Glossary term
Yield Strength
The stress at which a material begins to undergo permanent plastic deformation under specified test conditions.
Definition
quantityYield strength is the stress level at which a material begins to deform plastically under specified loading and test conditions.
Yield strength is a key material property for structural design because it marks the approximate transition from recoverable elastic deformation to permanent deformation. It is measured from stress-strain testing and may be defined by a distinct yield point or by an offset method such as 0.2 percent proof stress.
Yield strength is the stress at which a material begins to accumulate permanent strain. Below yield, an ideal elastic material returns to its original shape when unloaded. Above yield, plastic deformation remains after unloading. In design, this property often defines the limit for avoiding permanent set, excessive distortion, or plastic collapse.
Some materials show a clear yield point on the stress-strain curve. Others transition gradually from elastic to plastic behaviour, so engineers use an offset proof stress, often based on 0.2 percent plastic strain. The reported value is therefore tied to the test standard and definition.
Engineering use
Yield strength is used to size frames, shafts, pressure vessels, fasteners, springs, welded structures, aircraft parts, civil members, and machine components. It affects safety factors, allowable stress, load cases, buckling interaction, fatigue assessment, forming limits, and finite-element material models.
The value depends on alloy, heat treatment, grain size, cold work, temperature, strain rate, thickness, weld condition, manufacturing route, and loading direction. For anisotropic sheet, composite laminates, additively manufactured metals, or heavily worked material, one scalar yield value may be insufficient.
Common mistakes
A common mistake is confusing yield strength with ultimate tensile strength. Yield strength concerns the onset of plastic deformation; ultimate tensile strength is the maximum engineering stress reached in a tensile test. Another mistake is using a datasheet minimum without checking temperature, product form, orientation, strain rate, weld state, or design code basis. A strong material specification states test standard, product form, heat treatment, orientation, temperature, strain rate, statistical basis, and whether the value is nominal, minimum, or design allowable.