[TowerTalk] Steel vs Aluminum

[jmowery@nswc.navy.mil]

Tom,
     The property that you are trying to describe in Aluminum is
fatigue.  Fatigue failures typically occur in three stages.  First, a tiny
crack initiates at the surface, often at a time well after loading begins.
Next, the crack gradually propagates as the load continues to cycle.
Finally, sudden fracture of the material occurs when the remaining
cross-section of the material is too small to support the applied load.
     There are three design criteria when dealing with Aluminum
and fatigue.

Endurance limit, which is the stress below which there is a 50%
probability that failure by fatigue will never occur.

Fatigue life which tells up how long a component survives at a
particular stress.

Fatigue Strengh is the maximun stress for which fatigue will not occur
within a particular number of cycles.

Endurance ratio is equal to endurance limit divided by tensile strength
and is approximately 0.5


Good DX,
Joe