Why are heat shields dimpled? 10

[KirbyWan]

Howdy all,

I'm talking about nacelle system heat shields, such as on the inner surface of a thrust reverser or sometimes on exposed surfaces of the aft part of an inlet cowl. The heat shields are stainless steel foil sandwiching a kaowool or similar core, rarely thicker than a 1/4 inch. The stainless steel foil is dimpled. For small repairs we usually trim out the damaged foil and resistance spot weld some flat stainless steel foil and then seal the edge with sealant, a procedure used in several OEM repairs.

The question is, what's the benefit of the dimpling? Is it to add stiffness to the foil? Does it improve it's heat transfer properties? I want to be able to provide evidence that the flat repair foil I'm using is equivalent or better to the dimpled foil of the original heat shield. Generally these are for small repairs, but beyond the OEM limits as some permit repairs of only .25" diameter. We generally use a .004" thick foil to replace the .003" thick dimpled foil.

Thanks,

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

 

[MikeHalloran]

I'd GUESS that the dimples are intended to work as spacers, providing a controlled air gap between the protected component and the foil, with minimal direct contact and minimal 'heat short' at the apex of each dimple.

It would take relatively simple tooling (maybe just a ball pein hammer and a thick sheet of urethane) to dimple the flat foil, but can you then form the dimpled repair foil into the generally cylindrical shape I think you'd need? Or can you dimple the curved but 'flat' repair foil? Probably requires more skill than I have...



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

Dimples allow for free local expansion and contraction of the foil due to temperature changes, which prevents large area buckling (oil canning). Oil caning will lead to fatigue cracking from thermo-cycling.

 

[3DDave]

Dimpling or corrugation serves both purposes - it increases the section modulus so the part is less flexible than it would be if it was flat and it allows for out-of-plane expansion and contraction so the part doesn't tear or damage whatever it's mounted to, either from simple overload or through fatigue during differential thermal cycling.

The main problem is that these repairs add a stress discontinuity, so without knowing the load cases it's hard to tell the effect. Often a stress discontinuity of any kind will be a place to start a fatigue crack, but it may be the loading is sufficiently uniform that expected failure cycle will be beyond the useful life of the item, as most fatigue failures should be.

Proving this is very difficult and requires a lot of testing. What is the expected outcome of a failure?

 

[SWComposites]

Nacelles are a high vibration environment. The dimples increase flex stiffness inceaseing the sheet natural frequency, as well as helping thermally and for thermal expansion.

Improper repair or install of a TR heat blanket could lead to thermal damage to the inner wall.

 

[WKTaylor]

KW, all...

I have done similar repairs on fan engine heat-shields that look, and are assembled, exactly as You describe. These were made to conform to relatively complex shapes... to cover/surround/fasten-to various items. Typically they are made from I600, I625 and CRES321 foil with special patterns and sandwich the insulative materials seam/tack resistance welding seals the jacket.

Before I attempted repair, I had a long conversation with one of the vendors.

The impressed pattern of multi-side small diamond-like shapes with crossing lines on the metal foil is proprietary to each high-temp metal foil insulative-jacket vendor. These 'features' allow the annealed foil to readily deform out-of plane to form complex 3D-shapes from flat pattern-stock without balling-up and tearing/distorting in the process. Also, when installed, the patterning allows flexibility for thermal and sonic loads, to minimize foil stress cracking/tearing.

This foil-fab concept presented a dilemma: How to get raw foil material impressed with these patterns for repair-patches? NO ONE would sell it to us.. even for repairs!!! I had to salvage foil material form badly cracked/torn jackets, trim-to-fit over damage [torn-out holes, cracks, punctures, etc]... then 'poke-tack-weld' these delicate pieces into position on the old foil jacket... followed-up by sealing with DAPCO 2100 or 2200 sealant. What a pain... thankfully spares are plentiful... and are throw-away items after minor fire-wall sealant repair-limits are exceeded... and/or are replaced every PDM.

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]

 

[WKTaylor]

A long-time-ago-and-far-away...

Kaiser used to make 'quilted' [diamond textured] aluminum foil for kitchen/cooking use, claiming the benefits of toughness, formability and 'best sealing' properties. As I remember it, the aluminum foil of the 1950s/60s was ~slightly thicker than todays kitchen-grade foil. I suspect that the science of making smooth 'pretty slick' aluminum foil with better toughness/formability from thin material finally 'killed-off' the textured foil.

NOTE. This same general 'quilted' [textured] concept is why masking tapes have micro-textured [randomly cross-wrinkled/dimpled] surfaces: allows the tape to conformed to odd-surface shapes/textures much better than plain-slick-tape. Masking tape can not be made as strong as smooth tape made from paper, foil, plastic or fiber-reinforced tapes

For giggles...

https://www.youtube.com/watch?v=qa4weMG8F5Y

https://www.youtube.com/watch?v=23UVdJvl8LM

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]