What Is Yield Strength? Definition & Testing

Yield strength is the stress at which a material stops deforming elastically and begins to deform permanently. Below it, a loaded part springs back to its original shape when the load is removed; above it, the part stays bent or stretched. For metals it is usually measured with the 0.2% offset method and reported in MPa or psi.

In short: yield strength is the stress limit for any part that has to keep its shape.

What Does Yield Strength Mean?

A material under load first deforms elastically. In this region strain is proportional to stress (Hooke's law, σ = Eε), and the slope of that straight line is the modulus of elasticity, or stiffness. Remove the load and the material returns exactly to its original shape.

Push further and the atomic bonds start breaking and re-forming with new neighbors. From that point the deformation is permanent: the material has entered plastic deformation. The stress at which this becomes noticeable is the yield strength.

Past yield the material can usually carry more load, up to its tensile strength, but it will not return to its original dimensions. For most parts, such as a bracket or a clip, that permanent set is already a failure.

How Is Yield Strength Measured?

Yield strength comes from the same tensile test used for tensile strength.

Metals (0.2% offset method): the exact point where the curve stops being linear is hard to pinpoint, so a line is drawn parallel to the elastic part of the curve, offset by 0.002 strain (0.2%). Where that line crosses the curve is the yield strength. ASTM E8/E8M is a widely used standard for these tests and covers yield strength and yield point elongation as well as tensile strength.
Plastics: many plastics show a clear peak in stress just after the elastic region, and that peak is taken as the yield stress. Testing follows ASTM D638 (or ISO 527-1), and because plastics are highly sensitive to test speed and environment, their yield values depend strongly on temperature and loading rate.

For conversion, 1 MPa ≈ 145 psi.

Yield Strength vs Tensile Strength

	Yield strength	Tensile strength
What it marks	Where permanent (plastic) deformation begins	The maximum stress reached before fracture
Where on the stress-strain curve	Where the curve leaves its straight elastic line; for metals usually found by the 0.2% offset method	The highest point, where necking begins (metals)
What happens beyond it	The part no longer springs back; it stays bent or stretched	The part necks down and breaks
Metals	Lower than tensile strength	The higher of the two
Plastics	Often the peak stress just after the elastic region	Stress at break, which may be above or below yield
Used for	Sizing parts that must keep their shape	Comparing materials and judging margin against breaking

How Yield Strength Is Used in Design and Manufacturing

Size parts against yield, with margin. Keep the highest working stress comfortably below the yield strength so the part never takes a permanent set in service.
Use yield when forming metal. Bending sheet metal works only because the press brake takes the material past its yield strength; the small elastic portion that recovers afterward is springback, which the bend program must allow for.
Check the conditions behind the number. A datasheet yield value applies at the test temperature and speed. Plastics in particular can behave very differently when hot, loaded quickly or loaded for a long time.

On NICE Rapid's sheet metal fabrication projects, the sheet grade is confirmed against the part's load, environment and appearance requirements before cutting and CNC press brake bending.

Frequently Asked Questions