Formability
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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.
Processes affected by the formability of a material include: rolling, extrusion, forging, rollforming, stamping, and hydroforming
Fracture strain
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 (e. g. 80 mm 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 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 well 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.[1][2][3] 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 quasistable and instable deformation phase. This leads to material failure manifested by tearing. Forming limit curves depict the extreme but still possible deformation 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% and above for mild-strength materials and those with very good formability.
Deep drawability
A classic form of sheetforming is deep drawing. This is done by drawing a sheet using a punch tool (acting in the inner region of the sheet) whereas the material from the side which is held by a blankholder can draw inside. It has been observed that materials with outstanding deep drawability behave anisotropic (anisotropy). The plastic deformation in the surface is much more pronounced than in the thickness. The lankford coefficient ( r ) is a specific material property which displays the ratio of the width deformation versus the thickness deformation for the uniaxial tensile test. Materials with very good deep drawability have a r value of 2 and above. One can understand this positive aspect of formability with respect to the forming limit curve (forming limit diagram) in the way that the deformation paths of the material are concentrated on the very left side of the diagram where the forming limits become very large.
Ductility
Another failure mode that may occur without undergoing the tearing mode is ductile fracture after plastic deformation (ductility). This may happen due to shear deformation (inplane or shear through the thickness) or due to bending. The failure mechanism may be understood by void nucleation and expansion on a microscopic level. Microcracks and subsequent macrocracks may appear when deformation of the material between the voids has exceeded its limit. Research has been very active in recent years in order to understand and model ductile fracture. The approach taken is to identify ductile forming limits using various small scale tests which exhibit different strain ratios or stress triaxialities.[4][5] A good practical measure for this type of forming limit is a minimum radius for rollforming applications (e. g. half the sheet thickness for materials with good and 3 times the sheet thickness for materials with low formability).
Use of formability parameters
Knowledge of the material formability is very important for the layout and design of any industrial forming process. Here simulation with the finite element method along with the use of formability criteria like the forming limit curve (forming limit diagram) enhances and in some cases is indispensable for certain tool design processes (see also sheet metal forming analysis).
References
- ^ Pearce, R.: “Sheet Metal Forming”, Adam Hilger, 1991, ISBN 0-7503-0101-5.
- ^ Koistinen, D. P.; Wang, N.-M. edts.: „Mechanics of Sheet Metal Forming – Material Behavior and Deformation analysis“, Plenum Press, 1978, ISBN 0-306-40068-5.
- ^ Marciniak, Z.; Duncan, J.: “The Mechanics of Sheet Metal Forming”, Edward Arnold, 1992, ISBN 0-340-56405-9.
- ^ 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.