## Strain energy

by Marina Gandelsman

Strain energy is one of fundamental concepts in mechanics and its principles are widely used in practical applications to determine the response of a structure to loads.

#### Strain Energy in Uniaxial Loads

Consider a prismatic bar of length L subjected to a tensile force P. The load is applied slowly, so there are no effects due to motion. Such loads are called static loads. As the load reaches its full value P, the bar gradually elongates to L + d .

During this process, the load P gradually moves over the length d and does a certain amount of work. From physics we recall that

**W = F * d**

However, in this case, the force varies in magnitude (from F=0 to F=P). To find the value of work done under these conditions, we look at a load-displacement diagram to determine the manner in which the force varies.

The work done by the load is equal to the area under the curve. As the load is applied, strains are produced and their presence increases the energy of our bar. This strain energy is the energy absorbed by the bar as a result of its deformation under load. From the principle of conservation of energy we know that this energy is equal to the work done by the load, assuming no other energy transfer (such as heat) occurred.

**U = W = S _{L} P(x) dx**

Sometimes this energy is referred to as internal work, to distinguish it from work done by the load. The unit of strain energy is the same as work – J (SI) and ft-lb (British).

If the force P is gradually removed, the bar will shorten and at least a portion of the strain energy will be recovered in the form of work. If the material has not exceeded its elastic limit, the bar will return to its original length L, otherwise a permanent set will remain.

If the material of the bar follows Hooke’s Law, the load-displacement curve is a straight line, P = k d , and the strain energy stored in the bar is:

**U = W = k d 2 /2 = P d /2**

The complimentary energy in the preceding picture is used with Castigliano’s theorems which apply to linearly elastic systems for small deformations as well as Complementary Strain Energy theorems.

The total strain energy in a bar composed of several sections is equal to the sum of strain energies in each section; however, it is important to realize that the total strain energy for a bar with several loads is not equal to the sum of strain energies for each load.

The total strain energy determined from a load-deformation curve is not really indicative of material since the results will depend on the size of the test specimen. In order to eliminate size as a factor, we consider the strain energy per unit volume (also known as strain energy density). Since P = s A and d = e L, (2) can be rewritten as follows:

**U = ( se /2)*AL** **u = U/AL** **u = se /2 = s 2 /2E = E e 2 /2 (3)**

The unit of strain energy density are J/m3 (SI) and in-lb/in3 (British).

The area under a complete stress-strain diagram gives a measure of a material’s ability to absorb energy up to fracture and is called toughness. The larger the area under the diagram, the tougher the material. A high modulus of toughness is important when a material is subject to impact loads. In the inelastic range, only a small part of the energy absorbed by the material is recoverable. Most of the energy is dissipated in the form of heat. The energy that may be recovered when a specimen has been stressed to point A is represented by triangle ABC. AB is parallel to OD since all materials essentially behave elastically upon the release of stress. The area OABO represents the inelastic strain energy (dissipated). The strain-energy density of the material when it is stressed to the proportional limit (D on the diagram) is called modulus of resilience. It is found by substituting the proportional limit s _{pl} into one of elastic energy equations:

Resilience represents the ability of the material to absorb and release energy within the elastic range.

**Strain Energy in Torsion**

Consider a prismatic bar AB in pure torsion under the action of torque T. When the load is applied statically, the bar twists and the free end rotates through an angle f. Again, assume the material is linearly elastic and follows Hooke’s Law. The relationship between T and f will also be linear.

From this, we determine that

**U = W = T f /2**

Using the equation f = TL/GI _{p} , we can express strain energy as

**U = S _{L} T(x) 2 /2GI _{p} dx = T 2 L/2GI _{p} = GI _{p} f 2 /2L**

If the bar is subjected to non-uniform torsion, the total strain energy is equal to the sum of strain energies of each segment with constant torque, but again, the total strain energy of a structure supporting several loads is not equal to the sum of strain energies from each load.

**Strain Energy in Pure Shear**

Consider an element of dimensions x, y, and z subjected to shear load t . As this element is deformed, the force on top plane reaches a final value of t xz. The total displacement of this force for a small deformation of the element is g y. Therefore,

**W = U = 1/2 t xz * g y = 1/2 tg V = 1/2 t 2 V/G**

The strain-energy density in this case is

**u = 1/2 tg = 1/2 t 2 /G**

**Strain Energy in Bending**

Consider a beam in pure bending by couples of moment M. Its material follows Hooke’s Law and rotations are small. The normal stress varies linearly from the neutral axis and s = -My/I. >From U = s І/2E and the stress equation, we get

