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Informal testing for "minimum gauge" composite skin: hangar rash, handling

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Vigilant1

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We've had frequent discussion about the suitability of various composite skin types, thicknesses, and core materials with regard to damage tolerance. There seems to be general agreement that 21 oz per square yard fiberglass over solid 70 PSI XPS foam has a good record on many of the Rutan planes, and that thinner gauges of CF have held up well as an outer skin when applied to a PVC foam core with another layer of CF at the back of the sandwich. And then we have lots of KR's flying with 1" PU foam with thin epoxied polyester or FG as an outer face and bare foam on the inside.

This thread is a fork off from the "Not-so-solid-core" thread that is also running, and some other possible tests discussed there.

It's been observed frequently that handling/abuse tolerance, rather than other structural considerations, is frequently the factor that establishes the thickness of the outer skin.

If there are any studies or informal tests on this issue, please let me know. Otherwise, I'd like to do some simple (primitive?) tests to at least get an idea of the relative sturdiness of some laminate/core combinations.

So, any ideas on simple, repeatable tests that could tell us how well a composite skin can survive bumps, dropped fuel nozzles, clumsy hangar mates with tools, etc?

Billski has previously proposed a "ball-peen hammer test" as coming close to what might be useful. The ASTM D7766/D7766M impact testing of sandwich panels seems to come close to that. There are three "procedures" under that test, and I think the one that comes closes to our requirements is their "Procedure C" in which an edge-supported sandwich panel is tested with a dropped, smooth semi-hemispherical 16mm (5/8") impactor. I don't have the $60 ASTM document, but here's a study that describes it.

I'd make up samples, place them on a flat surface with a 4" dia hole in it, place another flat surface with a matching hole on top and clamp everything together, then drop the "impactor" straight down onto the center of the 4" testing area (probably riding in a PVC pipe). We can get the same energy with a low velocity/high mass or higher velocity and a lower mass but one might prove to be more damaging. For replicating the kinds of abuse a plane might take, I'd think relatively slow impacts (up to about 14 fps = 4.4 m/s = 10 MPH, which equals a drop from about 3 ft) at increasingly heavy weights would give the most useful information. Note if damage is visible, what type of damage, and the depth and diameter of any observable dents. Maybe a tap test for delamination (though the affected area will likely be small, maybe too small to detect with a coin and my ear).

Possible candidates for testing:
1) 21 oz FG over 3" 70 psi XPS foam ("solid core"), and over 1/2" XPS bare, 1/2" with laminate backing (tbd)
2) 12 oz CF over 3" 70 psi XPS foam ("solid core"), and over 1/2" XPS bare, 1" XPS bare, and 1/2" with laminate backing (tbd)
3) 12 oz CF over 6mm PVC with 6 oz CF backing
4) 6 oz CF facing over same XPS schedules as #2 above
5) 1mm plywood
6) .025" 6061 or 2024 aluminum

Depending on how this turns out, perhaps testing with more easily available, lower PSI XPS or even EPS would look promising.

Thoughts are solicited. My own interests are in seeing if there's a relatively light and inexpensive skin that is "tough enough" if we are willing to be careful with it, but that's a pretty squishy standard.
 
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wsimpso1

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I like it! The 5/8" ball is pretty common for this sort of thing. I prefer drop height and weight to define energy as they are pretty darned repeatable. And you can ramp up energy easily until you get damage.

I am now excitedly awaiting test results.

Billski
 

nickec

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What about when the impactor bounces?

Perhaps these multiple strikes, at smaller velocities, are OK - because you can still compare results.

Otherwise: an angled blow can serve. The target could be mounted at 45 degrees from vertical.

Further, it might take multiple impacts to truly answer.

Just because one impact is survived does not mean multiple impacts will not spoil the party.
 

Vigilant1

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What about when the impactor bounces?

Perhaps these multiple strikes, at smaller velocities, are OK - because you can still compare results.

Otherwise: an angled blow can serve. The target could be mounted at 45 degrees from vertical.
That's an interesting point. I'd rather keep things vertical if possible so the friction on any sliding surface or rails doesn't reduce repeatability.
Maybe a deadblow impactor? Lead shot or pea gravel behind the impactor point wouldn't bounce much, but I don't think the energy imparted to the tested surface would be the same, since the mass comes to rest over more time during the impact and peak load would be less..
 

Hephaestus

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5/8 ball peen, use a torsion spring (like repurposed from garage door) for repeatability without overhead / swing?

Torsion will limit the bounce back and hit a second time.
 

Vigilant1

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But if a guy in Australia wants to see how his panel compares to the ones already tested, how does he match the garage door spring I used? Gravity will be the same everywhere, and weight is easy to match.
 

Geraldc

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The Charpy impact test is repeatable and measurably.
A weight on a pendulum swings a set number of degrees to impact the sample to be tested.
 

Hephaestus

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But if a guy in Australia wants to see how his panel compares to the ones already tested, how does he match the garage door spring I used? Gravity will be the same everywhere, and weight is easy to match.
Torsion springs are pretty universal. It's just some headscratching math and lookup tables to sort out an IPPT and convert to what can be found cheaply in the local area / scrap pile.

