Cracks, and their conception

[Wedwin]

Hi all!

I am a glider technician, and every weekend the gang gathers for maintainance. Every so often the question arises:

Where do cracks come from?

Some background: We have a set of wings made of glassfiber reinforced plastic (epoxy), with a gelcoat finish. The gelcoat has cracked. Not in one place only, but millions of them, lined up in parallell, in the wind direction, from root and all the way to the tip. That leads me to believe it is a chemical thing because the loads at the tip is not that great. Cracks I can believe a yard or two from the root. This glider had a refinishing job done to these particular wings just a few years ago, and gelcoat normally holds fine through many years, at least in Scandinavia.

Anyway. My training says, a crack is a crack and it grows downward and eventually into the glass. Others say a crack can not under any circumstances go from a softer material to a harder.

Being the one with the responsibility, I would like to have an explanation to give as to why it is important to catch the cracks well before too late, not only considering the costs involved.

My main area is electronics, and the materials are fitted to an airplane. So this forum could be a bit off. But have any of You a good explanation for me to give?

 

[rerig]

Wedwin,

Boeing has the same cracking problems on Flight controls.
The paint cracks from exposure to the elements and the sun.
The the underlying surface, now exposed, cracks in the same locations as the paint cracks. (Graphite & Fiberglass)
Exposure causes resin starvation, which in turn eventually destroys the composite, which fails (cracks.

The answer here was to reduce the primer thickness.
Maybe a thinner layer of gelcoat might work?

I just know what we have to repair.

 

[Wedwin]

Thanks for the reply!

Weather in this area makes between little to less difference to our gelcoat. Thickness and elasticity of the wings do though. But the thickness in itself does not differ from any other glider.

One or two finer than hair cracks is easily ground down and filled in. But in this particular case, a total rework would cost us a fortune. And not grinding it down till the cracks end permanently will only induce new ones

I constantly tell my friends that the glass has no reason to crack under gelcoat that is not cracked.

The reply I get is:
"If You put a piece of tape on a steel beam and make a cut in the tape, will the steel beam crack there?"

If anyone has a scientific answer I would be soo grateful.

Wed

 

[Compositepro]

Your gel coat is micro-cracking due to flexing of the wings and numerous other contributing factors. The gel coat has residual stess from manufacturing. Anything tht increases this stress will increase propensity to crack because cracks relieve the stress. The gel coat shrinks during initial cure. Higher cure temperatures, and residual solvents increase shrinkage.

In service, colder temperatures and long term exposure to sunlight make the resin more brittle. One other factor could be the fiber orientation under the gelcoat. There is more tendancy to crack in the fiber direction because the strain of the gel coat will be greater cross-wise to the fiber during flexing.

Thickness of the gel coat is also a big factor. Thicker coatings are more prone to crack.

Cracks in the gel coat are critical with fiberglass because the exposed fiber will be attacked by water. With carbon it is much less citical.

 

[Wedwin]

Thanks a lot!

The rule of thumb that I was taught said that when the gelcoat is thick enough not to show any darkness of the underlaying structure, it was thick enough. However, the wing profile (airfoil?) has to be maintained.

Now, how worried should I be about the celcoatcracks growing down into the fiberglass?

Cracks as stress relievers I understand. The wings flex a bit in the air. In this particular case, there are a little gelcoat separating the cracks, so stress is very well relieved ...

A new finishing job would be very costly, even if we were able to sand it down ourselves

Also, how sensitive are the plastic and glass to water? That it is a bit hygroscopic I know, but how damaging is that? Is a crack to be seen as a direct access for water to the plastic? Does frequent waxing help at all?

The glider is stored in freezing temperatures during winter.

Most thankful for all answers, Wed

 

[jetmaker]

Wedwin,

The source of your cracks may be from something else... incorrect formulation of Gelcoat. If the Gelcost was made such that it is too brittle, it will crack along the lines of stress gradient. In your case, primarily bending, so cracks run chordwise. This is just another reason to add to the list that Compositepro provided.

Next, cracks are anything but stress relievers. At the tip of a crack is a very high stress concentration which drives the crack to grow at a high rate. My assumption would be that the crack will continue to grow into the matrix of the fiberglass, weakening the structural integrity of the composite.

Regards,

jetmaker

 

[rerig]

RE: The reply I get is:
"If You put a piece of tape on a steel beam and make a cut in the tape, will the steel beam crack there?"

