Stress-Strain Diagram
The results of tension test might be entirely presented in form of a stress-strain diagram in which stress is plotted as ordinate and strain as abscissa. It might, however, be emphasized here that in the definition of stress; it is supposed that area of cross-section will not change. It must have become clear by this time that throughout a tension test area of cross-section and length both change considerably. Thus, there is a need to reconsider the definition of stress.
The stress is described as ratio of load to original area of cross-section (mentions as A0). It will be explained as engineering stress. The prefix "engineering" is frequently dropped and whenever term stress is utilized, it is understood that it is engineering stress. The ratio of load to real or current area of cross-section is explained as true stress. Similarly, the ratio correspondingly the true strain is the sum of strains over small ranges of load upto the current load.
Engineering stress (σ) and strain (ε) shall be
σ = P/ A0
ε = δL/ L0
where, P is load at any moment. A0 the original area of cross-section, δL, the change in length when load changed from zero to P and L0 is the original length.