Modelling of ductile failure in metal forming

Damage and fracture are important criteria in the design of products and processes. Damage models can be used to predict ductile failure in metal forming processes. Nonlocal models avoid the mesh dependency problems of local damage models. A nonlocal damage model has been implemented in LSDYNA using the user-subroutines UMAT and UCTRL1. The implemented model will be compared with\ud results obtained with the available option in LS-DYNA to combine *MAT PLASTICITY WITH DAMAGE with *MAT NONLOCAL. Advantages and disadvantages of the different implementations will be discussed. The user nonlocal damage model has been applied to a bending and a blanking process. Results of these simulations will be shown

Edge Fracture Prediction Of Dual Phase Steels With Consideration Of Microstructures

In recent years, Advanced High Strength Steels (AHSS) have been used for the lightweight structural design and manufacturing in automotive industry. This class of sheet metals are prone to edge fracture during stamping production, and the fracture often occurs at much lower strain than that predicted based on the forming limit curves. The uncertainty in predicting edge fracture represents a great challenge in the application of AHSS. This dissertation is focused on the better understanding of edge fracture phenomenon through experimental observation and computer modeling with the consideration of microstructure effect. In this dissertation, Hole Expansion (HE) test was used to investigate the mechanisms of edge fracture of two AHSS, Dual Phase DP780 and DP980. The HE test includes two processing steps: hole punching and hole expanding. The punching process creates holes of various sheared edge morphologies, which are the input to the hole expanding process, along with the intrinsic property of material, produce a joint effect on the crack initiation and propagation on the sheared edges during HE test. The hole punching process was first investigated. The sheared edges were analyzed with image processing, and the relative heights of sheared four edge zones were measured, which were used to determine the strain distributions of sheared edges. The punching process was further simulated with FEA, and referenced with the experimental observations. Fracture mechanism of hole punching process was deduced. The process of crack initiation and propagation in hole expanding process was studied the next. Scanning Electron Microscope (SEM) was used to observe cracks on the sheared edges. FEA simulation was applied to investigate the effect of sheared edge geometry on the stress evolution in the hole expanding process. The interaction of two neighboring parallel cracks in the crack propagation process was analyzed to explain the effect of crack population on the main crack propagation. To investigate the microstructure effect on the edge fracture, the dual phase grain structure and its effect on mechanical properties and fracture behavior of DP steels were analyzed. Based on HE test results and observed DP steel grain structures, a criterion to predict edge fracture was proposed. This criterion includes mesoscale grain structure features of DP steels, and it can be applied to predict the HE test results of DP steels. The heterogeneous microstructures of DP steels were built into the Representative Volume Element (RVE) models to study the effects of microstructure parameters and fracture mechanisms on macroscale mechanical properties of DP steels, with damage based failure criteria applied to constituent ferritic and martensitic phases

Impulse-Based Manufacturing Technologies

In impulse-based manufacturing technologies, the energy required to form, join or cut components acts on the workpiece in a very short time and suddenly accelerates workpiece areas to very high velocities. The correspondingly high strain rates, together with inertia effects, affect the behavior of many materials, resulting in technological benefits such as improved formability, reduced localizing and springback, extended possibilities to produce high-quality multi material joints and burr-free cutting. This Special Issue of JMMP presents the current research findings, which focus on exploiting the full potential of these processes by providing a deeper understanding of the technology and the material behavior and detailed knowledge about the sophisticated process and equipment design. The range of processes that are considered covers electromagnetic forming, electrohydraulic forming, adiabatic cutting, forming by vaporizing foil actuators and other impulse-based manufacturing technologies. Papers show significant improvements in the aforementioned processes with regard to: Processes analysis; Measurement technique; Technology development; Materials and modelling; Tools and equipment; Industrial implementation

Hot semi-punching and cold scrap removing processes for hot stamping of ultra-high strength steel parts

Hot semi-punching and cold scrap removing processes for hot stamping of ultra-high strength steel parts were developed to eliminate laser cutting having low productivity after hot stamping. A quenchable steel sheet is semi-punched without separation of punching scraps during hot stamping, and subsequently, the scraps are removed from the hot-stamped part at room temperature. For hot semi-punching, additional channels for taking the punching scraps out of dies are not required. Minimum remainder without separation of punching scraps and no clearance between the die and punch were optimal for the hot semi-punching process. The hot punching and cold removing loads of the quenched sheet were considerably smaller than the cold punching load, the quality of the hole edge was high and the delayed fracture around the sheared edge was prevented. Hot semi-punching and cold scrap removal were applied to a hot hat-shaped bending process

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

Étude expérimentale et numérique des mécanismes d'endommagement ductile et rupture des bords découpés des aciers avancés pour l'automobile

