thin gauge repair doubler (thinner than original) 1

[IBD]

Hello,

I saw an interesting topic the other day - in which an external aircraft repair was done using a thinner gauge repair doubler.
The basics are:

- 0.100" thk original chem milled skin
- 0.100" filler
- 0.063" thk external repair doubler

- the reason is that a thicker doubler would simply not fit due to interference in this area.

how would one approach the analysis for this?

 

[WKTaylor]

IBD...

for conversation starters...

Pressurized, unpressurized or fuel-wet one-side, etc?

What was the purpose of the added doubler? material-removal beef-up? patch-over a hole? etc.

Why the added 0.100 thick filler? Is the filler 'floating' or structurally functional?

Is the added doubler countersunk or dimpled or 'NOT'?

What was the original sheet metal alloy-temper?

What is the repair sheet metal alloy-temper?

Is the original skin and the added doubler fully inspectable from at-least one side... or does the filler 'hide' some fastener holes?

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[IBD]

good questions. see below.


Pressurized, unpressurized or fuel-wet one-side, etc? - Pressurized - cargo door (external)

What was the purpose of the added doubler? material-removal beef-up? patch-over a hole? etc. material removal beef up (removed minor damage)

Why the added 0.100 thick filler? Is the filler 'floating' or structurally functional? - intended to be structurally functional

Is the added doubler countersunk or dimpled or 'NOT'? - machine CSK

What was the original sheet metal alloy-temper? 2024-T3

What is the repair sheet metal alloy-temper? 2024-T3

Is the original skin and the added doubler fully inspectable from at-least one side... or does the filler 'hide' some fastener holes? - inspectable from the inside, all rivets would be visible.

 

[rb1957]

you say "0.100" thk original chem milled skin" ... is that 0.1" minimum thickness, or the plate thickness before pocketing ?

how much repaired ? why repaired ? (corrosion?) how repaired ? (cut out the skin, hence the "filler" ? cut-out the skin within a stringer bay, or over several bays ??

on the face of it you'd expect the thinner dblr would have 50% higher stress in it, and lower buckling allowables and the load transfer between skin and dblr would need careful design.

commercial transport ?

as will asked ... pressure skin ??

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

 

[IBD]

0.1" thk prior to chem mill.
repair cut out roughly 1" x 1" due to some external damage (not sure entirely). Just one bay.
- commercial yes.

 

[rb1957]

ok, cargo door skin on a commercial jet. how thick are the chem-milled pockets ?

you're repairing a relatively small damage 1" dia, so the a/c skin has a hole in it that you've covered with the dblr, ok.
the chem-milled pocket should be close to your dblr thk.

not feeling the "filler", particularly a structural filler (if it is just filling the hole created by removing the damage).

FAR25.571 applies

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

 

[IBD]

Im assuming that the chem milled pocket thickness is 0.063"...

And yes agreed. dblr ok. and should be close or same as the pocket thkness.

And also agreed I assume the filler does nothing a structural filler.

I am wondering how would you write the stress to support 25.571?

As would your MS not be less than 0?

 

[WKTaylor]

IBD...

I have seen proposed repairs where thin sheet meal was added to an exterior surface of a thick sheet that was countersunk. The existing countersinks in the thick sheet were to have countersink fillers installed and the exterior sheet was then to be countersunk for the original flush tension head rivet/fastener.

A quick analysis revealed that the thin sheet was way-too-thin for a satisfactory csk and would have been a razor-sharp knife edge***. This is a truly unacceptable situation, since mechanical/fatigue allowables 'fall-off the table' when csks are too deep for carrying load.

***MS20426D5 = head is 0.055 deep X 100-deg... which is barely acceptable for a 0.063 thick sheet...

I will reject situations like this all-day-long.

Either the holes in the doubler need to be dimpled to nest into the original csks [to accept the original fasteners ]... or machine shallow-countersinks for flush-shear-head [FSH] HLs... or install protruding shear-head HLs without countersinks... or install flush tension head HLs into FSH csks and allow the extra head to ride above flush with a sealant fillet made from squeeze-out.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[IBD]

Thx WK.

