Basic Composites Knowledge
- Min Qian Lu (Deactivated)
Basic Definition of a Composite
A composite is generally a combination of 2 or more materials with the goal of creating one final unified material that has optimal qualities.
A common example is reinforced concrete! In this case, the rebar is the fiber/filament and the cement is the matrix. Concrete has particularly good compressive strength and rebar is good in tension. The combination of the two solves the weak point in concrete, its weak tensile strength.
Why we love Carbon Fiber
Carbon fiber is a two part composite. Generally, it is composed of carbon fabric and epoxy resin. In this case, the resin is good in compression and the fabric is good in tension. The combination of the two creates a material excellent in a variety of loading conditions.
In our case, we love carbon fiber because it's a beast. IF done properly, carbon fiber is five times stronger, twice as stiff and lighter than steel. This is optimal for a car with limited power and needs to minimize its mass.
Sandwiches
The typical carbon fiber panel resembles a very dry and crunchy sandwich.
It consists of 2 outer “skin” layers, in our case its carbon fiber (fabric+resin) and a middle “core” layer.
The main purpose of this core is to add additional stiffness by increasing thickness. This helps the panel resist bending. Below is the formula for deflection!
Thickness may not directly be in the equation but its actually embedded within the “I” term. For rectangular cross sections, which is the case for most of our panels, I is equal to the equation below.
Here, h is the thickness (height of the cross section). Thus we can see, we end up dividing the deflection by thickness^3. Note this theory is not crucial to understand. Simply think of it intuitively.
What is easier to break? A small thin pencil? Or a larger thicker pencil?
Link: https://youtu.be/f08Y39UiC-o?t=69
Basic Ideas
When you want a sandwich, there are many ways of going about it. You can either make it yourself, or you can buy it. This is kind of an analogy for the different types of carbon fiber?
When making a carbon fiber panel, you need to make sure the fibers of carbon and fully soaked or “wetted out” with epoxy. This is crucial to the panel’s strength. Without ensuring the proper mixture of the fabric + matrix in a composite, you expose a weak point.
There are two main ways of introducing the epoxy - either in its liquid form, or in a modified solid form. Liquid form encapsulates two methods - wet layups and resin infusion. Solid form is commonly known as pre-impregnated carbon fiber or prepreg for short.
In addition to introducing resin, another important factor is pressure. The pressure is used to allow resin to flow between all the layers of carbon fiber. By applying a constant downwards pressure on a part when the resin is liquid, it forces the resin to thin out in the bottom layers and then travel upwards into the upper layers as it has nowhere else to go.
Vacuum Bagging
Typically, uniform pressure is applied using a vacuum bag - similar to what is used when you move houses and want to pack your bedding and sheets etc. To make our own vacuum bags, we use sheets of plastic film and double sided tacky tape. There are two basic ways of vacuum bagging.
Envelope bagging is a type of vacuum bag where you envelope the entire part in the bag. Because your entire part needs to fit in the bag, its most commonly used for smaller parts.
Surface bagging is the other approach, where the surface of the mold is used as the bottom half of the envelope bag and a sheet of plastic film is used as the top half of the envelope bag.
Peel Ply
Peel ply is used as a barrier between the resin from the part and the vacuum bag. The reason why it’s called peel ply is because it's a thin fabric that can be easily peeled off of the composite part.
Short video: https://www.youtube.com/watch?v=dz_aJSfBLZI&ab_channel=FibreGlast
Breather/Breather Cloth
This is a fabric cloth used as the final layer before the vacuum bag. Its purpose is to ensure that air will always have a medium through which it can reach the vacuum hose. Without, you could have a section where the vacuum bag folds and blocks off the air’s path to the hose.
Video: https://www.youtube.com/watch?v=0QPu-qO_sAo&ab_channel=FibreGlast
Note, we don’t use aluminum foil in our layups.
Manufacturing Approaches
Wet Layups
This is typically the easiest method and usually the choice of small DIY projects. You essentially lay down a layer of carbon fabric and then introduce the liquid resin by hand, by pouring it on the surface of the fabric and then spreading it out with a squeegee.
This method has several strong points. It gives you control of the resin to fabric ratio, has far less prep work, and needs the least amount of equipment.
However, it is also possibly the messiest method, and many times there will be more resin that is needed - effectively deadweight as it no longer helps strengthen the part considerably.
