Material Science/Composites/Design Considerations

 

Practical Design with Fiber Reinforced Composites

Up until now, we’ve only discussed some of the theory behind CFRP and FR composites in general.  Let’s look at some of the practical considerations when designing with composites.  One of the issues with composites is that they are highly anisotropic, which means that they are sensitive to the direction that the load is applied in, relative to the orientation of the fibers.  A load that runs in the same direction as the fibers is called a longitudinal load.  This is like taking a fiber between two fingers and pulling on it – you’re loading it longitudinally, in tension.  It’s hard to imagine, but say you take that same fiber and pull sideways at one end and the other direction at the opposite end.  This is a transverse load.  Composites, generally speaking, are much weaker under transverse loading.  The strength of a composite can actually be lower than the tensile strength of just the plain ol’ matrix – meaning the fibers have made it weaker.  So you should start to appreciate just how important the direction of the fibers is within the composite compared to how it is loaded, because you won’t always be able to predict the direction of the loads perfectly in real life use, and more often than not the loads will come from a variety of directions – not just longitudinally.  Another way of thinking about it is like this: when you load the composite in the direction of the fibers, the stiffness and strength approaches that of the fibers.  When you load the composite perpendicular to the fibers, the matrix must carry the load, and so the strength and stiffness is near that of the much weaker matrix.  The matrix can be up to 100 times weaker than the fibers when measuring tensile strength.

Metals, as you may recall from earlier chapters, don’t have this problem.  They are generally isotropic (i.e. have the same properties in all directions, no matter the direction of loading).  The interesting thing about metals is that individual grains are anisotropic.  But the metal as a whole is isotropic because most metals are composed of many, many grains all randomly oriented, which effectively means that on average, grains are oriented in every direction equally.  Maybe you’re thinking, why not do the same thing with composites?  Of course this exists as an option which we discussed earlier – short fibers that are randomly aligned in the matrix.  But this comes at the cost of strength – composites with aligned long fibers are significantly stronger, if we can overcome the directionality issue.  Unfortunately continuous fiber composites are also much more expensive.

How do we orient the fibers?

The orientation of fibers is an interesting dilemma because it is FR composite material’s greatest asset (since fibers are so strong when loaded along their axis), but of course this comes at the cost of weakness in the other direction.   So do we design with composites given this critical ‘flaw’?  The answer (and it is somewhat obvious) is that these fibers should not be aligned in only one direction (unless it is absolutely certain that load will only be applied in a single direction).  But how can we accomplish this?

First, FR composites such as carbon fiber or fiberglass are usually manufactured into thin sheets called lamina.  Within each sheet, the fibers can be aligned in one direction (unidirectional) or woven into a weave pattern, in which the fibers run in two directions at 90 degrees – longitudinally and laterally (or transversely).  When fibers are oriented at 0 or 90 degrees to the applied load (parallel or perpendicular to the load) they are known as specially orthotropic lamina; fibers oriented in some other direction are known as general orthotropic lamina.  

Say you have a sheet of carbon fiber composite material with continuous fibers running in all the same direction.  You could take these sheets and stack several together, making sure that all sheets are oriented the same – so that at the end you have what is called a ‘lay-up’ of unidirectional sheets.  This piece of composite will be incredibly strong in that direction, but disturbingly weak in the other directions.  This is known as a unidirectional lay-up, and the entire thing is called a lamina (yes, the same name used for a single sheet – this only applies if the lay-up is unidirectional).

Lay-ups and Stacking Sequence

A better way in most cases to create a lay-up would be to take a unidirectional sheet of carbon fiber composite and stack a bunch of layers at different angles.  Relative to the first sheet, the second sheet might be oriented at 90 degrees, then +45 degrees, then -45 degrees.  In order for the composite to be symmetrical about the middle layer, you could reverse the stacking – add another -45 degree sheet, then +45 degrees, 90, and then 0.  The entire lay-up would be [0,90,+45,-45 | -45, +45, 90, 0].  You’ll notice that there are a total of four directions, with two sheets oriented in each of the four directions.  Which means that the strength and stiffness will be equal in those four directions.  Which also means that this composite would be able to handle more diverse loading than its unidirectional counterpart – which was only comfortable handling loads in one direction.  This design is known as a quasi-isotropic lay-up, and the eight sheet composite is called a laminate (not lamina this time!).  It is called quasi-isotropic which basically means ‘it sort of has the same properties in all directions.’  Except not quite, because if you load the laminate at 30 degrees it will be weaker than if you load it at 90 degrees (since there are no sheets aligned at 30 degrees but you have two sheets with fibers aligned to 90 degrees) – but at least you have fibers aligned at 45 degrees which is close to 30 degrees!  Keep in mind that if the direction of loading is well understood, then it makes sense to create a laminate with more sheets aligned in this direction, with fewer sheets aligned in other directions to slightly balance the load carrying capability of the composite in all directions.

When to use composites in your design?

Drawbacks

There’s a few key considerations to understand when designing a component and picking a material.  Let’s say you’re trying to decide between a composite such as CFRP, or something like aluminum or high strength steel.  Metals are much more ductile, meaning that they can plastically deform to a much greater extent without failure compared to composites.  This has some implications for design – stress concentrations (such as sharp notches or tight radii) will affect the composite more, since it is a brittle material.  A metal may yield at a stress concentration if the stress becomes high enough, but localized yielding will serve to make the material stronger (strain hardening) and as the metal deforms the shape of the stress concentration will be altered, and some of the excess load will be distributed to nearby material.  Composites won’t do this – they like to fracture, especially when impacted.  A metal component may deform when impacted, but fracture is not as common (unless the forces involved are sufficiently large).  The ability of metal to deform and absorb energy is extremely useful in the design of safe cars – the crumble zone is made of metal components and design to extensively deform, absorbing the impact of a crash.  Conversely, a composite laminate (without a honeycomb structure) would work poorly if used extensively in a crumble zone.  Of course, the major consideration of whether to use composites or not often comes down to cost.

Advantages

Some advantages of composites that are easily overlooked is their superior fatigue life and their resistance to corrosion.  Composites such as CFRP won’t experience fatigue unless the stress is relatively high compared to its static strength.  In other words, when designing with composite parts, fatigue isn’t as much of a concern as it is when you’re designing with metal.  If you’re designing a component to made of aluminum or steel, the vast majority of failures will be fatigue driven, and so the fatigue life is a critical consideration.  As well, composites are highly resistive to corrosion.  Rust causes havoc on metal components (especially in colder climates with salty roads, or near the ocean – think of fighter jets landing on aircraft carriers) but composites are not nearly as susceptible to this common issue.  While composites are more expensive initially, they may result in huge savings down the line compared to metal components, which will tire and rot – of course, the cost savings depend on the project.  For example, while steel and aluminum suspension components will begin to corrode, it is unlikely that you’ll see a major failure due to rust within a reasonable amount of time owning the car – you’ll likely have purchased a new car before rust on structural components becomes a serious issue (rust on the body on the other hand…).

Material Science/Composites/Design Considerations

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