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Über dieses Buch

- Overview of materials and treatment aspects of manufacturability of sheet metal

- Written by an industrial expert turned scientist

- Concentrates on the formability of sheet metal, one of the fundamental form material is used in metalworking

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction and Scope

Abstract
The ever ongoing development in both sheet metal forming and in forming simulations have demanded a better understanding of sheet metal formability: the ability to be formed into a desired shape.
Wilko C. Emmens

Chapter 2. Stating the Problem

Abstract
Comparing forming steel to construction steel illustrates the problem that arises when discussing formability: is it a material parameter in the strict sense?
Wilko C. Emmens

Chapter 3. Definition of Formability

Abstract
The definition of formability depends heavily on what it is used for: the press-shop uses another definition that the scientist, but both are relevant.
Wilko C. Emmens

Chapter 4. The Tensile Test

Abstract
The tensile test is the most widely used material test. By looking at the tensile test as a forming operation several lessons can be learned. Basic the tensile operation is unstable, and the forming is restricted by an instability that concentrates the formation into a small zone, the neck. The formability is directly related to the amount of work hardening of the material. The forming limit depends also on how much local thinning is allowed. When the instability can be suppressed by whatever means, much higher levels of deformation can be obtained. This is discussed in detail. The chapter ends with an overview of material parameters related to formability.
Wilko C. Emmens

Chapter 5. The Forming Limit Curve

Abstract
In a deforming sheet, the strain state is defined as the ratio between minor strain and major strain. The formability depends on this strain state and can be expressed as the so-called forming limit curve (FLC). In complex strain states the deformation is limited by an instability just as in a tensile test. In cases of negative minor strain this can be analyzed simply, in cases of positive major strain a more complex analysis is required. Two strain states allow high formability: deep-draw and equi-biaxial, in case of the first one the formality is infinite in theory. The FLC however is only valid under certain conditions: no bending, straight strain path, planar stress, no shear.
Wilko C. Emmens

Chapter 6. Bending

Abstract
Nearly all forming operations have some amount of bending. Bending can cause a multitude of effect, but raising the formability is the most relevant. In a situation of bending combined with tension the occurrence of compressive fibres at the concave side creates additional stability, therefore raising the formability. This also lowers the tension force. A normal stress at the tool contact has a similar effect, albeit of smaller magnitude. Bending can also create shear, or cause inter-crystalline fracture in some materials at the convex side that obviously lowers the formability.
Wilko C. Emmens

Chapter 7. Non-Straight Strain Path

Abstract
A non-straight strain path can both raise and lower the formability, depending on the character of the strain path changes. Abrupt changes create a stress overshoot that may cause premature fracture. The transient stress effect on the other hand can create a situation of quasi-hardening that creates additional formability. Pure cyclic straining can raise the formability as well.
Wilko C. Emmens

Chapter 8. Non-Planar Stress

Abstract
In all situations of contact between tool and sheet there is some contact stress (negative normal stress). This will raise the formability, but the effects are limited.
Wilko C. Emmens

Chapter 9. Shear

Abstract
There are three kinds of shear: in-plane shear, through-thickness shear, and out-of-plane shear, although fundamentally there is no difference. Forming by pure shear can in principle create infinite formability as there is not reduction in sheet thickness. The situation of shear combined with stretch is more complex, but several analyses have showed that it can raise the formability significantly.
Wilko C. Emmens

Chapter 10. Incremental Forming

Abstract
In incremental forming operations high levels of deformation are easily obtained. In shear spinning these are obtained by out-of-pale shear. In Incremental Sheet Forming (ISF) high levels of deformation are obtained by a combination of effects mentioned above: bending, cyclic straining, non-planar stress and shear.
Wilko C. Emmens

Chapter 11. Speed Effects

Abstract
The forming speed affects the forming operation by (in order of increasing magnitude): inertia effects, metallurgical effects, and tribological effects. Metallurgical effects (strain-rate hardening) can lower the uniform strain in a tensile test but slow down the development of necks. Enhanced formability in true high-speed forming operations however are not caused by strain-rate hardening, but by secondary inertia effects. At extreme speeds the formability can also be increased by viscous drag.
Wilko C. Emmens

