Forming forces need to exceed the yield strength for plastic deformation to occur and an engineered stamping to be produced. If a metal structure is loaded to a level below the yield strength, only elastic deformation occurs, and the load can be removed. With no permanent (plastic) deformation, the metal returns to its original shape.
On the stress-strain curve, yielding occurs where the initial linear region transitions to the non-linear portion. This transition does not occur always at a clearly visible well-defined point. Consistent yield strength measurement is facilitated by defining how this parameter should be determined. Two techniques are used when working with sheet metals. The most common method is to draw a line parallel to the modulus line at an offset strain of 0.2%. The intersection stress becomes what is defined at the “0.2% offset yield strength” (Figure 1). This value is referred to as Rp0.2. The second technique is drawing a vertical line at the 0.5% strain value until it crosses the stress-strain curve. This determines the “yield strength at 0.5% extension under load,” abbreviated as Rt0.5 (Figure 2). These techniques result in similar – but not identical – values for yield strength.
Figure 1: 0.2% offset yield strength, determined by offset of a line parallel to the modulus line by 0.2% strain.
Figure 2: Yield strength at 0.5% extension under load, determined by a vertical line offset from the origin by 0.5% strain
Some metals have yield point elongation (YPE) or Lüders bands. Deforming metal is locked in place by interstitial carbon and nitrogen atoms and other restrictive features of the microstructure. Load increases with little corresponding deformation – or put another way, stress increases with only an incremental increase in strain. The highest stress reached is known as the upper yield strength or upper yield point. Once a band of deformed (yielded) metal breaks free from being pinned by dislocations in the microstructure, the stress drops and there is an increase in strain. The lowest stress reached is known as the lower yield strength or lower yield point (Figure 3). The bands of deforming metal are known as Lüders bands, named after one of the people first observing the phenomenon. Lüders deformation continues at approximately a constant stress until the entire sample has yielded, and the sample begins to work harden. The total strain associated with this type of deformation is known as yield point elongation, or YPE. Stabilized, interstitial-free, vacuum degassed steel, such as ULC EDDS are not at risk of aging, and will not exhibit YPE. For those grades susceptible to YPE, leveling prior to sheet forming will minimize this tendency.
Figure 3: Defining upper yield stress, lower yield stress, and yield point elongation.
Since springback is proportional to the yield strength of the steel, knowing the yield strength allows some estimation of relative springback. Figure 4 compares mild steel, HSLA 700Y/800T, and MS 1500 AHSS having a 1400MPa yield strength. The relative magnitude of springback is indicated by the arrows shown on the horizontal axis, and reflects the increase of springback with yield strength.
Figure 4: Springback is proportional to yield strength.