STRAINDOE-HDBK-1017/1-93Properties of MetalsStrain may take two forms; elastic strain and plastic deformation.Elastic strainis a transitory dimensional change that exists only while the initiating stressis applied and disappears immediately upon removal of the stress. Elastic strain is alsocalled elastic deformation. The applied stresses cause the atoms in a crystal to move fromtheir equilibrium position. All the atoms are displaced the same amount and still maintaintheir relative geometry. When the stresses are removed, all the atoms return to theiroriginal positions and no permanent deformation occurs. Plastic deformation(or plastic strain) is a dimensional change that does not disappearwhen 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 andplasticity, respectively.At room temperature, most metals have some elasticity, which manifests itself as soon as theslightest stress is applied. Usually, they also possess some plasticity, but this may not becomeapparent until the stress has been raised appreciably. The magnitude of plastic strain, when itdoes appear, is likely to be much greater than that of the elastic strain for a given stressincrement. 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, elasticitywhen stressed in the annealed (heated and then cooled slowly to prevent brittleness) conditionat room temperature, but do exhibit marked plasticity. Some unalloyed metals and many alloyshave 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 theproportional limit, the deformation consists of both elastic and plastic strains.As mentioned previously in this chapter, strain measures the proportional dimensional changewith no load applied. Such values of strain are easily determined and only cease to besufficiently accurate when plastic strain becomes dominant.MS-02Page 8Rev. 0
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