Yes, if the entire steel beam is covered in tape and you cut the tape, the beam will corrode starting there and spread like an iceberg, eventually cracking the beam.

Glider...Good weather, low altitude (relatively speaking), stored in a hangar. But high stress concentration on the wing spar. I would keep an eye on the cracking gelcoat.
Yes, I would wax it to close the path to the fiberglass/graphite.

Rerig

 

[Wedwin]

Thanks jetmaker.

Flawed gelcoat is what I have ruled since the cracks have showed the way they have, in millions.

And thanks Rerig.

The example of tape on steel was my friends ridicule of cracks in gelcoat being able to dig into the fiberglass structure below. So corrosion is kinda not valid in this particular case.

We will keep an eye on our cracks because a new finishing job is rather expensive, both in labour and money. But watching the cracks at the surface gives little data about their depth, subsequently their damage to the structure below.

 

[jetmaker]

Wedwin,

Your friend's comment about the tape is interesting, and has given me something to ponder. However, this is the way I look at it.

The tape is really not an integral part of the structure. Similarly, one could fasten/glue (elmer's white, or wood glue prefereably) a steel plate to another. If you scratch the top plate, and cyclicly load it, the top plate will crack at the location of the scratch. However, continued cycling of the part will now cause a crack to grow in the lower part simply because it is now unreinforced compared to other regions, and has higher stresses now because of it.

Now, if you were to thermally weld the parts together so they were acting as a cohesive unit along the faying surface, the stress intensity field ahead of the crack would influence the lower part as it approached the faying surface. This in turn creates the stress concentration in the lower part, and helps the crack propogate.

The question here is: is the Gelcoat a structural component that is cohesively attached to the fiberglass laminate? If you think that it is strictly a protective coating, like wax, then the cracks are really not of concern from a strength point of view. It is still important for aerodynamics and environmental protection. But if it forms a cohesive bond with the laminate resin, then the cracks are likely to continue into the parent matrix.

Finally, as Rerig said, keep it waxed to seal the cracks and prevent environmental degredation of the glass laminate.

Hope this long winded explaination helps with your friend's question.

jetmaker

 

[Wedwin]

Jetmaker!

Your answer is exactly what I wanted, winded or not.

I wouldn't say that the gelcoat is part of the structure per se, but it is there and it sticks to the plastic (else it would fall off ...).

In the old days, the first layer of fiberglass was laid out in the wet gelcoat. Not so anymore.

Am I correct in guessing that a finnishing job with a softer, less brittle coating such as polyurethane, would not crack as easily and thus not grow down to the fiberglass?

 

[Compositepro]

The fibrous nature of composites is what makes them strong and fatigue resistant. Each fiber behaves as a crack stopper. But with enough time and the wrong conditions cracks will propagate. This would rarely occur is a catastrophic fashion, however. In this particular case I would say that the cracking must be monitored and inspected regularly because the cracks could grow to be a serious problem. But the gel coat cracks are much more analagous to scratched paint on a steel beam than to cracks in an aluminum wing skin.

 

[Gerrd]

Wedwin,

Polyurethane will not bond to epoxy or itself, use epoxy again. But think hard abount how to remove the wax befor you try reworking. if any is left in the crack tip it will form another stress consentration.

my understanding of a crack in any fibre composite is that the stress consentration at the crack tip will alow it to progress.
it may be that if they are due to chemical shrinkage and the crack profile is blunt and wont cause significant problems. However if its a fatigue crack then it may well cause the crack to propergate.

The tape on the steel beam is interesting, but what must be remenbered is that the gel coat is made of the same material as 55% of the structure.
generaly the crack will progress to the first fibre and A) delamanate up the syurface of the fibre. B)progeress through the fibre.
neither are good.
but neither are definatly going to happen in this situation.

i am an undergraduat studieing composite materials engineering and have little practical experiance, but i would recomend some form of rework as it is a safty critical component in cyclic loading.

 

[Wedwin]

Thanks a lot. Based on all Your contributions, I now feel a bit better about my situation. The border of my club don't really want to spend the money for a rework. And I as responsible for the maintainance would like to see no cracks at all. So a middle way solution by constant inspection and rigorous waxing is probably the one for now.

I like the atmosphere of Eng-tips, way to go!

Wedwin