Liquid Shim Process 1

[JCorsico]

Hello all.

We are working on a design where a carbon composite panel is fastened to a 4130 steel subframe. The subframe has flat mounting flanges for the carbon panel. The carbon will be attached to the mounting flanges using both an epoxy paste adhesive and Hi-Lok pins. Basically, the design is very similar to how a wing skin is attached to the stringers.

The mounting flanges are not perfectly flat (there is minor weld distortion). The carbon panels are also not perfectly flat (manufacturing tolerances). The largest gaps are about 0.030", but most gaps are 0.010" or less. We want to fill these gaps using a liquid shim paste adhesive.

What is the recommended process for installing the liquid shims? Some questions that we have:

1) Can we install the shim paste and the Hi-Loks at the same time, using the Hi-Loks as clamps? In other words, will our epoxy paste adhesive function as a shim? Or, do we need an entirely separate process to create and cure the shims first? And then after the shims are cured, then install the Hi-Loks and more paste adhesive?

2) If we need a separate process to create and cure the shims first, do we use mold release on the carbon panel? Or a release ply? How do we prevent the shim from adhering to the carbon panel?

3) If we need a separate process to create and cure the shims first, how do we clamp the carbon panels in place? Clecos? Does the shim material get all over the Clecos and glue them in place? Do we coat the Clecos in something so the shim material doesn't stick?

Thank you!
Jon

 

[IRstuff]

Is there some reason to suspect that your epoxy won't fill in the gaps?

Seems to me that you ought to run some tests.

Trying to custom fill the gaps sounds like a money losing proposition

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[rb1957]

I would want the liquid shim to cure before installing the final fasteners. I'd use clecos (in lieu of fasteners) for the cure. I'm pretty sure we've never had trouble removing clecos, but you could coat with release agent.

You want to limit the thickness of liquid shims, < 0.063" ?

We have use liquid shims as a template, 3D scan, then machine a metal shim.

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

 

[SWComposites]

Ok, assuming this “paste adhesive” is only intended as a shim, and not a load transfer adhesive. And assuming this is a permanent joint, not a removable panel.

Should have release agent on one of the two surface, to help prevent the shim from cracking under load.

Do Not torque the fasteners until after the shim has cured, otherwise you will have no clamp-up in the joint.

Apply release to one surface, apply liquid shim layer to opposite surface. Bring mating surfaces to position and clamp. Hold until cured. Separate joint and check for gaps or other defects. Bring parts back together. Drill fastener holes. Install fasteners.

 

[WKTaylor]

WHAT specific paste adhesive [company/industry/mil spec] formula are You considering for the epoxy 'liquid shim'?

What is the thermal environment [-xxF to +xxxF or -xxC to +xxXC]? Have You considered expansion/contraction effects between the composite and the steel?

WHAT is the 'filler'... to prevent excessive 'squeeze/thin-out'... IE: fibers? glass beads? glass bubbles? phenolic bubbles? etc?

WHAT is Your steel surface preparation?

WHAT is Your composite surface preparation?

WHAT is Your proposed fay-surface and filleting application method: Brush or Squeegee or injection or combination etc?

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]

 

[blakmax]

WKT is correct. If you expect the filler to transfer load then you MUST perform surface preparation as the last step and bond as soon as possible. Otherwise assume that the bond is ineffective because it will for sure be ineffective.

To get the filler in, we once modified one of the fasteners by drilling a hole down the length of the filler such that it was past the joint location. Then we drilled a twp holes at 90 degrees through the fastener at the location of the joint. we then manufactured a modified nut such that we could attach an injection nozzle to the end of the fastener. Finger tighten the modified nut, attach the injector and then inject into the bond until there was free flow at the splice joint. Remove the nozzle and let the resin set. Install new fasteners and tighten.

Regards
Blakmax

 

[rb1957]

"surely" liquid shim is non-structural (ie filling the space only) ?

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

 

[JCorsico]

All - thank you very much. Quite helpful.

To answer some of the questions:

1) The paste adhesive is a Hysol product, which Henkel recommends for use as both a general purpose adhesive and also as a liquid shim

2) The steel surface will be grit blasted shortly before adhesive application

3) The carbon composite surface will be grit blasted shortly before adhesive application

4) The operating temperature range is 40F through 200F. There is a thermal mismatch between the steel and the carbon, but we do not expect it to generate failures over this range of temperatures.

5) The epoxy will be applied via extrusion gun and brush.

6) The installation is permanent, and is not designed to be disassembled.

7) We are expecting the adhesive layer to transfer load. It's not merely a shim. It's both a shim and a load transfer path.

 

[rb1957]

7) ... ohh ... what's the shear strength (and stiffness) of liquid shim ? Adhesively bonded metallic structures are difficult to do (unless you've done them lots of times, and have figured out the wrinkles).

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

 

[JCorsico]

ohh ... what's the shear strength (and stiffness) of liquid shim ? Adhesively bonded metallic structures are difficult to do

It's a Hysol epoxy designed for bonding metals and serving as a liquid shim. 5000 psi lap shear at room temp.

 

[WKTaylor]

Bare steel needs a high quality surface preparation for long-terms adhesion and corrosion durability.

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]

 

[rb1957]

yes, there's a lot of precise preparation ... Phosphoric Acid Anodise is typical.

what I find odd is that adhesive works best when very thin ... which doesn't sound like a shim.

whatever, I would not count on liquid shim as a loadpath.

