What advantages does a composite have?

Previously, we talked about what composite materials are, in an engineering sense. To recapitulate, composites are materials comprising two or more constituents. The constituents are combined in a way such that they retain their distinct identities in the final material (unlike alloys, for example). In particular, we talked about composites with a homogenous ‘matrix’ material (such as epoxy resin in polymer composites, and metals such as aluminum in metal matrix composites) in which reinforcing fibers (such as carbon fibers or glass fibers) or particulates are embedded. The fibers are the reinforcing material that provides strength to the composite, while the matrix material serves other purposes such as: (a) protecting the fibers (b) binding the fibers together to actually create the composite (c) helping to redistribute stresses if a fiber breaks.

But the key question is: why use composite materials at all? Why not use metals as always? What advantages do composites provide? Turns out, quite a few.

For one, composites are stronger than traditional industrial materials. How is strength measured? We all intuitively know this—by the amount of load that a material can withstand. Of course, for a fair comparison, the area over which the load (force, in technical parlance) is applied must be the same. (A thicker piece of wood carries more load than a thinner piece, but the wood itself remains the same strength; only the area of loading—the thickness, in this case—changes.)

Mercedes-Benz Composite Bike

A Mercedes-Benz bicycle that has a composite frame. (Source)

For another, most composites are lighter than their traditional counterparts. This is measured by density, which is the weight of the material per unit volume (just like we had considered constant area in the case of force, we must consider constant volume when considering weight). Like the example before, a larger piece of wood weighs more than a smaller piece, but the density of the wood itself remains the same.

Comparison of material strengths

Comparison of material properties, normalized by density. (Longer is better. Notice the metallic materials at bottom left.) (Source)

Combine the two traits—lighter and stronger—and what we get is a material that can withstand the same load with a smaller amount of material, and the material itself weighs less for the same volume! This is a pretty neat arrangement, no?

It doesn’t end there; this is just the beginning.

Remember, most of the strength of a composite material comes from the fibers in the composite. Now consider a composite with all the fibers parallel to each other, i.e. pointing in the same direction. In which direction (or directions) does the composite have the most strength? Is it equally strong in all directions? Evidently not—the fibers provide tensile strength along its own direction, and so the composite itself will be much stronger in the ‘fiber direction’ as compared to other directions. We’ve discussed wood as an example before, and you’ll notice that the situation is very similar to this case of unidirectionally arranged fiber-reinforced composites.

Load on a table top

Load on a table top: notice the fiber direction! Here the requirements are bending strength for the table top, and ability to transfer the load to the legs of the table. (Source)

Now that we’re comfortable with the idea of a unidirectional composite, consider this material as a building block. If you had a unidirectional material, that you could orient any way you like, and stack them so that the final material had fibers in multiple orientations, could you design a material to be strong in any direction you wanted? Certainly you could! And this is exactly what is done.

Composite Ply Layup

Composite Ply Layup. (Source)

And this brings us to the engineering and design side of composites. Even though, at first look, this seems like a dubious idea—isn’t it better that the things we make are equally strong in all directions? What if it breaks in one of the weaker directions?–this makes good engineering sense. Whenever a new component is designed, the designer has already figured out what the weak points of the structure are, and has already planned a way (or ways) that the component should fail, should it become overloaded or reaches the end of its life. In other words, a good engineering design factors in, during the design process, the directions that the component needs to be strong in, and the ways the material can and should fail.

Of course, this implies that the components perform at their best when they are used as intended. If a golfer strikes his golf club in frustration against a tree trunk, should he be surprised if his prized club goes out of shape? (On the other hand, if many golfers do this exact same thing, the designer of the hi-end golf club might take this into consideration when he makes a new design—but it probably will make the club even more expensive!)

Bent Golf Club

A bent golf club, after it was struck against the ground. (Source)

And of course, what of the case where the component does actually need to be equally strong in all directions? Well, there’s nothing preventing the designer from using plies oriented in all directions, right? A component where the fibers are oriented so that the properties are approximately the same in all directions is called a quasi-isotropic material, i.e. a material that behaves sort of like an isotropic material, even though it actually isn’t.

Ply stacking sequence for composite laminates

A unidirectional and a quasi-isotropic laminate. (Source)

One final thing for this session. How to describe the ply sequence of composites? In brief (I’ll clarify as we come across actual examples), the most basic nomenclature is just a sequence of the play angles, with one of the fiber directions designated the ‘zero degree’ direction. Since it can get cumbersome to write the sequences of multi-layer composites (consider a 16-ply or 32-ply laminate, for example!), symmetries and repetitions in the order of the plies is used profitably. The subscript ‘S’ denotes symmetry, ‘N’ (where N is a number) denotes N repetitions of the same order, and ‘T’ conveys that no symmetries and repetitions are present (i.e. it is the ‘total’ sequence). Please see the figure below for some examples.

