Material Science/Composites/Intro to Composites

Intro to Composites

Metals and naturally occurring materials such as wood and rock have been used in an engineering capacity for a long, long time. More complex materials, such as alloys, also have a rich history.  For example, it was discovered around 3500 B.C that by combining copper and tin, you could create bronze, an alloy, which has better properties than either material. Indeed, this discovery triggered the Bronze Age.  But that was over 5000 years ago, and since then there has been a significant amount of materials research and development – and traditional engineering materials such as steel have more or less been fully developed.  In other words, it is likely that steel or other metals will not get much better than they are now, even with additional research.  

Composites, on the other hand, are more exciting, less mature, and less widespread.  The first real uses of carbon fibre, for example, began in the late 50s and early 60s.   Composites are simply materials that are made (‘composed’) from two or more materials. Usually, these materials (constituent materials) have quite different properties.  The resulting material doesn’t have the properties of any of the materials it was made from – rather, it has its own distinct properties.  The idea is that you can take a certain material – one which might have some good properties as well as some bad properties (because no material is perfect), and combine it with another material that also has some good and bad properties.  The hope is that if you combine them, the bad properties sort of cancel out, and the best properties emerge.  The resulting material is greater than the sum of the parts – it is better than either material on its own.

When you hear about composites, you might immediately think of carbon fibre – a material which is popular in many applications, particularly aerospace and automotive, but also for sporting equipment such as hockey sticks and tennis rackets. But as we stated before, a composite is really just two or more materials forming a unique one with different properties. Composites don’t have to be futuristic, advanced materials created in some laboratory; many composites are naturally occurring.  Usually, composites are formed with two specific materials that compliment each other and both have a job to do. You can’t just make any two materials into a composite, although you could certainly try. For example, wood and metal mixed together probably wouldn’t be very effective.

Typically, a composite is two materials: the matrix, and the reinforcement.  The matrix holds, or binds the material together.  The reinforcement material is usually strong and stiff. The binding material (the matrix) is weaker, but it is less brittle and can hold the stronger reinforcement materials together.  For example: mud reinforced with sticks.  The mud is the matrix; it binds the sticks together, which are stronger and stiffer.  The sticks on their own would be difficult to use – how would you keep them together?  And the mud is obviously weak, so it also needs the sticks.  Together, the two materials are better than either one on its own.

Common Examples of Composites

Concrete

Concrete, the most widely used material in the world, is a composite of an aggregate (reinforcement) and a cement binder (matrix). The cement is initially a fluid which hardens over time. The aggregate is usually larger chunks of material. It could be crushed rocks (such as limestone or granite) and finer sand. The matrix is initially dry. The aggregate is then added, so you have this dry mix consisting of cement and the gravel/rock/sand aggregate. Then water is added. The water reacts chemically with the cement, which hardens, and bonds all of the materials together. This chemical bonding process is called hydration.  Hydration goes something like this: take limestone and some other naturally occurring materials like clay and crush it down into smaller chunks. A few more materials are added in, like iron ore. At this point you have a controlled mix of calcium, silicon, aluminum, iron, etc. This is baked at a very high temperature, typically around 2700 F.  It’s cooled, ground down into a fine powder, and ready for use. At the construction site or wherever it’s being used, water is added, resulting in complex chemical reactions (the curing of concrete is not simply from the added water ‘evaporating’).

Adobe

You may have heard of Adobe houses. They are common in all parts of the world. Adobe is also the name of the material that the houses are made of, and is a composite material. For thousands of years, indigenous people in the southwest USA and parts of South America used Adobe to construct houses. And even though it is “low tech”, Adobe is gaining traction again due to its low cost and environmental friendliness, and it’s thermal properties. Adobe consists of basically mud and some other reinforcing component such as straw. The mud component is earth – often a mixture of clay, sand and silt – and water. Often, bricks of Adobe are made, which can be easily stacked. The straw helps to bind the matrix evenly together and helps in the brick drying evenly, reducing the chances of cracking as the brink shrinks when it loses water.  Walls made of Adobe are often structural, meaning it will carry the load of the building – and need to have sufficient compressive strength. Tensile strength is usually low, and so tensile or bending loads are often avoided when constructing an Adobe house or building. You can imagine that what is essentially dried mud would be weak when pulled, but strong when compressed.  What is particularly useful are the thermal properties of Adobe, and the fact that usually walls made of Adobe are relatively thick.  The result is a wall that regulates temperature well – it has a moderating effect, almost like a large body of water. Imagine a climate where it gets hot during the day and cold at night. The Adobe walls heat up slowly even in direct sunlight and sweltering heat, and the heat is slow to reach the inside of the wall (I.e. the inside of the house).  So during the day, the walls remain cool while it’s hot outside and slowly heat up as the day progresses. At night, it gets cold outside, and the walls are slow to cool so they keep the house warm. And this repeats every day. This moderating effect reduces the need for A/C and heat.

