Friday, January 07, 2011

Comparison of Tube Strength for Common Small Aircraft Tubing

Design for Strength

Comparative Tube Strength, 6061,5052,2024,6061,3003

Design for Endurance

Since aircraft are vibrating creatures, fatigue strength is also a limiting factor.
Tube Fatigue Strength

Most tubing-failures on aircraft are caused by fatigue.  Tubing on aircraft vibrates. How well your tubing endures when subjected to load reversals, impulses, and vibration is called "fatigue strength". Of the three popular tubing types (3003-0, 6061-0, 5052-0), 5052-0 has the best fatigue strength.

Originally aircraft used soft copper-tubing. There is even some of this still around. Although copper was strong enough, it was replaced with aluminum and stainless tube because of the high fatigue failures of copper. For lower pressures, 5052-0 became the tubing of choice because it has the best fatigue strength of any of the non heat-treat aluminum alloys. See "Fatigue Failures of Copper Alloy Fuel", AWB 28-007

Copper tubing on older aircraft should be removed and replaced with 5052-0 before it breaks. There is no warning when copper tubing breaks. One cannot "inspect" it and declare it ok. Another limitation on the use of copper tubing in aircraft engine compartments is that copper strength decreases rapidly with temperature.

Some experimental and light-sport aircraft have hydraulic and fuel lines built with 6061-0 or 3003-0-tubing. Low ultimate strength and low fatigue strength provide a narrow safety margin in dynamic (vibration or impulse) applications. Take extra care in clamping and preventing tube vibration. The aircraft industry's long experience with copper tubing failures proved the importance of fatigue strength.  5052-0 has higher strength and higher fatigue strength at a small price difference. Both 5052-0, 3003 have the same Cold Workability Rating of A (easy to work with). 6061-T6 has a far lower rating of C. "It hasn't failed yet," was the attitude at NASA that essentially led to both of the Space Shuttle disasters; the complacency arises from skirting the line and surviving.  But the law of large numbers will eventually get you.

The aluminum hydraulic lines on the Cessna-404 have experienced 5 reported failures due to metal fatigue. For the mechanic, this means that these lines cannot be inspected for fatigue failure. They will not show fatigue stress before failure. A replacement interval is the only method of prevening failure. The old adage that "if it flew in it will fly out"1. only works until the next failure. See NTSB Safety Board Recommendation A-83-1-2. Metal fluid lines in aircraft subject to vibration have a potential to fail due to metal fatigue. Using the proper alloy tube, combined with good fabrication techniques, and proper clamping, and hard-time replacement interval is the only protection from sudden failure due to metal fatigue.

Diameter Effects

The larger the tubing diameter, the less pressure it can withstand
A 1/4 inch (0.025) aluminum tubing can hold 3,500 psi of pressure. The same aluminum tubing, but in 1/2 inch can only hold 1,800 psi. If we made a business jet pressurized fuselage out of the same tubing, it could only handle 182 psi.

When working with large pressure vessels, such as aircraft fuselage, don't be fooled by the low pressures. Because of their large size, these pressure vessels contain a lot of energy.

Don't forget the bulkhead. The bulkhead constains the fuselage skin, sucking up the load. A good lesson to learn is why the rear bulkhead failed on  Japan Airline Flight 123.

Tubing and hose can be thought of as cylindrical thin-walled pressure-vessels. *

The strength of thin walled pressure vessels is determined by:

  1. The material strength
  2. The wall thickness, and
  3. The size of the tubing.
The formula is: strength, psi = yield*(wall thickness/radius)

This last item, tubing size, is unusual. One can understand how strength is related to how strong the material is and how thick it is but size (radius)? The relationship between tubing size and strength is inverse; the larger the tube diameter the less strength it has. When you look at pressure ratings for tubing and hose you will notice that for the same hose, maximum recommended operating pressure goes down as the size goes up.

You can use this property to your advantage. For example, you might have a choice of tubing or hose size for a particular application. Everything else being equal, a smaller diameter line holds more pressure than a larger diameter line. Another advantage is that a smaller size weights less.

When you inspect a hose or line, you are inspecting a pressure vessel. As with all pressure vessels, they should be protected from damage that reduces the wall strength. Inspect for nicks, cuts, chafing, and corrosion.  Make sure that the line does not vibrate.

*Pressurized aircraft are also pressure vessels.

1. "if it flew in it will fly out". When playing Russian roulette the fact that the first shot got off safely is little comfort for the next. 

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