Why does a square structure absorb more force?

Mechanical engineers at Clemson University pondering why the tail of a seahorse is square and how this adds to its strength referred back to research originally conducted at the University of San Diego to find their answer. This led to even more questions, like how this structural advantage can be used to create better medical devices and inspire flexible robotics.

Fig. 1 – Researchers also found that the tail’s square segments created more contact points with the surface that it is gripping when compared to a tail with round segments.
The seahorse tail is made of about 36 square-like segments, each composed of four L-shaped corner plates that progressively decrease in size along the length of the tail, creating a strong, yet flexible armor that excels at gripping and grasping. The plates are connected to the vertebrae by thick collagen layers of connective tissue. The joints between plates and vertebrae are extremely flexible with nearly six degrees of freedom.

While almost all animal tails have circular or oval cross-sections, the seahorse does not. These square plates are free to glide or pivot, the researchers found, and make the seahorse’s tail stiffer, stronger, and more resistant to strain at the same time. They said, usually strengthening any one of these characteristics will weaken at least one of the others.

They discovered that square plates move with only one degree of freedom when crushed—they slide. By contrast, circular plates both slide and rotate. As a result, square plates absorb more energy before permanent failure begins.

The team of scientists 3D printed a simplified model of the seahorse’s tail, which they then bent, twisted, compressed, and crushed. They also 3D printed and ran similar experiments on a tail model made of overlapping round segments that they designed, which is not found in nature. (See Figure 1)

Grasping and Gripping

When the researchers twisted their 3D-printed square seahorse tail model, they discovered that its plates interfered with one another, limiting its range of movement by about half when compared to the model made of round segments. In addition, after it was twisted, the square model returned to its original shape faster, while expending minimal energy. This may protect the tail from damage. By contrast, a tail made from round segments twists easily but requires more energy to return to its original shape. Researchers also found that the tail’s square segments created more contact points with the surface that it is gripping when compared to a tail with round segments. Since this is how the seahorse anchors itself, that gripping action is very important. In addition, a seahorse’s tail bends in a way that allows it to grasp objects within its line of sight.

Armor

Researchers compressed the 3D-printed model segments and compared their behavior to 3D-printed solid structures with square and circular cross-sections without segments. They found that a seahorse’s tail has joints at the exact locations where the solid structures fail when crushed. This allows the structures to absorb more energy on impact. Even more impressive, the square model outperformed the round one in all crushing tests. This is because square segments fail without changing their general shape. By contrast, round segments open up under the applied load, changing their shape from circular to elliptical.

Future Uses

The 3D printing technology allowed the scientists to create hypothetical models not found in nature to test against biological designs. They said that this may also lead to explanations of how biological systems may have evolved. Taking this one step further, they hypothesize that the structure could be used to create a gripping robotic arm. Another possible use is to scale it down to build a catheter.

For more information, visit http://ucsdnews.ucsd.edu.