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Strain energy is a form of potential energy that is produced by deforming a solid. As a solid item is deformed, the energy used to deform the item is stored. This energy is released when the deformed item returns to its original shape. This stored energy is called strain energy as a way of describing the effect the energy has on the solid. This energy is a part of a larger class of energy called potential energy; this group is made up of stored energy that is waiting to go into use.
When an item is strained, it is being forced to do something that it is not supposed to do. With strain energy, the strain is caused by forcing the item to hold a shape or position that is different from the one it wants to have. This can be as simple as bending a tree branch or as complex as the deformation that occurs in the systems of a landing fighter jet.
The strain energy within an object is generally equal to the amount of energy applied to the object that cased the strain. This basically means that a solid object will absorb and store the energies applied to it. When the energy is released, it is generally equal to the amount held. As a result, an object can have energy applied to it, then hold it until it is needed and finally release the total amount of the energy back into the system with no loss.
The only time that a loss will occur is when the strain energy approaches or passes the solid’s point of plastic deformation. At this point, the solid will undergo a permanent change into another shape. The energy stored in the item is used to catalyze the change. This deformation will slowly bleed off the strain energy until it has all gone into the transformation or into heat. If the object is released before the deformation is complete, a smaller amount of the energy will release as normal.
The best simple example of strain energy is a simple spring. When a person pushes the spring down, he is exerting force on the solid spring. This energy goes from the person and into the object. As long as the spring is held down all that energy is just sitting within the object. When the spring is released, the energy transforms from potential strain energy to kinetic energy. If the spring were held down so long that its shape changed, then some or all of the energy would be lost to deformation.