Austenitic Stainless Steel with Aluminum Extrusions in Aircraft Structure Subject to Cyclic Loads 3

[SAITAETGrad]

Has anyone here seen 1/2 hard 301 SS used for crack or corrosion related repairs to aluminum extrusions subject to cyclic loads?

Seems to me this is high risk for corrosion fatigue and maybe even SCC with any clamp-up issues. Scares me. Not sure where to start with a life knockdown.

I see a lot of repairs on one type that have been done this way. I've been told that some other types (Lockheed, Sikorsky) also use SS in high cyclic stress areas (but P-H maybe?).

AISI 301 is definitely easy to get and fabricate with tight bend radii.

Maybe it's just me. Passivate & prime - good to go?

 

[rb1957]

it does seem "odd" to me, but maybe the stress levels (ok for Al) are nothing for SS (particularly 1/2 hard SS) although I guess the cross-section is smaller in SS. Personally I'd use normalised SS (maybe no HT after forming ??).

for me the critical feature is the load transfer into the SS repair and the impact this has on the Al structure.

another day in paradise, or is paradise one day closer ?

 

[SAITAETGrad]

Hi RB...yes, difference in modulus - but that can easily be factored into analysis. I'm concerned primarily the dissimilar metal aspect.

I'm pretty sure the reasoning for using the 301 is to get the tight fits without extrusions which may not be on hand.

301 in annealed condition has poor yield strength so that's likely why 1/2 hard.

Thanks for your thoughts - appreciated.

 

[Sparweb]

What about at least having a fay sealant between the two materials? Install all fasteners "wet" too.
I can't think of any examples of mixed-material repairs, that I've come across before.
Where is this repair coming from? Existing on the aircraft in your hangar, or something puzzling found in a structural repair manual?
(I'm trying to gauge the urgency of your question.)

STF

 

[verymadmac]

What are the fasteners (material type & tensile capacity). I guess there is no practical way to inspect if there was any corrosion in service till a fastener broke or the doubler was deformed. What is the environmental exposure of this repair?

I have seen OEM repair schemes for M7 AC227 where the the emergency exit window was replaced with a SS301 that was passivated although that replaced a 4130 doubling that was prone to uninspectible corrosion (between the skins) that would show by popping rivets if you were lucky (I suspect they were fine if the fuselage skin paint wasn't cracked at the rivet heads). Don;t know how they went in service.

Someone does sell a 2024-T3 that apparently does have formidably close to that of O (never used, always wanted to)

 

[SAITAETGrad]

SparWeb - widespread use on one type in SRM & repairs. As noted above, some others have claimed to have seen similar practice on Sikorsky & Lockheed products. One favourable thing is that fay sealant has been specified - but I just feel that has some built in limitations. Workmanship can vary.

Verymadmac - environmental exposure should be considered humid air / salt water marine. Fasteners are aluminum rivets with a large number of steel & titanimum lockbolts.

I think this 'sanity check' has met my needs. That said, please speak up if you have some experience with this.

The next thing may be to gather some samples for metallurgical analysis.

Thanks again to those who responded!

 

[Sparweb]

Verymadmac,
Wow, I glanced at a Metro III SRM recently and I sure didn't see that! Now I have to back to look at the window repair again.


STF

 

[verymadmac]

Sparweb
The one I found were on the overwing emergency exits on a couple of sequential S/N 227CC's (metro 23). One showed up with a couple of missing rivets, due to corrosion between the doubler & skin which made us look a bit harder. The 2nd Airframe showed very minor internal surface corrosion (looked like paint prep issues) on 2 of the 3 center section exits, nothing too scary was found when we replaced those mainly minor corrosion starting from the rivet hole edges), unusually the call on replacing the 3rd doubler was mine, so while it looked perfect on the the principle that it would never be cheaper to replace (learning curve and all that), had it replaced. That doubler had patch of pitting corrosion, of which the depth was 1/2 the doubler thickness.

