Motorsport Composite Fundamentals: Core Materials

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Core Materials

10.08

00:00	Another possible constituent of composite parts are core materials.
00:04	And while these could be seen as a form of reinforcement, we'll separate them from our primary reinforcements to keep things simple.
00:11	We mentioned core materials briefly in the previous module when discussing sandwich panels consisting of a core material sandwiched between two thin sheets of composite, typically carbon fibre laminates.
00:23	The same idea goes for other composite parts, where we used a thick but lightweight core within a thin but stiff outer skin.
00:31	The purpose is to add structural or thickness to the part with minimal additional weight, cost or layup time.
00:38	We can think of this as bulking out the part without the extra weight or expense of doing this with our reinforcement.
00:45	The core material is usually quite low strength by itself, but the extra thickness provides an increase in bending stiffness of the part.
00:53	While sandwich panels are generally constructed by first curing the individual sheets and then bonding the core material between them, for more conventional parts, the core material will be included in the initial layup of the part and bonded in place as the part cures.
01:08	Before we get into the specific core materials, we need to cover off some related considerations and metrics that we can use to compare them.
01:16	The most basic and easy to understand are density, which dictates how much the part weighs, and compressive strength, essentially how much load the core can withstand before it's crushed.
01:28	Next, is how easy they are to work with.
01:30	This might be as simple as how easy the parts are to cut with basic tools, or how flexible they are.
01:36	If we're using core materials for parts that have contoured surfaces, as opposed to flat parts like in sandwich panels, then we'll need the core material that's conformable, meaning that it'll bend and form into the contours of the part.
01:50	Using thinner or lower density materials will help here, but certain materials, like PET foams for example, are more flexible and will conform better than the likes of styrofoam, even though they're much higher density.
02:03	We'll come back to these foams shortly though.
02:06	Other core products are designed specifically for the resin infusion process.
02:10	These could feature grooves or channels in the surface or through their structure to help with resin flow.
02:16	Alright, so what specifically are these core materials? Again, they're typically low weight, or more specifically, low density materials, so it might not surprise you that foam is a common core material.
02:29	Foams are one of the most inexpensive and easiest materials to work with, and they're available in quite a few different forms to suit our needs.
02:37	They're useful for making patterns and moulds as well.
02:40	For composites, we typically use closed cell foams, which is where each cell of the foam is completely closed, forming a little air pocket.
02:50	These restrict the flow of fluid, including air and resin through them.
02:54	This means the foam won't crash as much under vacuum and will maintain its structure, which is why we're using it.
03:01	It also means that the foam has no resin uptake, or at least it absorbs no resin during construction, other than a small amount on the surface texture.
03:10	This low uptake helps to reduce resin usage and, of course, the weight of the final part.
03:16	PVC foam is one of the most widely used core materials, and for good reason.
03:21	It's relatively cheap and has good mechanical properties, such as strength to weight ratio.
03:27	Different densities are available, but we'll often see PVC foam around 75 kgs per meter cubed.
03:33	Polystyrene, aka styrofoam, is even cheaper and has a very low density, usually around 33 kgs per meter cubed, so it's great for lightweight parts.
03:44	With that said, it can also be dissolved in solvents like acetone.
03:48	This makes it very suitable as a sacrificial core, where it's essentially used to create the geometry of the part and then removed after curing, meaning the final density is effectively zero.
03:59	However, it also means it's not suitable when working with solvent-based polyester and vinylester resins, as the resin will dissolve the foam.
04:08	While styrofoam is easy to cut with something like a hot wire, it's not as easy to form by hand, so it can be a bit trickier to work with.
04:16	Strength is also lacking compared to PVC and the other, more structural core foams we'll discuss next.
04:22	The first of these is PMI foam, which has a very thin cell structure, leading to almost no resin uptake.
04:30	Different densities are available, ranging from something comparable to PVC at the top end, to lower densities like styrofoam.
04:38	It also has higher compressive strength than PVC and high temperature resistance, so it's well suited to prepreg construction.
04:45	But just like other prepreg materials, it comes at a significantly higher cost.
04:50	PET foam is usually of higher density at around 100 kgs per cubic metre, so typically not ideal for lighter parts, but has higher compressive and shear strength, and it's also relatively inexpensive.
