|
For structures that need to be very stiff but don't need to endure a lot of flexing or vibration, aluminum is good stuff. But for structures that need to be able to absorb energy without suffering metal fatigue, steel or carbon fiber is usually used. A driveshaft is one of those structures. Aluminum has a very high ultimate tensile strength but it has a pretty low fatigue limit. This means that once you pass a pretty low threshold the metal begins to weaken each time it is stressed beyond that fatigue limit. Over time, the metal will weaken until it snaps at a stress point, which is usually a seam or weld. To compound this problem, aluminum is very rigid meaning it doesn't tend to flex much which would absorb energy, spread the strain out, and lessen the strain on the part. To the layman the high strength rating of aluminum looks very attractive, but usually when an engineer designs something out of aluminum they design around its fatigue limit.
For reference, this is what the terms mean:
Fatigue Limit- The maximum stress below which a materiel can presumable endure an infinite number of stress cycles. Ultimate Tensile Strength (UTS)- Another common term to describe mechanical strength. This metal property is expressed in thousands of pounds per square inch and denotes how much stress can be gradually applied to the metal before it fractures. Steel's fatigue limit is usually pretty close below its ultimate tensile strength, but aluminum's fatigue limit is way below it's ultimate tensile strength. To us that would mean this- A steel driveshaft that is able to hold 800 lbs of torque before it snaps might be able to repeatedly endure launches that stress it to 700 lbs of torque without weakening. An aluminum driveshaft might be able to hold the same 800 lbs of torque before it snaps but due to aluminum's low fatigue limit it may only be able to repeatedly endure cycles of 500 lbs of torque before it begins suffering metal fatigue. Enough cycles of 600+ lbs of torque and that shaft's going to eventually snap.
 |
