Strain Energy Formula

Strain energy is a sort of potential energy. It is stored in a structural part due to elastic deformation. For example, a bar is said to be deformed from its unstressed state when it is bent by applying force to it, and the amount of strain energy stored in it equals the work done on the bar by that force. Only if the stress is below the elastic limit can the strain energy stored in the material be translated into kinetic energy. Plastic deformation happens in the material when the stress exceeds the elastic limit, and some of the energy is wasted in this process.

An object made of a deformable material will change shape when a force is applied to it.

The object will continue to stretch as we apply greater force. The amount of force applied divided by the object's cross-sectional area equals stress.

Know About the Strain Energy Formula

As a result, strain energy is the stored energy in a body as a result of its deformation. Strain energy density is the term used to describe the amount of strain energy per unit volume. 

Also, the area beneath the stress-strain curve as it approaches the deformation point. When the applied force is released, the entire system returns to its original form.

Formula:

\[ U = \frac {F\delta}{2}\]

Where,

U = Strain Energy

δ = Compression

F = Force applied

• When stress σ is proportional to strain, the formula is:

\[ U = {\frac{1}{2}} V \sigma \varepsilon\]

Where,

U = Strain Energy

σ = Strain

V = Volume of body

• For young’s modulus E, the strain energy formula is 

\[ U = \frac{\sigma^{2}} {2E} \times {V} \]

Where,

U = Strain Energy

σ = Strain

V = Volume of body

E = young’s modulus

Stress

The same amount of force will be generated when we apply force to any material. These forces inside the material per unit area are called stress.

Relation Between Strain and Stress

According to Hooke's law, the strain is directly proportional to the stress for small deformations of elastic materials. In an elastic material, Hooke's law is only true for tiny deformations (up to the proportional limit).

Stress ∝ Strain Or Stress =E× Strain

The modulus of elasticity is equal to E, the proportionality constant.

Facts About Strain Energy Formula

When the force is exerted on a material, it deforms. The external force will exert force on the substance, which will be stored as strain energy in the material.

The work done by an external force is equal to the strain energy stored in the elastic limit (Work-energy theorem).

It can also be estimated by calculating the area under the curve of the Stress vs Strain graph until the elastic limit is reached. 

Resilience is the amount of strain energy that exists up to the elastic limit. N-m or Joules is the SI unit for strain energy.

Assumptions of Strain Energy Formula:

• The material should be able to stretch.

• Stress should be balanced, neither too high nor too low.

• Load should be applied slowly.

Density of Strain Energy

The strain energy density is the amount of strain energy per unit volume that is uniformly distributed inside a material. It's worth is determined by,

\[U = \frac{\text{Total Strain Energy}}{\text{Volume of the Material}} \]

\[ U = \frac{1}{2} \times {stress} \times {strain} \]

or

\[ U = \frac{1}{2} \times \frac{(stess)^{2}}{E}\]

More About the Strain Energy Formula

The strain energy is defined as the energy stored in any object which is loaded within its elastic limits. In other words, the strain energy is the energy stored in anybody due to its deformation. The strain energy is also known as Resilience. The unit of strain energy is N-m or Joules. Here, the derivation of the strain energy formula, its unit, equations, etc are discussed thoroughly. The concepts are explained with the help of relevant diagrams and symbols. 

Conditions for Strain Energy are:

The material should be loaded in its elastic limit.

The material shouldn’t undergo permanent deformation.

The amount of strain energy depends upon the amount of load and the type of load loaded.

Strain Energy Equation:

Consider a rod of length l and let the load be applied on either side of the rod gradually. Due to applied load, the rod deforms by a length δl. From the definition of strain energy, we know that it is the energy stored in a body when work is done on it to deform within the elastic limit. 