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

 

[SWComposites]

You CANNOT design to that 5000 psi vendor shear strength. It is based on ideal thin adherend short lap length specimens.

Why are you designing for bonding when also using fasteners?

 

[SWComposites]

And yes, that 5000 psi is for ideal thin adhesive layer. NOT for typical shim thicknesses.

 

[JCorsico]

Wow, so may naysayers! To address some of the specific questions:

Bare steel needs a high quality surface preparation for long-terms adhesion and corrosion durability.

Yes, we agree. There are additional steps we'll take for corrosion.

yes, there's a lot of precise preparation ... Phosphoric Acid Anodise is typical.

That's only for aluminum. Our stack is steel and carbon.

what I find odd is that adhesive works best when very thin ... which doesn't sound like a shim. whatever, I would not count on liquid shim as a loadpath.

At first blush, I understand your concern. But the shim is obviously part of the load path. It's between two structural members. How does Boeing and Airbus bond the wings on a 787 or A350? They must be using shims between the stringers and the face skins, as it would be impossible to manufacture an entire wing to tolerances that require no shims.

You CANNOT design to that 5000 psi vendor shear strength. It is based on ideal thin adherend short lap length specimens. Why are you designing for bonding when also using fasteners?

You answered your own question! The fasteners compensate for a less than ideal adhesive joint.

 

[SWComposites]

Sigh. You are not listening to us. Liquid shim is not used as an adhesive. In fact, you do not want it bonded to both joint surfaces.

 

[blakmax]

JCorsico

I agree with SWC. You are not listening, so let me explain. There are many unknowing soles such as you that thoroughly believe that grit blasting is used to roughen the surface so the adhesive can key into it. WRONG! Adhesive bonding is a chemical process that relies on molecular bonds to transfer loads. It is NEVER a keying mechanism. If you rely on this theory your bond will fail after a very short service life. The primary cause of bond failure is that the surface of metals form bonds to the surface oxides, which after grit blasting are thin, and produce apparent short term bond strength. But over time, the resins absorb moisture from the atmosphere, and that moisture causes hydration of the metal surface oxides.Fe2.03 converts to hydrated oxides. For that chemical reaction to occur, the weak chemical bonds to the old oxide layer dissociate to enable the hydrated oxides to form, This is disbonding. Load transfer will not occur.

Please understand that you MUST treat the metal to prevent that hydration. I suggest you consider using epoxy silane treatments that form covalent bonds to the newly formed metal oxides and prevent later hydration.

Next, please understand that the load cap[acity of fasteners and the load capacity of adhesive bonds are NEVER additive. I have data that shows that in one highly loaded bonded step lap joint, analysis showed that a fastened only joint carried approx 28000 lb/in load. The same joint fully bonded could carry approx 35000 lb/in but bonded and fastened joints carried approx 33500 lb/in. So the bonded and fastened joint was Weaker than just bonding (with correct surface preparation).

Next, the issue of designing on lap shear strength as measured by ASTM D1002 or similar tests. Convince yourself. Test with twice the overlap and the result will be vastly different. Test with dissimilar materials and the results will be different. Test at different temperatures and the results will be different. So how can you use a lap-shear average shear result for design?

Please, do not be aggressive to those who are trying to help you not make mistakes. Try reading

http://www.tc.faa.gov/its/worldpac/techrpt/artn0657.pdf

It might help you, just as we who know are trying to help you. We are not nay sayers. We have learnt our lessons from the same mistakes you are making.

I am happy to help if you are prepared to listen.

Regards

Blakmax

 

[JCorsico]

SWC and Blakmax - I am listening. 100%! I am just confused.

You are worried about the long term durability of the joint due to corrosion. You are also worried about how we are calculating the design loads and whether we should rely solely on adhesive or solely on fasteners or have both. Your concerns are entirely valid, but are not the problems we are trying to solve right now.

How do Boeing and Airbus bond composite skins onto metal stringers? They do this all the time with the 787 and A350. Many of those joints also use mechanical fasteners. I suppose you could tell me that shimming is not allowed, and that the parts must fit perfectly. But that doesn't seem plausible to me. They must be using shims over a structure the size of a 787 wing.

Thank you!!!
Jon

 

[rb1957]

There is a world of difference between structural adhesive and liquid shim.

Structural adhesive is very carefully designed and minimal gaps. and a tonne of testing. and huge environmental concerns. Please don't compare to liquid shims.

Shims are intended to fill spaces, liquid shim is just an easy way to form the shim (as opposed to peel-able or file-able metal shims. Our advice is not to count on the liquid shim for load transfer. Our advice is not "naysaying", even if it is contrary to your desires.

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

 

[JCorsico]

There is a world of difference between structural adhesive and liquid shim.

Structural adhesive is very carefully designed and minimal gaps. and a tonne of testing. and huge environmental concerns. Please don't compare to liquid shims.

Shims are intended to fill spaces, liquid shim is just an easy way to form the shim (as opposed to peel-able or file-able metal shims. Our advice is not to count on the liquid shim for load transfer. Our advice is not "naysaying", even if it is contrary to your desires.

Okay, so how do you adhere two components that don't fit perfectly together?

For example, I cannot imagine that Boeing and Airbus are manufacturing wing skin / stringer assemblies that have 0.005" or less gaps across the entire wing surface. There must be gaps. How are they filling the gaps before bonding?

This is a legitimate question. I'm not ignoring your advice.