What's the difference between engineering stress and strain and true stress and strain

he value of true stress or true strain is calculated from this instantaneous change in cross section area ... Conversely, since true strain is calculated by integrating the strain over the entire test, it is larger than engineering strain for the same value of displacement.

Explanation:

True strain and engineering strain? True stress is defined as the load divided by the cross-sectional area of the specimen at that instant and is a true indication of the internal pressures ... Engineering stress is defined as the load divided by the initial cross-sectional area of the specimen.