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.
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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