Researchers at the Indian Institute of Science and Rice University in Texas are learning how shells stand up to extraordinary pressures at the bottom of the sea. Their goal is to learn how seashells remain stable and redirect stress to minimize damage when failure is imminent and see how their mechanical principles may be adapted for use in human-scale structures such as vehicles and even buildings.

The team created computer simulations and printed 3D variants of two types of shells to run stress tests alongside real shells collected from beaches in India. Led by Chandra Sekhar Tiwary, a graduate student at the Indian Institute of Science and a visiting student at Rice, their results appeared in Science Advances.

Researchers say that the distinctive shape of seashells makes them nearly twice as good at bearing loads than nacre alone.Shells are made of nacre, also known as mother-of-pearl, a strong and resilient matrix of organic and inorganic materials recently studied by other Rice engineers as a model of strength, stiffness and toughness.

Tiwary and his colleagues took their research in a different direction to discover how seashells remain stable and redirect stress to minimize damage when failure is imminent. Their calculations showed their distinctive shapes make the shells nearly twice as good at bearing loads than nacre alone.

They examined two types of mollusk: Bivalves with two separate exoskeleton components joined at a hinge (as in clamshells) and terebridae that conceal themselves in screw-shaped shells. In the case of clamshells, the semicircular shape and curved ribs force stress to the hinge, while the screws direct the load toward the center and then the wide top.

They found such evolutionary optimization allows fractures to appear only where they are least likely to hurt the animal inside. The researchers noted engineers have made use of mechanical concepts from natural shapes such as beak shells and shark teeth to design protective shields, automotive parts that dampen impacts and even buildings. But seashells, they wrote, represent one of the best examples of evolutionary optimization to handle varied mechanical loads.

While biologists, mathematicians and artists have contributed to the literature about seashells, materials scientists "haven't tried to think about these complex shapes because making them is not easy," Tiwary says. But the development of 3D printing has made it much easier to replicate the shapes with materials that are strong. "With the help of 3D printing, these ideas can be extended to a larger reality," he says.

The researchers printed fan-shaped polymer shells, including some without their characteristic converging ribs. They also made cones that mimicked the screws but without the complexities.

They found the rib-less fans were far less effective at redirecting stress toward the base of the fan, spreading it to three separate regions across the shell. When cracks finally showed in the fans, they appeared in the same spots near the base in both the real shells and the realistic printed version.

Stress distribution in the more complex screws was "totally different," they write. The tough inner core of the shell took the most punishment, relieving stress from the outer surface and shunting it toward the top-most ring. In general, the researchers found the screw to be the better of the two shells at protecting its delicate contents.