Composite Nomenclature

Composite stacking sequence nomenclature. (Source)

To summarize, the advantage of composites is first in its most basic properties—it is both stronger and lighter than traditional metallic materials. (It must be noted that this is a simple generalization; ‘strength’ can be of various kinds. For example, composites are usually not very good in compression). Further, composites can be engineered as per design requirements of each component. A basic unidirectional building block can be used to prepare composites with a variety of effective material properties.


So, what are Composites, again?

In the broadest sense, a composite material is one that consists of two or more distinct materials—which retain their individual properties even when the final material is formed! In fact, we’ve all encountered various composite materials in our everyday lives.

Aggregate Concrete

Aggregate Concrete. (Source)

Take concrete, for example. Have you ever seen concrete being made for construction? It comprises cement (which binds everything together) and many other materials, including crushed stones. The properties of concrete are a combination of the properties of everything that went into it. Moreover, if concrete in its finished form is seen under the microscope, one finds that the components are essentially “mixed in”, and don’t interact chemically with each other in any way.

(This is, in contrast, to things such as alloys. Alloys, for example, are also made of multiple components—mostly metals, sometimes nonmetals such as carbon—but the final product has vastly different properties than the constituent materials. Also, the components themselves interact with each other at the smallest scale, and are not simply “mixed together”.)

Plywood is another example of a common composite material. In this case, you don’t even need a microscope: the individual layers making up the plywood are evident quite easily! In fact, while we’re on plywood, let’s remember the word ply—it refers to each layer of wood that makes up the plywood (yes, that’s how it gets its name). We’ll use the word ply plenty times later on.

Plywood

Plywood. (Source)

Okay, so that’s what composites are in the broadest sense.

But in an engineering and research perspective, a “composite” is something more specific. To wit, a composite material is an engineered material, in which one or more heterogenous components are used as reinforcement in a matrix of homogeneous material. If the material used as a matrix is a metal, it’s called a Metal Matrix Composite (MMC).

Okay, so let’s break that last paragraph up a little bit. First, the “matrix of homogeneous material”. This refers to any material that’s uniform (like cloth, gauze), which serves as the base for your composite, and between layers of which—or even into which—you can embed other things if you wanted to. (In concrete, the cement can be thought of as the matrix, into which you can add other things—including steel rods to create reinforced concrete!)

Reinforced Concrete

Reinforced Concrete. (Source)

As I already said, if the matrix material is a metal, the composite is called an MMC. But a variety of other materials are also used as the matrix material—different kinds of polymers, usually. The most popular one that I’ve come across is epoxy resin, but other than that, vinylester or polyesters are also used.

The reinforcing material in the composite is usually either a fiber (more common) or a particulate material (less common). Since fiber-reinforced polymers are more common—and what I mostly work with—let’s talk about fibers a little bit.

Silicon Carbide - Copper Metal Matrix Composite

Fracture surface of a SiC fibre-reinforced Cu metal matrix composite. (Source)

Fibers are a source of strength in composites, and a number of different kinds of fibers are used. Most common are carbon fibers, glass fibers, or even boron fibers. The question, of course, is—if the fibers add strength, why not build the entire material out of the same material as the fiber? Well, due to the way materials are manufactured, larger the size of the building block (i.e. the smallest unit used over and over again), lower is the overall strength. Hence, for example carbon in fiber form is much, much stronger than carbon in, say, sheet form (interested readers should look up “carbon fiber whiskers”–I could not find appropriate web pages to link to). This is why fibers are used for strength, and other materials (the matrix) are used to hold everything together around the fiber. (One can think of Metal Matrix Composites as special cases where even the matrix needs to be strong in its own right.)

Thus, to summarize, a composite material usually comprises a homogeneous matrix material, in which certain fibers are embedded to add strength and other advantages to the material. In some cases the matrix can be a metal, in which case the overall material is even stronger.


Getting up to speed…

Let us get introduced, first, to a class of materials which are highly directional in nature. What does directional mean? Let me give an example. Imagine that you have a sheet of thermocol in your hand. Try to pull the sheet apart—if it’s thin enough, you probably can. Does it matter in what direction you hold the sheet of thermocol? Top-bottom versus left-right? Of course it doesn’t. This is an example of a material that is not directional—it responds in identical fashion, whichever direction you choose to interact with it in.

Thermocol Sheet

Thermocol Sheets. (Source)

Now consider a log of wood. Have you seen pieces of wood closely? Have you noticed the striations that seem to run along the length of long pieces or logs of wood? Those are natural fibers present in the wood, and they provide the wood extremely high strength on the direction of the fibers. It’s pretty hard to cut wood across those fibers, and wherever possible, wood is cut along those striations.

