Phenolic Tube Failures

"A quick and dirty failure analysis"

When I tested the 3" phenolic tubing, I noticed that they would collapse in random areas. Shortly thereafter, I had a 3" phenolic tube break in two upon recovery. Examination of the fracture revealed apparent gaps within the tube wall. There were only a few wraps of phenolic paper at the point of fracture.

I sectioned a random scrap length of phenolic tubing to determine what may have been the cause of both the random crush locations during testing, and the failure of one of my airframes, or even if the two had the same root cause

The phenolic tube was sectioned in various random locations using a wafering saw. All samples were ground to 800 grit Si carbide paper to achieve a smooth surface to examine. It was apparent that there there were subsurface gaps in the wall of the samples. Two pieces were documented with photomacrographs.The gaps measured ~0.0605" x ~0.0095" using a filar recital eyepiece under 20X magnification. The tube was 7 layers of approximately 0.0095" phenolic-paper totaling a wall thickness of ~0.068".

A call to a manufacturer revealed that the cause of these gaps is in the manufacturing process. It was explained that the machine that wraps the Phenolic paper is adjusted such that there is a gap between the papers in order to avoid overlapping. It was also stated that as the machine wraps subsequent layers, it cannot determine where exactly each layer is and thus becomes random.

This then causes laws of probability to enter. The randomness of each wrap and its corresponding gap will occasionally create a condition where several of these gaps can be on top of each other or in very close proximity.

Thus, both the the random crush locations during testing, and the failure of one of my airframes had the same rot cause; concentrated subsurface gaps in the tube wall.

Click on pictures to enlarge.

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