Motorsport Composite Fundamentals: Section Cured Products

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Section Cured Products

07.48

00:00	Just like steel is available as tube, rod, square section and angle, so are composites.
00:05	And we can use these as raw materials to build composite automotive parts.
00:10	Let's dive straight in with carbon fibre tubing, starting with those that are circular in cross section, which is by far the most common form.
00:18	Roll-wrapped tubes are made from wrapping fibre reinforcement around a mandrel.
00:24	Put simply, a cylindrical tool like a metal tube.
00:27	For these tubes we often see a woven material used as the outer aesthetic layer, with a unidirectional layer down the length of the tube and also around the tube in the hoop direction, which is like wrapping a hose around a reel.
00:40	All these layers combined give relatively isotropic properties, where they have good longitudinal strength and stiffness, as well as torsional and hoop strength and stiffness.
00:50	But of course the layers of reinforcement can be to the application's requirements.
00:55	A side note here, longitudinal strength and stiffness are needed when a tube is loaded axially or in bending, like a suspension controller.
01:04	Torsional strength and stiffness are needed when a part is twisted, like a driveshaft.
01:09	And hoop strength and stiffness are needed when the part is subject to radial loads, like an intake pipe with forced induction.
01:17	Pultruded tubes are made by taking carbon fibre tow and passing it through a resin bath to wet it out, and then pulling it through a heated die, in a circular cross sectional shape.
01:28	As the reinforcement is unidirectional, so are the mechanical properties, and unsurprisingly, these tubes are much stiffer in tension and bending than when loaded in any other way.
01:39	Although just to be clear, we'll typically avoid loading these tubes in bending, as this means compressive stress on one side of the tube, and as we know, the fibres are prone to buckling.
01:49	To get an idea of how these properties compare to other common materials, for the same diameter and wall thicknesses, a roll wrapped tube is about 2.5 times the strength of stainless steel in pure tension, with pultruded tubes generally being another 10-20% stronger again.
02:07	Pultruded tubes are stiffer than roll wrapped tubes in tension, and while both are stiffer than aluminium, they do fall short of stainless steel here.
02:15	In bending however, the roll wrapped tube outperforms the pultruded tube, it's actually comparable in strength to stainless steel, although not as stiff.
02:24	In compression, the composites fall short of these metal parts as they're prone to buckling.
02:29	They're also significantly weaker and less stiff in torsion, although as we'd expect, the roll wrapped part performs significantly better than the pultruded tube here.
02:38	Something worth noting at this point is that the lack of stiffness doesn't tell the full story in all applications.
02:45	Carbon fibre driveshafts are often preferred due to how they will elastically deform in torsion and dampen the shock loads in the driveline for example.
02:53	What's also important to understand here is that this is based on tubing of the same dimension, not weight.
03:00	If we did account for weight, we'd find carbon fibre tubes outperform in all areas other than torsion.
03:05	Before moving on, there are other alternative construction methods we need to mention.
03:10	Filament winding is ideal for higher hoop strength and pull winding, which is somewhat of a cross between pultrusion and filament winding, provides more torsional strength.
03:20	Another alternative is laying up reinforcement in a split mould with overlapping edges.
03:25	This is typically done using prepreg in a vacuum bag with an internal channel.
03:30	This is best for creating more complex shapes where the tube follows a non-linear path like a curved intake tube.
03:37	Although more likely it would be something that we'd do ourselves for specific applications rather than buying pre-cured products.
03:44	So, let's get back to the main topic.
03:46	Pultruded tubes are generally available in smaller sizes from about 12mm outer diameter down to as low as 1mm or even slightly less.
03:55	Whereas roll wrap tubes typically start at around 5mm and go up to much larger sizes like 50mm or more.
04:03	The wall thickness of the tube will mostly be governed by the layers of reinforcement used but that will be advertised so we know what we're buying.
04:12	Let's get into some of the use cases.
04:14	At the lower end this could be for simple aesthetics where they're split and then wrapped around the door bars of a roll cage to prevent scratches when we're climbing in and out of the car.
04:24	We'll usually use a very thin material here and won't be too worried about the strength.
04:28	Naturally the tube also lends itself to plumbing applications.
04:33	Intake pipes for example, however these would generally be laid up in split moulds and formed with specific shapes for the application.
04:41	Pultruded tubes with their high tensile strength and stiffness are great for pull rods and linkages and braces for aero parts.
04:48	Non-aesthetic tubes like this are also often used as wing spars which are like the skeletons inside a wing running span-wise.
04:56	A more complex and demanding example and something seen in the higher levels of motorsport is carbon fibre control arms.
05:03	Here the tube is bonded to an aluminium or titanium end that houses spherical bearings for the suspension joints.
05:10	As you can imagine a lot of thought goes into the design of such components that we won't be able to cover in this course.
05:17	But one important differentiation worth noting is the cross section of the tube used.
05:22	For open wheel cars where we often see such designs used, we commonly see the use of what's called aero tube.
05:29	Essentially the cross section of the tube is an aerofoil or at least in a simple form it's an elliptical.
05:35	The main purpose of this is to reduce drag but at the pointy end of the spectrum they're also being used to guide airflow which is a very complex task for moving suspension parts.
05:46	Before we wrap up let's discuss a few of the forms of cured products that are similar to our tubes.
05:52	Rod is just like a tube except it's not hollow and as such it's made primarily using the pultrusion process through a simple circular die where a lamination process isn't well suited.
06:03	The same difference between a rod and tube for metal parts applies here where for the same weight a tube is stronger than a rod.
06:10	However, it should hopefully be obvious that for the same weight a tube will have a larger outer diameter.
06:17	So, if space is a limitation the rod may be necessary but otherwise a tube is generally a better choice.
06:24	Like we discussed for aerotube we aren't limited to circular cross sections and box or rhs or rectangular hollow sections are also available and these can be made from pultrusion process or a similar method to roll wrapping.
06:37	The benefit here is that these give nice flat surfaces for mounting with conventional hardware and brackets.
06:44	Likewise, we can purchase composite angle again providing a convenient form of the material with flat surfaces.
06:50	Just like metal angle used in traditional fabrication this has good stiffness in both directions compared to plate or strip products which would have poor stiffness in their thinner dimension.
07:00	With that let's recap the key points to remember.
07:03	Cured section products are available in many varieties to use in our composite projects.
07:08	Most common is hollow circular tubing and this is typically roll wrapped for more isotropic properties or pultruded for more directional properties.
07:17	It's important to understand the construction process and the layers of reinforcement used and therefore the resulting properties and suitability for our application.
07:27	Non-hollow pultruded rod is also available which although doesn't have the same strength to weight ratio as tubing can be suitable where space is a constraint.
07:36	We aren't limited to circular section so with aero tube box and angle section all being available and suitable for many different uses around the vehicle.