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Stress and Strain

What exactly are stress and strain?

Some of the objects you may have observed, such as a rubber band, maybe readily stretched. But can an iron rod be stretched? Isn't it impossible? In this article, we will study more about the characteristics of solids and how numbers like stress may help us comprehend the strength of solids.
Stress and strain are two words used in physics to describe the forces that cause things to deform. Deformation is the change in the shape of an item caused by the application of force. Deformation can also be caused by very tiny forces. The item feels it as a result of external forces, such as squeezing, squashing, twisting, shearing, ripping, or pushing the objects apart.

Define the terms "stress" and "strain"

Definition of stress

In mechanics, stress is the force delivered per unit area. It is represented by the formula below.
σ=FA
where,

  • F stands for the applied force.
  • The area of force applied is denoted by A.
  • The unit of stress is N/m2.

Stress is defined as the ratio of internal force F generated when a substance gets deformed to area A wherein force is applied. At equilibrium, the internal force equals the amount of the applied external force.

  • SI unit for stress = newton per square meter (Nm2)
  • CGS unit in which stress = Dyne-cm2
  • Dimensional formula for stress = ML-1T-2

Definition of strain

Strain is the ratio of the amount of deformation experienced by the body in the direction of force applied to the body's original sizes. The solid's deformation in terms of length is provided below.

ϵ=δlL
Where,

ϵ is the strain caused by the imposed tension
l represents a length change
L is the material's original length.
Strain is the ratio of form or size change to original shape or size. It is stated numerically since it has no dimensions.
Because strain is a dimensionless term that defines the relative change in form. Depending on the tension applied, the body might feel two forms of strain.

Explanation of the Stress-Strain Curve

The stress-strain curve for a material illustrates the connection between stress and strain. The strain values on the curve correspond to the stress incurred by various loads on the item.

Explanation of the Stress-Strain Graph

The stress-strain diagram has the following points or regions:

  • Proportional limit
  • Elastic limit
  • Point of yield
  • Ultimate stress point
  • Breaking point or fracture
  1. Proportional limit: The proportionate limit is the area of the stress-strain curve that obeys Hooke's Law. The ratio of stress and strain, according to this limit, gives us the proportionality constant known as Young's modulus. OA is known as the proportional limit in the graph point.
  2. Elastic limit: The elastic limit is the greatest stress that a substance can withstand before being irreversibly distorted. The elastic limit of an item is the point at which the load operating on it is entirely eliminated and the material returns to its original position.
  3. Point of yield: The yield point of a material is defined as the point at which it begins to distort plastically. Plastic deformation happens when an object's yield point is crossed. There are two kinds of yield points: upper yield points and lower yield points.
  4. Ultimate stress point: The Ultimate Stress point is the point at which a material experiences the greatest amount of stress before failing. The material will break after this point.
  5. Breaking Point or Fracture
    The point on the stress-strain curve where the material fails is known as the breaking point of the material.

Hooke's Theorem

While studying springs and elasticity in the nineteenth century, English scientist Robert Hooke found that numerous materials displayed a similar characteristic when the stress-strain connection was studied. Hooke's Law defined a linear area in which the force required to stretch material was proportionate to its extension.
Hooke's Law says that within the elastic limit of a material, the strain is proportional to the applied stress.
Hooke's law is usually stated mathematically as:
F = –k.x
Where F is the force, x is the extension length, and k is the proportionality constant known as the spring constant in N/m.

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