I am wondering how to approach the problem of carbon-fiber based bone replacements. Considering a femur, what downsides would carbon-fiber have under the types of torsion, compression, wear-and-tear, etc.. experienced by a biological femur?
It would be great if you happen to consider any methods for countering downsides, if any. Question assumes production is not an issue but design is.
So let's look at the facts. The bones in your body are made from material which has a tensile strength of 150MPa, a strain to failure of 2% and a fracture toughness of 4MPa(m)½. For a structural material that's not good. We can make alloy steels that are ten times better in all three of those properties. But of course there are some other factors we need to take account of in order to make a valid comparison. Bone is less dense than metals and this is important because the weight of our bones strongly affects the energy needed to move around. To do a quantitative analysis we need to consider the geometry and loading on the structure. The major bones are mostly tubular in shape, loaded in compression and bending. So a rational comparison is to imagine tubes made from different materials, all having the same length and diameter, with their thicknesses adjusted to give them all the same weight. Putting in some typical dimensions and material properties we find that the stresses in a bone made from titanium alloy, for example, would be about 1.3 times higher than in a bone of the same weight, made from bone. But the titanium alloy is 5 times stronger so obviously its safety factor is much higher.