MadRocketScientist
Well-Known Member
That doesn't surprise me at all, I think there is a very nasty corrosion tendency when those two materials are touching.
Shannon.
Shannon.
Yes, I've read that as well, which is why all of Rutan's A/C plans specify a particular type of dacron tape for use as peel-ply.Hi Topaz. I've heard that some peel-ply has some sort of coating which remains on the structure after the peel-ply is removed and causes some issues with bonding. This is only the case with some peel-ply, but not all. I forget what the treatment/sizing/contaminant is and I forget which brands have it and which don't, but it something that needs to be addressed if using peel-ply.
I second that, I am very interested in the bonding of aluminium as the CriCri I am building uses adhesives to hold the wing and tail skins on. That is something I don't want to get wrong. I was under the impression that you always needed to prime the aluminium or at least with the adhesive systems commonly used on the CriCri.Welcome and thank you Blakmax. The original PPRuNe thread seems to have been greatly diluted by the other posts. For the benefit of all here, would it be a great deal of trouble for you to copy the highlights and important points of your posts there and paste them in here as a whole?
Assuming that the peel-ply tape used was one of the varieties that does not contaminate the bonding surface (they're reputed to exist, and the original Rutan Aircraft Factory [RAF] plans specify a particular tape for that reason), would a light grit blast or mechanical sanding of a peel-ply-prepared surface then be acceptable to promote a better bond? Given homebuilder manufacturing processes, building some of these hand-layup structures would be quite a bit more difficult without the use of peel-ply....The problem I have with peel plies is that they are essentially based on the theory that you only need a clean surface to bond. I stress that it is essential that you get chemical reactions between the adhesive and the surface. A clean surfaces is a necessary but not sufficient condition for adhesion. The surface must be chemically active as well, so some form of abrasion is necessary. If the surface is contaminated with silicone, then abrasion simply spreads and embeds the contamination. I would use a heat-set uncoated peel ply and abrae the surface after removing the peel ply. Blow the dust off or dry wipe. Don't solvent clean after abrasion because that simply dissolves any residual contamination and spreads it over the surface. The best method for abrasion is a LIGHT grit blast but be careful to contain the dust and at all costs do not let the dust get into fuel systems because it will cause engine failures.
I can't argue with your experience and observations, and yet these aircraft have an admirable structural safety record. Apparently, the bond joint areas were either designed with this in mind or it's a byproduct of other design factors. Neither Lancairs, nor any of the aircraft using RAF methods, seem to be falling from the sky with any regularity, particularly for structural reasons. In every case of which I'm aware (admittedly, a small sample) if there was a structural failure it was due either to overload or the builder incorrectly assembling the structure, with deviations from the plans and/or instructions.As for the years of experience, it may be that the bond stresses are so low that you get away with a weak bond. However, some where you may encounter higher loads and there is no guarantee that the weak bond will be adequate. I have personally examined the bond surfaces from a Lancair crash (fuel starvation not structural cause) and every bond surface exhibited interfacial failure with the pattern of the peel ply replicated on the surface of the adhesive.
One of the links I followed indicated that one of the functions of peel-ply was to leave small jagged mounds of resin which acted as a roughened surface for the next layer to adhere to. Given that interpretation, wouldn't abrasion remove those mounds?The problem I have with peel plies is that they are essentially based on the theory that you only need a clean surface to bond. I stress that it is essential that you get chemical reactions between the adhesive and the surface. A clean surfaces is a necessary but not sufficient condition for adhesion. The surface must be chemically active as well, so some form of abrasion is necessary. If the surface is contaminated with silicone, then abrasion simply spreads and embeds the contamination. I would use a heat-set uncoated peel ply and abrade the surface after removing the peel ply. Blow the dust off or dry wipe. Don't solvent clean after abrasion because that simply dissolves any residual contamination and spreads it over the surface. The best method for abrasion is a LIGHT grit blast but be careful to contain the dust and at all costs do not let the dust get into fuel systems because it will cause engine failures.
Thank you for this bit of data!!!!With reference to the peel ply issue. We found out about this when we could not get strain gauges to stick to a composite specimen. Examination under SEM found extensive contamination with silicone. We then examined composite patches made with the same peel ply and found the bonding surface contaminated again. We ripped off the first ply and proved that the silicone was not part of the resin system, so it has to come from the peel ply.
Thanks Blakmax :ban:Originally Posted by blakmax
With regard to preparation of aluminium, the best option for a low production environment would be Boegel which is sold as AC130 and follow the instructions
Would you happen to know the brand/product name for that? I'd like to compare it to what Aircraft Spruce is selling. Supposedly that's the material that was called out in the RAF plans, but a little comparison never hurts......but my supplier assures me that they carry one of only two or three that remained OK to use.
I can confirm that there are small mounds which occur at the areas where the weave passes over and under each strand. The resin rich area there fractures leaving a rough surface which actually does bond because you expose a chemically active surface by fracturing the resin.One of the links I followed indicated that one of the functions of peel-ply was to leave small jagged mounds of resin which acted as a roughened surface for the next layer to adhere to. Given that interpretation, wouldn't abrasion remove those mounds?