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Strain is a measure of the magnitude of a deformation caused by a stress. Engineering strain is a slightly simplified measure of true strain, used by engineers in their calculations.

The principal difference between engineering and true
strain is:

- True strain is calculated at each point throughout the experiment and considers the change in cross-section of the deformed body, in turn giving a more accurate result.
- Engineering strain assumes a constant cross-section, giving slightly more inaccurate results but still acceptable for engineering applications.

Engineering strain is calculated as the change in length divided by the original length:

- ε = ΔL/L0 where:
- ε = engineering strain
- ΔL = change in length
- L0 = original length

Strain can also be given as the percentage increase in length compared to the material in its unstressed state.

https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/StressStrain.htm - full explanation of stress and strain.