Yield strength is the stress at which a material stops behaving elastically and begins to deform permanently. Below the yield point a material is in its elastic range: load it and it stretches, unload it and it springs all the way back to its original shape. Above the yield point the deformation is plastic — the material keeps a permanent set after the load is removed. On a stress–strain curve, yield strength is the stress where the line departs from straight, elastic behavior.
Because many metals do not show a sharp, obvious yield point, yield strength is conventionally defined by the 0.2% offset method: a line parallel to the elastic portion of the curve is drawn from 0.2% strain, and the stress where it intersects the curve is reported as the yield strength. This gives a consistent, repeatable number even for materials like aluminum and most steels that yield gradually.
Yield strength is the property that governs forming, and it is distinct from tensile strength, which marks the peak stress before fracture. To bend, draw, or stamp a part you must exceed yield somewhere in the material — that is what creates the permanent shape. But the part also stores elastic energy everywhere the stress is between zero and yield, and when the tool is removed that elastic portion recovers. That recovery is springback. So the higher the yield strength, the larger the elastic band the material rides through, and the more the bend opens up after forming. High-strength steels spring back far more than soft, low-yield steels for the same geometry.
Yield strength also drives bending force: the tonnage to form a bend scales with the material's strength, so a higher-yield material needs more press force for an identical bend. A practical consequence is that switching to a higher-strength grade to save weight usually means both more springback to compensate for and more tonnage to supply. Blank-holder force in drawing is likewise often set as a fraction of yield strength, making it a recurring input throughout die design.
One subtlety worth knowing is that yield strength is not a fixed property of an alloy but of its condition. Cold working raises it (that is exactly what work hardening does), while annealing lowers it back down, so the same chemistry can have very different yield strength depending on its temper. Some steels also show a distinct upper and lower yield point with a flat "yield plateau," which can leave visible stretcher-strain marks (Lüders bands) on a formed surface — a reason certain grades are temper-rolled before forming. For the die designer, the takeaway is to base bending-force and springback estimates on the yield strength of the material in the exact temper being run. For instance, a soft, fully annealed sheet bends with little force and little springback, while the same alloy in a hard temper, with a much higher yield strength, both fights the brake harder and opens up more after the punch lifts.