Quadcopter (drone) frames and carbon fiber strength: a discussion

Quadcopter (drone) frames and carbon fiber strength: a discussion

Cheap carbon (CF) fiber items such as quad frames coming out of china can be sketchy. It is useful to understand how carbon fiber works in order to identify cheap frames that are fundamentally good and bad.

First, it is important to understand what makes any material strong. Most materials have strength in two directions – tension and compression. Lightweight materials are strong only in one of the two directions. For example a piece of paper or fabric is strong under tension, however it is very weak under compression (i.e. you can hang a pretty heavy object from a piece of fabric, but it is completely incapable of supporting any weight when you try to compress it). On the other end of the spectrum is something like concrete, which has very strong compressive strength, but will fall apart if under tension. Composite materials such as carbon fiber are different, they combine materials that are strong under tension and compression into one (hence the composite). In case of CF, the carbon fabric provides high tensile strength, while epoxy that holds the composite together gives the material high compressive strength. Carbon fiber, however, is famous for being very strong and stiff only in certain direction. This is due to the way carbon is laid up. In most cases carbon fiber is made out of sheets which are on top of each other. Each sheet has a directional weave, so the material has most tensile strength in the direction of the fibers in the weave. CF is not as strong when tension is applied in a direction that is 45 degrees in relation to the fibers in the weave (see below).

Carbon fiber, however, is famous for being very strong and stiff only in certain direction. This is due to the way carbon is laid up. In most cases carbon fiber is made out of sheets which are on top of each other. Each sheet has a directional weave, so the material has most tensile strength in the direction of the fibers in the weave. CF is not as strong when tension is applied in a direction that is 45 degrees in relation to the fibers in the weave (see below).

On a quadcopter when an arm flexes downwards, the top of the arm is loaded under tension, while the bottom of the arm is loaded under compression. If too much force is applied or the arm flexes too much, it will fail catastrophically. Therefore it is very important for it to be strong in both compression and tension, making the way it is cut from CF extremely important. It is critical to cut the frame out of a sheet of carbon fiber in a way that maximizes the strength of the material to the areas where the frame is most likely to break (such as arms), as well as areas of the frame that would ideally be as stiff as possible.

Above are examples of some frames that are good and bad when it comes to their cut. Green lines indicate the direction in which the carbon would be stiffest and likely strongest under tension. Red lines would indicate direction into which carbon would be weakest under tension. The arm on the left would be very stiff and strong, while the arm on the right is weak and flexible due to direction of the fibers in the carbon composite. Basically, the easiest way to identify which direction the piece is strong is to look at direction of weave “zig-zags”. If you draw a straight line through the zig-zag, that is the direction in which the carbon fiber piece will be the weakest. Below is an example of the frame that is cut in completely wrong direction. You can see the CF zig-zags going the length of the arm. This results in a very flexible frame that can be very prone to delamination due to low tensile strength.


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