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Consider a rod with initial length L which is stretched to a length L'. The strain measure e, a dimensionless ratio, is defined as the ratio of elongation with respect to the original length,
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| The above strain measure is defined in a global
sense. The strain at each point may vary dramatically if the bar's
elastic modulus or cross-sectional area changes. To track down the
strain at each point, further refinement in the definition is
needed.
Consider an arbitrary point in the bar P, which has a position vector x, and its infinitesimal neighbor dx. Point P shifts to P', which has a position vector x', after the stretch. In the meantime, the small "step" dx is stretched to dx'. The strain at point p can be defined the same as in the global strain measure,
 Since the displacement
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, the strain can hence be
rewritten as,
