tensile test

[stacey Pu]

why is the 0.2%proof stress particularly useful for aluminium and how to calculate it?

[CFDFEAGURU]

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.

[Astronuc]

why is the 0.2%proof stress particularly useful for aluminium and how to calculate it? 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.