Bone

Bone (like the bones in your body) is also a composite material, and it has mechanical properties you might associate more with carbon fibre type composites – it’s relatively strong for its weight, and the material is also hard.  Of course, bones play a huge part in the functioning of the human body besides just being a composite material that provide structure – bones produce your red and white blood cells and store minerals.  They also allow you to move, which is a statement that may seem almost too obvious but also, I think, underappreciated.  Imagine trying to walk without bones.  They provide structure first and foremost.  Imagine taking a cross section of a bone – the exterior will be hard, while the interior will be softer (commonly known as spongy bone).  The exterior is known as cortical bone, and the interior material cancellous bone.  Note: here, we will discuss cortical bone exclusively, as that is more interesting from a mechanical perspective.  What is interesting is that these two types of bone tissue are biologically identical – they have the same compositions – but their microstructures are different, and so they have different mechanical properties.  This is something we see frequently with other materials, which we intentionally meddle with to create a spectrum of mechanical properties.  It’s not surprising that nature beat us to this idea.  Cortical bone (forming the cortex, aka outer shell) contributes to the majority of the weight of a human skeleton – up to 80%.  Bone has multiple levels of structure, and is quite complex compared to something like steel.  The cortical bone consists of these microscopic columns, called osteons.  Osteons are really just cylindrical rods, several mm long, and about 0.2mm in diameter, and can be thought of as the fundamental unit of the cortical bone – say you grab a handful of these osteons, lined them up and glued them together, and you might have something that resembles bone.  Kind of like if you took a bunch of trees, removed their branches, and packed them all together.  Let’s get into a little more detail about these osteons.  Each osteon is actually a bunch of concentric hollow cylinders, that get smaller and smaller, all fitted together.  These are called lamellae.   If you cut an osteon in half, you’d see these lamellae as concentric circles, like the rings of a tree trunk.   Lamellae is a generic word meaning a thin plate-like structure, and comprised mostly of collagen fibers.  Cells called osteoblasts actually create the lamellae by secreting dense and highly crosslinked layers of collagen along the along the axis of the bone (longitudinally), given the bone its tensile strength.  The collagen fibres usually aren’t perfectly aligned straight up and down, but rather run at a bit of an angle – they are oblique (which essentially means ‘slanting’), which gives them a bit more shear strength.   At the very center of the osteon is the haversian canal, which carries blood to the bone.   The exterior surface, or boundary, of the osteon is called the cement line.  There’s some other cells in these osteons, which comprise a network allowing the osteons to exchange nutrients and metabolic waste.   Since osteons are cylinders that run longitudinally, there’s some space between them.  This space is filled by interstitial lamellae.  Bone is constantly being replaced by newer bone (a process known as bone remodelling).  Interstitial lamellae are basically the remains of old osteons that were only partially resorbed during this process.  As fair as structural material goes, collagen is not the only one in the osteons, and it’s not even the dominate one.  The osteoblast also release calcium, magnesium, and phosphate ions, which combine to make bone mineral.  Bone mineral is a crystalline mineral; it’s a form of hydroxyapatite.  Bone mineral is a brittle material, about 5 on the Mohs hardness scale.  By comparison, steel has a Mohs hardness of about 4-4.5, whereas diamond is 10.  Bone mineral makes up about 50% of the volume of human bones, and 70% of the weight of human bones.

