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When a material has a force applied to it, it produces a stress. This stress deforms the material. Engineering stress is a slightly simplified version of true stress, used by engineers in their calculations.

The principal difference between engineering and true
stress is:

- True stress considers the change in cross-section of the deformed body throughout loading, which in turn gives a more accurate result.
- Engineering stress assumes a constant cross-section, giving slightly inaccurate results but still acceptable for most general engineering applications.

Stress is measured in N/mm2, the same as pressure.

Engineering stress is defined as:

- σ=F/A0 Where:
- σ=Stress
- F=Applied force
- A0=Original area of the plane on which the stress is acting.

The deformation of a material caused by stress is known as strain.