The mechanical properties of automotive structures made of advanced high strength steels (AHSS) is often seen reduced by the presence of cut edges. Here this phenomenon is investigated for ferrite-bainite steel (FB600) and martensite ferrite steel (DP600), the latter having higher work hardening and phase hardness gradient than FB600.Damage micromechanisms for these two base materials were assessed using in situ synchrotron tomography, in situ SEM and SEM on cross sections. It was revealed for the DP600 steel that damage nucleated from particles and ferrite-martensite interfaces. In addition, needle shaped voids, that are consistent with the presence of segregation lines, were seen. For the FB steel, the same observations hold true except that the decohesion on interfaces sets in at higher strains. Quantitative image analysis also showed that the initial number of voids and the number of nucleating voids was higher for DP steel than for FB steel which was also seen to be more damage tolerant.Punched and machined edges made of DP600 and FB600 steel were mechanically loaded during in situ laminography testing. It was found that the fracture zone of the punched edge was rough and that needle-shape voids at the surface and in the bulk followed material flow lines. During mechanical in situ testing the needle voids grew from the fracture zone surface and coalesced with the sheared zone. In contrast, for the machined edge the damage started away from the edge (~ 800 microns) where substantial necking has occurred. Three-dimensional image analysis was performed to quantify the initial damage and its evolution. The FB600 was more resistant to cut edges than the DP600 steel.3D elasto-plastic FE calculations were carried out to investigate mechanical fields, potentially affected by the edge profile and pre-hardening profile. These parameters were not found to substantially modify the mechanical fields. Finally, axisymmetric 2D simulations for hole expansion were carried out for different sheet thicknesses using a post-treated damage evaluation calibrated on in-situ tomography data.La performance mécanique des pièces de structures automobiles fabriquées à partir de tôles d'acier à très haute résistance (THR) est souvent réduite à cause des bords découpés. Ce phénomène a été étudié pour deux nuances d'aciers ferrite-bainite (FB600) et ferrite-martensite (DP600), ce dernier présente un écrouissage et un gradient de dureté entre les phases plus élevés que ceux de la nuance FB600. Les micromécanismes d'endommagement de ces matériaux de base ont été caractérisés en utilisant les techniques de tomographie in situ et MEB in situ. Pour l'acier DP600, la germination de cavités a eu lieu sur les inclusions et aux interfaces ferrite-martensite. De plus, des cavités sous forme d'aiguille ont été observées dans la zone centrale correspondant à la ligne de ségrégation. Les mêmes mécanismes de germination ont été observés dans le cas de l'acier FB en plus de la germination aux interfaces des carbures qui a eu lieu à des déformations élevées. L'analyse d'image a montré que l'acier DP présente une densité initiale de cavités et une densité de cavités germées plus élevées que celles de l'acier FB qui semblait plus tolérant à l'endommagement. Des bords poinçonnés et usinés des nuances DP et FB ont été caractérisés par laminographie in situ lors d'un chargement mécanique. Pour les bords poinçonnés, ces observations ont permis de constater que la zone rompue est rugueuse et qu'un micro-endommagement sous forme d'aiguille initié sur la surface et dans le volume suit les lignes d'écoulement. Lors du chargement mécanique, les cavités sous forme d'aiguilles croissent à partir de la zone rompue et coalescent avec la zone cisaillée. En revanche, pour les bords usinés, l'endommagement s'initie loin de la surface de bords (~800 microns). Une analyse des données 3D a été réalisée pour quantifier l'état initial de l'endommagement et son évolution. L'acier FB600 a été plus résistant aux bords découpés que l'acier DP600. Des simulations 3D par éléments finis ont été menées pour étudier les champs mécaniques potentiellement affectés par le profil du bord découpé et du pré-écrouissage. Cette analyse a permis de conclure que seuls ces paramètres ne modifient pas localement les champs mécaniques. Finalement, des simulations axisymétriques par éléments finis de l'essai d'expansion de trou ont été réalisées pour différentes épaisseurs de tôle en utilisant les critères d'endommagement identifiés sur les résultats expérimentaux de la tomographie in situ

An Investigation into Orthogonal and Oblique Cutting Processes

The cutting process may be divided into two major types. The one type is called as straight angle cutting, where blade is in continuous contact with metal sheet from one side of edge to another side of the edge at the beginning of the cutting process. The another type is known as shearing, where the blade edge is oblique and progressively proceed from one edge of metal sheet to other end of metal sheet during cutting process. The present study has investigated straight angle cutting and shearing processes by using virtual environment of software ANSYS 19-R. An explicit dynamic FEM analysis was used in various simulations of punching and shearing investigations. The current study has two four parts. In first part, impact of attack angle variation on stress generated on tool and work piece has been investigated. Both punching and shearing processes have been simulated in software environment to understand and differentiate both cutting processes. In second part, straight angle tool was used to create straight angle cutting environment. Various cutting parameters including tool, metal sheet and cutting process parameters have been investigated for straight angle cutting. In third part, oblique angle tool was used to create shearing environment. Various cutting parameters including tool, metal sheet and cutting process parameters have been investigated for shearing. In fourth part, straight angle cutting and oblique angle cutting process has been compared to find out impact of various parameters on straight angle cutting and shearing and how these may be differentiated. The cutting process is complex and depends upon tool, metal sheet and cutting process parameters. Tool parameters investigated in present study include tool edge angle, tool attack angle, tool material and tool thickness. The metal sheet parameters affecting cutting process have been investigated in the present study and include metal sheet material and metal sheet thickness. Cutting process parameters investigated in present study include cutting process speed and friction between metal sheet and tool. Both punching and shear cutting simulations has been designed in present study by considering guillotining machine