Consider a vertical bar of length ‘L’ and cross sectional area ‘A’ fixed at its bottom as shown in above image. Let a downward axial load, P is applied at its top (free end). The support at bottom now offers an upward reaction equal to P and keeps the bar in equilibrium. Now the applied load tends the bar to shorten by an amount ‘δL’. This load which causes shortening of the member is termed as a compressive load and the member is now said to be in compression.
Now, the internal stress developed in the bar (P/A) due to this compressive force is called the compressive stress (ϭc) and the corresponding strain in the bar (δL/L) is called the compressive strain.
It is now seen that compressive stress and compressive strain are developed in a member when two equal and opposite axial force act on it, inwards towards the centre of gravity of member.