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This paper is focussed on the mechanical and formability characterisation of wire-arc additive manufactured (WAAM) AISI 316-L stainless-steel tubes. The methodology to be presented involved carrying out tension and ring hoop tension tests on specimens extracted from the tube longitudinal, transverse and inclined directions. The force evolutions, acquired from the load cells, and the strain measurements, retrieved from digital image correlation and from thickness measurements along the cracks, allowed obtaining the stress-strain curves, the strain paths and the onset of failure by fracture for the three different tube directions. Special attention was paid to the ring hoop test, which was revisited to determine the appropriateness of using ring specimens with one or two dumbbell geometries. The originality of using the ring hoop tension test in WAAM tubes with strong anisotropic behaviour allowed obtaining strain loading paths that range from plane strain to pure shear deformation conditions. Resort to commercial AISI 316-L stainless-steel tubes during the presentation is included for reference purposes.

This article describes a methodology to characterize the failure limits by necking and fracture, and to determine the critical value of ductile damage in tube forming. The methodology makes use of digital image correlation, thickness measurements and force–displacement evolutions to obtain the strain loading paths and the strain values at the onsets of failure by necking and fracture. The onset of failure by necking is determined by a new technique that combines the strain and force–displacement evolutions whereas the fracture strains at the cracked regions of the tubes are obtained by a similar technique utilized by the authors in sheet metal forming. Transformation of the strain loading paths from principal strain space into the space of effective strain versus stress-triaxiality allows determining the critical experimental value of ductile damage at the onset of failure by necking. The methodology is applied to tube expansion with circular, elliptical and square cross-section punches and results confirm its importance and helpfulness for researchers and engineers involved in the development and optimization of industrial tube forming processes.

This article is focused on the formability limits by fracture obtained from standard bulk metal forming tests performed with cylindrical, tapered and flanged specimens. A total of two novel features are presented: the use of digital image correlation to determine strain paths and immersion of steel specimens in liquid nitrogen after the onset of crack formation to reveal the mode of fracture. A new methodology to determine the fracture loci in principal strain space is proposed based on the combination of experimental force–displacement evolutions with in-plane strain measurements. The experimental work is performed in cold-drawn steel AISI 1045 and two new formability tests with different values of stress triaxiality are proposed for obtaining strains at fracture in regions of principal strain space that are not sufficiently well covered by standard bulk metal forming tests.

This article investigates the possibility of failure by crack-opening mode III (out-of-plane shearing) in sheet–bulk metal forming processes. The investigation makes use of experimentally and theoretically determined fracture-forming limits of aluminium AA1050-H111 sheets with 1 mm thickness, experimental tests in incremental ploughing with a roll-tipped tool and numerical simulation using a commercial finite element programme. Results show that incremental ploughing of thin sheets with a roll-tipped tool under large indentation depths gives rise to transverse cracks that are triggered at the upper groove surface and propagate downward across thickness along an inclined direction to the sheet surface. In contrast to sheet–metal forming processes that only fail by fracture in crack-opening modes I and II, sheet–bulk metal forming processes present the unique ability of failing in all three possible crack-opening modes, namely, in mode III that is typical of bulk metal–forming processes.

This article presents a new experimental test for determining fracture toughness, in plane stress, in crack opening mode II based on the utilization of double-notched circular test specimens loaded in plane torsion. The proposed methodology for determining fracture toughness involves characterization of the evolution of torque with the degree of rotation for a number of test cases performed with specimens having different lengths of the ligaments between the notches. The work is supported by measurement of the in-plane and gauge length strains in aluminium AA1050-H111, and the overall experimental results show that the new proposed test provides an easy and effective way of evaluating the ability of a sheet metal to resist cracking under in-plane shear loading conditions.

This article presents an innovative testing machine for determining fracture toughness in double-notched cylindrical and prismatic test specimens loaded in shear over a wide range of applied strain rates and superimposed pressures. The equipment makes use of an electromagnetic actuator that is capable of accelerating the shearing punch against the specimens, with excellent repeatability and very precise control of the impact velocity, by means of the high pressure that is generated in a transient magnetic field produced by passing a pulse of electric current through a series of coils. Experiments performed in technically pure lead and aluminium AA1050-O give support to the presentation and allow understanding the influence of the loading rate and pressure on fracture toughness of ductile metals.

Multi-stage single-point incremental forming (SPIF) is a state-of-the-art manufacturing process that allows small-quantity production of complex sheet metal parts with vertical walls. This paper is focused on the application of multi-stage SPIF with the objective of producing cylindrical cups with vertical walls. The strategy consists of forming a conical cup with a taper angle in the first stage, followed by three subsequent stages that progressively move the conical shape towards the desired cylindrical geometry. The investigation includes material characterization, determination of forming-limit curves and fracture forming-limit curves (FFLCs), numerical simulation, and experimentation, namely the evaluation of strain paths and fracture strains in actual multi-stage parts. Assessment of numerical simulation with experimentation shows good agreement between computed and measured strain and strain paths. The results also reveal that the sequence of multi-stage forming has a large effect on the location of strain points in the principal strain space. Strain paths are linear in the first stage and highly non-linear in the subsequent forming stages. The overall results show that the experimentally determined FFLCs can successfully be employed to establish the forming limits of multi-stage SPIF.

In a previously published work, the current authors presented an analytical framework, built upon the combined utilization of membrane analysis and ductile damage mechanics, that is capable of modelling the fundamentals of single-point incremental forming (SPIF) of metallic sheets. The analytical framework accounts for the influence of major process parameters and their mutual interaction to be studied both qualitatively and quantitatively. It enables the conclusion to be drawn that the probable mode of material failure in SPIF is consistent with stretching, rather than shearing being the governing mode of deformation. The study of the morphology of the cracks combined with the experimentally observed suppression of neck formation enabled the authors to conclude that traditional forming limit curves are inapplicable for describing failure. Instead, fracture forming limit curves should be employed to evaluate the overall formability of the process. The aim of this paper is twofold: (a) to compare the mechanics of deformation of SPIF, namely the distribution of stresses and strains derived from the analytical framework with numerical estimates provided by finite element modelling; and (b) to compare the forming limits determined by the analytical framework with experimental values. It is shown that agreement between analytical, finite element, and experimental results is good, implying that the previously proposed analytical framework can be utilized to explain the mechanics of deformation and the forming limits of SPIF.

In recent work, the present authors constructed a closed‐form analytical model that is capable of dealing with the fundamentals of single‐point incremental forming (SPIF) and explaining the experimental and numerical results published in the literature over the past couple of years. The model is based on membrane analysis with in‐plane contact frictional forces but is limited to plane strain, rotationally symmetric conditions. The aim of the present paper is twofold: first, to extend the previous closed‐form analytical model into a theoretical framework that can easily be applied to the different modes of deformation that are commonly found in general single‐point incremental forming processes and, second, to investigate the formability limits of SPIF in terms of ductile damage mechanics and the question of whether necking does, or does not, precede fracture. Experimentation by the present authors, together with data retrieved from the literature, confirms that the proposed theoretical framework is capable of successfully addressing the influence of the major parameters of the SPIF process. It is demonstrated that neck formation is suppressed in SPIF, so that traditional forming limit diagrams are inapplicable to describe failure. Instead fracture forming limit diagrams should be employed.