An experiment I wish to attempt

[Anticept]

So, I want to see exactly how much damage UV and weather exposure does to composites, and I'm doing my preliminary research before moving forward.

My plan is to make multiple composite sandwiches using materials present in FD CTs. Some will be left outside on the roof (similar environment to storing a plane on the ramp), some will will be inside of a T-hangar, and some which will be inside of a controlled environment as a control (the purpose of the control is to prevent a misattribution that might result from a chemical breakdown affecting all groups).

I will also paint a couple of each using Acrylic Urethane (the type of paint used on CTs, which is UV resistant), while some will remain UNPAINTED.

Over time, I will take one set of sandwiches down, and document, ultrasound, and take pictures of the physical condition. I will then take them to the local engineering college to do shear bending testing to see if there is any strength degradation.

The plan is that this will be a long term experiment, over the course of 5 years, and each year taking down a set of sandwiches for testing and examination. My hypothesis is that there will be a significant difference in the painted and unpainted versions left on the roof, as well as between the unpainted ones on the roof vs those stored in the hangar and control. Additionally, the painted sandwiches on the roof will be moderately degraded in comparison to those left in the hangar/control. I do not believe there will be much of a difference between those in the hangar and those in the control in general.

Are there any issues that you guys see in my method which might lead to an error that would be introduced in the experiment's results?

[Roger Lee]

White paint. FD only uses this color because of heat build up when in the sun. Composites don't do well with dark colors in high temp and direct sunlight areas. If you live in England you could get away with a darker color.

[Jim]

A friend of mine is the VP of engineering at a company that makes AWOS stations.  They have a test station right on the Pacific coast that they do their long-term environmental tests of their enclosures.  Lots of sun, lots of salt spray, some rain.  I'm sure I could get him to put one of your samples out there if you're interested.

[Anticept]

Of course white paint . The idea is to eliminate as many variables aside from weather and sun exposure as possible!

 

 

A friend of mine is the VP of engineering at a company that makes AWOS stations.  They have a test station right on the Pacific coast that they do their long-term environmental tests of their enclosures.  Lots of sun, lots of salt spray, some rain.  I'm sure I could get him to put one of your samples out there if you're interested.

 

That would be neat to see the salty air effect on composites. I know salty air absolutely tears up metals, as we have one of our CTLS that used to be based in florida, and all the bolts have that really thin coat of corrosion.

[Tom Baker]

I would also make sure the foam core is protected. I could see some issues with deterioration if it was exposed.

[Ed Cesnalis]

They only use white paint to avoid paying me a 'design royalty'.

[WmInce]

. . . " florida, and all the bolts have that really thin coat of corrosion." . . .

 

"Boeshield T-9" is a wonderful product for that.  I use it generously on my CT and boat.

[Anticept]

I would also make sure the foam core is protected. I could see some issues with deterioration if it was exposed.

 

The entire sandwich will be encased in carbon fiber. The foam is being included to try and emulate the construction as best as possible. During the strength test, the edges will be trimmed off for consistency and inspection of the core.

 

 

"Boeshield T-9" is a wonderful product for that.  I use it generously on my CT and boat.

 

We're in the middle of ohio, so the exposure to any corrosive environment is basically nil. For our metal airplanes, we shoot corrosionx into the wings and airframe every couple years, simply because they sit on the ramp.

[GlennM]

Of course white paint . The idea is to eliminate as many variables aside from weather and sun exposure as possible!

 

 

 

 

 

That would be neat to see the salty air effect on composites. I know salty air absolutely tears up metals, as we have one of our CTLS that used to be based in florida, and all the bolts have that really thin coat of corrosion.

 

 

Florida Atlantic University has an Ocean Engineering program and they had composites sitting exposed to UV, salt air and water in the early 90's. I am sure they have results on the internet or you can contact them about their results. Once you know what composites our planes are made out of, they might have environmental-effect data on them.

[Anticept]

I will inquire about their results.

 

Our airframes use the epoxy MGS L 285. It is the epoxy component that is sensitive to UV, but the carbon fiber I use will be industry standard.

[ctfarmer]

Shear is shear and tensile is tensile and never the twain shall meet.

Not sure what you mean by

 

"shear testing to see if there is any tensile strength degradation"

 

I think you need compression, tensile and shear testing to correctly evaluate your hypothesis. My thoughts are that much of the loading on an air frame creates a tri-axial stress state. Ie tension or compression in two directions and shear all at the same time.

 

I suggest a tube made out of this stuff and subject to torsion and bending at the same time would better replicate the loads on say wings and fuselage. This would provide shear, compression and tensile tests all in one.

 

If you produce uni-directional fiber composites and perform only tensile tests in the direction of the fibre, the resin may have little contribution to the strength and give good results regardless of matrix degradation. When subject to loads at an angle to the weave, i suspect the resin will increase its contribution to the strength.

Compression is entirely another matter and we have compression and tension and shear in most structures. Compression is likely to be whole of matrix failure and likely to depend more heavily on the state of the resin.

 

peter

[Anticept]

When you subject a sandwich structure to shear stress, it puts the structure in tensile AND compressive stress, which is where the bulk of it's strength comes from. That is what makes it so strong, the forces are spread out over the entire part. We put in the cores because it's a cheap and lightweight method to amplify the physical characteristics of this compression and tensile strength combination, since you are spacing out the load bearing layers, thereby better transforming shear stresses into lateral stresses. You could just simply use a crapload of fiber reinforced resin layers instead of a core and it would be even stronger, but it's very heavy and expensive to do it that way, and the strength to weight gain would be negligible.

