Technically Speaking, if one had a 1/2 thick layup of Carbon fiber with Epoxy or Aramid with epoxy, given the same quantity of material (by weight) and same thickness of final product (8" x 8" and 1/2 inch thick square layup) and one were to drill a hole and install a bolt and start to tighten, which material would fail first?
Well if the thickness is the same and volume is the same, the kevlar should be lighter given its lower density than carbon ~1.45g/cc compared to 1.8g/cc with the carbon.
Tricky one which I dont know the answer to, surely shearing forces like that would be better resisted with the kevlar, since it is effectively designed to resist shearing forces. Im gonna guess carbon would fail first when loaded like that.
Kevlar will fail miserbley.
Moved to Engineering Talk due to content
If you are drilling at 90deg to the fibre direction you are looking at the transverse properties, which are the same for carbon and kevlar (140MPa). The driving factor will be the bond to the epoxy which will prevent micro buckling, as kevlar doesn’t bond to the resin then it will be much worse.
Maybe someone can clarify this. When hand laminating with epoxy, in what circumstance is kevlar better then carbon, when considering building a powerboat for example which is subjected to several twisting and “banging” stresses? As from what I’ve read, leaving the cost part out, carbon fiber seems to be more ideally suited for constructing a powerboat. I was dubious regarding the “bolt pressure test” (which was the first question I asked in this thread) But it seems that again, carbon is more suited. (I assumed aramid would have been better due to the fact that bullet proof vests are made out of aramid and not carbpn :/)
Are you talking about pressure exerted on the bolt/nut, or an external source of pressure that is attached to the bolt/nut (ie: mounting point)?
I think they will both be fine for a bolt hardpoint. However, the kevlar might crush slightly and hold the nut in place without vibration. Also on a side note, good luck drilling a blank of kevlar 
Now, if you want to protect a bolt mounting point, then adding kevlar will help take some of the moment off the carbon fibers. It will be more flexible and transfer the load along it’s plane, taking away the load forces from the more brittle carbon fiber.
As for banging forces, it’s the same idea. The kevlar will distribute the load throughout the layer, and allow a bit of flexibility that can fatigue the carbon fiber. It is the same line of thought on why you want a kevlar skin for impact resistance. Carbon will slowly crack, splinter, and when enough of the laminate fails, it will go BOOM. If you add some skin layers of kevlar, it will in essence hold the carbon in place, and take the impact.
as for the vest comment; the kevlar layers catch the bullet, and transfer loads along the fibers, taking the force from the impact and distributing it. Carbon is VERY brittle and breaks easily with high load. So an impact will just shatter the fibers and matrix and not allow much load to be transffered to the rest of the bulk material.
(ps: i put in laminate theory and textile theory in there, but it should make sense)
Now I’m no engineer and certainly no expert in the world of composites, but I’ve always worked under the thought that the use of Kevlar where needed is done so as an after effect if/when the epoxy resin binding the fibers fail…
IE…If I make a particular panel to cover a hole in a race car that needs to be light but also have penetration resistance, the benefit of using kevlar in the part for penetration protection wouldn’t come into effect until AFTER the part were impacted and the resin cracked/failed to the point that the fibers were all that were retaining the remaining structure of the part. (this obviously isn’t taking into account how the part is made nor how it’s fastened to the area in which it’s intended to cover)
Just me take on the whole subject…
I’m not sure if I understand your logic here…
Wouldn’t the lower modulus of kevlar cause less of the load to be carried through the kevlar fibers when compared to a layer of carbon? (assuming we don’t break anything)
Kevlar is great with bending and tension. It will not break as easily and brittley as carbon fiber. If you want a highly loaded bolt, the kevlar can help distribute the load without breaking. If you use several layers of kevlar in with the carbon fiber, or use a hybrid, it will help even everything out.
Again, I’m not sure what exactly OP is asking here. Yes, a powerboat out of carbon fiber will be light, and strong, but if you are wondering what forces there are on a bolted panel, a large impact on the bolt location will shatter the carbon. Adding kevlar will help minimize breakage. Making the whole boat from kevlar is NOT good, because it is not as stiff, and will deform easy, and bend/twist.
If there is a direct shear force on the bolt then the carbon will perform better than the Kevlar if the thickness is equal. Tested it myself.
Carbon is typically your first choice when stiffness is needed due to it’s significantly higher tensile modulus. Kevlar is tougher than carbon but less stiff. Toughness can be understood by comparing the area under the stress strain curve. Kevlar has better better strain properties as well. When it comes to impact you need to look at the rate of strain…i.e. how quickly the force is applied causing deformation. When the rate of strain in high then carbon will perform rather poorly.
The downside to Kevlar is it’s poor inter-laminar strength when fatigued and poor compression strength. This is why it’s often woven with carbon or s-glass. The inter-woven carbon or s-glass with improve the inter-laminar strength and improve it’s fatigue traits and compression strength.
Creating composite structures strong enough is much easier than making them stiff enough to resist deformation. This is why carbon is often used.
The only time that I use Kevlar is when radio frequencies need to pass through a portion of the skin/structure or puncture resistance is needed for safety. It’s stiffer than glass and you can get more thickness than glass for the same weight.
So would sandwiched layers of kevlar and carbon be an overall improvement with kevlar on the outer face than a mixed weave?
If you have concerns about impact on a boat hull then e-glass, s-glass, carbon on the outside and kevlar on the inside. Kevlar is great in tension and will minimize punch-through; tough stuff.
I’m making some stone guards at the moment. I did a test piece made with 520g of Diolen and 600g of glass. I had my mate shoot at it with a gas powered BB rifle. Half of it at the diolen side, half of it at the glass side. When shooting at the diolen side, it just dents. When shooting at the glass side, it creates tiny fractures and spider cracks appear on the diolen side.
Although Diolen is not Kevlar, it’s has similar properties. Yet my crude trials show it performs better on the outside.