Yield strength.

The stress where springback stops being complete.

Load a strip of metal and it stretches. Remove the load and it springs back — up to a point. Yield strength is that point: the stress beyond which some of the deformation stays.

Below yield, metal behaves like a very stiff spring. Atoms move apart a little and return when the load comes off; the slope of that straight line is Young's modulus. Above yield, planes of atoms start to slip past one another along dislocations. The slip is permanent, so the part keeps some of its new shape.

Stress is load divided by cross-sectional area, so yield strength is a material property, not a part property. A 6061-T6 strap yields at about 276 MPa (40 ksi) whether it is 1 inch wide or 10 inches wide; the wider strap simply needs ten times the force to get there.

Most metals do not have a sharp corner where yielding starts. The curve bends over gradually, so the number is defined by convention — which is why the test method matters as much as the figure.

Yield strength vs tensile strength: what is the difference?

Yield strength marks a specified level of permanent deformation. Ultimate tensile strength (UTS) is the highest engineering stress reached during a tensile test. A ductile metal can pass yield and continue carrying more load before reaching UTS.

A bracket that must keep its shape needs a design that stays below the applicable yield limit with an appropriate margin. Neither number alone is a finished-part load rating. See the strap example and calculator, or compare the ultimate tensile strength of individual materials.

Definitions: Instron tensile-testing FAQs. For the test method and the stress–strain diagram, continue to the 0.2% offset explanation.

How the tensile test finds it.

Yield strength comes from a tensile test (ASTM E8/E8M for metals, ISO 6892-1 internationally). A machined "dog-bone" specimen is pulled at a controlled rate while an extensometer records the stretch over a gauge length, typically 2 inches. The machine plots stress against strain.

The 0.2% offset method

Draw a line parallel to the straight elastic part of the curve, shifted 0.2% strain to the right. Where that line crosses the curve is the 0.2% offset yield strength, often written R_p0.2. Physically, it is the stress that leaves 0.002 in/in of permanent stretch once the load is removed — 0.004″ over a 2″ gauge.

Upper and lower yield points

Low-carbon steels such as A36 and A1008 often show a distinct drop: the stress peaks (upper yield point), falls, and then runs along a ragged plateau while bands of deformation, called Lüders bands, sweep along the specimen. Specifications usually report the lower yield or the offset value. Those same bands are the "stretcher strain" marks that can appear on formed parts made from steel that has aged.

Plastics behave differently. Many have a peak in the stress–strain curve that is called yield; brittle plastics and composites break before any clear yield. That is why the materials library plots tensile strength for those families.

How materials compare.

Yield strength spans almost two orders of magnitude across materials a fabricator touches — from about 20 MPa for UHMW polyethylene to roughly 1,400 MPa for AR500 wear plate. On a linear scale everything but the hardened steels would sit on the floor, so the chart uses a log scale.

Notice how wide each family is. The aluminum band runs from 1100-H14 at about 97 MPa to 7075-T6 at about 503 MPa — a factor of five — while every grade weighs about the same. Alloying, cold work (the H tempers) and heat treatment (the T tempers) are what move a grade up its band.

How engineers design with it.

For a part that must not bend permanently — a bracket, a mount, a hinge leaf — yield strength is the limit. The engineer works out the peak stress in the part and makes sure it stays below yield by a safety factor.

Choosing the safety factor

The factor covers uncertainty in loads, material and analysis. For well-understood static loads on ductile metal, 1.5 is common; machine design often uses 2–3; lifting equipment and pressure parts follow their own codes with prescribed factors. Use the specification minimum yield, not a typical value, when the number goes into a calculation. Typical values like the ones on this page are for choosing a material, not for signing off a design.

Where the stress actually peaks

The average stress in a strap is easy; the peak stress is not. Holes, notches, sharp inside corners and slots concentrate stress — a round hole in a wide plate roughly triples the local stress at its edge. Bending loads put the peak at the outer surface. Welds and heat-affected zones can be weaker than the base metal, especially in heat-treated aluminum.

Worked example

A 1.000″ × 0.125″ 6061-T6 strap has 0.125 in² of section. At 40 ksi yield it carries 5,000 lbf before taking a set; with a 1.5 safety factor the allowable load is about 3,330 lbf. The same strap in 5052-H32 (28 ksi) carries 3,500 lbf to yield. If a bolt hole sits in the strap, size the net section around the hole and allow for the concentration at its edge.

What it means on the shop floor.

Yield strength is not only a design limit. It is the stress the press brake has to exceed to make a bend at all, and it drives how the part behaves afterwards.

• Tonnage. Air-bending force rises roughly in proportion to the material's strength. Stainless and HSLAS need noticeably more force than mild steel at the same thickness; see the pressing force calculator.
• Springback. When the punch lifts, the elastic part of the strain recovers. The higher the yield relative to modulus, the more the bend opens up. 6061-T6 and stainless spring back more than 5052-H32 or mild steel, which is why angle control matters on those grades.
• Bend radius. High-strength tempers usually have less ductility to spare, so they need larger inside radii. 7075-T6 and AR500 are flat-part materials in our catalog for that reason.
• Welding and heat. Welding 6061-T6 locally overages the heat-treated structure; the heat-affected zone drops toward the strength of the annealed alloy. Cold-worked H-temper alloys lose their work hardening in the same way.

For each stocked gauge, the bend data on our material pages lists the radii and minimum flanges we actually run.

Common mistakes.

• Designing to tensile strength. A part loaded past yield has already failed if it has to keep its shape. Ultimate tensile strength is the peak engineering stress in the tensile test, not a working limit.
• Using a typical value as an allowable. Specifications guarantee a minimum; real lots usually exceed it, but the calculation should not count on that.
• Swapping to a stronger alloy to fix a flexible part. Strength and stiffness are different properties. 7075-T6 has more than twice the yield of 5052-H32 and almost exactly the same stiffness. If a panel deflects too much, change the shape — see stiffness comes from shape.
• Ignoring what fabrication does to it. Welding, heating and even powder-coat cure cycles can change temper-sensitive alloys. Ask before you assume the as-received number survives.