Carbon Fiber Tube

The different types of carbon fiber tube
There are two main types of tubes we are covering in this tutorial: Pultruded tubes and roll wrapped tubes.
Pultruded tubes tend to be in the smaller diameters and provide the maximum possible longitudinal strength but at the expense of being more vulnerable to crushing or torsional fracture. The inner and outer surface of these are smooth with the fibres visible if looked at closely. These tubes are formed by pulling the fibres through a resin bath and heated die to form the tube. The fibres are all orientated down their length. The pultrusion process does mean the tubes have a good level of stiffness but are easy to split.

Mechanical testing introduction
The purpose of these mechanical tests is to provide comparative data between the two types of carbon fiber tube, 304 stainless steel and 6063 aluminium. We are using the same profile sizes throughout the tests, namely 10mm outside diameter and 8mm inside diameter. This will show the performance relative to each other for identical profiles.

Tensile strength test
This is a simple tensile test where the tubes are pulled on until they fail. The tubes are pulled until their elastic limit and then further pulled until they break. The elastic limit is indicated by the constant gradient on the graph and when it begins to yield and move to plastic deformation, the gradient reduces until it breaks. As such the yield point is much more important to note.

Compressive strength test
This is a relatively simple compressive test pressing the tube between two grips. The metal samples behave pretty much as expected and the carbon tubes again fail at the grip showing the limitations of the test method. The steel tube is the best performer at 1.5 tons, followed up by the pultruded and roll wrapped tubes at just under a ton, with the aluminium tube failing at around half a ton.

Torsional stiffness test
We are comparing the tubes with a known torsional load to provide comparative data until the tubes slip in the grips or fail. It is particularly difficult to get a good grip in this kind of test to hold until failure, but we can test them to a known level. In this case 5 newton meters which is about what you can achieve with screwdriver, then continue until the grip or tube fails.

3-point bend test
This is a flexural 3 point bend test for the tubes. Using custom tool heads to show elastic and finally plastic deformation and eventually failure. As expected, both the metal tubes failed through plastic deformation and bending. The carbon tubes deflected before failing through cracking and crushing of the tube. The roll wrapped tube performed the best at around 200kg followed closely by the steel tube at 175kg. The pultruded tube failed at around 140kg followed by the aluminium at 50kg. This test particularly highlights the benefit of the additional hoop fibres on a roll wrapped tube compared to the pultruded tubes.

Pultruded tube results & considerations
In this section we consider the results and how it might influence the use of this type of tubes. In pure tensile and compressive loadings these tubes offer exceptional performance for their weight. Loaded down their length, these pultruded tubes are about as strong as a carbon tube can be but this is at expense of torsional performance, crushing and resistance to splitting.

Roll wrapped tube results & considerations
In this section we consider the results and how it might influence the use of this type of tubes. As seen in the tension and compression tests, these tubes perform almost as well as the pultruded. The three point bend test really did highlight the benefit of the hoop fibres and the crush resistance they offered over the pultruded tubes. It also highlighted how the hoop fibres do improve the torsional resistance slightly.

Limitations of carbon fiber tubes
Although the performance of carbon fiber tubes is very good for many applications, there are also limitations and uses where carbon fiber tubes may not be ideal or suitable. We shall cover some common examples here. A good example where carbon fibre may not be suitable is high temperature applications. Carbon fibre will severely deteriorate above 120°c. They are not suitable for applications involving high levels of abrasion such as sliding bearings and bushings where the surface will wear very quickly. Precision machining such as cutting of threads does not work well as the thread would be very weak - a better solution is to use a metal insert or similar with the thread in the metal for such applications.