Energy Methods

# Energy Methods | Strength of Materials (SOM) - Mechanical Engineering PDF Download

### Strain Energy

The work done by the load in straining the body is stored within the strained material in the form of strain energy.
Strain energy,

U = 1 / 2 P(Al)
U = P2L / 2AE
Put
P = AEAℓ / L

Or

U = σ/ 2E x V

Strain Energy Diagram

### Proof Resilience

The maximum strain energy that can be stored in a material is known as proof resilience.

U = σ/ 2E1

Where,

σ = p / A, u = 1 / 2 Ee2

Strain energy of prismatic bar with varying sections

Prismatic bar

Strain energy of non-prismatic bar with varying axial force

Ax = Cross-section of differential section.

Non-Prismatic Bar

Stresses due to

Work stored in the bar

.....(b)
By equating, stress will be

and If h is very small then

Strain Energy in Torsion

For solid shaft,

U = τ/ 4G x Volume of Shaft

For hollow shaft,
x Volume of Shaft

### Castigliano’s First Theorem

It the strain energy of an elastic structure can be expressed as a function of generalized displacement, then the partial derivative of the strain energy with respect to generalized displacement gives the generalized force

[Where M is function of W (load)]
Deflection:

Slope:

Theories of Failure

Theories of failure are defined as following groups:

1. Maximum Principal Stress Theory (Rankine theory)
• According to this theory, permanent set takes place under a state of complex stress, when the value of maximum principal stress is equal to that of yield point stress as found in a simple tensile test.
• For design, critical maximum principal stress (σ1) must not exceed the working stress (s1) for the material.
σ< σy

Note: For bittle material, it gives satisfactory result. Yield criteria for 3D stress system,

σ1 = σy or |σ3| = σry

Where, σy = Yield stress point in simple tension, and σy = Yield stress point in simple compression.

Stresses on rectangular Section

2. Maximum Principal Strain Theory (St. Venant’s theory)

According to this theory, a ductile material begins to yield when the maximum principal strain at which yielding occurs in simple tension.
For 3D stress system,

If ey = Yield point strain tensile σ/ E
ery = Yield point strain compressive σr/ E

According to theory, e1 = ey

Yield criteria:
And
For 2D system,

Rhombus

Note: This theory can estimate the elastic strength of ductile material.

3. Maximum Shear Stress Theory (Guest & Tresca’s theory)

According to this theory, failure of specimen subjected to any combination of loads when the maximum shearing stress at any point reaches the failure value equal to that developed at the yielding in an axial tensile or compressive test of the same material.
For 3D system:
Yielding criteria,
τmax = 1 / 2 (σ1 - σ3) = σ/ 2

In case of 2D: σ1 – σ3 = σy

Yielding criteria, σ1 - σ2 = σy
This theory gives well estimation for ductile material.

4. Maximum Strain Energy Theory (Haigh’s theory)
• According to this theory, a body under complex stress fails when the total strain energy on the body is equal to the strain energy at elastic limit in simple tension. For 3D stress system yield criteria,

For 2D stress system,

EllipseThis theory does not apply to brittle material for which elastic limit stress in tension and in compression are different.

5. Maximum shear strain energy/Distortion energy theory/Mises-Henky theory
• It states that inelastic action at any point in a body, under any combination of stress begins, when the strain energy of distortion per unit volume absorbed at the point is equal to the strain energy of distortion absorbed per unit volume at any point an a bar stressed to the elastic limit under the state of uniaxial stress as occurs in a simple tension/compression test.
1/2[(σ1 - σ2)2 + (σ1 - σ3)2 + (σ3 - σ1)2] ≤ σ2For no failure
1/2[(σ1 - σ2)2 + (σ2 - σ3)2 + (σ3 - σ1)2] ≤ (σ/ FOS)2 For no failure

The document Energy Methods | Strength of Materials (SOM) - Mechanical Engineering is a part of the Mechanical Engineering Course Strength of Materials (SOM).
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## Strength of Materials (SOM)

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## FAQs on Energy Methods - Strength of Materials (SOM) - Mechanical Engineering

 1. What are some energy methods used in the field of engineering?
Ans. Energy methods commonly used in engineering include the Finite Element Method (FEM), the Boundary Element Method (BEM), the Finite Difference Method (FDM), the Discrete Element Method (DEM), and the Rayleigh-Ritz Method. These methods are used to analyze and solve complex problems in various engineering disciplines.
 2. How does the Finite Element Method (FEM) work in energy analysis?
Ans. The Finite Element Method (FEM) is a numerical technique used to solve problems in engineering and physics. In energy analysis, FEM divides a complex system into smaller, simpler elements or subdomains. These elements are interconnected at discrete points called nodes. By applying mathematical equations and boundary conditions to these elements, FEM calculates the energy distribution and behavior of the system.
 3. What is the role of the Boundary Element Method (BEM) in energy analysis?
Ans. The Boundary Element Method (BEM) is a numerical technique used to solve problems in engineering and physics, particularly in energy analysis. BEM focuses on solving problems by discretizing only the boundary of a system instead of the entire volume. By applying mathematical equations and boundary conditions on the boundary elements, BEM determines the energy distribution and behavior of the system.
 4. How does the Finite Difference Method (FDM) contribute to energy analysis?
Ans. The Finite Difference Method (FDM) is a numerical technique used to solve partial differential equations in energy analysis. FDM discretizes the domain of the problem into a grid of points and approximates the derivatives of the energy variables using finite difference formulas. By solving these equations iteratively, FDM calculates the energy distribution and behavior of the system.
 5. What is the significance of the Rayleigh-Ritz Method in energy analysis?
Ans. The Rayleigh-Ritz Method is a numerical technique used to approximate the solutions of complex energy problems. It is particularly useful when analytical solutions are not feasible. In energy analysis, the Rayleigh-Ritz Method approximates the energy distribution and behavior of a system by representing the variables as a linear combination of known basis functions. By minimizing the energy functional, the Rayleigh-Ritz Method determines the most accurate approximation for the system's behavior.

## Strength of Materials (SOM)

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