M.B. Silva

Single‐point incremental forming and formability—failure diagrams

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.

Strategies and limits in multi-stage single-point incremental forming

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.

Revisiting single-point incremental forming and formability/failure diagrams by means of finite elements and experimentation:

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.

Incremental Forming of Hole-Flanges in Polymer Sheets

Hole-flanging by single-point incremental forming (SPIF) is an emerging sheet forming process with a high potential economic payoff for rapid prototyping and small quantity production. So far, applications have only been performed in metals but the aim of this article is to evaluate the feasibility of using polymers. Special emphasis is given to the deformation mechanics of the process and the physics behind the occurrence of failure by fracture along the wall flange. The investigation draws from the independent determination of the mechanical properties, fracture toughness, and formability limits of polyethylene terephthalate (PET) and polycarbonate (PC), to experimentation in a CNC machining center equipped with an apparatus for multi-stage SPIF. The work is supported by circle grid analysis and results show that PET is appropriate for producing hole-flanged parts with vertical walls due to its capability of withstanding high levels of deformation at room temperature.

Two-Point Incremental Forming with Partial Die: Theory and Experimentation

This paper proposes a new level of understanding of two-point incremental forming (TPIF) with partial die by means of a combined theoretical and experimental investigation. The theoretical developments include an innovative extension of the analytical model for rotational symmetric single point incremental forming (SPIF), originally developed by the authors, to address the influence of the major operating parameters of TPIF and to successfully explain the differences in formability between SPIF and TPIF. The experimental work comprised the mechanical characterization of the material and the determination of its formability limits at necking and fracture by means of circle grid analysis and benchmark incremental sheet forming tests. Results show the adequacy of the proposed analytical model to handle the deformation mechanics of SPIF and TPIF with partial die and demonstrate that neck formation is suppressed in TPIF, so that traditional forming limit curves are inapplicable to describe failure and must be replaced by fracture forming limits derived from ductile damage mechanics. The overall geometric accuracy of sheet metal parts produced by TPIF with partial die is found to be better than that of parts fabricated by SPIF due to smaller elastic recovery upon unloading.

Single point incremental forming of a facial implant.

Background: The investigation draws from the fundamentals of the mechanical behaviour of titanium grade 2 to the design and fabrication of facial implants by means of single point incremental forming. Objectives: To provide knowledge on the capabilities and limitations of a new manufacturing technology to fabricate low-cost, patient-specific medical implants. Study design: Rapid fabrication of a simplified model of a facial implant. Methods: Circle grid analysis and its graphical representation in the fracture forming limit diagram combined with finite element modelling are utilized to identify the failure limits and to assist the overall design of the facial implants. Results: Fabrication of facial implants without and with failure by cracking due to excessive thinning of the sheet from where the implant is to be cut. Conclusions: Identification of the major operative parameters that influence fabrication of sound facial implants by means of single point incremental forming. Clinical relevance Reduce the gap between production engineers and the medical community by presenting a state-of-the-art manufacturing technology to produce low-cost, patient-specific medical implants.

Towards square hole-flanging produced by single point incremental forming

This paper presents the first investigation on square hole-flanging produced by single point incremental forming. The aim and objective is to provide readers with a broad understanding on the deformation mechanics of the process that will enable them to understand plastic flow resulting from the interaction between the tool and the blank, to identify the influence of the pre-cut geometry in the overall formability, and to characterize the physics of failure. The methodology comprises the mechanical characterization of the material, the use of circle grid analysis, and the fabrication of square flanges with round corners by multistage single point incremental forming using blanks with different pre-cut hole geometries. The investigation is performed in aluminium AA1050-H111 and the overall results widens and enhances current research work in hole-flanging of cylindrical parts by giving the first contribution towards the understanding of plastic flow and failure in hole-flanging of square parts produced by single point incremental forming.

Revisiting the wrinkling limits in flexible roll forming

This article presents a new understanding on the deformation mechanics of flexible roll forming and is focused on the occurrence of flange wrinkling. The presentation draws from the experimental and numerical simulation of flexible roll forming to the determination of the wrinkling limits by means of a new theoretical and experimental methodology based on the utilization of rectangular test specimens loaded in axial compression. This research work is performed in mild steel sheets and the results show that the combined evolution of the effective strain with stress triaxiality obtained from finite element modelling of flexible roll forming and from the rectangular test specimens loaded in axial compression can be successfully utilized to predict the occurrence of flange wrinkling in variable cross-sectional profiles produced by flexible roll forming. The proposed methodology can, therefore, be considered an alternative to existing approaches based on simplified analytical and numerical procedures.

On the formability of hole-flanging by incremental sheet forming

Hole-flanging by incremental sheet forming is developing as an innovative metal forming technology for flexible small batch-production of cylindrical or conical flanges in blanks with pre-cut holes. The process is seen as an alternative to hole-flanging by conventional press-working due to significant cost savings through the replacement of complex press-tools by simple dieless forming apparatuses and to widespread belief that the limiting forming ratio of hole-flanging by incremental sheet forming is always higher than that of hole-flanging by conventional press-working. This article focuses on the aforementioned assumption and investigates the influence of material failure by necking and fracture on the limiting forming ratio. The experimental work is performed in a CNC machining center and a hydraulic press equipped with apparatuses for multi-stage single point incremental forming and conventional press-tooling, and the strain loading paths resulting from each forming process are determined by circle grid analysis. Results in Aluminium AA1050-H111 and Titanium (grade 2) blanks demonstrate that contrary to what has often been said in the literature there are process operating conditions leading to higher limiting forming ratio of hole-flanging by conventional press-working than by incremental sheet forming due to closeness of the forming limit curve and fracture forming limit line in the principal strain space.

A new test for determining fracture toughness in plane stress in mode II

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.