It draws its strength from hexagon-shaped scales and connections between them, University of California, SD engineers, have found. Lactoria cornuta “boxfish is small and yet it survives in the ocean where it is surrounded by bigger, aggressive fish, at a depth of 50 to 100 meters,” said Wen Yang,Swiss Federal Institute of Technology. “After I touched it, I realized why it can survive — it is so strong but at the same time so flexible.”
The hexagon-shaped scales are called scutes. They are connected by sutures, similar to the connections in a baby’s skull, which grow and fuse together as the baby grows. Most fish have overlapping scales = no weak points, should a bite from a predator land exactly in between scales. “We are currently investigating what mechanical advantage scutes and sutures might provide. We know that the boxfish has survived for 35 million years with this armor, so the design has proved very successful in nature.”
Each hexagonal scute in the boxfish’s armor has a raised, star-like structure in the center that distributes stress across the entire surface. Underneath the scutes lies a flexible, yet hard to penetrate, inner layer of interlocking collagen fibers.
Each scute has a raised, star-like structure in the center that 3distributes stress across the entire surface. Under the scutes is an inner layer that forms a complex structure in which collagen fibers interlock. This structure creates a flexible inner layer in the armor, difficult to penetrate due to the interlocking collagen fibers. Together, the outer and inner layers provide the fish with protection.
The team also tested scutes’ ability to withstand tension by pulling them apart both horizontally & vertically + ability to withstand penetration. “.. even if a predator manages to generate a crack in the outer layer, the collagen fibers will help to prevent the structure from failing,” said Yang.
Sutures make the armor even stronger. Upon impact, sutures’ zigzag patterns essentially lock in and keep the scutes from breaking apart. These sutures are different from many of those found elsewhere in nature, Naleway said. “The most common form of suture structures in nature are those that have a roughly triangular shape and consist of two important components: rigid suture teeth and a compliant interface,” he said. “To the best of our knowledge, there is no compliant phase in the interface of the boxfish’s sutures. In addition, the teeth themselves have a much lower aspect ratio – meaning that they are shorter and wider – than most other examples.”
Proposed mechanical advantage of sutured interface: (a) Straight interface with penetration by sharp tooth separating neighboring scutes; (b) Sutured interface with force F having tangential and normal components; (c) Tangential component of force, Fy, pushing opposite sides of teeth against each other in a locking-in mechanism.
They used scanning electron microscopy to characterize the surface structure of the scutes. They also took cross sections and used micro-computer tomography to characterize the dense regions. The results of mechanical testing left the researchers wanting to know why the boxfish would choose a design that excluded overlapping scales.
http://www.jacobsschool.ucsd.edu/news/news_releases/release.sfe?id=1781
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