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# Stress

stress on an object causes deformation. It is a measurement of the force inside an object over an area. Knowing this, it is no surprise that it is measured by Newtons per metre squared ($N/m^2$) or Pascals (Pa).

The symbol used for stress is a lower case sigma: $\sigma$

Engineers often need to calculate the stress on a particular object to determine if it will be strong enough to carry a load or to determine its maximum load. materials have a yield strength (point at which deformation becomes permanent) and an ultimate tensile strength (maximum amount of stress that can be applied to the material). The yield strength of a material is generally used when designing an object because you can stress an object up to its yield strength without any permanent deformation because up till this point deformation is purely plastic.

Under uniaxial loading (load applied along one axis of ab object), with an object of constant cross section, the stress through the entire object is constant and that is:
$\sigma = \frac{F}{A}$
Where A is the cross sectional area of the object

Stress is related to strain by the formula $\sigma = E\varepsilon$