Friday, March 19

Damage to Composites

In practice, as long as you don't do something dumb like take a torch to the composite, there are two main types of damage that the owner of a composite bike component needs to worry about. The first of these is impact damage and the second is delamination. I'll discuss each separately, though an impact may well cause delamination along with the more obvious symptoms of the impact.

Schematic of how Impact Can Propagate and Spread as it Travels through a Laminate
The Wiggly Lines are Broken Fibers (I don't CLAIM to be an Artist)
Think of the Damage to a Window from a BB Pellet

Impact Damage
100 foot pounds. It's an energy measurement. It's the energy that a 100 pound object the size of a 4 foot by 8 foot piece of wood would impart to whatever it hits if dropped from one foot in the air. It is also roughly the energy of a 22 caliber bullet as it leaves the muzzle of a gun. Clearly, not all impact energy is created equal. If you are going to get something on your carbon bike smacked, if it gets hit by something very small in diameter, you'll be able to see it long before it becomes a cause for concern. On the other hand, it it gets hit by something blunt, you may not be able to see the damage until after it has fatally compromised the structural integrity of the bike. That damage consists of broken fibers, and in delaminations (plies get unstuck from each other).

If you have ever seen the Zapruder film of the John Kennedy assasination and how the bullet takes half his head off on its exit, you can clearly see the problem with impact damage to composites - things are often MUCH worse on the back side. A minor pip on the front side may represent major broken fibers and extensive delamination on the back side - the side you cannot see if you are looking at a damaged fork or frame. Amazingly, a Google search on carbon impact damage doesn't show this very clearly. There's one, halfway decent photo of a fiberglass part, from this page, here, but even the better photos, here, really don't show how the impact spreads through the composite as it propagates through the material. In a nutshell, the damage may be small on the front and extensive on the back.

Photo from Wikipedia
This is NOT a Photo of a Simple Delamination, but Rather
of What Happens When the Part Takes Too Much Load
AFTER a Delamination
Delamination Damage
Delamination occurs when, often in response to an impact, the composite layers come "unstuck" from each other. Imagine, if you would, a half dozen 2X4's screwed together with screws every couple of inches. You could put one heck of a compression load on them. Now, imagine that all the screws are removed. The load capacity would be much less. Well, carbon is exactly the same way. If a delamination occurs, the part has a reduced compressive capability. The bigger the delamination, the more the reduction. The more layers that are delaminated, the more the reduction. Fortunately, delaminations usually have to be fairly big before a large compression strength reduction occurs.

Other Damage
There ARE other damages that occur to composites. I've already mentioned burning. Most composites used on bikes are like cookies rather than chocolate. If overheated, they burn rather than melt. In addition, the epoxy resin can be subject to deterioration by extended exposure to UV, leading to failure to support the strength giving fibers. Another potential problem is softening of the resin or dissolving of it via solvents or acids. Fortunately, most of these other forms of damage are readlily apparent from a casual visual inspection and they LOOK horrible long before they make the bike dangerous. Corrosion can also be a problem, but THAT will be another article, because it probably WON'T be the carbon in your carbon bike that rots away.

11 comments:

Doohickie said...

Fork damage.

Oldfool said...

Good informative post. You make it clear. I understand composites but I didn't know that carbon was just another one. Not that it will ever be IN my life.

Steve A said...

I would suspect the failure occurred as a result of a possibly unnoticed impact (during shipping, in the garage, or in use). The braking put the trailing edge of the fork in more compression than usual. Couple that with a pothole and the trailing edge of the weakened fork can cripple, causing the front of the fork to immediately fail in tension thereafter. Unfortunately, the picture angles do not allow a direct impact to be ruled out, and the failure progression through the laminate isn't visible in the photos.

There is a similar failure of a fork, at http://cozybeehive.blogspot.com/2008/12/bontrager-carbon-fiber-fork-failure.html

I do not agree with all of their analysis, however, particularly the "cancer and fatigue" stuff.

I would advise the guy to see what Trek will do for him, preferably starting with an examination via an independent lab. If he wishes to pursue it, a good ultrasonic and radiographic inspection might shed added light on the situation. In particular, if the laminate was porous, that is an indication of a manufacturing defect since porosity can't really be caused by the user.

Steve A said...

If you think of carbon as really fancy fiberglass or marine plywood, you won't go too far wrong.

Ham said...

(gulp) My carbon bike is LeMond/Bontrager/Trek. Logic says it shouldn't worry me just because the two pics of fork failure are both Trek but since when were humans any good at risk assessment?

Chuck Davis said...

Kcab in the mid 60's I wuz a principal in a quite dubious adventure called " Chuck Davis, Custom Surboards"

We used a UV stabilized polyester resin and I rather expect that today's epoxy resins are similarily formulated when a clear weave finish is desired

Often there can be "gaps" twixt composite design/inspiration/prototype and after execution (production) that can "surprise"

The Burrows (I think) designed Hotta frame/fork is a good example

Chuck Davis
Tulsa

Steve A said...

Ham, if it makes you feel any better, go to this link and you can see a LeMond seatstay. The part that ought to make you feel better is it GOT FIXED!

http://www.bustedcarbon.com/2010/01/broken-lemond-seatstay.html

twister said...

You are an engineer, aren't you?

Steve A said...

Twister. Yup!

Rantwick said...

I continue to thoroughly enjoy these pieces on carbon... well done Steve.

Ham said...

Don't reckon that you will have seen the women's scratch final in the World Championships (26th March, session 5), notable for a spectacular wipeout. A rider up the banking touched wheels, came off and slid down through the peleton. One bike split in half.

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