Fasteners on composite aircraft

smaragdz · Mon Oct 19, 2020 1:11 pm

Hi all,
I've fallen down a rabbit hole of looking at assembly of composite aircraft and was wondering if any maintenance or design engineers can help fill in a blank

Obviously galvanic corrosion is listed as an issue for CFRP to metallic joints, and so I'm curious as to what fasteners can be used for aluminium to CFRP joints, ensuring galvanic compatibility between the fastener, the CFRP and the aluminium? I'm having trouble seeing what sort of fastener can be used in such a case. Does anyone know what fasteners are used in such joints so I can figure out what the compatibility is like? Or is there a standard or specification that can also help figure out what is a good choice?
Thanks!
Smaragdz

Sokes · Mon Oct 19, 2020 1:28 pm

Not really qualified to answer.
I read about skyscrapers in which aluminium windows were attached to the walls with aluminium fasteners. These fasteners had to be replaced after some years as some windows started falling out.
I therefore assume that Al barrels are not joined with Al fasteners.

I assume with galvanic corrosion you have aluminium body and other metal fasteners in mind. So same problem should apply to aluminium barrel joints. My assumption is that paint is the solution. No moisture, no galvanic corrosion.

As I said, just my assumptions. I'm not an expert.

Strebav8or · Mon Oct 19, 2020 4:00 pm

Basic answer is using wet installation. The fastener is coated under the head, on the shank and under the fastening device with sealant to provide a barrier.

smaragdz · Mon Oct 19, 2020 4:45 pm

Strebav8or wrote:
Basic answer is using wet installation. The fastener is coated under the head, on the shank and under the fastening device with sealant to provide a barrier.

Really interesting. Any idea what type of sealant is typically used? I haven't heard of such a procedure before, so this sounds like my next rabbit hole to fall into

WIederling · Tue Oct 20, 2020 8:27 am

Strebav8or wrote:
Basic answer is using wet installation. The fastener is coated under the head, on the shank and under the fastening device with sealant to provide a barrier.

Can you be sure that sealant is not displaced to zero thickness ( and thus conductive contact ) on tightening?

on a side note: is this why Airbus uses those thermoplastic brackets to fasten fuselage skin to the frames?

gregorygoodwin · Tue Oct 20, 2020 11:40 pm

I agree with you in that it's a rabbit hole of sorts and I've worked heavy aircraft structures for over 35 years now. As stated in the B757 and B767 Structures Repair Manuals, the use of fasteners in a CFRP-to-CFRP application would be a titanium or CRES (corrosion resistant steel) fastener. The CRES is usually a A286 alloy. These fasteners would be installed dry, no sealant. If the panel is of a AFRP (aramid fiber reinforced plastic) or GFRP (glass fiber reinforced plastic) the fasteners would be installed "wet" with BMS 5-95 sealant. This is a common repair sealant used in structural repairs and is called PR1422. On the B757 & 767, the elevator and rudder trailing edges are secured with a fastener called a BACR15GA. This is a tubular rivet that is installed dry, no sealant, and the recommended installation is by way of a pneumatic squeezer set-up that minimizes impact to the trailing edge as it is not very thick and can be damaged easily. Although aluminum fasteners are not recommended in CFRP, they are used. For instance, on the B757 fan cowl leading and trailing edges there are plastics rub or chafe strips that protect the cowls where they overlap. These are installed with aluminum alloy (2117 AD) rivets through the carbon fiber and they are installed dry with no sealant. In cases where blind fasteners, CherryMax rivets, are used through carbon, they are usually made of monel alloy which is a high nickel corrosion resistant steel.
Hope this is of some help to you.

