Formability

Formability is the ability of a given metal workpiece to undergo plastic deformation without being damaged. The plastic deformation capacity of metallic materials, however, is limited to a certain extent, at which point, the material could experience tearing or fracture (breakage).

Processes affected by the formability of a material include: rolling, extrusion, forging, rollforming, stamping, and hydroforming.

A general parameter that indicates the formability and ductility of a material is the fracture strain which is determined by a uniaxial tensile test (see also fracture toughness). The strain identified by this test is defined by elongation with respect to a reference length. For example, a length of 80 mm (3.1 in) is used for the standardized uniaxial test of flat specimens, pursuant to EN 10002. It is important to note that deformation is homogeneous up to uniform elongation. Strain subsequently localizes until fracture occurs. Fracture strain is not an engineering strain since distribution of the deformation is inhomogeneous within the reference length. Fracture strain is nevertheless a rough indicator of the formability of a material. Typical values of the fracture strain are: 7% for ultra-high-strength material, and over 50% for mild-strength steel.

One main failure mode is caused by tearing of the material. This is typical for sheet-forming applications. A neck may appear at a certain forming stage. This is an indication of localized plastic deformation. Whereas more or less homogeneous deformation takes place in and around the subsequent neck location in the early stable deformation stage, almost all deformation is concentrated in the neck zone during the quasi-stable and instable deformation phase. This leads to material failure manifested by tearing. Forming-limit curves depict the extreme, but still possible, deformation which a sheet material may undergo during any stage of the stamping process. These limits depend on the deformation mode and the ratio of the surface strains. The major surface strain has a minimum value when plane strain deformation occurs, which means that the corresponding minor surface strain is zero. Forming limits are a specific material property. Typical plane strain values range from 10% for high-strength grades and 50% or above for mild-strength materials and those with very good formability. Forming limit diagrams are often used to graphically or mathematically represent formability. It is recognized by many authors that the nature of fracture and therefore the Forming limit diagrams are intrinsically non-deterministic, since large variations might be observed even within a single experimental campaign.

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