Compressive Strength - Deviation of Engineering Stress From True Stress

Deviation of Engineering Stress From True Stress

In engineering design practice we mostly rely on the engineering stress. In reality, the true stress is different from the engineering stress. Hence calculating the compressive strength of a material from the given equations will not yield an accurate result. This is of course because the cross sectional area A₀ changes and is some function of load A = φ(F).

The difference in values may therefore be summarized as follows:

• On compression, the specimen will shorten. The material will tend to spread in the lateral direction and hence increase the cross sectional area.

• In a compression test the specimen is clamped at the edges. For this reason, a frictional force arises which will oppose the lateral spread. This means that work has to be done to oppose this frictional force hence increasing the energy consumed during the process. This results in a slightly inaccurate value of stress which is obtained from the experiment.

As a final note, it should be mentioned that the frictional force mentioned in the second point is not constant for the entire cross section of the specimen. It varies from a minimum at the centre to a maximum at the edges. Due to this a phenomenon known as barrelling occurs where the specimen attains a barrel shape.