Dimpling process in cold roll metal forming by finite element modelling and experimental validation

The dimpling process is a novel cold-roll forming process that involves dimpling of a rolled flat strip prior to the roll forming operation. This is a process undertaken to enhance the material properties and subsequent products’ structural performance while maintaining a minimum strip thickness. In order to understand the complex and interrelated nonlinear changes in contact, geometry and material properties that occur in the process, it is necessary to accurately simulate the process and validate through physical tests. In this paper, 3D non-linear finite element analysis was employed to simulate the dimpling process and mechanical testing of the subsequent dimpled sheets, in which the dimple geometry and material properties data were directly transferred from the dimpling process. Physical measurements, tensile and bending tests on dimpled sheet steel were conducted to evaluate the simulation results. Simulation of the dimpling process identified the amount of non-uniform plastic strain introduced and the manner in which this was distributed through the sheet. The plastic strain resulted in strain hardening which could correlate to the increase in the strength of the dimpled steel when compared to plain steel originating from the same coil material. A parametric study revealed that the amount of plastic strain depends upon on the process parameters such as friction and overlapping gap between the two forming rolls. The results derived from simulations of the tensile and bending tests were in good agreement with the experimental ones. The validation indicates that the finite element analysis was able to successfully simulate the dimpling process and mechanical properties of the subsequent dimpled steel products

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

Review on the influence of process parameters in incremental sheet forming

Incremental sheet forming (ISF) is a relatively new flexible forming process. ISF has excellent adaptability to conventional milling machines and requires minimum use of complex tooling, dies and forming press, which makes the process cost-effective and easy to automate for various applications. In the past two decades, extensive research on ISF has resulted in significant advances being made in fundamental understanding and development of new processing and tooling solutions. However, ISF has yet to be fully implemented to mainstream high-value manufacturing industries due to a number of technical challenges, all of which are directly related to ISF process parameters. This paper aims to provide a detailed review of the current state-of-the-art of ISF processes in terms of its technological capabilities and specific limitations with discussions on the ISF process parameters and their effects on ISF processes. Particular attention is given to the ISF process parameters on the formability, deformation and failure mechanics, springback and accuracy and surface roughness. This leads to a number of recommendations that are considered essential for future research effort

Finite element modelling of material deformation and damage by tension under cyclic bending and compression test

Material deformation is determined by strain and stress states resulted from loading conditions applied on the material during the manufacturing process. Different testing methods, for example, uniaxial tensile test and dome test have been used to predict material deformation behavior during the manufacturing processes. However, under a complex deformation mode, materials display distinct deformation behavior. In double side incremental forming (DSIF) process, it has been widely acknowledged that the material deformation consists of stretching, bending, shearing, compression with cyclic loading. This leads to a significant material formability enhancement comparing to conventional sheet metal forming processes. This phenomenon cannot be explained by using the currently available testing methods because the complexity of the DSIF process prohibits a direct investigation of the influence of individual deformation modes. To simplify the loading conditions and to investigate their individual and interactive effects contributing to the formability enhancement in DSIF, in this study, a novel testing method of Tension under Cyclic Bending and Compression (TCBC) is proposed, through which the effect of stretching, bending, compression and cyclic loading can be independently evaluated. A finite element (FE) damage modelling of the TCBC test was developed by incorporating the shear-modified Gurson-Tvergaard-Needleman (GTN) model into the Abaqus/Explicit solver. The results showed that the damage accumulation in the material was suppressed due to the localized and cyclic material deformation. An enhanced material formability was obtained by using the FE damage modelling and the periodical accumulation of the damage showed that the TCBC test could be a possible representation of the material deformation in DSIF

Analysis and modelling of a rotary forming process for cast aluminum alloy A356

Spinning of a common aluminum automotive casting alloy A356 (Al-7Si-0.3 Mg) at elevated temperatures has been investigated experimentally with a novel industrial-scale apparatus. This has permitted the implementation of a fully coupled thermomechanical finite element model aimed at quantifying the processing history (stress, strain, strain-rate and temperature) and predicting the final geometry. The geometric predictions of this model have been compared directly to the geometry of the workpieces obtained experimentally. This study is novel in regards to both the size and shape of the component as well as the constitutive material representation employed. The model predictions are in reasonable agreement with experimental results for small deformations, but errors increase for large deformation conditions. The model has also enabled the characterization of the mechanical state which leads to a common spinning defect. Suggestions for improving the accuracy and robustness of the model to provide a predictive tool for industry are discussed

