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#1
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On Tuesday, September 24, 2019 at 9:11:31 AM UTC-4, son_of_flubber wrote:
How much does crazing affect Polar? Not at all (provided that the surface is being maintained). As far as "gelcoat cracks propagating into the structure", I'm terribly skeptical. It seems far more likely to this materials engineer that the reverse is what happened in cases where cracks in the structure have been associated with cracks in the finish. Polyester gelcoat simply isn't strong enough to provide meaningful stress concentration in the underlying structure. T8 |
#2
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On Tuesday, September 24, 2019 at 10:15:43 AM UTC-4, Tango Eight wrote:
On Tuesday, September 24, 2019 at 9:11:31 AM UTC-4, son_of_flubber wrote: How much does crazing affect Polar? Not at all (provided that the surface is being maintained). As far as "gelcoat cracks propagating into the structure", I'm terribly skeptical. It seems far more likely to this materials engineer that the reverse is what happened in cases where cracks in the structure have been associated with cracks in the finish. Polyester gelcoat simply isn't strong enough to provide meaningful stress concentration in the underlying structure.. T8 I have seen plenty of cases of imprinting on the top layer of the structural laminate under finish cracks. None have progressed beyond the top layer of the laminate. Some of those areas appeared to be compromised to a degree. These areas are also a path to moisture which can degrade the laminate. In very thin structures, like control surface trailing edges, the gelcoat does add stiffness. Finish cracks, usually just forward of the trailing edge joint are common on some types of ships. Move away from the crack and the surface is notably stiffer. Sand the gelcoat off and there usually is no underlying structural failure. Crazing is an indication that future cracking is coming. Crazing is mostly cosmetic. Cracking is not in my experience. FWIW UH |
#3
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To expand on Hank's comment, we've looked carefully at a couple of heavily crazed gliders (a Grob left tied out and an LS4 that was not well maintained) under magnification. After removing the failed gelcoat, the crazing lines are still visible in the glass/epoxy. Our resident materials scientist was able to see small voids in the epoxy (a few 10s of microns deep) and few failures of the glass fiber (individual strands) at the surface level.. Think of it as very shallow "pitting" of the epoxy in the outermost glass layer. There is no sign of it penetrating deeper than that. I've heard of some shops "painting" a warm coat of epoxy into particularly bad areas and others either peeling off the outer glass layer and replacing. Never saw the need to do either of these.
p3 On Tuesday, September 24, 2019 at 10:49:24 AM UTC-4, wrote: On Tuesday, September 24, 2019 at 10:15:43 AM UTC-4, Tango Eight wrote: On Tuesday, September 24, 2019 at 9:11:31 AM UTC-4, son_of_flubber wrote: How much does crazing affect Polar? Not at all (provided that the surface is being maintained). As far as "gelcoat cracks propagating into the structure", I'm terribly skeptical. It seems far more likely to this materials engineer that the reverse is what happened in cases where cracks in the structure have been associated with cracks in the finish. Polyester gelcoat simply isn't strong enough to provide meaningful stress concentration in the underlying structure. T8 I have seen plenty of cases of imprinting on the top layer of the structural laminate under finish cracks. None have progressed beyond the top layer of the laminate. Some of those areas appeared to be compromised to a degree. These areas are also a path to moisture which can degrade the laminate. In very thin structures, like control surface trailing edges, the gelcoat does add stiffness. Finish cracks, usually just forward of the trailing edge joint are common on some types of ships. Move away from the crack and the surface is notably stiffer. Sand the gelcoat off and there usually is no underlying structural failure. Crazing is an indication that future cracking is coming. Crazing is mostly cosmetic. Cracking is not in my experience. FWIW UH |
#4
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Bit of thread drift but it has to do with gel coat. I am currently doing a life extension on a Libelle (OD). It’s original gel coat is in far better shape then any of the ASW 20’s I looked at including the one I bought. It’s one of the earlier Libelle’s so 12 plus years older then a 20. I am not sure of why the difference? Did Glasflugel have better quality control than Schleicher? Or was it a different material?
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#5
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Glasflügel used Lesonal Schwabbellack instead of Vorgelat Gelcoat. They also did not paint the seams with a quick drying mixture, including too much hardener.