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Then, the elastic strain energy formula is given by,

$\Rightarrow$ Work Done = Elastic strain energy

$\Rightarrow U = W = F dx$ ……(1)

Here the force is the applied load and since the load was applied gradually we will consider the average of the load and the displacement is the deformation. Therefore equation (1) becomes,

$\Rightarrow U = F dx = \left(0 + \dfrac{F}{2}\right)\delta l$

$\Rightarrow U = \dfrac{F \delta l}{2}$ ……….(2)

Equation (2) is known as Strain Energy Formula or Strain Energy Equation.

Different Forms of Strain Energy or Strain Potential Energy Formula:

We know that from Hooke’s law,

$\Rightarrow E = \dfrac{\text{Stress}}{\text{Strain}}$

Where $E$ is Young’s modulus.

$\Rightarrow E = \left(\dfrac{F}{A}\right) \left(\dfrac{l}{\delta l}\right)$ …….(1)

On rearranging the above equation for $\delta l$ we get,

$\Rightarrow \delta l = \dfrac{Fl}{E}$ …….(2)

Substituting, the value of deformation in the expression of strain energy,

$\Rightarrow U = \dfrac{F^2A}{2EA}$……(3)

Multiply and divide by A in the equation (3), we get:

$U = \dfrac{F^2lA}{2EA^2}$

We know that $l \times A$ is the volume of the rod and $\dfrac{F}{A}$ is the stress(\sigma) applied to the material, substituting the same in the above expression we get:

$\Rightarrow U = \dfrac{\sigma^{2}V}{2E}$ ………(4)

OR

$\Rightarrow U = \dfrac{\sigma V \varepsilon}{2}   \left(\therefore E = \dfrac{\text{stress}}{\text{strain}} = \dfrac{\sigma}{\varepsilon}\right)$

Where,

$\sigma$ - Stress in the material.

$V$ - Volume of the material.

$\varepsilon$ - Strain in the material

Equations (3) and (4) are the two different forms of strain energy that we use while studying the strength of any given material. 

Strain Energy Per Unit Volume Formula:

Strain energy per unit volume is also known as the strain energy density formula is given by:

We know that the stress-energy formula is,

$\Rightarrow U = \dfrac{\sigma V \varepsilon}{2}$

Where,

$\sigma$ - Stress in the material.

$V$ - Volume of the material.

$\varepsilon$ - Strain in the material.

Then, strain energy per unit volume is,

$\Rightarrow \dfrac{U}{V} = \dfrac{\sigma \varepsilon V}{2V}$

$\Rightarrow$ Stress energy per unit volume $= \dfrac{\sigma \varepsilon}{2}$

This is the required elastic energy density formula.

Examples:

1. Calculate the Work Done in Stretching a Wire of Length 5m and its Cross-Sectional Area is Given by 1mm2 is Deformed by the Length of 1mm if Young’s Modulus is Given of the Wire is Given by $2 \times 10^{11}N/m^2$ 

Given,

Length of the wire $= l = 5m$

Area of the wire $= A= 1mm^2=10^{-6}m^2$

Deformation due to stretching $= \delta l = 1mm = 10^{-3}mA$

Young’s modulus of the wire $= E = 2 \times 10^{11} N/m^2$

Young’s Modulus of Elasticity is given by,

$E = \dfrac{F l}{A \delta l}$

\[\therefore F = EA \dfrac{\delta l}{l} = \dfrac{(2 \times 10^{11}N/m^2)(10^{-6}m^2)(10^{-3}m)}{5m}\]

\[\Rightarrow F = 40N\]

Now work done in stretching the wire is given by,

$\Rightarrow U = \dfrac{F \delta l}{2}$

$\Rightarrow U = \dfrac{40 \times 10^{-3}}{2}$

$\Rightarrow U = 0.02J$

Therefore, $0.02J$ of work is done in stretching $5m$ long wire.

The Strain Energy Formula has applications in several concepts. Hence it is important to learn and understand its derivation and features. The strain energy formula is derived and strain energy can be written in the form of the volumetric term, young’s modulus, etc.. depending upon the need of calculation and data available.