Upper & Lower Yield Strength

Some materials, particularly mild steels and certain low-carbon alloys, exhibit both an upper yield point and a lower yield point instead of a single, well-defined yield strength. This behavior is seen in the stress-strain diagram, where the transition from elastic to plastic deformation occurs in a distinct, two-step manner.

  • Upper yield point: The initial peak in the stress-strain curve, where the material first begins to yield. At this point, the material resists deformation momentarily before plastic flow begins.
  • Lower yield point: After the upper yield point, the stress drops to a relatively constant value known as the lower yield point, where plastic deformation continues at a nearly steady stress level.

Yield Point Elongation

After the lower yield point is reached, some materials exhibit a phenomenon called yield point elongation, where plastic deformation occurs at almost constant stress over a range of strain values before strain hardening begins. This results in a horizontal or plateau-like section in the stress-strain curve. Lüders bands, or localized regions of plastic deformation, often appear on the surface of a tensile specimen during this stage.