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Mini stress cracks in plexiglass

F

Flyyak

Well Known Member
Friend
Please see the attached photos of stress cracks I have just noticed. I have a tipup and my canopy and rear window are installed with sika flex. I have about 400 hours but these have recently appeared. I had sika that was on top of the bulkhead brace and glued the plexiglass to the brace just below the rear window.

I think the window is bonded to the skin on the outer side but was also bonded with brace on the bottom side of the window. I have cut the ska as best I can in between the brace and the bottom side of the plexiglass and hope that may reduce stress between the skin, plexiglass and the brace.

Hopefully you can see that there are multiple stress cracks. I do not know if they go through the total plexiglass top through to the bottom of the window. At this time, I will wait to see if that will alleviate the stress so that no other cracks will occur.

Are there any suggestions of other things that I can do?
 

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stress cracks

My 7A has been flying for 11 years now. Not long after finishing the build and beginning the flying, I noticed the same on the bottom forward section of my canopy and on both sides too. I put thin superglue on them and IIRC inside and out. No change in those stress cracks since. I do a close inspection for any signs for cracks on a regular basis. Have seen a few more always at the bottom and treat the same. Really don't know what else could be done.

I've worried a lot about cracks on these large and expensive canopies. A big crack would be a catastrophe. I avoid cold weather as well, (I'm in south Alabama so that helps). To eliminate the chances of a big crack, I would have been ok with a canopy design that had it in smaller chunks... Like maybe three pieces. Seems it would reduce the chance.
 
That almost looks like solvent cracking. Anything aromatic been used in that area? Maybe during installation or painting?

Since the pic won't show this, can you feel the crack on the surface, like with your fingernail?

Hopefully that myriad of small cracks indicates internal fissures that hopefully won't grow into a big one!

Cheers
 
You can feel the cracks with your fingernail. I cut the sika flex to try to eliminate any stress between top/skin bonding and bottom brace bonding.

Paul, what is IIRC? I can put superglue on the top but I cannot get anything between brace and bottom of window. Had to use a paint squeezie to create a little space to be able to get a razor knife in to cut sika free.
 
I agree with Bill and Walt - that looks like the kind of cracking you get from the application of solvents. I would hazard a guess that during the installation of the canopy somebody has used some kind of solvent to either clean the surfaces before the application of the sealant, or used solvent to wipe off excess and clean up the area after installation.

MEK, Acetone or paint thinners are three products that are very likely to cause this kind of cracking, along with some fuels. A guy two hangars down from me had to replace the perspex on his Savage Cub last week due to fuel spill which caused these kinds of cracks within 20 minutes of the spill. Not that you're likely to have experienced this here, of course.

The idea of using superglue is an interesting one. I suspect it wouldn't actually bond the perspex together however as its very thin capillery action would see it wick in and fill the gaps, disguising the cracks - like a diamond in a glass of water. Might be worth a go.
 
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A data point. I didn't know a chemical being able to cause any cracks. I do know that I've never used any kind of chemical on mine. (I am also the builder so no chemicals ever).

And another IIRC... seems like I could barley feel the cracks with fingernail on mine. I hoped that wicking action would solve my cracking and like I mentioned a decade later no changes. Also they're so low on the canopy I never notice the slightly uneven texture the super glue causes.
 
Also another data point.
I have always been very careful with chemicals on the canopy s. Dating back to the Cessna days. Had always been told no Windex.
However, during construction I was careless enough to get some gook on the canopy. Van's suggested being very careful and use MEK to remove gook.
I used plenty but did not get any near the edges. I had no issues with MEK.
 
Paul 5r4 said:
A data point. I didn't know a chemical being able to cause any cracks.
Click to expand...

From the Construction Manual itself:

CLEANING AND USE OF LIQUIDS
CAUTION: DO NOT use Loctite, aromatic solvents, acetone, benzene, ethyl acetate, carbon tetrachloride, lighter fluid,
lacquer thinners, gasoline, toluene, window sprays, concentrated alcohols, ketones, scouring compounds, ammonia, or
409 cleaner on or around acrylic or Lexan canopy materials.
Click to expand...
 
I would agree with posts above about solvents. Perspex (acrylic) does not play well with these types of chemicals. Another mistake is to ‘flame’ or heat the edges to round them off and make them smooth. Looks OK in the short term but this also creates stress cracks not unlike those in the OP’s photos.
Spilt fuel would have a more immediate effect on polycarbonate which is a different material. Some RV12s had problems with polycarbonate rear windows cracking from fuel spills.

Clive Whittfield
Auckland
New Zealand
RV6 built and flying
 
I spoke with Scott at Plasticfix and they do not have a product for this application but he suggested that weld-on and from research it looks like Weld 3 may be promising. It is a glue that uses methylene chroride that is a water like consistency and will follow the crack or craze. Applied with a needle like applicator just along the crack.

I have order some and will give it a try.

I have been extremely careful with the canopy and I have never used any chemicals on it. It took me about 6 months to cut, prep, and install it.

There appear to be few options and I am hopeful that the stress cracks have not penetrated totally through the window. I will post the result of weld3 after I apply it. Thanks
 
Weld On 4 Acrylic, Fast Set is what I used for a small repair on my canopy and many unrelated acrylic welding projects. The Weld On 3 may be a slower set.

This stuff will seep into any small crack and virtually disappear after melting the surfaces together. I would recommend keeping it off any shiny surface, avoiding run off as it is very thin. If you do manage to de-shine a surface, a treatment with Micro mesh will fix it right up.

Best of luck, awaiting your report!

Cheers
 
just thinking out loud... if these are cracks going all the way through, would pulling a vacuum help?...either on the opposite side or on the face like they do when repairing car windshields
 
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