What Is Yield Strength?

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 marks where permanent (plastic) deformation begins, while tensile strength is the maximum stress reached before fracture. On the stress-strain curve, yield strength is where the curve leaves its straight elastic line (for metals usually found by the 0.2% offset method), whereas tensile strength is the highest point, where necking begins in metals. Beyond yield strength the part no longer springs back; beyond tensile strength the part necks down and breaks. In metals, yield strength is lower than tensile strength. In plastics, yield is often the peak stress just after the elastic region, and tensile strength (stress at break) may be above or below yield. Engineers use yield strength for sizing parts that must keep their shape, and tensile strength for 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.

Frequently Asked Questions

What is yield strength? Yield strength is the stress at which a material begins to deform permanently. Below it, the material springs back when unloaded; above it, it stays deformed. For metals it is normally measured with the 0.2% offset method in a tensile test.

What is yield strength in simple terms? It is how much stress a material can take and still return to its original shape. Load a part beyond its yield strength and it stays bent or stretched, even if it has not broken.

Is yield strength higher than tensile strength? Not for metals: tensile strength is the maximum stress on the curve, so yield strength is lower. For many plastics the stress at break can be higher, equal or lower than the yield peak, which is why plastic datasheets often list both values.