**U = M 2 L/2EI**

Substituting q = ML/EI, we get

**U = EI q 2 /2L**

If the bending moment in a beam varies along its length, we can obtain the total strain energy by applying one of the preceding equations to an element of the beam and integrating along its length. From d q = 1/ r dx = d 2 v/dx 2 dx, we get

**U = S _{L} M(x) 2 /2EI dx = S _{L} EI/2 (d 2 v/dx 2 ) 2 dx**

The previous equations only consider the effect on bending on the beam. If shear forces are also present, additional strain energy will be stored in the beam, however, this energy is negligible in beams where L >> t. If a beam supports a single load (either point load P or moment M _{0} , we can determine either deflection d (for P) or angle of rotation q (for M _{0} ) from strain energy. The deflection is measured along the line of action of the load and is positive in the direction of the load. The angle of rotation is the angle of rotation of the beam axis at the point where the moment is applied. We can obtain the following equations:

**U = W = P d /2 U = W = M _{0} q /2**

This method is limited in its applications because only one deflection (or angle) can be found and that deflection (or angle) must correspond to the load (or couple).

The equations demonstrated here are fairly basic and simplified in several ways. Only the perfectly elastic bodies do not dissipate any energy and thus can store all of the work as recoverable energy. Most real-life materials are not elastic. A lot of research is concerned with what happens in the postelastic regions, however, this discussion is beyond the scope of this paper.

**Bibliography**

Mechanics of Materials, Gere & Timoshenko, 4th edition, PWS Publishing **Engineering Mechanics of Materials**, B.B. Muvdi and J.W. McNabb, Macmillan Publishing Co. **Engineering Mechanics of Solids**, E. Popov, 2nd edition, Prentice Hall **Engineering Mechanics of Deformable Bodies**, E.F. Byars and R.D. Snyder, 3rd edition, Intext Educational Publishers

© 1999 by Marina Gandelsman. Distribute freely (I borrowed quite liberally from my sources, obviously 🙂

Strain energy by Marina Gandelsman Strain energy is one of fundamental concepts in mechanics and its principles are widely used in practical applications to determine the response of a

## Discussion Forum

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### Elastic strain energy

Posted 18 авг. 2014 г., 15:14 GMT+3 Results & Visualization, Structural Mechanics & Thermal Stresses Version 4.3b 8 Replies

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Is there a way to output the elastic strain energy (and not its density at certain points), as a number for my whole structure? i.e. the half of the volume integral of (stress x strain)?

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Do you mean the total elastic strain energy? This is called “solid.Ws_tot” in COMSOL and can be accessed under global evaluation for instance.

Note that you can also just create integrals manually under component>>definitions>>component couplings.

Best regards,

Frank

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I suspect that’s exactly what I want, but I can’t find how to access solid.Ws_tot. Accessing just solid.Ws is no problem, but I cannot find where to input the “tot” part, it doesn’t recognise it. I have version 4.2a, was it introduced later maybe?

Thanks very much for your help,

Alex

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that explains it. In 4.2a that was not yet introduced.

You can however take the volume integral over the whole domain, of solid.Ws and this gives you the same result as solid.Ws_tot.

best regards,

Frank

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Great, that clears it up, I have to upgrade. just to make sure, can I take the volume integral by starting an integration definition (intop1) in the model, and then doing intop1(solid.Ws)? Just asking because this gives a strange tiny number, far from what I expected.

Thanks again,

Alex

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Make sure that all the domains are included in the intop1.

Also, perhaps you can share the result that you have now, and the result what you expect (and why). Perhaps we can find out where this is coming from.

best regards,

Frank

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I have a question concerning to the strain density. I have an elastoplastic analysis and I want to investigate the the total (included plastic) strain energy at a certain point of the structure. I found only the elastic strain energy density option, however, I carried out my calculation with plastic material model.

Please, could you inform me how can i manage the plastic (or total) strain energy?

Thank you in advance!

BR

Balazs

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This functionality is available in version 4.4. To calculate plastic dissipation, you will first need to enable “Advanced Physics Options” from the “show” icon in the “model builder”. Then, navigate to the branch “Linear elastic Material” in “solid Mechanics” physics interface and enable the “calculate dissipated energy” option in the settings window.

To evaluate the plastic dissipation, for example, you can right click on “Derived values” and choose “Global Evaluation” and then select “Total Plastic dissipation” from the “replace expression” menu.

—

Best regards,

Josh Thomas

AltaSim Technologies

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Discussion Forum Note: This discussion is about an older version of the COMSOL Multiphysics ® software. The information provided may be out of date. Discussion Closed This discussion was