why is the 0.2%proof stress particularly useful for aluminium and how to calculate it?
2% offset on a stress strain diagram is used for many materials other than aluminum. See any book on strength of materials to see the calculation.
The use of 0.2% offset for proof stress (or yield stress) is quite common. Some organizations/national standards specify 0.1%. It's used because the stress at which the stress-strain (load-displacement) relationship departs from the linear relation (Hooke's law) is not precisely known.
Stresses are not measures, but rather inferred or calculated based on the stress-strain relationship, which is obtained from load(force)-displacement test data taken from a tensile test.
Yield or tensile stress (strength) provide technical limits in design. Usually one designs an object taking into account some margin to the stress limit.
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