Wilko C. Emmens

Strain in Shear, and Material Behaviour in Incremental Forming

This paper discusses some consequences of forming by shear, a situation that is sometimes claimed to occur in incremental forming. The determination of the principal strains and principal directions is discussed in detail. Two methods are presented: using a circular grid (although simulated on the computer), and by deriving formulae from the theory; both yield identical results. The strains assuming forming by shear are found to be (much) higher than in situations of forming by stretch. This affects notably more fundamental studies on material behaviour in incremental forming. The effects are illustrated using experimental data obtained with pre-stressed material.

Incremental Sheet Forming Analysed by Tensile Tests

To study material behaviour under conditions encountered in ISF operations tensile tests have been carried out on material taken from the walls of pyramidal products. The shape of the stress-strain curves depend on orientation. Tests in the direction of punch movement show an overshoot indicating a change in strain path, tests across that direction do not. From this it is concluded that the major direction of deformation in the walls is perpendicular to the direction of punch movement. There is no indication of a severe deformation in the direction of punch movement, either stretch or shear. The level of hardening in the material is less than expected from the macroscopic changes in dimensions. Apparently the forming operation in ISF causes additional softening of the material

Detailed Incremental Forming of Steel Beverage Cans by a High-Pressure Water Jet

This paper describes the process of shaping fully formed beer & beverage cans with a rotating high-pressure water jet. In detail the paper discusses situations where forming is restricted to a limited area by using a special mould. This causes the strain to shift to biaxial causing quite different properties. The process is compared to ‘normal’ incremental forming showing many similarities that are discussed in more general terms and can be summarized into four propositions.

Stating the Problem

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

Appendix: Measuring Strains and The FLC

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.

Appendix: Some Basic Concepts of Stress and Strain

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

Appendix: Directions of Zero Strain

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.

Testing of Formability

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

The Tensile Test

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.

Back to the Press Shop

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.