Klaus Pöhlandt

New experiments for determining yield loci of sheet metal

Abstract Two new methods for determining yield loci of sheet metal are described. In the cross tensile test, by varying longitudinal and transverse stress, the yield locus can be determined in the range of biaxial tensile stress. Starting from a specimen described by Kreiβig the test piece geometry was varied by finite element calculations and optimized for obtaining a large zone of homogeneous deformation and high strain before instability occurs. The optimized geometry was verified through photoelastic tests. In the inclined tensile test with suppressed lateral contraction, by varying the angle between the clamps and the direction of movement, states of stress ranging from pure shear to biaxial tensile stress can be obtained. Cross tensile tests were carried out in which the temperature was measured as a function of strain. After a small decrease due to thermoelastic cooling, the beginning of plastic deformation is indicated by the dissipation of deformation heat. By applying this principle, yield loci were determined for various metals. The results were compared with those obtained from uniaxial tensile tests assuming yield criteria by Tresca, v. Mises, Hosford-Backofen and Hill.

Determining Stress-Strain Curves of Sheet Metal in the Plane Torsion Test

The plane torsion test has been studied for determining the flow curve k f (φ) of sheet metal. This test was first proposed by Marciniak and Kolodziejski for a determination of the n-value assuming Ludwiks law for the flow curve. In the present work, a new procedure is proposed by which k f (φ) can be determined allowing for deviations from the Ludwik equation. A new test apparatus and limits of application are described. First experimental results are reported, and it is shown that k f (φ) can be determined up to much higher strains than is possible by tensile tests.

Improvement of the Plane-Strain Compression Test for Determining Flow Curves

Summary The plane-strain compression test can be applied for determining flow curves up to high-strains. In order to improve the accuracy of the test the use of dies with rounded edges was studied both theoretically and by experiments. The effects of influencing factors were studied and the actual effective area of the dies and the coefficient of friction were determined. The flow curve is determined by eliminating the contribution of friction and shear deformation from the resistance to deformation using the methods of elementary plastomechanics.

A New Approach to the Torsion Test for Determining Flow Curves

Summary A new approximative method is applied for calculating stress-stain curves from torsion test results. The method is idependent of the material and the strain rate. For obtaining best accuracy the use of thinwalled tubular specimens is recommended especially when the flow curve deviates strongly from the exponential Ludwig-Hollomon law. The calculation also makes use of the concept of the “effective” length during deformation for short specimens. The test results are compared with those of upsetting tests with low friction and homogeneous deformation. The test is also applied for assessing the formability of the material. The calculated formability is compared with the experimental results by measuring the local shear strain of the surface.

Concepts for the description of plastic anisotropy in cold bulk metal forming

Abstract Similarly to normal and planar anisotropy of sheet metal the axial and radial anisotropy of round bars having axisymmetric properties are defined. The axial anisotropy is a measure of the deviation of plastic behaviour in axial direction from that vertical to the axis. It can be evaluated by upsetting tests on Rastegaev specimens machined out of a bar vertically to the axis whereby the specimens obtain elliptic cross-sections. This is demonstrated by test results for hexagonal metals. The radial anisotropy is a measure of the deviation of plastic behaviour in radial direction from that in tangential direction at some distance from the axis. Possible experiments for determining the radial anisotropy ana effects of radial anisotropy to be expected in metal forming processes are discussed.

Testing The Plastic Behaviour of Bars and thin Sheet by Torsion Tests

The (cold and hot) torsion test on cylindrical bars and the recently developed (cold) plane torsion test on thin sheet are discussed with respect to their communi ties and differences. In torsion of bars the test results are conventionally used for calculating stress and strain at the specimen surface. However, since the material properties are distorted at the surface it is recommended to calculate stress and strain at a “critical radius”, where shear stress at a given torque is almost 1ndependent of the shape of flow curve: so it can be calculated with good accuracy by a new series expansion method. The plane torsion test 1S evaluated in similar way. Sources of error as well as the uncertainty of the yield criterion are discussed.

