Yield strength

Yield strength is the stress at which a material begins to deform permanently. Below it, deformation is predominantly elastic; above it, plastic deformation remains after unloading.

Stress–strain curve: A graph showing how a material's strain changes as applied stress increases. Its transition from linear response marks the onset of yielding.
Stress (mechanics): Internal force per unit area within a material, expressed as a tensor in general cases. Yield strength is a threshold expressed in units of stress.
Factor of safety: The ratio between a structure's failure capacity and its expected demand under specified conditions. Designers apply a margin to keep working stress below yield strength.
Ultimate tensile strength: The maximum engineering stress reached during a tensile test before necking or fracture. It marks a later point in a tensile test than the onset of yielding.
Elastic deformation: Reversible deformation that disappears when the applied load is removed. It describes the material's response below yield strength.
Strain: A dimensionless measure of deformation, commonly defined as change in length divided by original length. It records the deformation whose permanent onset defines yielding.
Structural engineering: The engineering discipline concerned with designing structures that safely resist loads and environmental effects. Structural members are sized to avoid yielding under expected loads.
Fracture strength: The stress at which a material specimen fractures under a specified loading condition. Fracture is a separate failure threshold that may lie far above yield strength.
Plastic deformation: Permanent change in shape caused by loading beyond a material's elastic range. Yield strength marks the onset of this lasting deformation.
Elastic modulus: A measure of a material's stiffness, relating stress to strain in its elastic range. It sets the slope before yielding on a simple stress–strain curve.

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