13.1
The proof strength (non-proportional extension) is determined from the force-extension diagram by drawing a line parallel to the straight portion of the curve and at a distance from this equivalent to the prescribed non-proportional percentage, for example 0,2 %. The point at which this line intersects the curve gives the force corresponding to the desired proof strength (non-proportional extension). The latter is obtained by dividing this force by the original cross-sectional area of the test piece (S₀) (see figure 6).
Accuracy in drawing the force-extension diagram is essential.
If the straight portion of the force-extension diagram is not clearly defined, thereby preventing drawing the parallel line with sufficient precision, the following procedure is recommended (see figure 6).
When the presumed proof strength has been exceeded, the force is reduced to a value equal to about 10 % of the force obtained. The force is then increased again until it exceeds the value obtained originally. To determine the desired proof strength a line is drawn through the hysteresis loop. A line is then drawn parallel to this line, at a distance from the corrected origin of the curve, measured along the abscissa, equal to the prescribed non-proportional percentage. The intersection of this parallel line and the force-extension curve gives the force corresponding to the proof strength. The latter is obtained by dividing this force by the original cross-sectional area of the test piece ( ) (see figure 6).
NOTE — The correction of the origin of the curve can be done by various methods. The following method is generally used: draw a line parallel to the line defined by the hysteresis loop which crosses the rising elastic part of the diagram, the slope of which is nearest to that of the loop. The point at which this line intersects the abscissa gives the corrected origin of the curve.
13.2
The property may be obtained without plotting the force-extension curve by using automatic devices (eg. microprocessor).