14.1

The proof strength (total extension) is determined on the force-extension diagram by drawing a line parallel to the ordinate axis (force axis) and at a distance from this equivalent to the prescribed total percentage extension. The point at which this line intersects the curve gives the force corresponding to the desired proof strength. The latter is obtained by dividing this force by the original cross-sectional area of the test piece (S₀) (see figure 4).

14.2

The property may be obtained without plotting the force-extension diagram by using automatic devices.