On the Use of Maximum Force Criteria to Predict Localised Necking in Metal Sheets under Stretch-Bending

The maximum force criteria and their derivatives, the Swift and Hill criteria, have been extensively used in the past to study sheet formability. Many extensions or modifications of these criteria have been proposed to improve necking predictions under only stretching conditions. This work analyses the maximum force principle under stretch-bending conditions and develops two different approaches to predict necking. The first is a generalisation of classical maximum force criteria to stretch-bending processes. The second approach is an extension of a previous work of the authors based on critical distance concepts, suggesting that necking of the sheet is controlled by the damage of a critical material volume located at the inner side of the sheet. An analytical deformation model is proposed to characterise the stretch-bending process under plane-strain conditions. Different parameters are considered, such as the thickness reduction, the gradient of variables through the sheet thickness, the thickness stress and the anisotropy of the material. The proposed necking models have been successfully applied to predict the failure in different materials, such as steel, brass and aluminiumGobierno español DPI2015-64047-

On the Use of Strain Path Independent Metrics and Critical Distance Rule for Predicting Failure of AA7075-O Stretch-Bend Sheets

The strain-based forming limit curve is the traditional tool to assess the formability of metal sheets. However, its application should be restricted to proportional loading processes under uniform strain conditions. Several works have focused on overcoming this limitation to characterize the safe process windows in industrial stretch-bend forming processes. In this paper, the use of critical distance rule and two path-independent stress-based metrics are explored to numerically predict failure of AA7075-O stretch-bend sheets with 1.6 mm thickness. Formability limits of the material were experimentally obtained by means of a series of Nakazima and stretch-bending tests at di erent thickness-over-radius ratios for inducing controlled non-uniform strain distributions across the sheet thickness. By using a 3D calibrated finite element model, the strain-based forming limit curve was numerically transformed into the path-independent stress and equivalent plastic strain polar spaces. The numerical predictions of necking strains in the stretch-bending simulations using the above approaches were successfully compared and critically discussed with the experimental results for di erent values of the critical distance. It was found that failure was triggered by a critical material volume of around the half thickness, measured from the inner surface, for the both path-independent metrics analyzed.Gobierno de España PGC2018-095508-B-I0

On the study of the single-stage hole-flanging process by SPIF

Recent studies show the capability of single-point incremental forming to perform successfully hole-flanging operations using multi-stage strategies. The aim of this work is to investigate the ability of the SPIF process to perform hole-flanges in a single stage, contributing to a better understanding of the formability of the sheet in this demanding situation. To this end, a series of experimental tests in AA7075-O metal sheets are performed in order to evaluate the limiting forming ratio. The physical mechanisms controlling sheet failure during the process are analyzed and discussed. In the test conditions studied this failure is postponed necking followed by ductile fracture in the wall of the flange.Ministerio de Economía y Competitividad DPI2012-3291

Preliminary investigation on homogenization of the thickness distribution in hole-flanging by SPIF

A drawback of the hole-flanging process by single-stage SPIF is the non-uniform thickness obtained along the flange. Multi-stage strategies have been used to improve it, however they increase notably the manufacturing time. This work presents a preliminary study of the tool paths for a hole-flanging process by SPIF in two stages. An intermediate geometry of the piece is proposed from the analysis of the thickness distribution observed in previous single-stage process. A simple optimization procedure is used to automate the intermediate part design, the NC code generation for the tool path and the validation of the optimal forming strategy by means of FEA

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

Numerical explicit analysis of hole flanging by single-stage incremental forming

The use of Single-Point Incremental Forming (SPIF) technology in hole flanging operations using multi-stages strategies have been widely studied in the last few years. However, these strategies are very time-consuming, limiting its industrial application.In a very recent work of the authors, the capability of SPIF process to successfully perform hole-flanges using a single-stage strategy has been experimentally investigated. The aim of the present work is to develop a numerical model of this process to beable to predict the sheet failure as a function of the size of the pre-cut hole. The numerical results are compared and discussed in the light of experimental tests over AA7075-O metal sheets with 1.6mm thickness.Ministerio de Economía y Competitividad DPI2015-64047-

Experimental and numerical analysis of the flanging process by SPIF

This paper analyses the flangeability of AA2024-T3 sheets using single point incremental forming (SPIF). With this purpose, a series of process parameters is considered including flange length and width, tool radius and spindle speed. An initial experimental campaign is carried out for the evaluation of the limiting strain states of the flanges within the material forming limit diagram (FLD). Numerical modelling through finite element analysis (FEA) is used in order to provide a better understanding of the sheets flangeability and forming conditions that either allow manufacturing or lead to failure in this process using concave dies. The capability of the SPIF process to improve formability is discussed.Altair Engineering, Inc.,Amada Foundation,AutoForm Engineering GmbH as Platinum and Dinner,et al.,ITOCHU Techno-Solutions Corporation,JSOL Corporatio

Preliminary Study on the Onset of Necking Detection Using DIT in Tensile Tests

The experimental detection of localized necking is an important issue in sheet metal forming. Today, the most common and extended techniques are strain-based methods using digital image correlation (DIC). The present work discuses a thermal methodology to detect the onset of necking in metals based on the analysis of the temperature gradient using digital infrared thermography (DIT). A series of tensile tests of H240LA-O3 high-strength steel of 1.2mm thickness is analysed using DIC and DIT techniques. It is proposed that necking initiates when the temperature difference at a reference distance from the necking point reaches a critical value, which allows identifying the necking time and estimating the limit strains from the visible images using circle grid analysis

Experimental Study on the Evaluation of Necking and Fracture Strains in Sheet Metal Forming Processes

In this paper the formability of AA2024-T3 metal sheets is experimentally analyzed. For this purpose, a series of StretchBending and Incremental Sheet Forming (ISF) tests are carried out. The former tests allow determine the formability limits through the evaluation of necking and fracture using the optical deformation measurement system ARAMIS® and measuring the thickness strains along the fracture line. The latter are performed with the aim of confirming the validity of these limits. In this case, the spifability, formability in Single Point Incremental Forming (SPIF), was studied in the light of circle grid analysis by means of the 3D deformation digital measurement system ARGUS®. Different punch diameters are used in both processes. The results exhibit the importance of the accuracy in the setting of the formability limits as well as the variability that these limits present depending on the forming process or some variables such as the tool radius.Ministerio de Ciencia e Innovación DPI 2009-1333

Optimization of hole-flanging by single point incremental forming in two stages

Special Issue of the Manufacturing Engineering Society (MES)Single point incremental forming (SPIF) has been demonstrated to accomplish current trends and requirements in industry. Recent studies have applied this technology to hole-flanging by performing different forming strategies using one or multiple stages. In this work, an optimization procedure is proposed to balance fabrication time and thickness distribution along the produced flange in a two-stage variant. A detailed analytical, numerical and experimental investigation is carried out to provide, evaluate and corroborate the optimal strategy. The methodology begins by analysing the single-stage process to understand the deformation and failure mechanisms. Accordingly, a parametric two-stage SPIF strategy is proposed and evaluated by an explicit Finite Element Analysis to find the optimal parameters. The study is focused on AA7075-O sheets with different pre-cut hole diameters and considering a variety of forming tool radii. The study exposes the relevant role of the tool radius in finding the optimal hole-flanging process by the proposed two-stage SPI