I believe the chem milled pockets are actually thinner (0.040 to 0.050) however I am not 100% sure.
I am just curious as to the analysis approach. However saying that for a 0.063" thk doubler - if reduced head csks where used - NAS1097E?

Again I am merely intrigued by the repair scheme. (Wondering what they know that I dont)...

 

[rb1957]

good point will, maybe they used LZ4 or 5 ?

the MS is not necessarily <1 by default; are you sure the original was =1 ? or just a conservative (ie no data). if the pocket thick is close to 0.063" you should be ok. A "small" 1" dia hole doesn't really worry the structure a whole lot. A good repair dblr, 2 rows of rivets should be ok. The door probably reacts pressure (easy-ish to calc) and manoeuvre loads much less easy.

Maybe the best way to attack is to say "equivalent strength", you can calc allowable loads for the pre-repair and repaired skins without referring to the loads actually carried (which I assume you don't know).

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

 

[IBD]

thx rb.

I have 'no data' to the original strength. just the material data that I provided above. i.e. 0.1" thk chem milled to 0.040" pockets. Repair extends from 0.1" edge into Pocket area of 0.040" thk.

I would agree and use 'equivalent strength' for calculations. However what would you use for original skin strength in this case? i.e. what would the allowable be in this case for the original skin be based off of? the thickness of the chem milled? or would you us a combination?

 

[rb1957]

sounds like it'd be the combination. I assume the 0.1" thk is under a stringer and maybe there's something extra added there to help ? is that where the filler goes ? note the increased bending in the fasteners.

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

 

[WKTaylor]

IBD...

Be aware that when chem-milling is employed, there is usually NEVER any intent to fasten thru the thin-chem-milled-pockets... only thru the thick lands [with adequate edge margin to the chem-milled-step] where fastener countersink depth and shear/tension/fatigue durability can be assured.

NOTE.
Single-step chem-milled pockets of 0.040 in a 0.100 thick sheet are very unusual. My company and personal policy would be to have at least one-chem-milled step/land [~0.5-wide] of 0.070 in-transition to the 0.040 pocket. The stress concentration of 0.100-step to 0.040 [line] with a standard chem-milled-radius [relatively rough/irregular] would be pretty significant.

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[IBD]

Thanks for that Will.
I noticed that the OE drawing indicates that the chem-mill pocket is 0.050" however it measures around 0.042"thk.

When you do such a repair analysis would you consider the strength of the chem-milled pocket area for equivalent strength?
Or how else would you approach it?

 

[WKTaylor]

IBD...

Just to be clear...

0.042" remaining thickness measured... or 0.042" depth of chem mill resulting in 0.058" remaining thickness????

Is this measured with or without coatings [primer, topcoat]?

NOTE. Remaining material thickness 'thicker than spec' is usually far-better than 'remaining material thinner than spec'. Quality of milled surface and the fillet radii are also important Kt factors: Rrough/irregular -> 'poor'; smooth/low-roughness/precise line control -> 'better'.

FYI if interested.

SAE
AMS-C-81769 CHEMICAL MILLING OF METALS, SPECIFICATION FOR
ARP1331 THE CHEMICAL MILLING PROCESS
AS6507 TEST METHOD AND LIMITS FOR BOW HEIGHT OF ALUMINUM SHEET SAMPLES AFTER CHEMICAL MILLING

FAA
There are several 'AD's" related to: "Detecting And Correcting Fatigue Cracking Of xxxxx Skin Panels At The Chem-Mill Steps"


Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[IBD]

Thanks Will.

I did come across a few specs.

Do you mind pointing me to where I can find these FAA 'AD's"??

As for the thickness it measured at 0.042" remaining thickness (Im sure primer scratched off) but none the less yes it was under the material spec which should be (0.050" thk remaining thickness)....

This is an OEM part with no prior modifications to it... FYI.

Will is there a PM function in this forum so I can message you directly? I have some other questions.

Cheers.