Tube and Sheet Metal Forming Processes and Applications

At present, the manufacturing industry is focused on the production of lighter weight components with better mechanical properties and always fulfilling all the environmental requirements. These challenges have caused a need for developing manufacturing processes in general, including obviously those devoted in particular to the development of thin-walled metallic shapes, as is the case with tubular and sheet metal parts and devices.This Special Issue is thus devoted to research in the fields of sheet metal forming and tube forming, and their applications, including both experimental and numerical approaches and using a variety of scientific and technological tools, such as forming limit diagrams (FLDs), analysis on formability and failure, strain analysis based on circle grids or digital image correlation (DIC), and finite element analysis (FEA), among others.In this context, we are pleased to present this Special Issue dealing with recent studies in the field of tube and sheet metal forming processes and their main applications within different high-tech industries, such as the aerospace, automotive, or medical sectors, among others

Cracking in asphalt materials

This chapter provides a comprehensive review of both laboratory characterization and modelling of bulk material fracture in asphalt mixtures. For the purpose of organization, this chapter is divided into a section on laboratory tests and a section on models. The laboratory characterization section is further subdivided on the basis of predominant loading conditions (monotonic vs. cyclic). The section on constitutive models is subdivided into two sections, the first one containing fracture mechanics based models for crack initiation and propagation that do not include material degradation due to cyclic loading conditions. The second section discusses phenomenological models that have been developed for crack growth through the use of dissipated energy and damage accumulation concepts. These latter models have the capability to simulate degradation of material capacity upon exceeding a threshold number of loading cycles.Peer ReviewedPostprint (author's final draft

Investigation on a new hole-flanging approach by incremental sheet forming through a featured tool

One of the major challenges in conventional incremental sheet forming (ISF) is the extreme sheet thinning resulted in an uneven thickness distribution of formed part. This is also the case for incrementally formed parts with hole-flanging features. To overcome this problem, a new ISF based hole-flanging processing method is proposed by developing a new ISF flanging tool. Comparative studies are conducted by performing hole-flanging tests using both ISF conventional ball-nose tool and the new flanging tool to evaluate the sheet deformation behavior and the quality of the final part. Stress distribution and strain variation are investigated by analytical approach and numerical simulation. Experiments have been conducted to validate the analytical model and simulation results, and to further study the fracture behavior. Results show that the new flanging tool generates greater meridional bending than stretching deformation in conventional ISF. The combination of bending-dominated deformation mode with localized deformation of ISF ensures more uniform thickness distribution on hole-flanging part with better resistance to fracture

Double-sided incremental forming: a review

Incremental sheet forming (ISF) or single point incremental forming (SPIF) processes have been developed rapidly in the past three decades. Its high flexibility and easy operability have a significant appeal for industrial applications and substantial progress has been made in fundamental understanding and demonstration of practical implementation. However, there are a number of obstacles including achievable accuracy and instability in material deformation which are considered as a main contributing factor for preventing the ISF process to be widely used in industry. As a variant of the general ISF process, Double-Sided Incremental Forming (DSIF) uses an additional supporting tool in the opposite side of the workpiece, maintains the flexibility and at the same time improves the material deformation stability and reduces material thinning. In recent years, there has been increased research interest in looking into DSIF specific material deformation mechanisms and DISF investigation. This paper aims to provide a technical review of the DSIF process as benchmarked with SPIF. It starts with a brief overview of the current state of the art of both SPIF and DSIF. This is followed by a comparative study between SPIF and DSIF with the key research challenges identified. This leads to a recommendation of key research challenges for DSIF focused research

King post truss as a motif for internal structure of (meta)material with controlled elastic properties

One of the most interesting challenges in the modern theory of materials consists in the determination of those microstructures which produce, at the macro-level, a class of metamaterials whose elastic range is many orders of magnitude wider than the one exhibited by ‘standard’ materials. In Dell’Isola et al. (2015 Zeitschrift für angewandte Mathematik und Physik 66, 3473- 3498. (doi: 10.1007/s00033-015-0556-4)), it was proved that, with a pantographic microstructure constituted by ‘long’ microbeams it is possible to obtainmetamaterials whose elastic range spans up to an elongation exceeding 30%. In this paper, we demonstrate that the same behaviour can be obtained bymeans of an internal microstructure based on a king post motif. This solution shows many advantages: it involves only microbeams; all constituting beams are undergoing only extension or compression; all internal constraints are terminal pivots. While the elastic deformation energy can be determined as easily as in the case of long-beam microstructure, the proposed design seems to have obvious remarkable advantages: it seems to be more damage resistant and therefore to be able to have a wider elastic range; it can be realized with the same three-dimensional printing technology; it seems to be less subject to compression buckling. The analysis which we present here includes: (i) the determination of Hencky-type discrete models for king post trusses, (ii) the application of an effective integration scheme to a class of relevant deformation tests for the proposed metamaterial and (iii) the numerical determination of an equivalent second gradient continuum model. The numerical tools which we have developed and which are presented here can be readily used to develop an extensive measurement campaign for the proposed metamaterial