Material Science/Composites/Particle Reinforced

Large Particle Reinforced Composites

For large particle reinforced composites, like concrete, the reinforcing particle is usually stiffer and harder.  It’s simple to calculate the new elastic modulus of the composite using what is known as the rule of mixtures. It’s difficult to precisely calculate it however, and so there are a couple of different equations used – one to estimate the upper bound, and one to estimate the lower bound.  The new elastic modulus should fall somewhere between these.  The elasticity of the new material really depends on how much of each material is in the composite, which makes sense.  In the case of concrete – if it was mostly cement (the matrix), then it would have an elastic modulus close to that of cement.  Conversely, if it was mostly gravel, it would have an elastic modulus close to that of gravel (stone).  Obviously, concrete isn’t the only example of particle-reinforced composites.  There’s actually a whole category of materials called cermets (so closely spelt to cement that it is a little confusing).  Cermets are composites consisting of ceramics and metals.  These composites can have some interesting properties.  The name comes from [cer]amic + [met]al.  The idea with cermets is that you’ll get the hardness, high temperature resistance, and oxidation resistance that ceramics are known for, but you’ll also get the toughness and plastic deformation that are so useful with metals.  A common cermet is cemented carbide, which is basically small particles of carbide embedded in a metal matrix, just like gravel and sand in cement to make concrete.  The carbide is usually something like tungsten carbide, or titanium carbide. Cemented carbide is used extensively for industrial cutting, as it is able to better resist the extremely high temperatures that occur when cutting something hard like stainless steel.  Because it can withstand high temperatures, it can cut faster than standard steel tools.  Because carbide is brittle and expensive, and prone to chipping, usually just an insert of cemented carbide is used on the cutting tip while the bulk of the cutting tool is some type of steel.  This makes it easier to replace, and less costly than making the whole cutting tool out of cemented carbide.  The reason the carbide needs to be in a metal matrix is that it is too brittle to do the job alone – an insert made of a pure carbide would instantly break when subjected to the stresses involved in cutting metal.

Reinforced Concrete

You may have also heard of reinforced concrete.  Concrete is terrible at withstanding tensile forces compared to compressive forces.  It is roughly ten times weaker when stretched to failure compared to when it is compressed to failure.  We know that the main principle of composites is to add two or more materials together to get a better material, so what can we do with concrete to improve its tensile strength?  This can be achieved by adding steel into the mix – steel is quite good in tension.  Usually, steel is added either as rods, wires, mesh or as rebar.  Steel is also an ideal material because its coefficient of thermal expansion is similar to concrete.  If it wasn’t, this could be an issue.  If the reinforced concrete was exposed to sub-zero temperatures (which it commonly is in many cities) and either the concrete or steel contracted more (due to different thermal expansion coefficients) it could weaken the material or cause cracking, and in the worst case, failure.  Steel is also a good choice as it bonds well to the concrete matrix, although usually the steel is shaped in such a way to improve this bond.  If you’ve ever seen rebar, you can see that it is ribbed – which makes it difficult to remove from concrete once bonded.  Steel is not the only material used to reinforce concrete – others include glass, nylon, and some plastics.  Care is taken with these types of fibers – while steel won’t corrode in concrete, some materials will.  Reinforced concrete first saw use in the nineteenth century, in Paris.

 

Material Science/Composites/Particle Reinforced

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