Determination of Proof Stress

Most ductile materials do not yield and show curve of as in Figure. For such materials the end of elastic behaviour is defined as attaining a certain plastic strain. The method of determination of this strength value is to measure a certain offset strain from origin as in Figure represented by oA.

Figure 7: Determination of Proof Stress

From point A a line parallel to elastic strength line part is drawn to meet the curve at d. the stress corresponding to point d is defined as the "yielding strength" or "proof strength" of the material. The values of offset strain have been standardised for different materials. The yield strength resolute by the offset method is always explained as yield strength for an offset. Table 1 describes the values of offset strains commonly used.
                                                         Table 1 : Commonly used Offset Values