Stress strain behavior for brittle material

Consider any brittle material, like cast iron, aluminium, concrete, etc. is subjected with uniformly increasing tensile load up to breaking point. It behave differently at different conditions of increasing load. It behavior can be easily understood by graphical representation.
If we plot graph as mentioned above it seem like as following.

In above graph we can easily see that, behavior of brittle materials under load subjected to tension. In brittle materials we can not define the yield point. In such cases where yield point is not clearly defined it is taken as point of some definite amount of permanent strain generally 0.2%. It is obtained by offset method as shown in above fig.
The stress corresponding to that point is called as "Yield Stress or Proof Stress". For brittle materials graph obtained is continuous curve form origin.