Materials Testing for the Metal Forming Industry

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.- 1.1 The System of Metal Forming.- 1.2 The Material before the Forming Process.- 1.2.1 Overview.- 1.2.2 Testing the Forming Behavior.- 1.2.3 Further Testing Methods.- 1.3 Concluding Remarks.- 1.4 References.- 2 Determination of Flow Curves for Bulk Metal Forming.- 2.1 Basic Concepts.- 2.1.1 Flow Curves of Single Crystals.- 2.1.2 Flow Curves of Polycrystals.- 2.1.2.1 Effect of Grain Size.- 2.1.2.2 Flow Curves at Room Temperature.- 2.1.2.3 Effect of Temperature and Strain Rate.- 2.2 Tensile Test.- 2.2.1 Introduction.- 2.2.2 Standardized Tensile Test.- 2.2.3 Tensile Test beyond Uniform Elongation.- 2.2.4 Approximative Determination of Flow Curves from Characteristic Values Obtained by Tensile Tests.- 2.3 Upsetting Test.- 2.3.1 Fundamentals.- 2.3.2 Effect of Friction.- 2.3.2.1 Overview.- 2.3.2.2 Modifications of the Upsetting Test.- 2.3.3 Discontinuous Upsetting Test.- 2.3.4 Rastegaev Test.- 2.3.4.1 Upsetting Cylindrical Specimens with Conventional Lubrication.- 2.3.4.2 Principle of the Rastegaev Test.- 2.3.4.3 Optimum Geometry of Rastegaev Specimens.- 2.3.4.4 Errors in the Rastegaev Test.- 2.3.4.5 Measurement of Diameter in the Rastegaev Test.- 2.3.5 Upsetting Noncylindrical Test Pieces.- 2.3.6 Plane Strain Upsetting Test.- 2.3.7 Priliminary Comparison of the Modifications of the Upsetting Test.- 2.4 Torsion Test.- 2.4.1 Fundamentals.- 2.4.2 Calculation of the Flow Curve from the Test Results.- 2.4.3 Choice of the Yield Criterion.- 2.5 Determination of Flow Curves at Elevated Temperatures.- 2.5.1 General Conditions.- 2.5.2 Hot Tensile Test.- 2.5.3 Hot Compression Test.- 2.5.4 Hot Torsion Test.- 2.6 Further Testing Methods for Determining Flow Curves.- 2.6.1 Overview.- 2.6.2 Tests at Extreme Strain Rate.- 2.6.3 Tests at Superimposed Hydrostatic Compressive Stress.- 2.6.4 Indentation Tests.- 2.6.5 Testing Unconventional Materials.- 2.7 Critical Comparison of Testing Methods.- 2.7.1 Overview.- 2.7.2 The Three Basic Tests.- 2.7.2.1 Factors Causing Systematic Errors.- 2.7.2.2 Further Criteria of Valuating.- 2.7.3 Remarks on the Special Testing Methods.- 2.8 References.- 3 Determining Flow Curves of Sheet Metal.- 3.1 Special Properties of Thin Sheet Metal.- 3.2 Tensile Test on Thin Sheet Metal.- 3.3 Plane Strain Deformation Tests.- 3.3.1 Tensile Test with Supressed Lateral Contraction.- 3.3.2 Bending Test.- 3.3.3 Plane Strain Compression.- 3.4 Hydraulic Bulge Test.- 3.5 Plane Torsion Test.- 3.5.1 Principle of the Test.- 3.5.2 Test Evaluation.- 3.5.3 Effect of the Choice of the Yield Criterion.- 3.5.4 Limits of Application.- 3.6 Effects of Strain Rate and Temperature.- 3.7 Testing Superplastic Materials.- 3.8 Comparison of the Methods.- 3.9 Anisotropy.- 3.9.1 Background.- 3.9.2 Determination of the r-Value of Sheet Metal.- 3.10 References.- 4 Transferability of Results.- 4.1 Basic Problem.- 4.2 Uncertainty of Experimentally Determined Flow Curves.