Recent Approaches for the Determination of Forming Limits by Necking and Fracture in Sheet Metal Forming

Forming limit diagrams (FLD’s) are used to evaluate the workability of metal sheets. FLD’s provide the failure locus at which plastic instability occurs and localized necking develops (commonly designated as the forming limit curve - FLC), and the failure loci at the onset of fracture by tension (FFL) or by in-plane shear (SFFL). The interest of metal formers in controlling localized necking is understandable because the consequence of plastic instability is an undesirable surface blemish in components. However, because under certain loading conditions fracture can precede necking in sheet metal forming processes, there is a growing interest in characterizing the forming limits by necking and fracture in the FLD’s. This paper gathers together a number of recently developed methodologies for detecting the onset of local necking and fracture by in-plane tension or in-plane shear, and discusses their applicability to determine experimentally the FLC’s, FFL’s and SFFL’s.Ministerio de Economía y Competitividad DPI2012-3291

Tube Expansion by Single Point Incremental Forming: An Experimental and Numerical Investigation

In this paper, we revisit the formability of tube expansion by single point incremental forming to account for the material strain hardening and the non-proportional loading paths that were not taken into consideration in a previously published analytical model of the process built upon a rigid perfectly plastic material. The objective is to provide a new insight on the reason why the critical strains at failure of tube expansion by single point incremental forming are far superior to those of conventional tube expansion by rigid tapered conical punches. For this purpose, we replaced the stress triaxiality ratio that is responsible for the accumulation of damage and cracking by tension in monotonic, proportional loading paths, by integral forms of the stress triaxiality ratio that are more adequate for the non-proportional paths resulting from the loading and unloading cycles of incremental tube expansion. Experimental and numerical simulation results plotted in the effective strain vs. stress triaxiality space confirm the validity of the new damage accumulation approach for handling the non-proportional loading paths that oscillate cyclically from shearing to biaxial stretching, as the single point hemispherical tool approaches, contacts and moves away from a specific location of the incrementally expanded tube surface.Junta de Andalucía (Consejería de Economía y Conocimiento) US-1263138Fundação para a Ciência e da Tecnologia of Portugal UIDB/50022/202

Innovative cold joining technologies based on tube forming

This paper is focused on innovative cold joining technologies for connecting tubes and fixing tubes to sheets. The proposed technologies are based on the utilization of plastic instability waves in thin-walled tubes subjected to axial compression and may be seen as an alternative to conventional joining technologies based on mechanical fixing with fasteners, welding and structural adhesive bonding. Besides allowing connecting dissimilar materials and being successfully employed in fixture conditions that are difficult and costly to achieve by means of conventional joining the new proposed technologies also cope with the growing concerns on the demand, lifecycle and recycling of materials

Innovative cold joining technologies based on tube forming

This paper is focused on innovative cold joining technologies for connecting tubes and fixing tubes to sheets. The proposed technologies are based on the utilization of plastic instability waves in thin-walled tubes subjected to axial compression and may be seen as an alternative to conventional joining technologies based on mechanical fixing with fasteners, welding and structural adhesive bonding. Besides allowing connecting dissimilar materials and being successfully employed in fixture conditions that are difficult and costly to achieve by means of conventional joining the new proposed technologies also cope with the growing concerns on the demand, lifecycle and recycling of materials