SAITAETGrad
That is an interesting range of fasteners unless someone spent quite of bit of time on fastener stiffness considerations.

 

[WKTaylor]

SAITAETGrad...

1. I am not a fan of inserting 3xx CRES sheet 1/4H and above in any repair with aluminum structure for reasons not mentioned.

The dissimilar mix of CRES and aluminum can be dealt with using coatings for isolation,

Aluminum: CCC or anodic coatings + epoxy primer W corrosion protective pigments.

CRES Sheet preferred.
Cadmium-plating + post-plating chromate treatment [or phosphate] + epoxy primer W corrosion protective pigments.
(or)
Zinc-nickel alloy plating + post-plating chromate treatment [or phosphate] + epoxy primer W corrosion protective pigments
(or)
CRES Sheet OK...
Abrasive clean/passivate per AMS2700 + epoxy primer W corrosion protective pigments.

Assemble structure and install fasteners 'wet' with sealant [threaded or lock-bolt]... except install aluminum** rivets 'wet' with epoxy primer W corrosion protective pigments. Fillet around repair edges with sealant squeeze-out.

**WARNINGS.

Solid rivets should always be installed 'wet' with primer'. Sealant is thick/gooey and can interfere with rivet swelling during bucking... resulting in a loose bucked rivet.

RE: installation of CRES or any hard metal into joints with aluminum. Steel/CRES/Ti burrs are extremely sharp and damaging to aluminum structure and aluminum rivets shanks [solids or blinds]. Plated or aluminum coated steel CRES or Ti solid-shank fasteners can take a beating/scoring from CRES burrs... but are far tougher than aluminum.

CAREFUL deburring [but not knife-edging] of holes and edges is required to ensure repair 'works' as designed/intended.

The modulus of elasticity of CRES is 3X the modulus of aluminum... careful thickness matching is required to ensure CRES 'plays well' in the joint. Unfortunately the mix of high modulus and the low yield and ultimate strength CRES sheet means that the CRES can actually attract load... then be over stressed.

3. I personally prefer using Titanium 6-Al-4V annealed sheet [common], inserted into joints for the following reasons.

Titanium is just as hard to cut/drill as CRES sheet... and produces sharp burrs also... but that is 'easy' to take care of thru workmanship.

Ti-sheet is 'more compatible' with aluminum... but still needs dissimilar isolation as described above. Typical finish for titanium is chemical cleaning + epoxy primer W corrosion protective pigments.

The modulus of Ti-6Al-4V is ~1.6 X 2xxx & 7xxx aluminum alloys... which results in a better stiffness match... or at least 'not as bad as CRES'. Also the yield and ultimate of this Ti alloy is ~1.8--2.0X that of most high strength aluminum alloys.

4. Some observations/comments.

I worked on F-15s which have a high percentage of titanium alloys in the structure/systems... so titanium sheet/tubing/bar/plate was readily stocked and sheet-metal/machining techs dealt with-it regularly... and NOT so-much CRES.

However... in shops that deal primarily with primarily older aluminum structure, the techs prefer using CRES sheet/tube/bar/plate etc because it is familiar, available, cheep, reliable, etc. The techs and engineers I have worked with in these shops generally 'push-back' against using Ti sheet for repairs... too exotic.

Drilling thru a stack of hard-and soft alloys can produce terrible quality hole in the soft alloys. There are many reasons for this but it is a fact [explanation is a difficult discussion]. Drill-bits geometry/alloys for Steel/CRES/Ti [hard alloys] are generally different relative to drill-bits intended for aluminum [soft alloys]. I recommend drilling/reaming the stack-up with drill bits intended for the hard metal. Also I have MUCH ‘better luck’ [no such-thing] step-up-drilling/reaming thru these stack-ups thru pilot-holes and/or with ‘piloted’ drill-bits and reamers. This approach attains ‘best’ hole quality. See NAS897/NAS898 [reamers], NAS907 [drill bits] and NAS937 [stepped drill-bits].


Regards, Wil Taylor

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