Stress vs Strain: All You Need to Know

Stress and strain are two terms that are different and related at the same time. It is crucial to understand their relationship so that the material selection can be made for different relevant applications.

The article below explores the differences between the terms stress vs strain and their significance in materials selection and layout of the design. So, let's read.

What is Stress?

Stress is the force per unit area that acts on a given material. It also defines the way materials react when they are under certain loading conditions. Different types of stress occur within an object.

Types of Stress

The two most common forms of stress are the following:

Tensile Stress

Tensile stress takes place when an object is being pulled out as it causes an increase in length. For instance, when a rod is pulled or wire is stretched by forces in the opposite direction from both ends, it stretches outwards.

Compressive Stress

Compressive stress occurs when the object is pushed, and the length is decreased. For instance, when a rod or wire is squeezed when pushed from the two ends by opposite and equal forces.

How Stress in Materials Works

Stress in a material is created when the material passes through a deforming force. It creates an applied force, and this could be either elastic deformation, which is reversible, or a performance deformation that takes place.

The material’s atoms resist the external forces due to the bonding present, which creates a force in the opposite reaction.

In the case of a simple tensile force, even when a material is stretched from one end, the reaction is observed in the form of stress in the two dimensions. Hence, it is stretched in one direction and contracted in another.

How Stress of Materials is Measured

It is impossible to measure stress directly, so the deformations and applied forces are measured instead. In order to measure deformation, understanding the relationship between the deformation and force is very important.

There are different methods applicable to measuring the deformation, which can be done through strain gauges, load cells, photoelasticity and ultrasonic testing.

Once you know the force magnitude, you can use the stress equation to calculate stress. The equation is σ=F/A.

The analysis of stress affects the forces that an object can tolerate. It is very important to identify them so that the material selection can be made accordingly.

What is Strain?

Strain measures the material deformation in material that takes place under the external force influence. It also considers the level of deformation that takes place in a material that passes through the stress.

Strain is the measure of change in the length of a material to the original dimension and is a unitless quantity.

Types of Strain

The two forms of strain are as follows:

Tensile Strain

The tensile strain takes place when a material is elongated or stretched.

Compressive Strain

Compressive strain takes place when the material is shortened or compressed.

How Strain in Materials Works

On the application of external force, the material deforms, and such deformation is strain. The behavior or deformation of the material when under stress is based on many factors like material type, composition, direction, and magnitude.

There are three ways in which the materials can deform when subjected to stress; these are as follows:

• Elastic deformation takes place when a material deforms when subjected to stress but gains its actual space back when the stress is released.
• Plastic deformation is a permanent deformation, and when the shape of a material alters under stress, it does not recover.
• Fracture takes place when the applied stress is greater than the strength of the material, which leads to failure or break.

How Strain of Materials is Measured

There are many methods through which the strain can be measured, and the most popular ones are extensometers and strain gauges.

They are both directional methods, and apart from this, other methods are piezoelectric sensors and digital image correlation.

Examples of Stress vs Strain of Different Materials

Every material has a different reaction to stress and strain, and this section highlights the response of metals and polymers:

Metals

Some ductile metals like stainless steel and other alloys tend to deform under stress. Meanwhile, brittle metals, like high-carbon steel, are subjected to fracture even if there is a minor deformation.

Meanwhile, low-carbon steel bends when subjected to stress till a yield point is achieved; it is deformed, which is strain-hardened.

Polymers

The behavior of stress and strain in polymers is highly diverse. A brittle polymer is elastically deformed, whereas if it is pulled more, a fracture may take place. This point represents the tensile strength in the form of stress value.

Some elastomers, like rubber, can restore to their actual point after being subjected to stress, and if extended to the fracture point, they break.

How Stress and Strain Relate to Each Other

The stress and strain are associated with one another such that one causes the other. Young's modulus describes how stress and strain are related to one another.

The elastic model relates to the stress applied to a material, which results in strain, and this relationship is also described by Hooke’s law. According to this law, stress and strain are directly proportional to one another, provided that the material offers elasticity.

This relationship is mathematically expressed as follows:

σ = E* ε

The Stress-Strain Curve

The stress and strain relationship has some important terms that are a part of the stress and strain curve, and these are as follows:

The Elastic Region

Elastic regions are the regions of deformation, and in the same region, the materials return to their actual shape when stress is eliminated.

Yield Point

It describes the point of the material deformation. The stress present at this point is called the yield strength.

The Plastic Region

The plastic region is that region that starts after the yield point, and the deformation that takes place in this region is permanent.

Ultimate Tensile Strength

Ultimate tensile strength is the material's maximum stress-handling limit. This point on the curve can be seen where the necking starts to occur.

Necking

Necking is the region in the stress and strain curve that features a larger deformation.

Fracture

At this point, the material tends to fracture.

Conclusion

The guide above highlights a detailed difference between stress vs. strain and how they both relate. If you are looking to get some components manufactured with high accuracy and no deformation, you can get in touch with us at DEK.

We understand the principle of stress and strain; hence, we will choose the right material with the right process for your required applications.