05:04	Okay, let's move on to a group of products known as core mat.
05:07	This is another cost effective type of core material, and as the name suggests, it's in the form of a mat, more similar to a felted cloth than a woven fabric.
05:18	These are usually available in thicknesses under 5 mm, and are very flexible, so conform well to the contours in the parts.
05:26	The key difference is that they'll absorb a certain amount of resin, creating a layer of core material that will harden when cured.
05:34	While this adds some strength to the core, it does increase the weight.
05:37	Realistically, core mat is more of a cost and time saver than a weight saver.
05:42	Like our foams, some core mats are designed with structures that won't collapse under vacuum, and some also have channels to aid in the resin flow for the infusion process.
05:52	Let's move on to natural core materials.
05:55	In most cases, this will be balsa wood.
05:58	For a wood, this is very soft, and in other words, it's easy to work with, and more importantly, lightweight.
06:05	Balsa wood cores are a good combination of mechanical performance, boasting one of the highest strength to weight ratios of all our core materials, as well as being cost effective and sustainable.
06:16	Although this last point does tend to be more important for larger scale manufacturing operations, and if our parts aren't sealed, the wood will decompose from ongoing exposure to moisture.
06:27	Lastly, let's discuss honeycomb structures for core materials.
06:31	The first thing to note here is that they don't have to be specifically the hexagonal cells of honeycomb.
06:37	Other polygon structures are possible.
06:40	However, we usually see honeycomb hexagonal structure used due to its efficiency, essentially providing great structure with minimum weight.
06:48	Of all the core materials, these have the best strength to weight ratio, and are great for high performance applications.
06:55	Unfortunately, the downside is that they're much harder to work with.
06:59	While thinner offerings, say under 3mm, can conform to curved surfaces, anything thicker than this is near impossible.
07:07	On top of this, the cells also don't allow the resin to flow throughout the part during construction.
07:13	So, for these reasons, the skin is often cured individually, and then the honeycomb core is bonded between them with an adhesive, similar to how sandwich panels are made.
07:24	These materials are available in different cell sizes, in different thicknesses, and in different materials.
07:30	The two most common materials we see here are aluminium and nomex.
07:35	It's no secret that aluminium has great strength to weight ratio, which is why it's used extensively in motorsport.
07:41	This, combined with the benefits of honeycomb structures, means it's the core material often utilised in the highest performance use cases, like aerospace and F1.
07:51	Unsurprisingly, though, it's not the most affordable option.
07:54	And then there's nomex.
07:56	This is a non-metallic and slightly cheaper choice for honeycomb core structures.
08:00	Nomex is a trademark name of Dupont, and is an aramid similar to the Kevlar we're familiar with.
08:06	While nomex core materials don't tend to be as ultimately strong as aluminium, they are lighter and have great durability, similar to Kevlar, and are usually a bit easier to work with.
08:17	Again, they're commonly used in F1, as well as WRC cars and the splitters, which you can imagine are subject to some fairly rough and abrasive conditions.
08:27	The added durability goes a long way.
08:30	It's also inherently flame retardant, offering a barrier when things go really wrong, justifying its use in race suits or in chassis bulkheads.
08:38	What we've covered in this module isn't an exhaustive list of every possible core material.
08:43	In reality we could use almost anything for core material, however these are some commonly used options.
08:49	We'll look at using these core materials and worked examples later in the course, so for now let's summarize what we've covered.
08:56	A core material is any medium used between the outer and inner skins of a composite part.
09:01	The main idea is to add thickness, structure and stiffness to a composite with minimum weight, expense and layup time.
09:08	Core mat is probably the cheapest and easiest material to work with, but isn't particularly high performing as its resin uptake usually leads to a heavier part.
09:18	Close cell foams prevent this resin uptake.
09:21	Styrofoam is cheap and light.
09:23	PVC foams are easy to work with and have relatively good mechanical properties.
09:28	PET is another step up for mechanical properties, as is PMI which is best to use with prepregs.
09:35	Balsa wood is a sustainable option and also has good mechanical properties but will decompose if exposed to moisture.
09:42	Honeycomb structures are the gold standard for efficiency in minimizing weight while providing structure, but require an alternative construction method of bonding pre-cured skins together.
09:53	Aluminium honeycomb provides the best strength but is the most expensive, where nomex is slightly cheaper, easier to work with and provides great durability and flame retardant properties.