INTRODUCTION TO STRESS AND STRAIN

There are certain behaviours of all materials under the influence of external force. Stress and strain are one of the measures to show these behaviours. Stress is a resistive force per unit area, which is developed internally to oppose the external force subjected to the material. Strain is a measure of deformation of the material per unit dimensions. If the stress developed in the material is perpendicular to cross-section, it is known as direct stress and if it is tangential or parallel to cross-section, it is known as shear stress.

In Figure 12.1 (a), a cylindrical job is subjected by a load P. If the specimen is broken at AB, same amount of stress will be developed for equilibrium at the section AB which is known as direct stress. Direct stress can be explained as average stress and normal stress. Since stress is force developed per unit area, it is not necessary that amount of force on every point on cross-section AB will be same. It depends on direction and point of application of external force P, so it is known as average stress (σ).

Figure 12.1 (a) Cylindrical Job Subjected to Axial Load and (b) Cylindrical Job Subjected to External Load Parallel to Cross-section

If the stress distribution along the cross-section is uniform, it is known as simple or normal stress. σ = P/A. To develop normal stress, the external force P must be passed through the centroidal axis.

In Figure 12.1 (b), an external load P is subjected to the specimen parallel to the cross-section. There will be resistance just opposite to the external load which is parallel to the cross-section and known as shear force. Again shear stress distribution may or may not be uniform along the cross-section.

Strain is a way to show the deformation in the material. In Figure 12.2, the deformations are shown in two directions: one in the direction of load application and other in the direction perpendicular to load application. The strain produced in the direction of load is known as longitudinal strain and the strain produced perpendicular to load application is known as lateral strain.