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Mechanical stress: physical condition which exists within any material because of strain or deformation by external forces or by non-uniform thermal expansion.
Definitions of stress and strain. Nominal stress: the force on the object divided by the original area. True stress: the force on the object divided by the actual area. E.g., when necking occurs the true stress is the force applied divided by the area of the neck. The unit for stress is Nm-2, or Pa. Nominal strain: change in dimension / original dimension. This is also known as the engineering strain, and is strictly only true for very small displacements.