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Über dieses Buch

This book is addressed to both research scientists at universities and technical institutes and to engineers in the metal forming industry. It is based upon the author's experience as head of the Materials Science Department of the In­ stitut fUr Umformtechnik at the University of Stuttgart. The book deals with materials testing for the special demands of the metal for­ ming industry. The general methods of materials testing, as far as they are not directly related to metal forming, are not considered in detail since many books are available on this subject. Emphasis is put on the determination of processing properties of metallic materials in metal forming, i. e. the forming behavior. This includes the evaluation of stress-strain curves by tensile, up­ setting or torsion tests as well as determining the limits of formability. Among these subjects, special emphasis has been laid upon recent developments in the field of compression and torsion testing. The transferability of test results is discussed. Some testing methods for the functional properties of workpieces in the final state after metal forming are described. Finally, methods of testing tool materials for bulk metal forming are treated. Testing methods for surface properties and tribological parameters have not been included. The emphasis is put on the deformation of the specimens. Prob­ lems related to the testing machines and measuring techniques as well as the use of computers are only considered in very few cases deemed necessary.



1. Introduction

Remark: in the subsequent chapters only new symbols or such ones that have been used in a different meaning have been listed. Most of the symbols agree with those in the “Handbook of Metal Forming” /1.1/ which in some cases are based on ISO R 31/III, IV and V and on CIRP “Recommended Symbols in Forming Technology” (1976); in cases when they are related to materials testing, however, they are mainly based on the ASM Metals Handbook, Vol. 8 /1.2/, rather than on ISO R 82 or ISO/TC 17 N 1093. To a large part, these symbols agree with those used in the ASTM standards.
Klaus Pöhlandt

2. Determination of Flow Curves for Bulk Metal Forming

In the simplest case the flow curve is obtained by measuring the force F and the elongation A L in the tensile test. In the range of strain below uniform elongation it is assumed that the stress is constant over the cross-section of the specimen. The ratio F/A where A is the actual cross-section is called flow stress σf:
$$ {\sigma _f} = \frac{F}{A}$$
For the plastic deformation of most metals the condition of volume conservation is fulfilled with good accuracy. Therefore the actual cross-section A can be calculated from the measured elongation:
$$A = \frac{{\pi r_0^2{L_0}}}{{{L_0} + \Delta L}} $$
where L0 is the initial gage length of the test piece. Flow stress is plotted vs. strain
$$ \phi = In\frac{{{L_0} + \Delta L}}{{{L_0}}}$$
The function
$$ {\sigma _t} = {\sigma _f}\left(\phi\right) $$
is called the flow curve. It indicates the stress required for plastic deformation to occur under uniaxial stress. The relation between an uniaxial and a multiaxial state of stress is described by the concepts of equivalent stress and equivalent strain. So in the general case of a triaxial state of stress in Eq. (2.4) φ must be replaced by φ̄.
Klaus Pöhlandt

3. 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:
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.
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.
Klaus Pöhlandt

4. Transferability of Results

For enabling a calculation of the force and work requirement of metal forming processes and the strain hardening of the material from the flow curve obtained by any experiment /4.1, 4.2/, the test conditions must simulate those of the actual metal forming process. This holds both for parameters like temperature, strain rate and the state of stress and for the specimens which shall be representative of the material or to be formed. The problems arising from these basic requirements shall now be discussed in detail. It should be taken in mind, however, that besides these basic requirements the number, size and location of specimens may also be determined by product specifications for the material to be tested.
Klaus Pöhlandt

5. Determining the Limits of Formability

This chapter ist widely based on refs. /5.1/ and /2.1/. Compared to the methods for determining the force and work requirements, the evaluation of the limits of metal forming processes has not yet been studied thoroughly.
Klaus Pöhlandt

6. Material and Workpiece after the Forming Process

After the (last step of the) metal forming process the formability of the material is no longer of interest. Generally, however, a succeeding production process of a different kind must be taken into account. This subject is treated in Sec. 6.2 using machining as an example.
Klaus Pöhlandt

7. Testing Tool Materials for Bulk Metal Forming

This chapter is widely based on ref. /7.1/ (see also /7.2/). Tool materials for sheet material forming are not included but basically some of the testing methods described below can also be applied to sheet metal forming tools.
Klaus Pöhlandt


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