An overview of stabilizing deformation mechanisms in incremental sheet forming

doi:10.1016/j.jmatprotec.2008.10.003

Journal of Materials Processing Technology

Volume 209, Issue 8, 21 April 2009, Pages 3688-3695

https://doi.org/10.1016/j.jmatprotec.2008.10.003 Get rights and content

Abstract

In incremental sheet forming (ISF) strains can be obtained well above the forming limit curve (FLC) that is applicable to common sheet forming operations like deep drawing and stretching. This paper presents an overview of mechanisms that have been suggested to explain the enhanced formability. The difference between fracture limit and necking limit in sheet metal forming is discussed. The necking limit represents a localized geometrical instability. Localized deformation is an essential characteristic of ISF and proposed mechanisms should stabilize the localization before it leads to fracture. In literature six mechanisms are mentioned in relation to ISF: contact stress; bending-under-tension; shear; cyclic straining; geometrical inability to grow and hydrostatic stress. The first three are able to localize deformation and all but the last, are found to be able to postpone unstable growth of a neck. Hydrostatic pressure may influence the final failure, but cannot explain stability above the FLC.

Section snippets

Introduction and scope

The last decade has shown an increasing interest in a new class of forming processes known as incremental sheet forming (ISF). The name incremental forming is used for a variety of processes, all characterized by the fact that at any time only a small part of the product is actually being formed, and that area of local deformation is moving over the entire product. This definition covers many processes, including spinning but also for example rolling. The paper will however focus on incremental

Forming limits in conventional stamping

Any metal that is continuously being deformed will finally fail. The continuous deformation creates more and more dislocations that move through the material, interact with each other and create voids that finally result in a crack. The limit strain before failure is called the fracture limit and depends on the stress state: a high level of hydrostatic compressive stress squeezes the voids and slows down damage development. This is the reason why forming processes that are largely compressive

Effects of shear

This section reviews shear as a stabilizing mechanism. It starts with an explanation of the principle of the mechanism, followed by the relation to ISF and a review of testing.

Effects of contact stress

This section reviews contact stress as a stabilizing mechanism following the same structure as Section 3. It starts with an explanation of the principle of the mechanism, followed by the relation to ISF and a review of testing.

Bending-under-tension

This section reviews bending-under-tension (BUT) as a stabilizing mechanism following the same structure as Sections 3 Effects of shear, 4 Effects of contact stress. It starts with an explanation of the principle of the mechanism, followed by the relation to ISF and a review of testing.

Cyclic effects

In a practical ISF operation the punch passes a certain material point several times, up to a few dozen. Each pass causes bending and unbending with possible strain reversal so the material is subjected to cyclic straining, see Fig. 8. Bambach has noticed this cyclic straining and the possible effect on material behaviour, and suggests that this “motivates further investigation of the process using a constitutive law that takes into account path-dependent damage” (Bambach et al., 2003). Eyckens

Other mechanisms

This section presents some mechanisms that have been proposed in the literature but are not discussed there extensively.

Final discussion

In the previous sections several mechanisms have been discussed that may enable stable deformation above the FLC. The mechanisms are based on theoretical considerations and some of them have been validated experimentally. The relation between the mechanisms and ISF was ascertained and it can safely be assumed that if the isolated mechanism explains stability it will also contribute to the stability of deformation in ISF. The question which mechanism plays a major role and which contribution is

Acknowledgements

The authors are thankful for the extensive comments from the reviewers on an earlier version and their encouragement to continue the work on this paper. The elaborate discussion about possible mechanisms goes beyond an ordinary review and is very much appreciated.

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ReviewAn overview of stabilizing deformation mechanisms in incremental sheet forming

Abstract

Section snippets

Introduction and scope

Forming limits in conventional stamping

Effects of shear

Effects of contact stress

Bending-under-tension

Cyclic effects

Other mechanisms

Final discussion

Acknowledgements

Annals of the CIRP

Journal of Materials Processing Technology

International Journal of Machine Tools & Manufacture

Journal of Materials Processing Technology

CIRP Annals - Manufacturing Technology

International Journal of Mechanical Sciences

International Journal of Machine Tools & Manufacture

The increased forming limits of incremental sheet forming processes

Key Engineering Materials

Modelling and experimental evaluation of the incremental CNC sheet metal forming process

On the effect of the normal pressure upon the forming limit strains

A method for increasing elongation values for ferrous and nonferrous sheet metals

Journal of Materials

Forming limit predictions for the serrated strain paths in single point incremental forming

Review
An overview of stabilizing deformation mechanisms in incremental sheet forming