Gregory

extender · Wed Oct 21, 2020 11:57 am

Sealant doesn't really work in interference-fit holes. Sometimes there is an intermediate composite such as fiberglass between CFRP and an aluminum detail.
gregorygoodwin has a good reply above.

smaragdz · Wed Oct 21, 2020 6:26 pm

gregorygoodwin wrote:
I agree with you in that it's a rabbit hole of sorts and I've worked heavy aircraft structures for over 35 years now. As stated in the B757 and B767 Structures Repair Manuals, the use of fasteners in a CFRP-to-CFRP application would be a titanium or CRES (corrosion resistant steel) fastener. The CRES is usually a A286 alloy. These fasteners would be installed dry, no sealant. If the panel is of a AFRP (aramid fiber reinforced plastic) or GFRP (glass fiber reinforced plastic) the fasteners would be installed "wet" with BMS 5-95 sealant. This is a common repair sealant used in structural repairs and is called PR1422. On the B757 & 767, the elevator and rudder trailing edges are secured with a fastener called a BACR15GA. This is a tubular rivet that is installed dry, no sealant, and the recommended installation is by way of a pneumatic squeezer set-up that minimizes impact to the trailing edge as it is not very thick and can be damaged easily. Although aluminum fasteners are not recommended in CFRP, they are used. For instance, on the B757 fan cowl leading and trailing edges there are plastics rub or chafe strips that protect the cowls where they overlap. These are installed with aluminum alloy (2117 AD) rivets through the carbon fiber and they are installed dry with no sealant. In cases where blind fasteners, CherryMax rivets, are used through carbon, they are usually made of monel alloy which is a high nickel corrosion resistant steel.
Hope this is of some help to you.

Gregory

Really interesting and great info -- thanks a million for detailing all that out. Do you happen to know if the aluminium rivets used on the B757 cowling are solid aluiminium, or are they coated? If it's the latter then perhaps the coating is treated as 'sacrificial', which would solve the alu-cfrp compatibility issue? Just a guess.

Thanks all for the info on sealants as well, I was also confused about how an interference fit could be sealed up effectively without squeezing everything out of the way. It looks like the answer that they 'live' with a non-optimal set up by the looks of things?

gregorygoodwin · Wed Oct 21, 2020 9:33 pm

In my experiences with solid shank aluminum rivets, they are not coated. The rivets we install on the 757 fan cowl edges are a bare 2117 AD alloy. You can buy 1100 A series rivets that are pure unalloyed aluminum. These are not used in a structural load bearing setting as they are a very soft low strength fastener. Pure aluminum will form a outer oxide coating that becomes somewhat inert to further corrosion.This is why you see aircraft parts and repairs made from Alclad aluminum sheet metal which has a very thin (.001"-.002") thick pure aluminum coating. One problem with installing fasteners through composites is that if you use a compressive fastener such as a solid shank rivet or a blind fastener such as a CherryMax, is that it expands the fastener hole slightly and will do so every time you remove and install a new fastener. Eventually you have to step up to a larger diameter fastener. Threaded fasteners such as Hi-Loks can prevent some of this if you can use them.

As for sealant in interference fit holes, we do use it if the SRM or engineering drawing calls for it. You will get a lot of squeeze out around the fastener head and thread shank.
You may want to see what the SAE has on this subject. I believe it stands for Society of Automotive Engineers and I have seen carbon composite articles authored by them.

Gregory

Dalmd88 · Wed Oct 21, 2020 10:23 pm

It's been a while since I did structural work and I didn't do too much with composite panels, but I think for most panels there were never tolerance fit fasteners through composite. I think like Ti, they were always clearance fit through the composite panel. Seems like they were always 'wet install' with BMS5-95.

gregorygoodwin · Thu Oct 22, 2020 8:42 pm

With further reading of the various Boeing SRM's, I found that to install aluminum fasteners into dissimilar material, the preferred method is to coat the fastener or the fastener hole with BMS 5-95 sealant (PR1422). The secondary preferred method is to coat with BMS 10-11 primer (epoxy primer). The fastener must be installed before the cure time of either material. So, for aluminum rivets through CFRP, you can apparently use either the sealant or primer as a barrier.

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