Originally Posted by
Canaboo
I can't really agree with that. The matrix glue has to complement the fiber choice.
The highest modulus Carbon with a soft curing epoxy isn't going to be a stiff frame.
Different types of epoxy have wildly different strength ratios. West epoxy is very highly thought of as a binding resin but something like Poly Epoxy has double the strength.
If the carbon is really only providing tensile strength then it has to be bracing itself against something with good compression resistance to show that strength.
West epoxy has about 15,000 psi of compressive strength while Poly Epoxy is 32,000.
I think you are a bit confused about stiffness and strength.
Since the material as a whole must equalise strain for a given stress, the load taken by the different materials depends on their ratios of strain to stress, which is their stiffness.
CF is approximately 100 times as stiff as the epoxy matrix, the exact number is obviously dependent on fibre type and resin but epoxies are around 3.5 GPa and CF ranges from roughly 200 to 400 GPa*. The law of volumes applies**, so for a standard 60% content of 300 GPa fibre the load carried by the fibre is 0.6 x 300 / (0.6 x 300 + 0.4 x 3.5) which is 99.3%.
The function of the matrix is to bind the fibres together so they share the load. Problems arise when the soft matrix allows the fibres to bend so they are no longer loaded optimally. This is what makes CFRP weaker in compression than in tension: a tension load pulls the fibres straight so the material will hold until the fibres snap which takes a huge load. In compression the applied load tends to bend the fibres so the material will hold until the fibres start to buckle which takes a smaller load, how much smaller being dependent on a number of factors such as layup.
Problems also arise arise when the matrix can't transfer the load between layers properly, typically because of interlaminar shear. When this occurs the various layers can move against each other and are thus independently loaded and the overall properties suffer.
* I've seen some references to UHM carbon at up to 800 GPa but I don't know if anyone gets to use it.
** If the fibre isn't completely wet by the matrix, a modified law of volumes applies where a diminution factor is applied to the fibre volume. One good example of this is flax fibre composites where the diminution factor can be 1/3 or less so flax fibre composites aren't very strong or stiff at all: Although flax itself is about 70 - 80 GPa so it's about 1/3 as stiff as a medium modulus carbon, a 50% Flax composite struggles to reach 15 GPa which is about 1/10 what you could achieve with said medium modulus carbon.