gel coat

The ANs represent all the lessons learnt by the GFA ON various airworthiness issues, over the last 40+ years, lots of good stuff in there in others too.

On Monday, October 21, 2019 at 4:15:52 AM UTC-4, Charlie Quebec wrote:
The ANs represent all the lessons learnt by the GFA ON various airworthiness issues, over the last 40+ years, lots of good stuff in there in others too.

The AN is an interesting read--and an alarming one given the warning that:

"Irrespective of the initial cause of the gel coat cracking, all gel coat cracks can, given time:
....
(4) Travel completely through the skin layers of wings, tailplanes, fins, rudders, elevators, flap and ailerons.
(5) Transfer into and through the foam layers of sandwich skins.
(6) Transfer from skin layers into spar caps."

However, I'm not sure I understand the basis for these warnings given a statement earlier in the AN that:

"It must be noted that at June 1987 the "cracking" seems restricted to the epoxy resin in the skins, the question being - how long can cracked gel coat be left before it will induce glass or carbon fibre breakdown?"

That's the real question, apparently, but one the AN seems to sidestep. Has there been more study since 1987 then to validate the claims made above?

Chip Bearden
JB

On Monday, October 21, 2019 at 3:37:59 PM UTC-4, wrote:
On Monday, October 21, 2019 at 4:15:52 AM UTC-4, Charlie Quebec wrote:
The ANs represent all the lessons learnt by the GFA ON various airworthiness issues, over the last 40+ years, lots of good stuff in there in others too.

The AN is an interesting read--and an alarming one given the warning that:

"Irrespective of the initial cause of the gel coat cracking, all gel coat cracks can, given time:
...
(4) Travel completely through the skin layers of wings, tailplanes, fins, rudders, elevators, flap and ailerons.
(5) Transfer into and through the foam layers of sandwich skins.
(6) Transfer from skin layers into spar caps."

However, I'm not sure I understand the basis for these warnings given a statement earlier in the AN that:

"It must be noted that at June 1987 the "cracking" seems restricted to the epoxy resin in the skins, the question being - how long can cracked gel coat be left before it will induce glass or carbon fibre breakdown?"

That's the real question, apparently, but one the AN seems to sidestep. Has there been more study since 1987 then to validate the claims made above?

Chip Bearden
JB

I can see no basis to an expectation that a crack propagating into an outer skin would continue through a core foam and then start again through inner skin.
I've seen imprinting of gelcoat cracks in the outer layer of skin laminate, but have not seen one that progressed beyond the outer(usually 110) layer.
The real message is that if the glider is in gelcoat failure(deep cracking, cupping, peeling) real attention needs to be paid.

FWIW
UH

I concur with Uncle Hank’s assessment. After grinding off all gelcoat, I have seen what looks like a ghost of the crack that was there, but close examination with strong light and magnification, showed some resin degradation in the outside layer, but I could not see broken fibers. That said, I believe deep cracks should be ground out, filled, contoured and re-finished.
Refinishing the whole ship is the hardest work I have ever done, my hat’s off to anyone willing to take on that nasty job!
JJ

I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.

On Wednesday, October 23, 2019 at 1:45:51 PM UTC-7, Charlie Quebec wrote:
I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.

What, exactly, is "dry penetration testing?"

At 03:05 24 October 2019, 2G wrote:
On Wednesday, October 23, 2019 at 1:45:51 PM UTC-7, Charlie Quebec
wrote:
I know of several cases where the top layers of glass had to be
replaced
due to crack propagation. This was only confirmed by dry penetration
testing.

What, exactly, is "dry penetration testing?"

I think that it is Aussie code for "dye penetrant"

On Wednesday, October 23, 2019 at 4:45:51 PM UTC-4, Charlie Quebec wrote:
I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.

I have never had to replace glass due to crack propagation. I have had to replace glass due to damage during finish removal countless times. Sometimes the top layer is in a condition that is makes sense to peel the first layer and replace it.
UH

On Wednesday, October 23, 2019 at 4:45:51 PM UTC-4, Charlie Quebec wrote:
I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.

I'd like to see those results.

T8

On Thursday, October 24, 2019 at 10:15:44 AM UTC-4, Tango Eight wrote:
On Wednesday, October 23, 2019 at 4:45:51 PM UTC-4, Charlie Quebec wrote:
I know of several cases where the top layers of glass had to be replaced due to crack propagation. This was only confirmed by dry penetration testing.

I'd like to see those results.

T8

That makes 2 of us. The reason may (may) be because it's just easier to achieve a smooth finish by peeling off the top layer and replacing rather than structural concerns.

Back in the dark ages when I was in school (30 years ago), the materials science lab had an ongoing experiment related to aging of gelcoated fiberglass. At that time (around the same time as the Aussie report), I believe they had yet to see any significant degradation of structural properties even with significant failures of the gelcoat. In fact, I believe (can't recall exactly) that the test surface was taken from a wrecked glider!) I'll see if I can dig up the professor's name and see if there was any output published.

p3