Repairing Cracks

Talk Composites - The Forum for Advanced Composites

Back

Repairing Cracks

Author	Message
fibernoob	fibernoob posted 8 Years Ago HOT Topic Details
Group: Forum Members Posts: 34, Visits: 134	Hi folks. I need to repair a cracked CF part (automotive rear spoiler /wing) and prefer to seek some wisdom before I start the job. The crack seems to be on the surface, but I am not certain if it has gone through the part (I do not want to pressure it to make it worse). There are in fact 2 cracks - one larger exposed one and another hair line crack below (see image) Unfortunately this part is NOT accessible from behind as it is one closed piece. I am thinking of sanding the crack with a small file by hand rather than using a Dremel not to risk cutting through it. Then filling the crack with a clear lower viscosity epoxy resin (using a syringe) and then polishing it. Any other alternatives to this ? I am concerned with cracks growing so I have considered drilling small holes on their ends to prevent further opening. Does this method (often used in metal) also work for Carbon Fiber ? (See image) I also will need to fill the holes afterwards. Many Thanks in advance Tags c/f cracked cracks repair repairing Edited 8 Years Ago by fibernoob
	Reply Like 6
Hanaldo	Hanaldo posted 8 Years Ago HOT Post Details
Group: Forum Members Posts: 2.5K, Visits: 28K	The hairline crack is just the clear coat so that's fine, but I would be inclined to say that the larger crack is damaged fibres. Considering it is in such a load bearing area, I would want to be doing a structural repair there rather than just cosmetic. Unfortunately because it is a closed piece, there won't really be any way of doing a structural repair that is also cosmetically perfect. If it were me, I would scarf in the crack with a Dremel either until the crack is gone or you start to break through the skin. Avoid going all the way through if you can. Then lay some fibres into the crack and wet it out with resin, or chop up some fibres and mix them with resin to make a 'paste' which you then smear into the crevice. Either way, the use of some release film on top will help both in getting some compaction and with getting a flatter surface again. Also yes, drilling holes on either side will prevent the crack from propagating - the downside being that you will be left without any reinforcement in those holes. Best to use a filler powder or milled fibre to give the resin more strength and hopefully hold up.
	Reply Like 5
fibernoob	fibernoob posted 8 Years Ago HOT Post Details
Group: Forum Members Posts: 34, Visits: 134	Thanks again Hanaldo.
	Reply Like 6
f1rob	f1rob posted 8 Years Ago HOT Post Details
Group: Forum Members Posts: 237, Visits: 4.8K	As said you cant really do anything from the outside "but" when you first look at doing a repair you should first look at what caused it in the first place and aim the repair towards this. without a picture of the whole item its hard to be certain but in going for mounting pylon on a rear wing element. Normally you would have ribs,spars an reinforcement in load areas but on the cheapy stuff they don't bother. wings flexing and pushing down on the pillars under load an its snapped in the area where there is least flex. Myself I would drill out the two rivets shown in the pic and use them as your inlet points and inject with worth or john burn two part repair resin using the mixer nozzle in the river hole. Its fairly thick (think p38) an if you get the quick one it sets in 1.5 minutes so doesn't really flow away it stays where you want it. hold the wing vertical (trailing edge down) an inject away up to an beyond your rivet holes. The area of the crack is now solid so no more flex. refit a couple of rivets an do a cosmetic on the outside with fast epoxy/lacquer whatever your weapon of choice is
	Reply Like 6
fibernoob	fibernoob posted 8 Years Ago HOT Post Details
Group: Forum Members Posts: 34, Visits: 134	Thank-you all for your help. Just posting this pic to show the end result. Given my time frame and some technical challenges, I am pleased with the end result.
	Reply Like 6