Freeze Fit bushes in composite 2

[martymcp]

Hi,

My name is Martin and I am a junior stress engineer working in research. I need some help as to what are the effects of freeze fit bushes in composite structures. Can any one help?

Cheers

Martin

 

[SWComposites]

Interference fit (mechanical, freeze fit, etc.) bushings/fasteners/etc can cause delaminations on installation if the amount of expansion is too high. What type of composite structure (material, thickness, etc.) and what type of bushes (material, diameter, etc)?

Steve

 

[plasgears]

Freeze fit seems OK. Don't overlook thermal stress and final fitup dimensions. You're intention should be to maintain desired clearance under operations. I had misery with this under hot conditions.

 

[martymcp]

The stucture itself is a large double lug arrangement ideally 20/60/20 composite about 90mm thick with an intial Al-Brz bush of inside diameter around 105mm. I will be looking at the thermal effects also to check the feasibility of the design but I am not sure what to expect. Has there been much work on this topic before?

 

[MechanicalChef]

Thinking out loud, I'm curious to the benefit of a freee-fit. Composites aren't so hot in bearing strength, since you're more or less trying to push a string. That leaves your matrix (a polymer) to take the thermal loading. But, an issue of polymers is stress relaxation. Hence, the bushing will loosen with time. Hmmm....

 

[rb1957]

i think the point of the bushing is to distribute the loading into the composite, particularly with a soft bushing. ... there's a thought, what's the material of the pin, steel ? is that a good idea ? what about material compatability ... is composite (maybe graphite) compatable with Al-Brz ??

i hope you're not going to lay-up the fibre, then cut the bore through the cured material (cutting all those fibers). I think it'd be better to lay-up the fibers around the hole.

curious that this big of a job is given to a junior

 

[berkshire]

martymcp
You will have better results with a potted in bushing, than a press in bushing, especially if you are putting this into a bored hole. If the bushing can be molded in during the layup that would be even better.
Most aircraft that I work on that use this system put a light knurl on the outside of the bushing to improve the retention.

 

[MechanicalChef]

http://www.metal-mart.com/Guides/Galvanic.htm

Al-bronze falls at approximately the lower quarter of the list. So depending on the specs you work with, it may or may not be possible to use that material for fear of it being devoured galvanically.

 

[RPstress]

With that layup and a close tolerance pin in carbon with no bush you should achieve at least 400 MPa/60 ksi bearing strength, and it could easily be 1.5 times that. It depends on material form (cloth vs. tape), thickness to diameter ratio, pin clearance and any clamp-up (and a little bit on fiber volume fraction). 90 mm is thick for carbon. This is maybe glass?

Theoretically you should get some benefit from the more even distribution of stress from a shrink fit that will offset the residual radial stress a little. The only sensible way to allow for this is to use an allowable bearing stress from tests with a shrink fitted bush. If this isn't available then you'll just have to use the bearing allowable data for a clearance fit pin. Non-optimal but conservative (safe).

If the joint isn't clamped up make sure that the allowable bearing stress is for a plain pin. Quite a lot of bearing data is for use with bolts and may well be for a finger tight torque-up. This makes a surprising amount of difference.

You have to be sure that the peak bearing stress from the squashing of the bush plus the peak pin loading is less than your bearing allowable. Be sure to allow for the max service temperature making the bush try to expand still more, and make sure you check for the maximum possible bush thickness and diameter and the smallest possible hole diameter. Normally I would ignore the bush expanding the composite (and so reducing the shrink fit bearing stress a little). However, in glass this might just be a significant reduction. Ignoring it is conservative. NB: the peak bearing stress should be less than this because the pin load will elongate the hole and reduce the shrink fit bearing stress. However, this is difficult to allow for.

Ignoring lug expansion, elastic bearing stress from the shrink fit is from:
Circumferential_strain_in_bush = (bush_OD - hole_ID) / bush_OD
Circumferential_force_in_bush =~ bush_thickness * bush_length * circumferential_strain_in_bush * bush_E
(The above is a slight approximation: strictly we should use the compressed dimensions of the bush.)
Bearing_stress = circumferential_force_in_bush * 2 / bearing_area

Usually
bearing_area = hole_ID*bush_length

To allow for the max service temperature add in
Circumferential_thermal_stress_in_bush = delta_T * bush_CTE * bush_E
Extra_thermal_bearing_stress = circumferential_thermal_stress_in_bush * bush_t * bush_length * 2 / bearing_area


In carbon I would ignore any thermal expansion of the composite. This also avoids the slightly thorny topic of what to do if your laminate has different CTEs in the X and Y directions. In glass, the composite thermal expansion might just be significant. However, ignoring it is, again, conservative.

If the bush yields (unlikely) all this would be an over-estimate.

Note that with liquid nitrogen it can be quite hard to achieve the necessary tolerance on the bush O/D and hole ID. Too big with hole too small and it'll damage the composite, too small with hole too big and minimum service temperature (which is when the bush is smallest) and it'll not have interference at all. However, judging by the thickness this is going to be a big hole, which makes the tolerancing issue easier.

The bush will also cause circumferential tension around the hole. This may be relieved by the hole's elongation under pin load, but to be safe it should be left in (at least to start with) and be added to the net section tension from the max lug tension load.

Things may depend a bit on how the lug is being stressed. You can make safe predictions with methods such as the point stress criterion, but you need quite a lot of pin-in-hole and net section failure test data to do it well. To properly optimise a lug you really need test data on lugs. However, as I said above you can produce a safe-but-somewhat-non-optimal part reasonably easily. If you're using the point stress criterion or similar you can add the force from 2 * circumferential_force_in_bush from above to the pin load safely.

Fatigue won't be an issue in carbon (probably), but may well be in glass. Here the residual circumferential stress should prolong the fatigue life in the usual way (by upping the mean stress and reducing the alternating). In this case, the bush tolerances need to be considered at the lower end, with the residual circumferential stress from smallest and thinnest possible bush with the minimum likely average service temperature.

Re corrosion, if it's carbon you'll want to paint the outside and ends of the bush and use plenty of sealant in the shrink fit op, and also paint the outside of the carbon around the bush if it's going to get wet. If it's glass there're no real corrosion concerns that I'm aware of.

NB: an Al-bronze may well have a bearing allowable less than that of the composite. You probably don't want it yielding in bearing too much, so remember to check the bush ID with the pin load.