[Physics Class Notes] on Stress and Strain Pdf for Exam

Some of the items you might have noticed can be easily stretched, for example, like a rubber band. But can you stretch an iron rod? It is impossible, right? On this page, we will learn about the properties of solids in greater detail also, how quantities like stress will help us to understand the strength of solids.

Stress and Strain are the two terms in Physics that describe the forces causing the deformation of objects. Deformation is known as the change of the shape of an object by applications of force. The object experiences it due to external forces; for example, the forces might be like squeezing, squashing, twisting, shearing, ripping, or pulling the objects apart. 

Types of Strain

A person’s body can be stressed in one of two ways, depending on how much stress is applied:

Tensile Strain: Tensile strain is defined as a change in the length (or area) of a body caused by tensile tension.

Compressive Strain: The change in length (or area) of a body caused by compressive strain is known as compressive strain.

Use of Stress-Strain Graph

The stress-strain diagram is a graphical representation of the material’s strength and elasticity. The stress-strain diagram can also be used to study the behaviour of the materials, which simplifies the application of these materials.

Relationship Between Stress and Strain

Stress and strain have a straight proportional relationship up to an elastic limit. The relationship between stress and strain is explained by Hooke’s law. Hooke’s law states that the strain in a solid is proportional to the applied stress, which must be within the solid’s elastic limit.

Yield Point in Stress-Strain Graph

The yield point of a material is the point at which it begins to distort plastically. Permanent plastic deformation happens after the yield point is passed.

How to Create a Stress-Strain Curve?

The stress-strain curve is created by progressively adding load to a test coupon and monitoring the deformation, which allows the stress and strain to be calculated.

Hooke’s Law

When English scientist Robert Hooke was studying springs and elasticity in the 19th century, he observed that numerous materials had a similar feature when the stress-strain connection was analysed. Hooke’s Law defined a linear zone in which the force required to stretch material was proportionate to the extension of the material.

Define Stress and Strain

The force applied per unit area in mechanics is known as stress. The following formula represents it

Σ =F/A

where,

σ is stress applied

F is force applied

A is the area of force applied

Stress is measured by unit N/m2

The ratio of internal force F, produced when a substance is deformed, to the area A where force is applied is known as stress. At equilibrium, the internal force is equal to the magnitude of the externally applied force.

The newton per square meter (Nm2) is the SI unit for stress. Dyne-cm2 is the CGS unit in which stress is measured. ML-1T-2 is the dimensional formula for stress.

What is Strain?

Strain is the ratio of the amount of deformation experienced by the body in the direction of force applied to the initial sizes of the body. The relation of deformation in terms of the length of the solid is given below

ϵ=δlL

where,

ϵ is strain due to stress applied

δl is changed in length

L is the original length of the material.

Strain is the ratio for change of shape or size to the original shape or size. It is expressed in number as it doesn’t have any dimensions. Since strain defines the relative change in shape and it is a dimensionless quantity. A body can experience two types of strain depending upon the stress application.

Stress-Strain Curve Explanation

The material’s stress-strain curve represents the relationship between stress and strain for materials. The strain values are plotted on the curve corresponding to the stress incurred by different loads on the object. 

Explaining Stress-Strain Graph

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

(i) Proportional Limit

The region in the stress-strain curve that observes Hooke’s Law is known as the proportional limit. According to this limit, the ratio of stress and strain provides us with the proportionality constant known as young’s modulus. In the graph point, OA is known as the proportional limit.

(ii) Elastic Limit

Elastic limit is the maximum stress that a substance can endure before permanently being deformed. When the load acting on the object is completely removed and the material returns to its original position, that point is known as the object’s elastic limit. 

(iii) Yield Point

The point at which the material starts showing to deform plastically is known as the yield point of the material. Once the yield point of an object is crossed, plastic deformation occurs. There are two types of yield points (i) upper yield point (ii) lower yield point.

(iv) Ultimate Stress Point

The point at which a material endures maximum stress before failure is known as the Ultimate Stress point. After this point, the material will break.

(v) Fracture or Breaking Point

In the stress-strain curve, the point at which the failure of the material takes place is known as the breaking point of the material.