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  • Writer's pictureNed Patton

Composites as Biological Analogs

I want to expand on the notion of composites being biological analogs. Nearly every type of composite material we have created was motivated by man attempting to replicate the extraordinary feats of structural engineering that we see around us every day.

The best example of this for the string and glue type of composites, which is the most common form of composites, is a tree. Whether you have noticed this or not, an oak tree is an extraordinary example of the finest bit of structural engineering that you will ever come across. Aside from being absolutely beautiful examples of what mother nature gives us to look at, it is also an extraordinary feat of structural engineering. Using only cellulose as the string and lignin with pitch as the glue, an oak tree can reach 80 feet tall, and in North America can live for up to about 500 years. The oldest living oak tree in the world is in Sherwood Forest – Major Oak is estimated to be as old as 1000 years, and it is enormous and quite beautiful.

But to bring this back to composites, let’s talk about the structure of the trunk of an oak tree with an eye to understanding how just cellulose and lignin can produce such a thing as the wood of an oak tree. First – why is oak so hard to split for firewood, and woods like alder and pine are so easy to split. They are both made of basically the same string – cellulose, and the same glue – lignin and pitch. The difference is in how the strings are arranged and what mix of glue each tree uses as its basic building blocks.

The pic to the right is a little visual example of this. This pic has three pieces of wood, all the same exact size. From left to right, we have a piece of alder, a piece of oak, and another piece of oak. But, why is the piece of oak on the far right heavier and redder than the piece of oak in the middle of the pic? And why is the piece of alder the lightest of the three? This is an example of why I say that mother nature is a better engineer than any human being ever could be. The oak on the right is from the heart of the oak tree where the tree needs to have its compressive strength to hold up the weight of the canopy and all of the branches, leaves, acorns, etc. The red color comes from the minerals that the center of the trunk of the tree pulls up out of the surrounding soils. The minerals help strengthen the cellulose fibers as well as the lignin and pitch that make up the glue and give that part of the tree higher compressive strength. The piece of wood in the middle is from farther out on the tree where compressive strength is not as important as is the flexibility of the wood. So, if you notice, the tree rings in the middle piece of wood are a bit farther apart and are slightly more curved, so they can flex and extend as the canopy of the tree sways in the wind. There is some interweaving of fibers between the layers. Most of the fibers go in the up-down direction of the tree, but lots of them also aren’t completely straight up and down. This is what makes oak so hard to split – the fibers aren’t all aligned along the up-down direction of the trunk of the tree. It is also what give the oak tree trunk enough strength across the trunk to be able to support the weight of a very wide spread canopy.

If you think of this difference as a difference in the design requirements or structural problem that the tree is trying to solve, this arrangement of strings and glues makes sense. Putting compressive strength where it is needed and flexibility and tensile strength where it is needed in any structure demonstrates that mother nature is an extraordinary engineer.

But, what about the alder piece on the left? Why is it so light? This is another example of a completely different set of what engineers call design requirements or design architecture. The basic architecture of the wood of an oak is different than the wood of an alder because of the difference in each tree’s reproductive and growth strategy. An oak tree grows slowly, and typically has a life span of 300 to 500 years. It becomes mature so that it can reproduce in about 20 years, and can produce acorns for more than 200 years. An alder tree only lives about 70 years and can bear seeds in 3-4 years, so it is considered a fast growing and fast spreading tree species. Since it is so fast growing, its wood is fairly light weight and the growth rings in the tree are thicker than they are in an oak tree. Also, the cellulose fiber or grain of an alder tree is fairly straight, because the alder tree is trying to grow height faster than all of the other species around it and doesn’t grow a large canopy like an oak tree. So, as the alder trees die off, the slower growing oak trees eventually take over the canopy.

Well, that’s about it for my tree analogy in this blog about composites, so I hope that you got at least some understanding of the connection between what structural problem needs to be solved and how mother nature goes about solving each problem in a different way. This is important in the beginning of a composites design – to know at a high level what problem actually needs to be solved.

Next time I am going to talk a bit about the evolution of the industry through the lens of what has happened to the carbon fiber bicycle business in the last 35 years or so. Some very cool technology has been developed just to shave mere hundredths of a second off of the time it takes to ride the Tour de France and win it.


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