engineering stress-strain curve

Ultimate strength refers to the point on the engineering stress-strain curve corresponding to the maximum stress.

The reversal point is the maximum stress on the engineering stress-strain curve, and the engineering stress coordinate of this point is the ultimate tensile strength, given by point 1.

Material strength refers to the point on the engineering stress-strain curve (yield stress) beyond which the material experiences deformations that will not be completely reversed upon removal of the loading and as a result the member will have a permanent deflection.

In a sufficiently ductile material, when necking becomes substantial, it causes a reversal of the engineering stress-strain curve (curve A, right figure); this is because the engineering stress is calculated assuming the original cross-sectional area before necking.