Video of a wet layup: https://youtu.be/cj26c3V54SQ
Resin Infusion
This was the manufacturing technique that was used for the previous car, MSXII. The resin is introduced using tubing and pumps and is drawn throughout the entire part with a vacuum pump.
This method requires a good amount of setup work but doesn’t really require too much specialized equipment. It also ensures an excellent surface finish as typically, there will be more resin than necessary and thus the part will resemble a laminate table. However, the drawback is the same as the wet layup, the part could be heavier than needed.
Video of resin infusion layup: https://youtu.be/VodfQcrXpxc
Prepreg
This is the current manufacturing technique for MSXIV. Prepreg contains resin in a solid-state that becomes liquid and slowly cures at room temperature. However, room temperature cures over the span of a few weeks. Instead, higher temperatures are used (70+ Celsius) in order to turn the resin into liquid more easily.
When curing prepreg, both temperature and pressure are extremely important. Each manufacturer’s prepreg has different specs and they must be strictly followed in order to ensure the same mechanical properties stated by the manufacturer.
An autoclave is a chamber that can control temperature and pressure.
Prepreg is usually the most optimal method of manufacturing for established companies. It doesn’t require a lot of preparation work.
Video of prepreg layup: https://youtu.be/HfrFaKDsJxc
Carbon Fiber Layers
Before I hop into this topic, a few concepts need to be understood
Tension and Compression in Bending
When a beam or part is put in bending, there will be tension (material below the green plane), compression, and a neutral axis (green plane). In our case, we’re really only concerned with tension forces. This is because failure in a carbon fiber part is when the fibers exceed their tensile strength and begin to snap/fray.
Isotropic vs. Anisotropic materials
Isotropic materials are defined as one that is equal in strength in all axes. This means that if I were to grab a blank sheet or block of raw material, I could machine my part in any orientation and the strength would be the same.
Anisotropic materials are materials that are stronger in one axes vs another! A common example is wood - it's much stronger if you push it downwards and allow the fibers to bend.
Here’s a good video for a more in-depth explanation: https://youtu.be/Hu4zjPIeDq8
Applications in Carbon Fiber
In the majority of carbon fiber parts, we want the panel to be isotropic. This is cause the load cases are not always fully defined. For example, the floor panels of the car, we cannot guarantee that the passenger will step on the exact same spot every time. Therefore, we need to ensure our panels are able to sustain loads wherever they COULD be applied.
To do this, we leverage the fact that carbon fiber strands are really strong in tension! We orient the fibers such that when the part is bent, the fabric is put into tension as opposed to the epoxy.
Here’s a quick visual
In this first picture, the fibers run towards the load/force. In this case, when the load is applied, the part bends and puts the fibers in tension. Since carbon strands are strong in tension, this part would be very good at sustaining loads similar to the one shown.
In this second picture, however, the strands of carbon are running perpendicular to the load. In this case, when the part is loaded, the epoxy is put in tension as none of the fabrics run in the correct direction to sustain the tensile forces. This means that the part would snap much earlier
For those that are keen, you may have noticed that the first case could easily turn into the second scenario if you were to relocate the load such that the fiber directions run away from the load. This is why we stack up multiple layers of carbon fiber with strands running in multiple different directions!
By having strands of carbon running in multiple directions, we ensure that when a part is bent from an arbitrary location, the strands are able to sustain the tensile loads!
Common Fiber Arrangement
Shown above are the 2 most common fabric arrangements. At Midnight Sun, we do 6 layers instead of 5 so that “stack up” looks a bit different. However, the concepts still hold.
0/90 ply infers that the strands of fabric run length and widthwise on the part. -45/45 infers that the strands run 45 degrees offset from length and width. 45-degree layers allow a part to be more versatile for all load cases but less specialized for a particular load case.
A 0/90 layup is the colloquial term for a layup containing only 0/90 layers and a quasi-isotropic layup is a term used to describe the picture on the right. It’s called quasi-isotropic as it is only part isotropic.
Below is a picture showing differences in deflection of 2 parts, both under the same load. One being a 0/90 layup and one being quasi-isotropic.
As you can see, the 0/90 layup deflects far more as there are no fabrics in the direction necessary to sustain the load. This leads to the 0/90 fabrics only being able to contribute a “component” of strength, similar to vector addition.
This world of fabric orientation customization can be used anywhere, you can add strength to a part in a very specific direction!! So long as you have a defined load case, you can optimize your part, structurally, to handle that load!