Non-conventional extrusion of less-common materials

Abstract The theoretical background of metal forming is improving rapidly, finite-element-based methods, especially, giving an important impact in this area. Despite these developments, the production of difficult — and hence industrially interesting — parts still requires a lot of experience and ingenuity. Particularly, cold and warm extrusion is a domain of metal forming in which theoretical methods cannot aid the production of extrudates that are either of complex shape or made of less-common materials or even both. The present paper aims to give mostly unknown technological details of the production of such workpieces. The described know-how is the outcome of approximately 300 man-years experience gathered at the Institut fur Umformtechnik of the Universitat Stuttgart. The examples given represent the state-of-the-art of extrusion technology.

Consistent Parameters for Plastic Anisotropy of Sheet Metal (Part 2- Plane-strain and Compression Tests)

To include the case of deep‐drawing (without blank‐holder), states of combined tensile and compressive stress have to be considered whereby it is necessary to define two more anisotropy parameters. They are called “tensile‐compressive anisotropy” in rolling and transverse direction. Finally, a new consistent system of “true” anisotropy parameters is presented. They are defined as the difference between the experimentally determined anisotropy parameters and the values which would be obtained in case of isotropy. They all are zero for isotropic materials.

Consistent Parameters for Plastic Anisotropy of Sheet Metal (Part 1‐Uniaxial and Biaxial Tests)

The anisotropy parameters for sheet metal used hitherto are mainly determined by uniaxial tensile tests. Such tests, however, do not give sufficient information about the yield locus and the forming behaviour in that range where the two principal tensile stresses are of similar magnitude like in stretch forming. The same applies for combined tensile and compressive stress like in deep‐drawing. To fill these gaps, new parameters are defined. Their experimental determination is briefly discussed.The “equibiaxial yield stress” and “equibiaxial anisotropy” which refer to equibiaxial tensile stress can be determined by cross tensile tests. However, these require a special apparatus. Alternatively experiments for obtaining plane strain can be applied for determining the equibiaxial parameters indirectly. This is possible using conventional tensile testing machines. In this case also anisotropy parameters for plane‐strain deformation, the “semibiaxial anisotropy” in rolling and transverse direction, can be determined.The anisotropy parameters for sheet metal used hitherto are mainly determined by uniaxial tensile tests. Such tests, however, do not give sufficient information about the yield locus and the forming behaviour in that range where the two principal tensile stresses are of similar magnitude like in stretch forming. The same applies for combined tensile and compressive stress like in deep‐drawing. To fill these gaps, new parameters are defined. Their experimental determination is briefly discussed.The “equibiaxial yield stress” and “equibiaxial anisotropy” which refer to equibiaxial tensile stress can be determined by cross tensile tests. However, these require a special apparatus. Alternatively experiments for obtaining plane strain can be applied for determining the equibiaxial parameters indirectly. This is possible using conventional tensile testing machines. In this case also anisotropy parameters for plane‐strain deformation, the “semibiaxial anisotropy” in rolling and transverse direction, can be deter...

Determining Flow Curves of Sheet Metal

While there are many different methods known for determining the flow curves of materials for bulk metal forming, only a limited number of experiments can be applied for thin sheet metal. There are several reasons for this: 1. For sheet metal the plastic anisotropy usually is of greater importance than for bulk metal forming. Therefore methods for determining flow curves of sheet metal should also enable one to obtain some information on anisotropy. 2. The tensile test on sheet metal allows for the determination of the flow curve only for strains below uniform elongation because it is not possible to measure the contour of the neck with good accuracy. The upsetting test on sheet metal is limited to sheet thicker than 5 to 7 mm because otherwise the relative error of the measurements is too large. In ASTM E 9–81 /2.17/ also the compression of rectangular sheet specimens in planar direction is included whereby a jig is needed for lateral support of the test piece. Such tests, however, are somewhat complicated and the result may be influenced by friction.