not have wings, so its elytra and connective suture help to distribute an applied force more evenly throughout its body.

"The suture kind of acts like a jigsaw puzzle. It connects various exoskeletal blades…in the abdomen under the elytra," said Pablo Zavattieri, Purdue's Jerry M. and Lynda T. Engelhardt Professor of Civil Engineering.

This jigsaw puzzle comes to the rescue in several different ways depending on the amount of force applied, Zavattieri said in a statement.

To uncover these strategies, a team led by UCI professor David Kisailus first tested the limits of the beetle's exoskeleton and characterised the various structural components with CT scans.

Using compressive steel plates, UCI researchers found that the diabolical ironclad beetle can take on an applied force of about 150N before the exoskeleton begins to fracture.

Zavattieri's lab followed up these experiments with computer simulations and 3D-printed models that isolated certain structures to better understand their role in saving the beetle's life.

All these studies combined revealed that when under a compressive load, the diabolical ironclad beetle's jigsaw-like suture offers two lines of defence.

First, the interconnecting blades lock to prevent themselves from pulling out of the suture like puzzle pieces. Second, the suture and blades delaminate, which leads to a more graceful deformation that mitigates catastrophic failure of the exoskeleton. Each strategy dissipates energy to circumvent a fatal impact at the neck, where the beetle's exoskeleton is most likely to fracture.

Even if a maximum force is applied to the beetle's exoskeleton, delamination allows the interconnecting blades to pull out from the suture more gently. If the blades were to interlock too much or too little, the sudden release of energy would cause the beetle's neck to snap.

"An active engineering challenge is joining together different materials without limiting their ability to support loads. The diabolical ironclad beetle has strategies to circumvent these limitations," said David Restrepo, an assistant professor at the University of Texas at San Antonio who worked on this project as a postdoctoral researcher.

In the gas turbines of aircraft, for example, metals and composite materials are joined with a mechanical fastener that adds weight and introduces stress that could lead to fractures and corrosion.

"These fasteners ultimately decrease the performance of the system and need to be replaced every so often. But the interfacial sutures of the diabolical ironclad beetle provide a robust and more predictable failure that could help solve these problems," said Maryam Hosseini, who worked on this project as a Ph.D. student and postdoctoral researcher.

UCI researchers built a carbon fibre composite fastener mimicking a diabolical ironclad beetle's suture. Purdue researchers found through loading tests that this fastener is as strong as a standard aerospace fastener, but significantly tougher.

"This work shows that we may be able to shift from using strong, brittle materials to ones that can be both strong and tough by dissipating energy as they break. That is what nature has enabled the diabolical ironclad beetle to do," Zavattieri said.