Gentlemen i am having a thought i would like to share with you.
The normal lay up resin usually has better or equal specifications than the infusion spec resin - probably due to the low viscosity of the infusion resin and the compromises that need to be done in order to achieve it. But i think this is also the reason why the end result of the resin infused part is of better strength - the low viscosity of the resin helps the resin to reach and bond with the carbon fiber more significantly than the more viscous resin does . And this is a very important part ! Do you agree with this or not ??
Epoxy is only 1% of the strenght of carbon and yes it’s important.
Michiel is mostly right, matrix-dominated properties are shear (in-plane and interlaminar) and some compressive effects but overall it’s largest effect on the strength (usually talking about tensile when referring to carbon) of the laminate comes though transferring loads to the fibers. In doing that the processing effects are hugely significant. I would say that it isn’t the viscosity of the resin that is allowing a significant strength increase, although it is important in penetrating the fiber bundles, but the vacuum infusion process eliminating void content that would otherwise be present in a wet-layup process. Void content (introduced during processing) has a significant effect on the ILSS which leads to delamination and/or fiber pullout before fiber failure, which manifests as lower “strength” in both “hit it with a hammer” testing and more rigorous tests.
Sorry for the text wall, it seems my coffee has finally kicked in this morning.
Summary - You are correct but I think a different mechanism (void content) is responsible for the increase in strength rather than viscosity
I would say it is simply the fibre/resin ratio.
No reason a hand laid job should have any significant air voids but the resin content of infusion layups would be much less.
I’d like to butt in for a second and ask a somewhat related question that deals with volume fractions and the like.
Lets supposed we’re looking at a strictly unidirection ply in the 0*, and all tensile tests are done in this 0* (lets be theoretical and say the fibers are aligned perfectly and the test coupon is aligned perfectly). The uni has no crimp.
How come, when you take, lets say a 650KSI (~4500MPa) fiber strength fabric, and then impregnate it to, lets say, a 70% fiber volume fraction (30% resin), the drop in strength is so dramatic? Even if the resin had 0 strength, shouldn’t the tensile strength only drop by ~30% since the cross section is now comprised of 30% resin (which, in this example, we can assume has 0 TS)?
Instead, most unidirectional laminates have real world ratings of something more along the lines of 150-200 KSI (1000-1400MPa)… what gives? How come the dry fabrics lab ratings are so high but real, useful laminates are so much weaker?
Thanks!
You mean the tensile strength or maybe a 3 point test ?
Because if you talk about a 3 point test the the force applied to the fibers is brings in the inertia of the outer spline of deformation which
is in 2nd F*f where f is force…so it is way much more