A tank beetle reveals secrets

This beetle cannot be crushed. (Image: David Kisailus / UCI)

It even survives if a car runs over it: Researchers report on a Beetle whose astonishingly stable tank construction could inspire the development of technical structures with increased resilience. According to the investigations, the secret of the robust crawler are ingenious “puzzle piece elements” that form seam structures that ensure the stability of its shell. This concept can also be imitated technically, the study shows.

It is well known that many beetles protect themselves from the stresses of their respective way of life with a hard shell. The exoskeleton consists of resistant layers of the fibrous material chitin and a protein matrix. This concept was taken to extremes by the North American Phloeodes diabolicus, report the researchers working with David Kisailus from the University of California in Riverside. In order to withstand the pressure of the beaks and mouths of predators, the two centimeter long crawler has developed an exoskeleton that is one of the hardest and most resilient structures in the biological world.

The scientists were able to impressively document this as part of their study using squeeze experiments. The beetle’s shell can therefore still withstand a force that is roughly 39,000 times its body weight. He cannot be trodden on with his shoe and even after being run over by a car tire he happily crawls on. The scientists then analyzed what lies behind this astonishing resilience through microscopic examinations of sections of the armor and through spectroscopy.

A cleverly closed slot

As they explain, the basis of the Phloeodes beetle’s stability is a feature that distinguishes it from other species of beetle: it does not have any hinged cover wings (elytra). With others

The puzzle piece elements in the seam of the tank. (Image: Jesus Rivera / UCI)

Beetles can open these two elements on the insect’s back to reveal the wings below for flying. The Phloeodes beetle, which lives on the ground, has given up its ability to fly in the course of evolution in favor of its resilience: The two cover wings are united by a solid seam to form a single and therefore more resistant protective shield.

The ingenious connection concept of the seam itself plays a key role in the resistance of the armor to pressure loads, the structural analyzes revealed. The seam connects the two parts of the wing covers via bulge structures that are reminiscent of interlocking puzzle pieces. How these compounds react to pressure loads, the researchers investigated using electron microscopy in special devices. One could assume that these structures break when the maximum load is exceeded in the “neck area” of the bulging part of the puzzle. “But we don’t see these kinds of abrupt breaks in the beetle system,” says Kisailus.

If it breaks, it will leaf

Click for animation: The researchers simulated the delamination mechanism using 3D printed versions of the beetle structures. (Credit: Purdue University / Maryam Hosseini and Pablo Zavattieri)

As the researchers explain, the special characteristics of the material instead lead to so-called delamination when exposed to heavy loads: layers of the substance gradually give way. This results in a delay effect and a pressure distribution in the breaking process, the researchers explain. Further investigation results also confirmed that this structural concept in the area of ​​the seam of the cover wings forms the central secret of the enormous resistance of the tank. Further elements to increase the resistance to breakage and a particularly strong chitin protein matrix round off the system, according to the test results.

The scientists now see considerable potential for the development of technical applications in the ingenious concept of the connecting elements. As they explain, the connection of dissimilar materials such as plastics and metals in technical structures is still a challenge. This is because conventional techniques such as mechanical fastening by riveting and joining by welding or gluing can lead to premature and possibly dangerously abrupt material failure.

Technical potential

In order to sound out the potential of the structure in the beetle shell as a robust mechanical connecting element, the researchers constructed a series of connections made of metals and composite materials that imitated the biological model: Even with them delamination occurs when overloaded. “Our study should build a bridge between the fields of biology, physics, mechanics and materials science and technical applications,” says Kisailus. He and his colleagues found that their constructions actually imparted increased strength and toughness compared to a commonly used technical compound.

“Our results show that it is possible to move from using strong materials that are prone to sudden breakage to materials that are both hard and tough. The key here is that energy is distributed in a targeted manner during the process of overloading. This is exactly what nature made this armored beetle possible, ”concludes co-author Pablo Zavattieris from Purdue University in West Lafayette.

Source: Purdue University, University of California, article: Nature, doi: 10.1038 / s41586-020-2813-8

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