 

I should clarify that the method used for shear testing is to support the structure with two "sawhorse" like structures, each from the edge roughly 1/3rd of the diameter of the part and press down in the middle of the part using a hydraulic press with psi gauge, so that it "bends". Like this:

 

 

Sandwich structure is not designed for torsional loads at all, and it's actually not a realistic test, as it will more than likely damage the core before the resin or fibers give way. Wings do not twist much, and the design of aircraft cause torsional stress to be spread out across the entirety of the skin and structure as uniformly as possible. Since spars are not exposed to the elements, I am not testing them. That, and to construct the box spar design that FD uses will be prohibitively expensive.

 

As for the suggestion of testing in the bias direction: that I believe would be a good idea. It would put the most stress on the resin, without excluding the properties of the rest of the composite sandwich components. Also, it would be closer to FD's design, as the fibers in the wing skins are laid out in the bias direction to the lateral axis.

 

Oh and, i removed the "tensile" word from my original post, since in reality, I am testing multiple types of stress, and that's what I really meant.

[ctfarmer]

Understood! Terminology.

 

That is a bending test not shear. There is a shear component in a bending test as well as compression in the top fibres as per you diagram and tension in the bottom. So which way up are you planning to test relative to the weathered side?

 

I agree that to test a sample of foam sandwich such as you plan to make in torsion is an incorrect application however members made out of foam sandwich composite are often designed to be loaded in torsion thus giving a resultant tensile or compressive force at an angle to the unidirectional weave. Eg closed structural members like a racing car chassis and parts of the ct. They are made as closed structures (they may still have openings for access eg to put the engine in or seating) to handle torsional loads.

 

Correct me if I am wrong but from what I can see many structures on the ct do not appear to be foam sandwich. The wing skins for example and the entire fuselage from behind the cabin to the empennage?

 

Nevertheless, subjecting to bending alone for the foam sandwich parts, you might need to design them so point loads do not crush the structure at the point of load application and support points or spread the load to be uniformly distributed. Depends on how thick or strong your test structure is and how thin are the skins.

 

I would keenly await your results but feel non foam sandwich parts won't relate in the same manner. Even the weathering will be a interesting exercise.

Peter

[Anticept]

Understood! Terminology.

 

That is a bending test not shear. There is a shear component in a bending test as well as compression in the top fibres as per you diagram and tension in the bottom. So which way up are you planning to test relative to the weathered side?

 

I agree that to test a sample of foam sandwich such as you plan to make in torsion is an incorrect application however members made out of foam sandwich composite are often designed to be loaded in torsion thus giving a resultant tensile or compressive force at an angle to the unidirectional weave. Eg closed structural members like a racing car chassis and parts of the ct. They are made as closed structures (they may still have openings for access eg to put the engine in or seating) to handle torsional loads.

 

Correct me if I am wrong but from what I can see many structures on the ct do not appear to be foam sandwich. The wing skins for example and the entire fuselage from behind the cabin to the empennage?

 

Nevertheless, subjecting to bending alone for the foam sandwich parts, you might need to design them so point loads do not crush the structure at the point of load application and support points or spread the load to be uniformly distributed. Depends on how thick or strong your test structure is and how thin are the skins.

 

I would keenly await your results but feel non foam sandwich parts won't relate in the same manner. Even the weathering will be a interesting exercise.

Peter

 

Thanks for the better word, I'm going to call it a bending test (I couldn't think of a better term than shear).

 

It shouldn't really matter which side is tested, as long as it is the same side throughout the experiment. However, I think it would be best to test with the sun-exposed side in compression, as this would simulate flight loads.

 

As for the torsional design: The box spars are about the only item in the CT that would be able to sustain resistance to torsional forces in isolation due to the way they are made. The rest of the skin surfaces spread and translate torsional forces into tensile and compressive forces as much as possible, as fiber reinforced resins are weakest in shear and twisting forces.

 

The CTs use sandwich structure almost entirely throughout, even in the tail underfin. The core is a thin layer, about an 8th of an inch. It consists of primarily Rohacell and Airex, but it can also consist of Lange & Ritter and Gerlingen. Here's a photo of two holes that I had to drill due to some minor skin damage. (link will remain valid for a while).

 

As for the bending test: The Ohio State University's aeronautical engineering program has the tools needed for testing this material. They actually asked the maintenance program at Columbus State Community College (where I attended) to provide some testing material, as they began teaching advanced composites in their program. They were provided with a sandwich structure consisting of 1 layer of fiberglass and 1 layer of aramid, and 1 layer of fiberglass and 1 layer of carbon fiber, all bidirectional, around a honeycomb core. I believe it was about 8x8 inches. They put it on a shear testing machine (actual shear testing, not bending) which applied 55,000 pounds of force before the safety kicked in, with no damage to the structure. (let me see if I can get the actual test data, so don't quote me on those numbers, I'm only pulling them out of memory)

[Ian]

....... If you live in England you could get away with a darker color.

 

Only if it's waterproof

[Roger Lee]

I have seen pictures of dark colored CT's in country's with a much cooler climate and less sunlit days. I saw a picture of a black one in England.

[FlyingMonkey]

I would not mind having an orange CT, but that might not be light enough.