Yes you're correct hard to beat gravity, but making a clean release without a push to hit a specific target precisely gets to be the fun part.
 

Marc Zeitlin

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There seems to be general agreement that 21 oz per square yard fiberglass over solid 70 PSI XPS foam has a good record on many of the Rutan planes...
I'm not sure where you're getting those #'s from, but that's pretty conservative. Assuming you're talking about the 2 lb/ft^3 blue foam, there are some places on the these planes that have 15 - 19 oz/yd^2 on them (2 plies of 7715 UNI, or 2 plies of 7725 BID) and are reasonably robust, as long as you're not hacking at them with ice picks.
 

Vigilant1

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I'm not sure where you're getting those #'s from, but that's pretty conservative. Assuming you're talking about the 2 lb/ft^3 blue foam, there are some places on the these planes that have 15 - 19 oz/yd^2 on them...
Thanks, Marc. Do you know where I could find the layup schedule for a Long-Eze main wing upper surface? Thanks.

Mark.
 

Hephaestus

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Rutan terf cd should be easily found online for download - all the instructions plans and schedules.
 

karmarepair

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That's an interesting point. I'd rather keep things vertical if possible so the friction on any sliding surface or rails doesn't reduce repeatability.
The way I understand the commenters point, the "chute" for the impactor is still vertical and essentially frictionless. The ball impacts the panel, once, a glancing blow, and then rolls across the shop floor under the Snap-On rollaway so heavily loaded with tools you have to break out the "yellow gear" tug to move it.
 

Vigilant1

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The way I understand the commenters point, the "chute" for the impactor is still vertical and essentially frictionless. The ball impacts the panel, once, a glancing blow, and then rolls across the shop floor under the Snap-On rollaway so heavily loaded with tools you have to break out the "yellow gear" tug to move it.
Ahh, I see now. That would prevent a bounce, though only a portion of the KE goes into the test panel.
I suspect the energy in the bounce-and-second-strike won't be much, but it won't be zero, either.
 

Marc Zeitlin

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Thanks, Marc. Do you know where I could find the layup schedule for a Long-Eze main wing upper surface?
Right here:

1606881310322.png\

This is the bottom skin - 2 UNI. Then for the top skin:

1606881353596.png

And the fuselage sides are thus:

1606886215515.png

two ply UNI over most of it, and a 3rd ply in the front.

And with respect to the strakes:
1606886322313.png

most of it is 2 UNI, with another ply on the top of the left strake since that's the one that people climb on to get in and out of the rear seat. Another ply would have been better, as many LE's have knee dents in the strake top.

So you can see that there's a fair amount of the plane that's just 14.5 oz, give or take.

Most of Voyager was 1 ply skin, but if you sneezed at it sideways, you'd poke a hole in it.

Hope this helps.
 

stanislavz

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Just loudly thinking - but it is a "unfair test" from carbon fibre vs glass fibre. Ie - some fishing rods made form carbon, was broken under minimum of load, after being dropped on some stone..

And foam/no foam test - samples with foam will be considered dead, after foam/skin delamination ? single skin when we have a hole ?
 

aeromomentum

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When working on another project we did some test panels (and products) with a cored skin over a lighter weight foam core. In this case it was FG, 2mm Soric, slightly thinner FG, thick (1") and light foam core and then FG. The Soric cored skin to the foam cored panel provided very good impact resistance, strength and low weight.
 

Vigilant1

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There are limitations in the impact test as spelled out in the OP. Among them, it wouldn't replicate repeated impacts in the same spot that result in cumulative damage (e.g. from where a wing storage cradle loads the same place every time and the wing gets banged around when inside a trailer. Or where the pilot always puts his foot when boarding the plane, etc). The good thing about these loads is that they affect a limited area and can largely be anticipated and designed for. It is a little different with regard to the errant clipboard or a hangarmate's wingtip.

Just loudly thinking - but it is a "unfair test" from carbon fibre vs glass fibre. Ie - some fishing rods made form carbon, was broken under minimum of load, after being dropped on some stone..
I think the tests (if something so informal can be called that) will just indicate the degree of observable damage a particular set of "challenges" will produce on various composite skin types. The importance of those mars and dents is an issue left to the designer. We could take things a bit further, maybe bend the panels after the abuse and see if the skins fracture broadly or not. Maybe that would tell us something useful, maybe not. To some degree this is cosmetics: if the dropped headphones dimple my wing, I'm not concerned that the wing may crack off later as a result. Now, if the lineboy's golf cart smacks my CF propeller or CF gear strut at speed, it is a different situation.
And foam/no foam test - samples with foam will be considered dead, after foam/skin delamination ? single skin when we have a hole ?
The tests will probably just give an indication of what type of abuse will result in what type of damage. I suspect different designers on different projects will have different thresholds for what is acceptable.
 

stanislavz

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+ I think we shell ask some glider boys of their experience. They are removing wings before/after flying and skins are way thinner, than in most aircraft. 200 gsm cf + foam + 200 gsm cf.
 
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