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The yield strength is the stress applied to a material that just causes permanent plastic deformation—the stress which marks the onset of permanent plastic deformation in a material. Commonly abbreviated by Sy or σy.
Details
The yield strength is the stress at which plastic deformation becomes noticeable and yield is initiated. In metals, this is usually the stress required for dislocations to slip. The yield strength therefore is the stress that divides the elastic and plastic behavior of the material. When designing a part that will not plastically deform, a material must be selected that has a high yield strength or make the component so large so that the applied force produces a stress that is below the yield strength.