Why Composite Materials (and why maybe not)?
When it comes to performance, CFRP (Carbon Fibre Reinforced Plastics) offer an unbeatable strength to weight ratio. It is no suprise this material class has found its way to aerospace, automotive and industrial applications. Even if this perspective is valid, the core of utilizing Composite Materials is not fully represented.
Composite Materials are what we call "Engineered Materials", which means you can customize their behavior according to your engineering needs. In Fiber Composites like CFRP this is done by utilizing the design freedom of the material.
Composites are mainly made out of two components: The Fibers and the Matrix. Usually the Fibers is not used directly. Instead so called "semifinished" textiles are used to facilitate manufacturing. There are probably endless ways of varying the mixture, orientation, stacking and shape of composites.
So in result we have a material class with endless modificatio npossibilities. But this also makes it complicated to find a starting point. Therefore I want to offer some advice from my experience as a mechanical engineer.
1. Tip: Start with Costs and Mechanics
In Lightweighting, the core target is to reduce weight at a reasonable cost. Therefore, having an understanding about the cost target of your product is essential for engineering suitable products and materials. This should give you an early estimation if CFRP is an option at all.
If you feel that there is an allowance for higher costs to increase performance, CFRP might be a good option. As mentioned above, the orientation of the plies is a defining factor for a lot of characteristics in this material class. Start thinking about load pathes: Where are loads inserted to my structure and where is my structure mounted. Now connect these points. This will give you an early indicator of where you need to put fibers in which direction.
Once you are done, start building simple FEA models of your structure, representing the anisotropy of your material. Play around with orientations, thicknesses, shapes and other influencing factors until you find a valid setup.
2. Tip: Define the manufacturing process by looking at the topology
As mentioned above, composites are a mixture of fibers and the resin. There are again endless ways to bring these two components together. Unless you have good insights into the process variants, it is a good starting point to look at the topology of your structure. Here are some examples of where this might lead you:
- Tubes: Tubes are usually made by filament winding or pultrusion (with each having subvariants)
- 3D-Shells are usually made by Prepreg/Autoclave process for high quality and low production volume or by infusion for big parts in low volume. Automotive parts which are usually made in large volumes are usually produced by HP-RTM or Wetpregging.
- Flat panels: Those are widely available and can be cut into the desired 2D-shape by CNC-machining.
3. Tip: Design according to #1 and #2
Finally it comes to designing the part. Unlike most other processes, in composite materials you would not want to start with the design and then go to simulation as the design is mostly defined by the mechanics.
Make sure to design according to the manufacturability of the chosen process and that the fiber orientations you defined in step #1 can be achieved with the given process.
Besides the 3D-CAD, you also need to define a "plybook" which guides the manufacture on where to position which ply or patch at which orientation. If you are doing it the first time, make sure to at least share the following information:
- Fiber, resin and textile type of every patch or ply
- Orientation of every patch or ply
- Area density of your textile
- Manufacturing process
- Quality inspection method and documentation requirements
Too complicated? Get expert advice.
Congratulations. You are on your way to designing your first composite part. If material choice, anisotropy or regular problems of manufacturing composites should get over your head, don't hesitate to ask for expert advice. Consultants and Engineering companies in the Composite field are well aware of issues that might arise and are (mostly) happy to help. Experience is a everything in Composites, so don't be shy to ask for it.