The collagen fibres (mainly protein) are soft and flexible, and have a reasonably high tensile strength.  But the compressive strength of collagen is low.  As an extreme example for illustration, it would be like pushing on a rope.  Bones made of collagen fibres alone would be terrible – it would be like having bones made of rubber.  If we add bone mineral as the (inorganic) matrix, which is hard and brittle and can bind the collagen fibers together, we get bone, which has better properties than either two material individually.  Because the osteons run longitudinally along the long axis of the bone, bone is considered to be anisotropic – a common characteristic of composites.  This, you might recall, means that it has different properties depending on the the direction of how you load it up.  Most metals, on the other hand, are isotropic – their properties are essentially uniform, no matter how they are oriented.  This has implications for the mechanical properties of bone – first of all, it is stiffer longitudinally – in the same direction that the osteons are oriented.  Young’s modulus of collagen has been found to be about 6 GPa (as a reference, steel is 210 GPa, meaning it is 35 times stiffer, and the modulus of wood is about 11 GPa).  Bone mineral has a modulus of about 80 GPa.  Combined into cortical bone tissue, the modulus is about 11-20 GPa longitudinally, and about half that – 5 – 10 MPa – in the transverse direction.  The tensile strength in the longitudinal direction is about 130 MPa, and the compressive strength is about 190 MPa.  The values for the transverse direction are about 50 MPa and 130 MPa.   Bones are brittle – anyone who has broken one can probably appreciate that – as they can only elongate anywhere from 0.5 – 3% before fracture, which classifies it as a brittle material (generally anything under 5% maximum elongation is considered to be brittle).  Because of the high bone mineral content, bone is strongest is compression, which makes sense if you think of what the human body is designed to do: walk, run, jump – all activities that will cause compressive forces.  Bones are most vulnerable when loaded in the transverse direction, such as bending your arm.  Again, if you have ever broken a bone (or even not), you probably intuitively understand this.  The rate of loading also matters.  The faster the force is applied to your bone, the more brittle it behaves, which is bad for us.  Falling on your arm is a sudden shock, and your arm will behave in a more brittle way than if the bone was loaded nice and slowly in a lab test.  And while bone is self healing, and we don’t need to worry about fatigue failure, we do need to worry about aging.  Bone becomes less strong, less stiff, and more brittle as we age.  

Principle of Combined Action

Composites work because of something called the principle of combined action, which is a fancy way of saying that the resultant material will have better properties than either of the two (or more) individual materials.  Of course, some trade-offs will be made.  Like with carbon fibre – the resulting composite will not have a higher tensile strength then the fibres themselves, but you will get a material that is less brittle and is actually applicable, since a bunch of loose fibres are impossible to use in a practical application.  So keep that in mind.  There are trade-offs – some properties will worsen – but overall, from multiple measures, the resulting material will have better properties and will likely be more usable.

Most composites are simply two materials, and the materials either act as the matrix phase or the dispersed phase.  The matrix phase is what binds everything together, and the dispersed phase is also known as the reinforcement.  The matrix phase is continuous – it occupies the space around the reinforcement phase.  

Factors Affecting Composite Properties

Let’s say you do decide to create some adobe for a house you’re making, and you have some mud and some straw.  Speaking broadly, there are three things that will affect the property of your composite:

1. The properties of the individual materials.  How strong is the straw/mud?  Are they strong in tension, or compression?  What is their modulus?  Their density?
2. Amounts of each material.  Is the composite 50% straw and 50% mud?  Or 90/10?  
3. The spatial and geometrical arrangement of the reinforcing material. As you can imagine, there are many different ways to adjust the reinforcements inside the matrix: you could alter their shape, size, orientation, concentration, and distribution.  The mud is more or less mud.  It is continuous, and apart from altering its composition (e.g. more or less sand, or earth, or water), and apart from the shape and size of the component we are building (say the wall of a house) – we cannot alter the shape of the mud, or the size of it.  But we can drastically alter how the sticks are arranged, how many sticks there are, how large they are, what shape they are, etc.  

You can see that building composite materials is a very deliberate task, because the many choices that are made about the geometrical and spatial characteristics of the reinforcement alone could drastically affect the performance of the material, and you must make a decision about of much of each material to include.  Of course, the matrix must serve its purpose of binding everything together.  A matrix that just crumbles and falls apart like dry mud won’t create a good composite, no matter how great your reinforcing phase is.

Composite Classifications

Since there’s multiple ways to make composites, material scientists like to classify them:

• Particle-reinforced.  An example is putting stones in cement to create concrete.  The particles can either be large (like stones) or fine particles dispersed throughout.  
• Fiber-reinforced: fiber indicates that the geometry of the reinforced phase has a much smaller diameter relative to its length, like human hair.  Carbon fiber composites are fiber-reinforced, because, if the name didn’t give it away, the carbon fibers which act as the reinforcement are relatively long and thin.  The same goes for fiberglass.  There are a few more ways to characterize fiber-reinforcement composites.  The fibers can either be continuous and aligned, which is the type of composites where the fibers are bundled together and neatly aligned in the same direction.  In this context, continuous really means ‘long’.  The fibers can also be short.  With short fibers, you can either randomly chuck them into the matrix (‘randomly oriented’) or neatly order them (‘aligned’).
• Structural composites.  An example of this is a ‘sandwich construction’ – like a honeycomb core placed between two sheets of material.

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