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At 10:12 AM 3/29/2005 -0600, you wrote:
I was perusing the Velocity manual online (sorry, not a public site) and came across a statement with regards to AN fittings needing to be steel if they are situated ON the engine.
Fatigue is likely the motivation for this caution.
It is not that aluminum has no fatigue life, it is that is has no fatigue limit. That is, no matter how lightly you load it, it will fatigue crack "eventually."
Steel, on the other hand, has a "fatigue limit". If the peak load always stays below this critical stress value, the part will never fail, ever.
The word "eventually" is the key. A good example is aluminum connecting rods. They are subjected to severe cycling. They ALL will fatigue crack and fail eventually. If you run the engine at high load, they will fail well before the rest of the engine wears out. If you load the engine lightly, the rods will (nearly) always outlast the rest of the engine.
If the cycling rate is low and the cycling load low, the AN aluminum fittings will last a long time. If the cycle rate is high, and the load is high, they will fail much sooner.
For small fittings, the mass, stiffness, and length of the hose (or pipe) is typically large compared with the diameter of the fitting. Thus, the peak stress is typically higher in small fittings than it is in large fittings.
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