Material Science/Composites/Fiber Reinforced

Fiber Reinforced Composites

The types of composites with fibers might be the first type of composite that comes to mind.  If you play sports, or drive expensive supercars on a regular basis, or feel the need to navigate rapids, no doubt you’ve experienced the proliferation of composites such as fiberglass and carbon fiber reinforced polymer (CFRP).  Fiber reinforced composites (which we’ll call FR composites) are often designed to be very strong and stiff, but relatively lightweight.  The fibers are very strong, and can carry high loads in both tension and compression.  The matrix distributes the load between the fibers, and prevents them from buckling in compression.  In simple terms, the matrix basically acts as a medium in which the fibers can reside.  A material with those properties can find uses in many, many applications.  What ends up restricting the use of such materials isn’t too surprising: cost.  Difficulties in manufacturing certain components based on shape, and also based on volume (the number of parts) also reduces the applications of FR composites.  As well, it can be more difficult to properly join composite pieces – often with steel parts you can simply weld them together.  It becomes more difficult to create assemblies (multiple components assembled together) with composites – difficult, but certainly not impossible.

Fiber Length

One of the big influences in the properties of FR composites is the length of the fiber.  A fiber needs to be strong, but it also needs to be able to transmit load to the matrix phase.  It is better if they can transfer more load to the matrix, as the load can then be spread out across the material.  A thought experiment: where there is no fiber, there can be no load transmittal to the matrix from the fiber.  Which is incredibly obvious, but it goes to show that the interface between the fiber and the matrix is important.  Particularly, the length of the interface between the fiber and the matrix.  In fact, there is a critical (‘minimum’) length of fiber that is necessary to ensure the composite is sufficiently strong and stiff.  This critical length depends on the diameter of the fiber, the strength of the fiber, and the strength of the bond between the fiber and the matrix (the shear strength of the matrix can be substituted).  We could get into more detail about this and show graphs and such, but the take away here is this:  If the fiber is too short, the matrix will deform and the fiber will not offer any reinforcement; it is essentially useless.  In order to be sure that the fibers are actually reinforcing the composite, they must be larger than the critical length.  To illustrate this, imagine taking long fibers and blending them up into dust, and then dispersing this into the matrix.  When you pull on this ‘fiber reinforced’ composite, the fibers are too short to do anything – the matrix will just deform, and the fibers will go along for the ride without providing any reinforcement – there will be no transfer of stress from the matrix to the fiber, and so what good are the fibers?

Fiber Orientation

Apart from fiber length, the orientation of the fibers greatly affects the properties of the composite.  So does the distribution and concentration of fibers, but this is somewhat more obvious – you aren’t going to clump all the fibers together in the matrix and have large areas of matrix with no fibers.  With respect to orientation, there are two extremes: all fibers run in the same direction, or all fibers are completely randomly oriented.  With short fibers, either case is possible.  With long fibers – longer than the critical length – usually the fibers are aligned nicely.

Fiber Diameter

Another note about fibers – composites are generally stronger if you can make the fibers smaller in diameter.  When carbon fibers were first being manufactured, they were larger in diameter – around 20 micrometers, whereas human hair ranges from about 20 to 180 micrometers (let’s call it 100 micrometers on average).  With better technology, recent carbon fibers are in the 5 to 10 micrometer range.

Fiber Reinforced Stress Strain Curve

Let’s say that we’ve aligned fibers in a sample of FR composite to run in the longitudinal direction – the same direction that we’ll apply a load on the sample in order to generate a stress strain curve.  We know what the stress strain curve of an individual fiber will look like – it will fracture at a very high stress, and a low strain.  The curve will be more or less linear – it will elastically deform slightly (not too much because the fibers are very stiff) – and then fracture, as brittle materials do.  The matrix, on the other hand, will be elastic for a little bit, but begin to plastically deform at a relatively low stress, generating a nice curve on the stress strain graph.  WIth increasing stress, the matrix will continue to deform and elongate, before fracturing at some elongation significantly greater than the fiber.  This much is known – this is why we picked the two materials to create our composite, since the fiber is strong and stiff but brittle, and the matrix is weaker and less stiff but is also tougher and can elongate more.  They work well together, like opposites attracting.  Now the stress strain curve of the composite will sort of resemble the fiber – it will fail at the same strain (elongation won’t have improved much) but at a lower stress, so overall the slope will be a bit shallower.  Imagine the FR composite test sample is being stretched: first, the matrix will start to plastically deform, while the fibers are still stretching elastically, since the matrix is much weaker than the fibers.  Now, since the matrix is plastically deforming and elongating, more of the load will be transferred to the fibers.  This will continue until the fibers begin to fracture, which is considered to be the failure point of the composite.  Note that since there are bound to be small variations in the fiber properties and shapes and sizes (not to mention slight variations in the stresses of each fiber), not every fiber will fail at this point – some will remain intact.  As well, although the matrix has plastically deformed, it hasn’t actually fractured yet – since it’s maximum elongation is much larger than that of the fibers.  Which may seem a little odd that the fibers fail before the matrix, because they’re so much stronger.  But remember – the fibers mean that it can reach that high of a load in the first place.  If we had a material that was just matrix, it would fail at a much lower load.  This is the whole point of the fibers reinforcing the matrix.

Material Science/Composites/Fiber Reinforced

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