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It is not true to say that aluminium alloys have a fatigue life.
The only materials that really exhibit this beaviour are ferritic or martensitic steels. (Certain manganese bronzes can also behave in the same manner but these alloys are not too common)
The 'Endurance Limit' of a material is the stress range that can be repeatedly applied without causing failure.
In steels the crystal structure allows dislocations to be pinned and this reduces the formation of the slip band intrusions and extrusions that cause fatigue crack initiation.
Neither Aluminium or Titanium have a crystal structre that can behave in this manner.
The Endurance Limit of Aluminium and Titanium is often quoted as the stress range which will allow a life of 10^7 (10 Million) cycles, however, even if a component manufactured from these alloys is stressed at below this level failure will eventually occur.
The problem with old titanium parts is that 95% of the components life will be taken up in accumulating the surface damage needed to cause crack initiation. Once the crack initiates it will rapidly propogate and cause failure.
The only way to 'life' these types of component is to estimate the number of significant fatigue cycles per hour of use, apply a conservative 'Safety Factor' and then throw them away.
There isn't really any inspection technique that can measure how much of the life has been used.
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