Any tips on inspecting carbon?

[harmonix1234]

Before I get into I it I will say that I am taking it to the shop for a qualified mech to look at.
However, does anyone have any good techniques for a home inspection?
Basic diagnostics tools?

I figured we should all have these skills in our bag as the world of biked becomes carbon dominated.

I found a crack on my carbon frame, but unsure if it's just the paint.
It's about as big as a 5 cent coin and shaped like a corn flake.
I can get my fingernail just under the thin layer of what seems like paint and it appears to go no deeper than the paint layer.
There has been no impact, it's on the side of the down tube a few inches from where it joins the head tube.

What I have used to determine it's OK is
a) The coin test. - Tap the area around it with a coin and listen to the tone. It should be a nice hollw resonating 'Donk'. Then tap the suspect area and listen to the sound. If you get a dull thud or change in sound then it's suss, but in this case it sounds just the same.

b) Press test. - With my thumb, press firmly on the suspect area, if there is a crack it may either depress slightly, or make a creaking noise. Neither of which happened.

c) Crack pattern. - The fact that it's a round shape only a fraction of a mm thick, and doesn't seem to go anywhere it makes me think that it is not a crack in the carbon as there is no linear seam that goes along any join of stress line, it's a really strong part of the frame and probably where the most carbon layers are.

d) Exactly the same thing happened on the chain stay. An area about the same size and shape flaked off, I got my fingernail under it and it just fell off like a scab. Underneath was smooth carbon. I got some 2000grit wet and dry and just smoothed off the dip between the painted edge and the carbon underneath and then hit it with a little clear nail polish and a year later it's just the same. Only a paint chip, but a large one.

Any suggestions for other diagnostic tricks I can do before I take it to lbs?

I'd post pics but not at home at the moment.

 

[eastie]

Roadie or mtb? (sorry - had to ask...i.e. different stiffness and material behaviour).

There's a couple of things you can do yourself to try to quantify damage like you've mentioned, but they are pretty crude. My take on it is below having worked through a few issues one on one with a great bike mech and santa cruz dealer...

Visual - is there any damage to underlying weave visible / can you flex the section and does the facture grow/shrink / push on the fracture and either side while looking and feeling for any movement. You can also rub a crayon of contrasting colour over the crack to increase contrast and help pick up any visual movement. Take a good quality close up photo of the area - that will let you compare it over time if you don't opt for repair.

Noise - repeat above listening very closely for movement noise; any noise is not good. Next is tap test: take a tea spoon/screw driver handle/... and tap the area, then perform similar around the area, opposite it etc. Go back and do it with a different material (coin then screw driver) again listening for any discernible difference. I found it's really obvious dull/deadened sound as opposed to the solid ping everywhere else. As for cracking at joints, you'd have to wiegh up the build, if it is a tube and joint construction or one piece layup? I'd imagine paint cracking at tube/glue frames would be common? Would that mean bond failure? (unlikely me thinks)

Paint chips - not so sure? The paint chipped on mine (under frame skins) and that was the sign of underlying damage (not near any join). If it's chipping in a roundish area I would start to think impact damage of some kind, again carbon is pretty forgiving.

I fractured the rear triangle at melrose last year, first ride in to a 2 week trip across SA/Vic and bang/crack, most stupid low speed crash I've had. Full thickness on the impact, partial thickness around the radius of the seat stay. Rode it for the entire 2 weeks, then for a month on return before getting the frame inspected/tested and opting for a new rear triangle through SC crash replacement policy which was marginally more expensive than a repair.

If you go to a dealer to help with the initial assessment, make sure they deal in composite bikes, and by that I mean they move them often, not just have one or two on show. While they most likely wont have ultrasound inspection gear, they will likely be more attuned to picking up on possible issues from the basic visual and noise checks and know what to send off v's what's most probably ok.

If you are concerned beyond curiosity, get it ultrasounded