# What is Poisson’s Ratio : Strength of Material

Poisson’s ratio measures the Poisson effect. It is the phenomenon where a material tends to Compress in directions perpendicular to the direction of tensile force. Whereas material extends in the direction perpendicular to the direction of compression force,

#### What is Poisson’s Ratio

When (Compression or Tension) force is applied to a material, material length changes in the direction and perpendicular to applied force. This change in length can represented by Poisson’s Ratio. Mathematically Poisson’s Ratio is the ratio of Lateral Strain to the Longitudinal Strain within Elastic Limits. It is denoted by a symbol “ν” . It’s Value lies in between -1 to +0.5. Refer this article to read more about lateral and longitudinal strain. Example

When a tensile force is applied to a 10 mm diameter and 100 mm length metal rod. It’s length will increase and diameter will reduce. This change in diameter of the metal road is calculated using Poisson Ratio.

#### Poisson’s Ratio Values For common Materials

It’s value lies in between 0 to 0.5 for most of materials. For common materials its values given below for indicative purpose only:

Material Poisson’s Ratio (indicative Purpose Only)
Steel 0.27-0.3
Brass 0.33
Copper 0.35
Rubber 0.499

#### Positive Poisson Ratio

If material length increases in the direction of applied tensile force. Whereas it reduces in the direction perpendicular to applied force. This types of material behavior indicates a positive Poisson Ratio.  For Example: Material like plastic and steel has a positive Poisson ratio.

#### Negative Poisson Ratio If material length increases in the direction perpendicular to the applied tensile force. Whereas it reduces in the direction of applied force. This types of material behavior indicates a negative Poisson Ratio.

Negative Poisson ratio materials are called Auxetics. They exhibit high energy absorption and resistance to fracture properties. They have applications in packing material, medical knee pads, footwear etc.

#### Summery

To sum up, Poisson’s ratio is very useful while selecting material for an application. It’s Value is unit-less and  constant within elastic limit. For perfectly isotropic material, Poisson ratio is 0.25.

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