Chapter 12. Ultrasonic Forming

Abstract
The application of vibrating tools can enhance the formability significantly. A major cause is the decrease of friction between workpiece and tool, but also the formability in the strict sense can be increased.
Wilko C. Emmens

Chapter 13. Testing of Formability

Abstract
There are three types of formability testing: direct testing, simulative testing, and indirect testing. All are relevant for the classification of material formability.
Wilko C. Emmens

Chapter 14. Instabilities Again and Not

Abstract
The absence if instabilities, if possible, does not create infinite formability, the material will always fail finally by damage development. In sheet metal forming this forming limit is called fracture limit, contrary to the necking limit that is caused by instability.
Wilko C. Emmens

Chapter 15. Back to the Press Shop

Abstract
The formability may be restricted by either a strain limit, or a force limit. In case of the latter the process may be repeated for increased formability. In an actual press forming operation the product may be rejected by numerous causes, not only fracture. This illustrates that formability is a more complex concept that is not a straightforward material property in the strict sense. However for a certain prescribed process/product the formability can be related to a set of material parameters.
Wilko C. Emmens

Chapter 16. Summary

Abstract
There is a general rule of thumb (but only that): if a mechanism makes it easier for a material to be stretched, meaning by a lower force, than that material can be stretched further. Keeping this in mind we can make the following categories for formability enhancing mechanisms. This may help the reader for further research.
Wilko C. Emmens

Chapter 17. Closure

Abstract
We have learned a lot about the different aspects of formability, and which properties control formability. One thing is still missing: a proper definition of formability. The practical definition as stated in {A} is still the best we can find, but unsatisfactory for the researcher. However, that is the situation, and it has to be accepted just like that. Formability cannot be treated as a simple material property, and hence not determined by a simple test. This makes things complicated, the formability as defined in {B} depends heavily on the actual forming process. Luckily, there is another side. Because formability is such a complex phenomenon there is room for improvement. As formability depends heavily on the process there is a possibility that the formability of the material can be increased by proper process settings. An extreme example of this has been met.
Wilko C. Emmens

Chapter 18. Appendix: Some Basic Concepts of Stress and Strain

18.1
This Appendix presents only a very brief introduction to plasticity. The reader is referred to text books for a more detailed treatment. Note that most expressions presented here are only valid for isotropic materials, materials of which the properties are the same in every direction. Anisotropy is discussed in Sect. 18.3
Wilko C. Emmens

Chapter 19. Appendix: The Considère Condition

Abstract
In Sect. 4.2 the Considère condition was presented as the basic condition that determines stability in the tensile test. This Appendix discusses that condition in detail.
Wilko C. Emmens

Chapter 20. Appendix: Measuring Strains and The FLC

Abstract
To determine strains they have to be measured. This is done by marking the surface in some way, and to compare the situation after forming to that before forming. These markings can easily be made by painting, but there is a possibility that the paint wears off when the surface slides over the tool. Another way is to etch the markings chemically into the surface. This supplies very robust markings, but the procedure is tedious. Also, a deep etching may act as a stress concentrator initiating early failure.
Wilko C. Emmens

Chapter 21. Appendix: Directions of Zero Strain

Abstract
There is a general rule of thumb (but only that): if a mechanism makes it easier for a material to be stretched, meaning by a lower force, than that material can be stretched further. Keeping this in mind we can make the following categories for formability enhancing mechanisms. This may help the reader for further research.
Wilko C. Emmens

Chapter 22. Appendix: Speed Effects of Lubrication

Abstract
If in a practical forming operation lubrication is applied (as is most often), speed affects may be created by the lubricant. This is caused by the fact that movement of the sheet over the tool can generate a pressure in the lubricant by hydrodynamic effects. As a result the pressurized lubricant carries part of the load thereby reducing the load on the roughness asperities. This will reduce friction. Under ideal conditions the influence of speed on friction may be presented.
Wilko C. Emmens

Chapter 23. Appendix: Influence of Parameters on Press Behaviour

Abstract
The following table presents an overview of the influence of some material parameters on press behaviour found in literature, as compiled by Alan Daglish
Wilko C. Emmens
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