Stress

In almost all engineering applications of materials, they are subjected to forces over time. It becomes imperative to keep in check the behaviour of materials under these varying circumstances. Engineers and material scientists try to understand how these materials deform i.e., whether they elongate, compress, twist or break, as a function of applied external force, time, temperature and other conditions.

The first step in doing this is to test these materials on their mechanical properties. Understanding stress and its properties are where it all begins.

Concept of stress

Let’s first understand the stress definition.

• When an external force is applied to a material, stress is the measure of what it feels. So when you stretch a rubber band, the force you apply with your thumbs divided by the cross-sectional area of the rubber band on which the force is applied is called stress.

• Stress has units of force per area: N/m2. 
• Stress is usually denoted by the symbol σ.

The formula of stress is as follows: σ = F/A

Where:

• σ: Stress
• F: Force applied
• A: Area on which the force is applied

Types of stress

Two basic types of stress exist: normal stress and shear stress.

While there are other stress types, they are either similar to these or are a combination of these two. For example, bending stress is a combination of tensile, compressive and shear stress. 

When a shaft is twisted, the stress-induced is shearing stress.

Let’s define some of these stresses.

Normal stress

Stress is defined as force per unit area. If these applied forces are perpendicular to the areas (cross-sectional) of material, then this stress is termed normal stress.

This stress is also known as the uniaxial state of stress, as the stress acts only in one direction. But in engineering applications, such a scenario is uncommon and thus we have something called a biaxial and triaxial state of stress. When external forces are applied in such a way that either two or three mutually perpendicular stresses exist in the material.

Compressive stress

Normal stress that acts into the area of the material subjected to external forces is called compressive stress.

Bearing stress

Another type of compressive stress that comes in the picture is when two objects are pressed against each other. The resulting stress induced is bearing stress.

Shear stress

Consider a situation where a material whose cross-sectional area is subjected to external forces that are parallel to the cross-sectional area. The stress induced in such a scenario is called shear stress.

Thermal stress

As the name suggests, any stress induced because of a change in the temperature of the given material is called thermal stress.

Rapid heating or cooling of material causes a difference between the surface temperatures and the internal temperatures of the material, causing expansion or contraction. The relative movement of the material induces thermal stress.

Conclusion

The key to understanding the mechanical properties of materials is to understand the stress-strain curve and for this a firm grasp of the concept of stress is crucial. To summarise the concept of stress, remember that it is force per unit cross-sectional area and has a unit of N/m2. There are different types of stress, but the two basic kinds of stress are normal stress and shear stress. If the applied forces are perpendicular to the areas (cross-sectional) of material, then this stress is termed normal stress. When the applied force is parallel to the cross-sectional area then the induced stress is called shear stress. All other kinds are similar to these or are a combination of these.