Natural structures have often done better than carefully designed man-made shells of the best materials. The structure of cellulose, the material of wood, the shells and wings of insects, all better iron and steel in their strength, given their weight.
But nothing seems to compare with the shell of a diminutive wood insect which is found in the drier parts of western US. This is Phloeodes diabolicus, now known as Nosoderma diabolicum. It may be quite right that “devilish” is its second name, as the beetle has a shell so hard that steel pins used to mount insect specimens are ineffective.
The shell protects the beetle from bird-pecks and all predators, and is so strong that being run over by a car does it no harm. Jesus Rivera, Maryam Sadat Hosseini, David Restrepo, Satoshi Murata, Drago Vasile, Dilworth Y Parkinson, Harold S Barnard, Atsushi Arakaki, Pablo Zavattieri and David Kisailus, from the Universities of California at Riverside and Irvine, Purdue University, University of Texas, Lawrence Berkeley National Laboratory and Tokyo University of Agriculture and Technology, describe in the Journal, Nature, their study of what makes the shell of the diabolicum, or the diabolical ironclad beetle, so strong, and whether we can borrow the principles to design our own engineering structures.
Through the ages of evolution, animals that lack speed and the arsenal for defence have evolved protective armour – one such is the group of arthropods, or animals that have an exoskeleton. Beetles, of which there are over 350,000 species, are the best example – their outer shell provides structural support, water collection and retention, and defence. In particular, the paper says, is the Zopherinae (Ironclad) family, which can resist being crushed and whose forewings are so hard that steel pins bend before they can pierce them.
These beetles no longer have the membranous hind wings which enabled their ancestors, and most other current day insects, to fly and evade their predators. Instead, the beetles have adapted by hardening the forewing, to fuse with the hind parts and form an outer covering, more robust, with density of 0.97 gm/cc, compared to 0.51 gm/cc which is more commonly found, as a shield or protection.
The rough exterior acts as camouflage – the beetle looks a lot like a bit of rock, and is so hard that it can withstand piercing strikes by predators, or even heavy impact, like being run over by a car, the paper says.
To get a hold on how well P.diabolicus dealt with loads that it encountered, the team carried out compression tests and compared the results with what other beetles which had the same needs, of resisting crushing and pecks by predators, could do.
They found that the P.diabolicus shell increases its stiffness, by over two and a half times, when the load is put on and can stand a load as high as about 15 kg. Considering its own weight, this is equivalent to a load, on a human who weighs 60kg, of more than 2,300 tonnes! Other beetles of the same kind do pretty well too, but only half as well as P.diabolicus.
There was a species that showed comparable stiffness at the start of the loading but was not better than the others when the load increased.
This suggests that the shell of P.diabolicus has a different composition or structure, the paper says. The shells, wings, and outer coverings of insects are known to have evolved microstructure that multiplies strength and makes the material more hardy than metal sheets or other human fabrications.
The Fullerian geodesic dome was a sally into this world, it represents architecture that is present at the nano-scale in materials like graphite, but its application is more in the design of large structures.
The arrangement of the molecules that make up natural materials follow similar principles, and are formed in layers, which gives the materials great capacity to absorb impact and resist damage. All of this however, is not good enough to explain the much greater resilience of the P.diabolicus shell.
The team went into the details of how P.diabolicus’ shell is built, using methods like micro-CT scans and scanning electron microscopy. While the normal X-ray only throws a shadow, to help make out the kind of tissue the radiation has traversed, the CT scan is a series of X-ray images of slices of an organ.
By viewing the slices together, one can build a 3D picture of the internals of an organ. And the scanning electron microscope produces surface images of fineness that is not possible with optical microscopes.
The investigation revealed that the secret of P.diabolicus is a pair of hardy, left and right halves, connected by a central suture on top and with supports to connect the upper shell to the shell on the underside. While the material of the shell has a complex structure and is formed in layers, the connections have their own complexity.
The supports themselves stiffen and stay strong under compression, while some of the connections allow an extent of deformation, which enables impact to be distributed, so that no one portion bears a large impact, which could make it collapse. The study has revealed an air-filled cavity within the shell, which enables the shell to deform, to absorb shock, without damage to internal organs. The paper describes analysis of the jointing of the shell to the underside and between the two halves of the shell along the central suture.
The connections to the base are found to vary along its length, with maximum stiffness in the region of the thorax. The connections in the central suture and to the underside use “mechanically interlocking jigsaw blades”, a connection method that is found in other beetle shells too.
The shapes and numbers of the blades in P.diabolicus, however, makes for greater distribution of stress and “maximum tensile and shear stiffness, strength and fracture toughness,” the paper says.
The connectors that keep together the parts of the P.diabolicus shell, the paper finds, create “robust joints with more predictable failure than in other beetles.” For failure to be “predictable”, rather than “sudden” is a great advantage in the design of structures.
In the case of the beetle, which needs to squeeze into cracks and crevices, it can judge when to stop increasing the pressure. Even if there is damage, the “layered” structure localises the effect and prevents spread.
The authors draw a parallel of comparable shaping of turbine blades or the landing gear of aircraft. These are devices that need to take high loads and sudden failure can have serious consequences.
The authors hence constructed models which mimicked biological materials, in shapes and the layered structure.
The mimics were found to do significantly better than the same devices made of standard materials. While composites that mimicked the P.diabolicus suture were positively stronger, they also distributed stress more evenly.
And a layered architecture reduced the possibility of a local failure that could lead to the collapse of the device.