# Deformation (engineering)

## Change in the shape or size of an object / From Wikipedia, the free encyclopedia

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In engineering, **deformation** refers to the change in size or shape of an object.

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**Displacements** are the *absolute* change in position of a point on the object.

**Deflection** is the relative change in *external* displacements on an object.

**Strain** is the *relative* *internal* change in shape of an infinitesimal cube of material and can be expressed as a non-dimensional change in length or angle of distortion of the cube. Mechanical strains are caused by mechanical stress, *see stress-strain curve*. The relationship between stress and strain is generally linear and reversible up until the yield point and the deformation is elastic. Elasticity in materials occurs when applied stress does not surpass the energy required to break molecular bonds, allowing the material to deform reversibly and return to its original shape once the stress is removed. The linear relationship for a material is known as Young's modulus. Above the yield point, some degree of permanent distortion remains after unloading and is termed plastic deformation. The determination of the stress and strain throughout a solid object is given by the field of strength of materials and for a structure by structural analysis.

**Engineering stress** and **engineering strain** are approximations to the internal state that may be determined from the external forces and deformations of an object, provided that there is no significant change in size. When there is a significant change in size, the **true stress** and **true strain** can be derived from the instantaneous size of the object.

In the figure it can be seen that the compressive loading (indicated by the arrow) has caused deformation in the cylinder so that the original shape (dashed lines) has changed (deformed) into one with bulging sides. The sides bulge because the material, although strong enough to not crack or otherwise fail, is not strong enough to support the load without change. As a result, the material is forced out laterally. Internal forces (in this case at right angles to the deformation) resist the applied load.

The concept of a rigid body can be applied if the deformation is negligible.