Wood Grain

Wood Grain. (Source)

Wood, then, is an example of a material that’s directional—its response to external effects (such as a saw cutting through it) differs along different directions. That brings us to our first set of technical terms: isotropy and anisotropy. The thermocol sheet (or Aluminum, or Steel, or most other metals) is not directional—it’s isotropic. Wood is directional—and is anisotropic.

Why is this important? Because composite materials gain significance in large part because they’re anisotropic! Which means, they’re strong in one direction; not-so-strong in another. Plus, they can be engineered to be strong in any direction you want!


It doesn’t matter if global warming is man-made

Earth’s climate is changing. If you’re not wearing blinkers, and usually follow the news, this should no longer be a controversial statement to you. (Of course, climate change is a better descriptor than global warming. Earth’s temperatures will not literally rise everywhere all the time. Instead, extremes of climates will become more extreme, and the overall nature of Earth’s climate will shift dramatically.)

For example, from this great New York Times piece:

Especially lately. China is enduring its coldest winter in nearly 30 years. Brazil is in the grip of a dreadful heat spell. Eastern Russia is so freezing — minus 50 degrees Fahrenheit, and counting — that the traffic lights recently stopped working in the city of Yakutsk.

Bush fires are raging across Australia, fueled by a record-shattering heat wave. Pakistan was inundated by unexpected flooding in September. A vicious storm bringing rain, snow and floods just struck the Middle East. And in the United States, scientists confirmed this week what people could have figured out simply by going outside: last year was the hottest since records began.

“Each year we have extreme weather, but it’s unusual to have so many extreme events around the world at once,” said Omar Baddour, chief of the data management applications division at the World Meteorological Organization, in Geneva. “The heat wave in Australia; the flooding in the U.K., and most recently the flooding and extensive snowstorm in the Middle East — it’s already a big year in terms of extreme weather calamity.”

The question, then, apparently shifts to: ‘Is this climate shift man-made? Or at the least, is the climate shift exacerbated by human contribution?’ People who are ‘skeptics’ on this matter hold forth on how Earth’s climate has changed many times before. On the one hand, they doubt claims made by climate scientists—that the Earth is becoming hotter—based on their research, and on the other hand cite the very same scientists’ results on how temperatures on Earth have varied over the millenia.

A crude example—in the New York Times article cited above, a commenter writes:

Hasn’t “extreme weather” raged world wide since time began.? I think the dinosaurs might have something to say about that. It is time to put your words in the proper respective in regards to how long the Earth has existed………Solve our current pressing problems, then worry about how to pay for “climate control”. If you can.

So very true. The question is—where did said climate change, so naturally occuring on Earth over millenia, leave species that existed in those times? Thriving and healthy? Or as dust covered fossils scattered around the world? About 90-99% of all species of life to have existed on Earth are now extinct. Yes, that many.

What many people don’t realize is—it doesn’t matter if the climate shift is man-made. The Earth does not need humankind; we need the Earth to maintain a certain kind of climate for us to thrive, and, indeed, survive. If the Earth’s climate changes to an extreme, and we are not able to adapt to the new conditions quickly enough, we will run a very real risk of joining those 99% of extinct species. The Earth will happily continue to exist, and a new burst of evolution will spring forth a new dominant species to rule the Earth, just like homo sapiens now, and the dinosaurs before us.

Let’s stop fighting over whether we are the reason climate change is accelerating. (Most likely, we are, but as I said, that’s besides the point.) There’s absolutely no doubt human activities at least contribute to global warming (via burning fossil fuels, for example), and there’s no doubt a drastic change in climate is not good at all for the overall health of the human species. We already know what a runaway global warming process leads to: just look at Venus! It’s currently so toxic that we have a hard time even getting our space craft to operate on its surface. We will be extinct long before Earth’s global warming reaches even a percentage point of that of Venus. Not in 10 years, not in 100, not even in 1000, but the seeds of the far future are being planted now.

So let’s do something about it! I can understand the influence exerted by industries that would suffer if we changed our energy usage, but seriously, the health of the human species should take precedence over the relatively shorter term goals and ambitions of interested parties. Let’s stop being tone deaf; let’s take our collective hands off of our ears and eyes and start believing our own data. And please, let’s not selectively trust our scientists. They are experts in their field, and know what they’re talking about.

And no, climate science is not the same as economics and statistics.

The Earth is the only home we have; let’s not burn it down. Even if we are not the primary cause of the fire, we have to make our best effort to contain it, and if possible, put it out. Our survival depends on it.

P.S.: While looking for a suitable link for global warming on Venus, I found more links disputing the comparison than affirming it. Interestingly, most of such articles, with analyses, are written by, for example, Economists, and medical doctors; articles by climate scientists seem to be curiously missing. I will address this “dispute”, since I brought up the comparison with Venus, but in a separate post—let’s only focus on Earth for now.