Simple stress

It is simply defined as the load applied per unit area. It is denoted by σ (sigma).

∴ σ = ^P / _A

Where,
             P - Load,
             A - Area.
It's unit is ^N / _mm2.

Explanation:

Consider any material, say one bar of having length (l) and having cross sectional area (A) is subjected to load (P). Due to this loading length of bar will increase, this increase in length is (δl). This increasing length is resisted by internal resisting force (R).
After continuous increase in load (P); increase in length (δl) is continued till internal resisting force(R) equal to external applied load (P). After increasing load (P) than internal resisting force (R) bar will break.
And the situation before failure of bar, internal resisting force(R) equal to external applied force(P); and this internal resisting force per unit area is called as stress (s).