- 4.2.1 Error of Measurement and Uncertainty of the Yield Criterion.- 4.2.2 Requirements Concerning the Test Pieces.- 4.2.2.1 Location and Number of Specimens.- 4.2.2.2 Effect of Size.- 4.3 Estimation of Flow Curves without Experiments.- 4.4 References.- 5 Determining the Limits of Formability.- 5.1 Basic Concepts.- 5.2 The Concept of “Ductility”.- 5.2.1 Survey.- 5.2.2 Notched Tensile Test.- 5.3 The Forming Limit.- 5.3.1 Introduction.- 5.3.2 Forming Limit in Bulk Metal Forming.- 5.3.3 Forming Limit in Sheet Metal Forming.- 5.3.4 Discussion of the Forming Limit Diagram.- 5.3.5 Strain Analysis.- 5.4 Process Simulation Testing Methods.- 5.4.1 Preliminary Remarks.- 5.4.2 Simulating Testing Methods for Bulk Metal Forming.- 5.4.3 Simulating Testing Methods for Sheet Metal Forming.- 5.4.3.1 General Remarks.- 5.4.3.2 Stretch-Forming Tests.- 5.4.3.3 Deep-Drawing Tests.- 5.4.3.4 Bending Tests.- 5.4.3.5 Closed-Die Bending and Aging Tests.- 5.4.4 Combination of Several Process Simulation Testing Methods.- 5.5 References.- 6 Material and Workpiece after the Forming Process.- 6.1 Overview.- 6.2 Material Behavior during Production Processes after Metal Forming.- 6.3 Material Properties after Macroscopically Homogeneous Deformation.- 6.4 Properties of Workpieces after Metal Forming.- 6.4.1 Visioplasticity Method.- 6.4.2 Measurements of the Hardness Distribution.- 6.4.3 Determination of Macroscopic Residual Stresses.- 6.4.4 Testing Functional Properties of Workpieces after Metal Formingl6l.- 6.4.4.1 Overviewl6l.- 6.4.4.2 Fatigue Testing.- 6.4.4.3 Corrosive Testing.- 6.4.4.4 Concluding Remarks.- 6.5 References.- 7 Testing Tool Materials for Bulk Metal Forming.- 7.1 Introduction.- 7.2 State of Knowledge about Tool Fracture for Bulk Metal Forming.- 7.2.1 Overview.- 7.2.2 Tool Fracture when Hot Forming.- 7.2.3 Tool Fracture when Cold Forming.- 7.2.4 Tool Inspection for Bulk Metal Forming.- 7.3 Materials Testing for Tool Life Estimation.- 7.3.1 General Remarks.- 7.3.2 Methods of Fracture Mechanics.- 7.3.2.1 Fracture Toughness.- 7.3.2.2 Crack Growth Rate.- 7.4 Application of Fracture Mechanics for Evaluating Tool Materials for Hot Forming.- 7.5 Examples of Tool Life Estimation for Cold Bulk Metal Forming.- 7.5.1 Applications of Fracture Mechanics.- 7.5.2 Tool Life Estimation for Cold Extrusion.- 7.6 References.- Appendix A: Supplements to the Theory and Practice of Torsion Tests.- A.1 Torsion Test on Bars.- A. 1.1 Determination of the Zero Approximation of the Flow Curve.- A. 1.2 Taylor Series Expansion of the “Correction Function”.- A. 1.3 Use of Extremely Short Specimens.- A. 1.4 Recommended Specimen Geometry.- A. 1.5 Possible Sources of Error.- A.1.6 Determination of the Strain Rate Sensitivity.- A.2 Plane Torsion Test.- A.3 References.- Appendix B: Standards and Recommended Testing Procedures.- Appendix C: Chemical Composition and Comparative Designations of Steels.- Appendix D: Conversion Factors for Units.- Appendix E: Recommendations for an Uniform Procedure of Determining Flow Curves by Upsetting Cylindrical Specimens.