Steel has two strenght ratings.
Yield strength which is where one can stress the metal up to that point and it will "bounce back". Over the yield point the metal is never the same.
Ultimate strength is where the designer will design within these parameters and the steel will have infinite life. It is always under yield strength.
Aluminum on the other hand has no ultimate life. From the day that it is manufactured and exposed to stress, it will fail at some point in time depending on the amount of stresses applied to it. That is why aluminum bicycle frames eventually crack and have to be scrapped.
Copper, depending on what alloy and hardness, has an Ultimate Strength and a Yield Strength. Manufacturers design the torque settings so they remain in the ultimate strength range.
For a description of both let's use an antenna on a car.
Ultimate strength range is when you bend it and it returns to its original position with no deformation in shape and position.
Yield strength range is exceeded when you bend it and it develops a "kink" and you cannot ever bring it back to its original position and also have the same strength at that point in the metal. On a molecular level, the lattice in the metal has been distorted.
Copper is mallable and depending on the alloy used can have an impact on the life of the connection when heat, mechanical and torsional stresses are applied.
Hopes this makes sense to all who wondered about this topic.
[This message has been edited by Currently (edited 01-26-2003).]