## Introduction to Stress and Strain

You may have noticed that certain objects can stretch easily, but stretching objects like an iron rod sounds impossible, right? In this article, we will help you understand why few objects are more malleable than others. Mainly, we will be discussing Stress-strain curves because they are useful in understanding the tensile strength of a given material. We will also learn how does a force applied on a body generates stress and know the stress-strain relationship with the help of a stress-strain curve.

## What is Stress?

In mechanics, stress is defined as a force applied per unit area. It is given by the formula

\(\sigma = \frac{F}{A}\)

where,

\(\sigma\) is the stress applied

** F** is the force applied

*is the area of force application*

**A**The unit of stress is \(N/m^{2}\)

Stress applied to a material can be of two types. They are:

● **Tensile Stress**: It is the force applied per unit area which results in the increase in length (or area) of a body. Objects under tensile stress become thinner and longer.

●** Compressive Stress**: It is the force applied per unit area which results in the decrease in length (or area) of a body. The object under compressive stress becomes thicker and shorter.

### What is Strain?

The strain is the amount of deformation experienced by the body in the direction of force applied, divided by initial dimensions of the body. The relation for deformation in terms of length of a solid is given below.

\(\epsilon = \frac{\delta l}{L}\)

where,

\(\epsilon\) is the strain due to stress applied

\(\delta l\) is the change in length

L is the original length of the material.

The strain is a dimensionless quantity as it just defines the relative change in shape.

Depending on stress application, strain experienced in a body can be of two types. They are:

● **Tensile Strain**: It is the change in length (or area) of a body due to the application of tensile stress.

● **Compressive Strain**: It is the change in length (or area) of a body due to the application of compressive strain

When we study solids and their mechanical properties, information regarding their elastic properties is most important. These can be obtained by studying the stress-strain relationships, under different loads, in these materials.

## Stress-Strain Curve

### Explaning Stress-Strain Graph

The stress-strain graph has different points or regions as follows:

- Proportional limit
- Elastic limit
- Yield point
- Ultimate stress point
- Fracture or breaking point

#### (i) Proportional Limit

It is the region in the stress-strain curve that obeys the Hooke’s Law. In this limit, the ratio of stress with strain gives us proportionality constant known as young’s modulus. The point OA in the graph is called the proportional limit.

#### (ii) Elastic Limit

It is the point in the graph up to which the material returns to its original position when the load acting on it is completely removed. Beyond this limit, the material doesn’t return to its original position and a plastic deformation starts to appear in it.

#### (iii) Yield Point

The yield point is defined as the point at which the material starts to deform plastically. After the yield point is passed, permanent plastic deformation occurs. There are two yield points (i) upper yield point (ii) lower yield point.

#### (iv) Ultimate Stress Point

It is a point that represents the maximum stress that a material can endure before failure. Beyond this point, failure occurs.

#### (v) Fracture or Breaking Point

It is the point in the stress-strain curve at which the failure of the material takes place.

## Hooke’s Law

In 19th-century, while studying springs and elasticity, English scientist Robert Hooke noticed that many materials exhibited a similar property when the stress-strain relationship was studied. There was a linear region where the force required to stretch the material was proportional to the extension of the material. This is known as Hooke’s Law.

Hooke’s Law states that the strain of the material is proportional to the applied stress within the elastic limit of that material.

Mathematically, Hooke’s law is commonly expressed as:

**F = –k.x**

- F is the force
- x is the extension length
- k is the constant of proportionality known as spring constant in N/m

**Read more:** Hooke’s Law

Also explore the terms related to stress-strain such as:

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### Elastic Moduli of Materials

Following is the table with Young’s modulus, shear modulus, and bulk modulus for the common materials that we use every day in life:

Material |
Young’s modulus (E) in GPa |
Shear modulus (G) in GPa |
Bulk modulus (K) in GPA |

Glass | 55 | 23 | 37 |

Steel | 200 | 84 | 160 |

Iron | 91 | 70 | 100 |

Lead | 16 | 5.6 | 7.7 |

Aluminium | 70 | 24 | 70 |

Note: GPa is gigapascal and 1 GPa = 1,00,00,00,000 Pa.

## Frequently Asked Questions – FAQs

### What are the SI units for stress and strain?

The SI unit of stress is Newton per square metre. Or we can express the same in terms of Pascal.

1 Pascal = 1 Pa = 1 N.m-2.

While there is no unit for strain. It is a dimensionless quantity. This is because it is the ratio of change of length to the original length, and therefore it is unitless.

### What is the use of the stress-strain diagram?

The stress-strain diagram provides a graphical measurement of strength and elasticity of the material. Also, the behaviour of the materials can be studied with the help of the stress-strain diagram which makes it easy with the application of these materials.

### What is the relation between stress and strain?

The relation between stress and strain is that they are directly proportional to each other up to an elastic limit. Hooke’s law explains the relationship between stress and strain. According to Hooke’s law, the strain in a solid is proportional to the applied stress and this should be within the elastic limit of that solid.

### What are the types of strain?

There are three types of strain and they are:

- Normal strain: Normal strain is defined as the ratio of change in dimension to its original dimension. Consider an iron bar of length L and let dL be the change in the length of the bar. According to normal strain, e = dL/L.
- Shear strain: The shear strain is defined as the angular distortion between two mutually perpendicular planes. It is represented by ℽ.
- Volumetric strain: Volumetric strain is defined as the ratio of change in actual volume to original volume.

### What is the difference between stress and strain?

- Stress is defined as the force experienced by the object which causes a change in the object while strain is defined as the change in the shape of an object when stress is applied.
- Stress is measurable and has a unit while a strain is a dimensionless quantity and has no unit.
- The words stress and strain are derived from the Latin words “strictus” and “stringere” which means “to draw tight” and “to bind tightly” respectively.
- Stress can occur in the absence of strain whereas strain does not occur in the absence of stress.