Carbon forks won't be a problem. It's hard to find a decent modern road bike that doesn't have carbon forks, and riders of all weights fail to break them. Failure or a metal steered is far far more common than failure of carbon fork blades
Carbon has great strength per weight and fatigure resistance. The problem for a mountain bike is delamination.
This means that after a crash a fork can look fine, but the bonding between layers has weakened and that at some point in the future the fork will suddenly snap, usually while under stress. This is a problem for crit riders too and there is some guidance for diy checking here (I suspect many bike shops will always tell you to buy a new fork, because the legal risk of doing anything else is too high - plus, sell fork = good):
http://www.bikeforums.net/archive/index ... 70450.html
08-10-10, 11:57 AM
If there's no obvious cracking damage showing there's still the issue of internal delamination that is hard to find. Two methods of checking it yourself come to mind. First off hold the fork by the steerer and using a screwdriver handle tap the legs all over and listen to the sound. Taps on each leg in the same place should sound the same. Any sort of drastic change in the sound is likely caused by some delamination. Assuming the fork passes the sound check then I'd move on to a flexing check. For this you need to grip the steerer in some manner that will hold it very firmly. Something like a block of wood with a 1 1/8 hole that is sawn through and then the blocks and fork are clamped to a door frame or in a vise attached to a heavy immovable bench. Once secured I'd flex the legs inwards, outwards, to the front and to the rear. What you're looking for is for both legs to flex equally. If one flexes more than the other then again there's very likely internal delamination. And if it snaps like a dry turkey wishbone then again you have your answer. Don't be gentle with it. You should be using enough pressure to flex the legs by about 1/2 inch to properly simulate the sort of load it'll see on the road. This will be quite a bit of tension and it may be hard for you to do this and still look for even flexing in both legs. So it may help to have a spotter watching and measuring things.
...As BCRider explained there's no reliable way to test. If it were anywhere else on the bike, I'd follow his methodology and make a judgment call, but it's a fork and I hold those to the highest standard
So basically, understand the issues - if you don't really need the weight saving and are an aggressive rider who pushes it and has the odd off, consider staying away from carbon unless you're willing to either chance snaps or replace forks. Otoh if you never crash and are loaded, much more attractive!