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#6
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Le mardi 24 septembre 2019 23:48:48 UTC+2, Papa3 a écritÂ*:
To expand on Hank's comment, we've looked carefully at a couple of heavily crazed gliders (a Grob left tied out and an LS4 that was not well maintained) under magnification. After removing the failed gelcoat, the crazing lines are still visible in the glass/epoxy. Our resident materials scientist was able to see small voids in the epoxy (a few 10s of microns deep) and few failures of the glass fiber (individual strands) at the surface level. Think of it as very shallow "pitting" of the epoxy in the outermost glass layer. There is no sign of it penetrating deeper than that. I've heard of some shops "painting" a warm coat of epoxy into particularly bad areas and others either peeling off the outer glass layer and replacing. Never saw the need to do either of these. p3 These small voids on the outmost epoxy layer are absolutely normal and due to the manufacturing method. The outmost glass fiber layer is laminated into the mould which has been spray-coated with the gelcoat. This process will always trap air between this glass layer and the gelcoat (although curing under vacuum does get rid of most of it). These small voids are main reason that the first step on a sanded-down wing is to laminate a very thin layer of glass fibers. This layer does not help the structure - it efficiently fills up those small voids. Otherwise, any sprayed-on filler or gelcoat won't be able to fill up the voids due to the relatively high surface tension, creating thousands of nasty litte craters on the filler/gelcoat surface. Bert Ventus cM "TW" |
#7
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On Thursday, September 26, 2019 at 10:24:16 AM UTC-4, Tango Whisky wrote:
Le mardi 24 septembre 2019 23:48:48 UTC+2, Papa3 a écritÂ*: To expand on Hank's comment, we've looked carefully at a couple of heavily crazed gliders (a Grob left tied out and an LS4 that was not well maintained) under magnification. After removing the failed gelcoat, the crazing lines are still visible in the glass/epoxy. Our resident materials scientist was able to see small voids in the epoxy (a few 10s of microns deep) and few failures of the glass fiber (individual strands) at the surface level. Think of it as very shallow "pitting" of the epoxy in the outermost glass layer. There is no sign of it penetrating deeper than that. I've heard of some shops "painting" a warm coat of epoxy into particularly bad areas and others either peeling off the outer glass layer and replacing. Never saw the need to do either of these. p3 These small voids on the outmost epoxy layer are absolutely normal and due to the manufacturing method. The outmost glass fiber layer is laminated into the mould which has been spray-coated with the gelcoat. This process will always trap air between this glass layer and the gelcoat (although curing under vacuum does get rid of most of it). These small voids are main reason that the first step on a sanded-down wing is to laminate a very thin layer of glass fibers. This layer does not help the structure - it efficiently fills up those small voids. Otherwise, any sprayed-on filler or gelcoat won't be able to fill up the voids due to the relatively high surface tension, creating thousands of nasty litte craters on the filler/gelcoat surface. Bert Ventus cM "TW" If the structure is not compromised we fill the "million pinholes" using polyester surface filler applied with a foam roller. This works filler into the voids and avoids the bridging that results if filler is sprayed. Laminating another layer, without peeling the top ply,can result in undesired shape change, and when sanded smooth, can leave defects that still require detail filling. FWIW UH |
#8
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Actually no - these are not the normal voids. Being a sergeant in the "pinhole patrol army", I'm very familiar with these voids.
The pitting I'm talking about is 100%, directly related to the failure of the gelcoat. The crazing lines literally transfer down into the outermost layer of the substrate. When you put a 6x loupe on the crazing lines after the gelcoat is sanded off, you see that there is a) slight discoloration and b) tiny chunks of epoxy coming out of the glass matrix. I have pictures, but not under magnification. Trust me - the key guy who looked at this is a materials scientist who works for one of the top electron microscopy vendors specializing in materials failure analysis. Cheers, P3 On Thursday, September 26, 2019 at 10:24:16 AM UTC-4, Tango Whisky wrote: Le mardi 24 septembre 2019 23:48:48 UTC+2, Papa3 a écritÂ*: To expand on Hank's comment, we've looked carefully at a couple of heavily crazed gliders (a Grob left tied out and an LS4 that was not well maintained) under magnification. After removing the failed gelcoat, the crazing lines are still visible in the glass/epoxy. Our resident materials scientist was able to see small voids in the epoxy (a few 10s of microns deep) and few failures of the glass fiber (individual strands) at the surface level. Think of it as very shallow "pitting" of the epoxy in the outermost glass layer. There is no sign of it penetrating deeper than that. I've heard of some shops "painting" a warm coat of epoxy into particularly bad areas and others either peeling off the outer glass layer and replacing. Never saw the need to do either of these. p3 These small voids on the outmost epoxy layer are absolutely normal and due to the manufacturing method. The outmost glass fiber layer is laminated into the mould which has been spray-coated with the gelcoat. This process will always trap air between this glass layer and the gelcoat (although curing under vacuum does get rid of most of it). These small voids are main reason that the first step on a sanded-down wing is to laminate a very thin layer of glass fibers. This layer does not help the structure - it efficiently fills up those small voids. Otherwise, any sprayed-on filler or gelcoat won't be able to fill up the voids due to the relatively high surface tension, creating thousands of nasty litte craters on the filler/gelcoat surface. Bert Ventus cM "TW" |
#9
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How many wing failures have composite gliders had over the years?
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#10
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Excellent Question
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