Understanding Strain and True Strain


Strain in engineering and materials science is the measure of deformation representing the elongation or compression of a material. The most common type is engineering strain (also known as nominal strain), which is the change in length divided by the original length.

Engineering Strain is given by the formula:

\(\epsilon_e = \frac{\Delta L}{L_0}\)


  • \(\Delta L\) is the change in length (final length minus original length).
  • \(L_0\) is the original length.

This strain is an approximation, assuming the deformation is small relative to the object's overall dimensions.

True strain, or logarithmic strain, provides a more accurate measure for large deformations. It is defined as:

\(\epsilon_t = \ln\left(\frac{l}{L_0}\right)\)

The difference between engineering strain and true strain becomes significant with large deformations. For small deformations, they are approximately equal since the natural logarithm of a value close to 1 is roughly equivalent to the value minus 1.

In summary:

  • Engineering strain: Used for small deformations, simpler to calculate.
  • True strain: Used for large deformations, provides a more accurate description.

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