The work, which could eventually have applications in supersonic aircraft, was led by Ling Li, assistant professor in the department of mechanical engineering at Virginia Tech.
Ceramics are highly resistant to heat, making them a favourite choice for managing the brutal thermal demands of aircraft that travel faster than the speed of sound. At those speeds, compressed air creates significant friction against the vehicle, resulting in a rapid increase in temperature.
The materials are susceptible to damage, however – a single pinpoint impact in a ceramic plate can result in a rapidly spreading crack that causes total structural failure. Ceramics become even less tolerant to damage when they are made porous for weight reduction, but decreasing weight is a critical requirement for many structural applications, including in high-speed vehicles.
The US Air Force, one of the sponsors of Li’s research, has long been interested in improving the mechanical performance of ceramic materials.
Li and colleagues set out to find new design principles from the natural ceramic cellular solids formed by organisms such as sea urchins. A sea urchin’s exoskeleton has a ‘foam’ microstructure with an assembly of open cells with solid edges or faces, and gaps between them make the material porous and more mechanically efficient than dense structures.
“In this work, we think we found some of the key strategies that enable the sea urchin to be strong and tough while offering weight reduction with its porous microstructure,” said Li.
The spines of sea urchins are stiff, strong, and lightweight. They are made of a brittle mineral called calcium carbonate, which is similar to synthetic ceramics, but the creature has a much higher tolerance for damage when receiving weight or force.
The team tested this principle by pressing the spines mechanically, simulating the same kind of condition that an engineering ceramic might need to endure. The sea urchin spines deformed gracefully under the force, in contrast to catastrophic failure of synthetic ceramic cellular solids. This ‘graceful failure’ allows the sea urchin spines to withstand damage with significant energy absorption capability.
“There are a couple of secrets in the structural features of sea urchin spines. One is related to the connection of branches,” said Li. “The second is the size of the pores.”
The researchers observed an architecture of interconnected short branches under a microscope. A network of nodes holds these branches together, and one of the secrets to the urchin’s damage tolerance is the balance between the number of nodes and branches.
“That number is critical because nodes with too many connected branches will cause the structure to become more brittle and breakable,” a research announcement said. “The nodes in the porous structure in sea urchin spines are connected to three branches on average, which means the network of branches will undergo bending-induced fracture instead of more catastrophic stretching-induced fracture.”
The second secret lies in the size of the pores between branches. The team discovered that the gaps within the porous structure of sea urchin spines are just slightly smaller than the size of the branches, meaning that branches can be ‘locked’ in place after they fracture. Broken branches stack on top of each other on the pores, creating a dense region that is still able to sustain load.
Sea urchins also have a different surface morphology to synthetic ceramics. Manufactured cellular ceramics have microscopic defects across their surfaces and internally, making them more susceptible to failure, but the sea urchin spines have an almost glasslike surface, smooth down to the nanometre scale.
With branches, pores and a smooth surface, the lightweight sea urchin spines achieve high strength and damage tolerance by uniformly distributing the stress within the structure and absorbing energy more efficiently.
Despite the desirable characteristics, recreating those features in synthetic ceramic is not yet possible with current manufacturing methods, the researchers said. They are typically formed in a two-step process – first creating their shape, then firing the piece so it hardens and gains strength. That sintering process also leads to the formation of microscopic defects, however.
“In my lab, we are also interested in how organisms such as sea urchins form these natural ceramic cellular solids,” said Li. “Hopefully one day, we can not only integrate the material design principles to bio-inspired lightweight ceramic materials, but also the material processing strategies learned from natural systems.”
The team also received funding from the US National Science Foundation.
The work was published in Nature Communications.
Navigate a turbulent future by attending Aerospace & Defence (28 November – 2 December). Register for FREE today.
Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.