Formability is the ability of a given

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typica ...

workpiece to undergo

plastic deformation In engineering, deformation refers to the change in size or shape of an object. ''Displacements'' are the ''absolute'' change in position of a point on the object. Deflection is the relative change in external displacements on an object. Strai ...

without being damaged. The plastic deformation capacity of

lic 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 Rolling is a type of motion that combines rotation (commonly, of an axially symmetric object) and translation of that object with respect to a surface (either one or the other moves), such that, if ideal conditions exist, the two are in conta ...

extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex ...

forging Forging is a manufacturing process involving the shaping of metal using localized compressive forces. The blows are delivered with a hammer (often a power hammer) or a die. Forging is often classified according to the temperature at whi ...

rollforming Roll forming, also spelled roll-forming or rollforming, is a type of rolling involving the continuous bending of a long strip of sheet metal (typically coiled steel) into a desired cross-section. The strip passes through sets of rolls mounted ...

, stamping, and

hydroforming Hydroforming is a cost-effective way of shaping ductile metals such as aluminium, brass, low alloy steel, and stainless steel into lightweight, structurally stiff and strong pieces. One of the largest applications of hydroforming is the automoti ...

Fracture strain

A general parameter that indicates the formability and

ductility Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

of a material is the

fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displ ...

strain which is determined by a uniaxial

tensile test Tensile testing, also known as tension testing, is a fundamental materials science and engineering test in which a sample is subjected to a controlled tension until failure. Properties that are directly measured via a tensile test are ultimat ...

(see also

fracture toughness In materials science, fracture toughness is the critical stress intensity factor of a sharp crack where propagation of the crack suddenly becomes rapid and unlimited. A component's thickness affects the constraint conditions at the tip of a ...

). The strain identified by this test is defined by elongation with respect to a reference length. For example, a length of 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.

Forming limits for sheet forming

One main failure mode is caused by tearing of the material. This is typical for sheet-forming applications.Pearce, R.: “Sheet Metal Forming”, Adam Hilger, 1991, .Koistinen, D. P.; Wang, N.-M. eds.: „Mechanics of Sheet Metal Forming – Material Behavior and Deformation analysis“, Plenum Press, 1978, .Marciniak, Z.; Duncan, J.: “The Mechanics of Sheet Metal Forming”, Edward Arnold, 1992, . A neck may appear at a certain forming stage. This is an indication of localized

. 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 unstable 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 diagram A forming limit diagram, also known as a forming limit curve, is used in sheet metal forming for predicting forming behavior of sheet metal. The diagram attempts to provide a graphical description of material failure tests, such as a punched dome ...

s are often used to graphically or mathematically represent formability. It is recognized by many authors that the nature of fracture and therefore the

s are intrinsically non-deterministic since large variations might be observed even within a single experimental campaign.{{cite journal, last=Strano, first=M., author2=Colosimo, B.M., title=Logistic regression analysis for experimental determination of forming limit diagrams, journal=International Journal of Machine Tools and Manufacture, date=30 April 2006, volume=46, issue=6, pages=673–682, doi=10.1016/j.ijmachtools.2005.07.005

Deep drawability

A classic form of sheetforming is

deep drawing Deep drawing is a sheet metal forming process in which a sheet metal blank is radially drawn into a forming die by the mechanical action of a punch. It is thus a shape transformation process with material retention. The process is considered "dee ...

, which is done by drawing a sheet by means of a punch tool pressing on the inner region of the sheet, whereas the side material held by a blankholder can be drawn toward the center. It has been observed that materials with outstanding deep drawability behave anisotropically (see:

anisotropy Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physi ...

). Plastic deformation in the surface is much more pronounced than in the thickness. The

lankford coefficient The Lankford coefficient (also called Lankford value, R-value, or plastic strain ratio) is a measure of the plastic anisotropy of a rolled sheet metal. This scalar quantity is used extensively as an indicator of the formability of recrystalliz ...

(r) is a specific material property indicating the ratio between width deformation and thickness deformation in the uniaxial tensile test. Materials with very good deep drawability have an ''r'' value of 2 or below. The positive aspect of formability with respect to the forming limit curve (

forming limit diagram A forming limit diagram, also known as a forming limit curve, is used in sheet metal forming for predicting forming behavior of sheet metal. The diagram attempts to provide a graphical description of material failure tests, such as a punched dome ...

) is seen in the deformation paths of the material that are concentrated in the extreme left of the diagram, where the forming limits become very large.

Ductility

Another failure mode that may occur without any tearing is

ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...

fracture after plastic deformation (

). This may occur as a result of bending or shear deformation (inplane or through the thickness). The failure mechanism may be due to void

nucleation In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...

and expansion on a microscopic level. Microcracks and subsequent macrocracks may appear when deformation of the material between the voids has exceeded the limit. Extensive research has focused in recent years on understanding and modeling

fracture. The approach has been to identify

forming limits using various small-scale tests that show different strain ratios or stress triaxialities.Hooputra, H.; Gese, H.; Dell, H.; Werner, H.: "A comprehensive failure model for crashworthiness simulation of aluminium extrusions", IJ Crash 2004 Vol 9, No. 5, pp. 449–463.Wierzbicki, T.; Bao, Y.; Lee, Y.-W.; Bai, Y.: “Calibration and Evaluation of Seven Fracture Models”, Int. J. Mech. Sci., Vol. 47, 719–743, 2005. An effective measure of this type of forming limit is the minimum radius in roll-forming applications (half the sheet thickness for materials with good and three times the sheet thickness for materials with low formability).

Use of formability parameters

Knowledge of the material formability is very important to the layout and design of any industrial forming process. Simulations using the

finite-element method The finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical modeling. Typical problem areas of interest include the traditional fields of structural analysis, heat t ...

and use of formability criteria such as the forming limit curve (

) enhance and, in some cases, are indispensable to certain tool design processes (also see:

Sheet metal forming simulation Today the metal forming industry is making increasing use of simulation to evaluate the performing of dies, processes and blanks prior to building try-out tooling. Finite element analysis (FEA) is the most common method of simulating sheet metal for ...

and

Sheet metal forming analysis For sheet metal forming analysis within the metal forming process, a successful technique requires a non-contact optical 3D deformation measuring system. The system analyzes, calculates and documents deformations of sheet metal parts, for example ...

IDDRG

One major objective of the International Deep Drawing Research Group (

IDDRG International Deep Drawing Research Group (IDDRG) Background The International Deep Drawing Research GroupIDDRG focuses on sheet metal research in industry and academia. It was started 1957 as an organization of national groups.Col, A.: “How s ...

, from 1957) is the investigation, exchange and dissemination of knowledge and experience about the formability of sheet materials.

References

Metal forming Mechanical engineering