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

Abstract

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

Numerical Modelling of Conventional and Incremental Forming of Thin-Walled Tube

“…where the tube thickness decreases as approaching the tube end. On the other hand, the section plotted on the numerical model of the expanded tool ends at the same precise point ("point A", see more details in [12] for the assessment of this point location), whereas the experimental strains contain the pairs of principal strains along 3 parallel sections up to the very tube edge. Finally, the pair of in-plane principal strains at fracture (red triangle in Fig.…”

“…Finally, the pair of in-plane principal strains at fracture (red triangle in Fig. 3) was calculated as an average of the fracture strains of the different specimens tested following the procedure explain in [12]. This fracture strain is characterized by a value of 𝜀𝜀̅ = 1.17 of equivalent strain that defines the Mises iso-equivalent fracture elliptical locus plotted in dotted line in Figure 3.…”

“…1a). The process parameters of the experiments on multi-stage expansion by SPIF can be found in [12,13].…”

“…In this scientific context, and following the very recently published work by Suntaxi et al [12], this paper aimed providing a novel perspective of formability and failure in the non-proportional process of multi-stage tube expansion by SPIF, discussing the validity of the conventional forming limits for incremental forming processes. The study was carried out by means of numerical modelling of the SPIF process combined with a numerical analysis of the strain paths within the equivalent strain versus stress triaxiality space, including a comparison with conventional tube expansion.…”

Tube and Sheet Metal Forming Processes and Applications

“…Incremental sheet forming (ISF) was another topic of relevance in this Special Issue, dealt with in the work by Bautista-Monsalve et al [8] through a novel machine-learningbased procedure for determining the surface finish quality of parts obtained by heat-assisted SPIF. Additionally, the work by Suntaxi et al [9] dealt with ISF, although in this case, concerning the multistage SPIF of thin-walled tubes from a numerical perspective. Other papers analyzing tube forming were carried out by Standley and Knezevic [10] dealing with the manufacturing of ultrafine metallic tubular structures by accumulative extrusion bonding, or the paper by Kishimoto et al [11] which analyzed the deformation behavior causing the excessive thinning of micro metal tubes in hollow sinking.…”