ascaro wrote: ↑
Tue Feb 12, 2019 1:11 pm
has anybody got some experience regarding the inspection or the simply maintenance of a full carbon frame for a flex wing? How do you do if a nose in occur, or a bad landing happens? I know that the carbon fiber is way more and more stronger than any aluminum equivalent part, but how do we do with the inspection? Somebody says that you can test a frame by gently tapping on it with a coin or something that could 'sound' to give you an idea if the part is completely solid and filled, but it seems such an empiric thing that I can't help being doubtful... What do you say?
You have hit on one of the main issues (beyond original cost) that I have with CF, or any composite structures for HGs. How can you test this stuff, after a few whacks?
I have done some good work in that arena, using an infrared camera where I worked. I was using a lab-grade IR camera, but it is possible that lesser (and far less expensive) gear MAY
give you decent results. I used heat lamps (such as the fast-food places use to keep food warm) as a heat source for composites. I aimed the heat at one side of the part, with the camera looking from a 90 degree angle to the heat rays. As the part warms up, the heat should spread smoothly and evenly through the composite material. Any crack or void in the structure would show up as a line with warm material on one side and cooler material on the other side. You would need to repeat the test from fully cold (room temperature) in three axis - rotate the target object 90 degrees (without moving the lamps or camera) to view front, side, and top images of the target. For long structures, such as a main spar or keel, you would need to pass the target through the camera viewpoint at a slow and steady speed. I never had that requirement, so you may need some creativity there. In all cases, I used image capturing software to allow repeated views of any test done. The factory that made our composites told us to tap with a coin and listen; we told them to go take a flying leap at a rolling doughnut.
Any faults found by the camera could only be verified by cutting the part into pieces, but our testing never called a bad part that was not verified by cutting.
If you are still with me, the heat lamps do NOT need to cook the target; usually you could barely feel a warmth in the target object, but the camera showed a rainbow of "false" colors, as any section became "warm." Attached you will see"false colors" of a transmission line tower as an infrared image - one side is the everyday view of normal metal, and the other side shown with heated parts (as "false colors" in infrared). Each part fades
from one color to the next, in infrared normally. If any part was cracked or broken, the heat would stop at the crack. I am guessing that the Sun was providing the heat on the tower that you see in the picture. Electrical power was heating the transmission lines.