## Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

## Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

(OP)

hey,

I have ULTEM 1000 (CTE=55e-6 1/c) part that is attached (using adhesive) to a steel part (CTE=10e-6 1/c).

The ULTEM 1000 part is the orange ring in the photos below:

By the way, the parts' materials and the joining method by adhesion are constraints.

We are very worrid about stresses that will develop in the interface (adhesive) when exposing these parts to temperature change.

Someone claimed that splitting the ULTEM 1000 ring to some segments may help to reduce the thermal stresses, but I am not sure about it.

What can you say about it? an explanation would be great.

I have ULTEM 1000 (CTE=55e-6 1/c) part that is attached (using adhesive) to a steel part (CTE=10e-6 1/c).

The ULTEM 1000 part is the orange ring in the photos below:

By the way, the parts' materials and the joining method by adhesion are constraints.

We are very worrid about stresses that will develop in the interface (adhesive) when exposing these parts to temperature change.

Someone claimed that splitting the ULTEM 1000 ring to some segments may help to reduce the thermal stresses, but I am not sure about it.

What can you say about it? an explanation would be great.

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

TTFN (ta ta for now)

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## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

About the question if I did a thermal stress analysis - so, as a one that isn't deeply familiar with simulations (except for simple parts cases) I wasn't sure how to correctly model the adhesive layer, so I assumed a bonded relation between the parts and tried to see the interface stresses and it seemed quite the same in both cases, something that made me raise the question here...

I originally thought that splitting the ring may help...if I take the simplified case of adhesion a short simple straight part Versus a long one, the answer is clear...but the geometry here and the analysis results made me confused a little bit. Do you think the splitted ting is better?

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

I regard this analysis as simple. You have a

ΔT. You have a circumference. You can work out the strain due to the temperature, and you can work out the resulting stresses. Your adhesive and design either works, or it doesn't.If it doesn't, either you need to select materials with matching CTE, or you need a new way to retain the part. Can you cut a retaining slot?

--

JHG

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

= = = = = = = = = = = = = = = = = = = =

P.E. Metallurgy, consulting work welcomed

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

I recall glueing moderately large mirrors to metal substrates. Glass has a CTE of 9×10

^{-6}K°^{-1}. Our preferred material, aluminium, is around 23×10^{-6}K°^{-1}. 300 grade stainless is about 18×10^{-6}K°^{-1}. 400 grade stainless is around 10×10^{-6}K°^{-1}. I called up 416MX, and I added an inspection note stating that the desired stainless steel grade was magnetic.--

JHG

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

My colleagues and I had a discussion today regarding this question, and I came out a little confused. They insisted that splitting the ring doesn't decrease the thermal stresses in the adhesive... they said that in each case the mechanical strain equals to the thermal strain, and as a result a same stress would develop.

About the suggestion of making a simple analysis - I don't know how to correctly model the adhesive, and as a result the interfacial stresses may be wrong.

EdStainless, do you have any example of how to manually calculate the expected stresses in each case mentioned above?

To be honest - I am a little embarrassed how simple problem makes me wonder a lot how to properly analyze this joint.

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

The length of your split ring closely approximates the circumference of an un‑split ring.

For each diameter, you have a circumference

C_{T0}andC_{T1}. Your adhesive has a thickness, an elastic modulus and a shear strength.Can you retain with a thick layer of something flexible, like RTV?

Can you split your ring, and apply a small dab of adhesive opposite to the split? This works better if your ring is a little oversized, and it sits in an orthogonal pocket.

--

JHG

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

An epoxy adhesive can easily bond a 1 meter square of aluminum foil to a 1 meter square of 1 cm thick steel plate through a 100C temperature cycle. Yet, there is no adhesive that can work if the aluminum is also 1 cm thick.

A better example might be bonding aluminum foil to steel foil in alternating layers. No problem.

Now try bonding alternating layers of one inch thick plates. It cannot be done. The shear stresses between plates is greater than any adhesive can bear.

You have to consider the exact stresses and strains each material will will experience due to the particular geometry in each case. At the ends of bonded joints tensile and compression forces are zero (by definition) while shear stresses are at maximum. This is the reason why splits in the ring might help.

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

However, boundary conditions are a different matter.

The unrestricted/unbonded top surface of that plastic ring is attempting to shrink, just as the plastic is at the adhesive layer. The difference is the directly bonded material is being resisted by the adhesive's grip on the metal. This produces a torque where the plastic at the top is trying to peel the ring loose.

That unrestricted shrinkage is largely resisted at the edge of the ring. Splitting it may help enough with that, but it does add more edges which means more peeling-failure initiation sites.

If the ring had a rectangular section and was fully in the tube, as a bushing might be installed, the unbonded surface would uniformly add to the tension load when it shrinks from cold.

Typically the adhesive layer is too thin to matter. If you have a layer that is 0.001 inch thick and the unrestricted delta in diameter is 0.030 inches, that's above any elastic limit one might find**. IOW, it may be worthwhile to ignore the adhesive. In addition if the adhesive CTE and elastic modulus are similar to the plastic, it won't matter anyway.

I haven't seen the delta-T expected for this application, nor if it is large enough to change the elastic modulus of the plastic.

**I've seen failures in thin adhesive joints between much more closely matched CTE metals using silicone elastomer adhesives. The problem was fixed by bumping the joint thickness to a more reasonable 0.020 inch.

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

You mentioned good things I wasn't familiar with. Thank you

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

Would it be possible to form a counter bore

At the end of the tube and nest the ring in it.with two surfaces to apply adhesive.

Instead of an angle.

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

"Unfortunately I can't machine the part"

is this in the developing stage or completed parts?

it would be a simple forming operation

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

Have you picked an adhesive? do you have detailed properties for it (strength and modulus)?

So take sizes at 25C and then apply your delta of 25C. Now just figure out the force to squeeze the plastic ring to the new size of the metal part. That force will be distributed across you adhesive joint.

= = = = = = = = = = = = = = = = = = = =

P.E. Metallurgy, consulting work welcomed

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

elinah34, you might trywishing. It works occasionally.Regards.

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

Some offered using a counterbore or even a square shoulder. A cross section sketch might be blessed for making sure I understood you well.

Thanks again all of you!

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts

## RE: Undesirable stresses due to attaching (with adhesive) 2 different CTE parts