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STRAIN
Deformation of Cubic Structures

Material Science Volume 1 of 2
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STRAIN DOE-HDBK-1017/1-93 Properties of Metals Strain may take two forms; elastic strain and plastic deformation. Elastic strain is a transitory dimensional change that exists only while the initiating stress is applied and disappears immediately upon removal of the stress.   Elastic strain is also called elastic deformation.  The applied stresses cause the atoms in a crystal to move from their equilibrium position.  All the atoms are displaced the same amount and still maintain their  relative  geometry.    When  the  stresses  are  removed,  all  the  atoms  return  to  their original positions and no permanent deformation occurs. Plastic  deformation  (or  plastic strain)  is  a  dimensional change  that does  not disappear when the initiating stress is removed.   It is usually accompanied by some elastic strain. The phenomenon of elastic strain and plastic deformation in a material are called elasticity and plasticity, respectively. At room temperature, most metals have some elasticity, which manifests itself as  soon as the slightest stress is applied.   Usually, they also possess some plasticity, but this may not become apparent until the stress has been raised appreciably.   The magnitude of plastic strain, when it does  appear,  is  likely  to  be  much  greater  than  that  of  the  elastic  strain  for  a  given  stress increment.  Metals are likely to exhibit less elasticity and more plasticity at elevated temperatures. A few pure unalloyed metals (notably aluminum, copper and gold) show little, if any, elasticity when stressed in the annealed (heated and then cooled slowly to prevent brittleness) condition at room temperature, but do exhibit marked plasticity.  Some unalloyed metals and many alloys have marked elasticity at room temperature, but no plasticity. The state of stress just before plastic strain begins to appear is known as the proportional limit, or elastic limit, and is defined by the stress level and the corresponding value of elastic strain. The proportional limit is expressed in pounds per square inch.   For load intensities beyond the proportional limit, the deformation consists of both elastic and plastic strains. As  mentioned previously in this  chapter, strain measures the proportional dimensional change with  no  load  applied.    Such  values  of  strain  are  easily  determined  and  only  cease  to  be sufficiently accurate when plastic strain becomes dominant. MS-02 Page 8 Rev. 0







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