Niko Manopulo

Zero Failure Production Methods Based on a Process Integrated Virtual Control

Although the virtual methods are nowadays fully established as a widely used tool in the planning and optimization of forming processes they are still completely omitted for a direct, “intelligent” process control in the later production of the parts. The paper presents a proposal for a Process-Integrated-Virtual-Control (PIVC) considering the real process perturbations induced by deviations of material properties as well as by further time dependent process parameters like the tool temperature. For the detection of the material deviations an in-line eddy-current measurement method and the appropriate evaluation method for the definition of the stochastic yield curves will be presented. The paper closes with a virtually taught control system modifying the blank-holder forces in dependency of thermal conditions and material deviations. The goal of this PIVC coupled to in-line process controls is to achieve a Zero Failure production even under alternative time dependent process conditions.

An ALE Based FE Formulation for the 3D Numerical Simulation of Fineblanking Processes

Fineblanking is a manufacturing process which allows the mass production of blanked products with superior surface quality. The 3D numerical simulation of this particularly precise process is however challenging. This is because quality-critical tool features such as the die clearance and the shape of the cutting edges have dimensions up to two orders of magnitude smaller than the average part dimensions. If conventional Updated Lagrange codes are used, a very high FE mesh resolution becomes a must in order to accurately represent the surface evolution along the edge, which in turn makes the computation unfeasible. The methodology presented in this paper makes use of the Arbitrary Lagrangian Eulerian FE Formulation in order to keep control over the mesh region in contact with the tools. This way an optimal FE mesh can be guaranteed throughout the computation. This not only reduces the computational cost considerably, but also avoids mesh distortion along the cutting edge, allowing an accurate representation of the tool features. This approach will be used in conjunction to the stress limit criterion delineated in order to predict material failure in fine blanked products. Numerical results will be validated against the experiments carried out with a specially designed fineblanking tool in use at our institute.

Failure Prediction in Fine Blanking Process with Stress Limit Model

Extremely small blanking clearance and nearly sharp edges of blanking tool are the characteristics of fine blanking that produces near net shape parts. The extreme forming conditions make the failure prediction for fine blanking more difficult than for ordinary forming processes. Stress Limit Criterion (SLC) is adopted in this work to perform the failure prediction of 3D fine blanking process. In comparison with the stress triaxiality diagram, SLC is not sensitively affected by complex nonlinear deformation paths and can perform the task as well. However, the parameters that support the model have to be obtained with combination of dedicatedly designed experiments and numerical simulation. The FEM simulation must also be able to provide stable and reliable results.

On the modelling of strength differential and anisotropy exhibited by titanium

Modelling the response of titanium alloys under plastic deformation is challenging. In addition to strong anisotropy, these materials exhibit tension-compression asymmetry as well as anisotropic hardening even under monotonous loading conditions. The present contribution aims to propose a new modelling approach for titanium materials, by a homogeneous introduction of asymmetry into existing symmetric yield functions. Furthermore, anisotropic hardening effects are tracked describing the strain dependant yield surface evolution. The results are validated using the earing profile of deep drawn cups.

Stretch bending - the plane within the sheet where strains reach the forming limit curve

Finite element analysis (FEA) was used to model the angular stretch bend test, where a strip of sheet metal is locked at both ends and a tool with a radius stretches and bends the center of the strip until failure. The FEA program used in the study was Abaqus. The FEA model was verified by experimental work using a dual phase steel (DP600) and with a simplified analytical analysis. The FEA model was used to simulate the experimental test for various frictional conditions and various radii of an upward moving tool. The primary objective of the study was to evaluate the concave-side rule, which states that during stretch bending the forming limit occurs when the strains on the concave surface plane of the bent sheet (i.e. bottom plane) reach the forming limit curve (FLC). The verification with experimental data indicates that the FEA model represents the process very well. Only conditions where failure occurred on or near the tooling are included in the results. The FEA simulations showed that the actual forming limit of the sheet occurs when the strains on the bottom plane of the sheet (i.e. concave side of the bend) reach the forming limit curve for high friction and small tool radii. For lower friction and for larger tool radii the actual forming limit occurs when strains on other planes in the sheet (i.e. mid planes or top surface plane) reach the forming limit curve. The implications of these results suggest that care must be taken in assessing forming operations when both stretch and bending occur. Although it is known that the FLC cannot predict the forming limit for small bend radii, the common assumption that the forming limit occurs when the strains for the middle thickness plane of the sheet reach the forming limit curve or that the concave side rule is often made. Understanding the limits of this assumption needs to be carefully and critically evaluated.

Analysis of strain paths of sheared edges during hole expansion tests

One of the limitations to the widespread use of advanced high strength steel (AHSS) sheets is the cracking of sheared edges during subsequent stretching operations, as nearly all stamped parts are sheared prior to sheet forming. Cracking at the edge occurs at levels below the conventional forming limit criteria. Understanding the strain path of the sheared edge during a hole expansion test should provide insight into the strain path of a sheared edge when it is stretched during production. As a result, experimental as well as finite element simulations are used for analyzing the strain path behavior of a sheared edge during hole expansion tests. The shearing process changes the global behavior in the sheet during a hole expansion, and the finite element results indicate that the strain paths for points near the edge of the hole during expansion are non-linear due to the presence of the shear affected zone (SAZ). These results are consistent with previously measured experimental values for the strain path.

Combination of the strain dependent Yld2000 model with an extended HAH model

In addition to the initial anisotropy, materials used in sheet metal usually show anisotropic hardening effects. These are mainly direction dependent hardening, the Bauschinger effect as well as latent hardening and softening effects. To take into account all these different effects, a strain dependent Yld2000 model, which is able to describe the direction dependent hardening is combined with an extended version of the HAH model, which describes the Bauschinger and the latent effects. The model is implemented in LS-Dyna as a user-defined material subroutine. The present work gives an overview and shows the advantages of this combined model by means of stress responses to prescribed deformation paths.

On the role of Anisotropy and Bauschinger-Effect in Sheet Metal Spinning

Incremental sheet forming is known for its local forming character where the material is continuously bent and unbent in a multitude of tool passes. By this nature, anisotropy and Bauschinger-Effect might play a significant role in numerical modelling of the process. This paper aims to assess different material modelling techniques by comparing the resulting stress and strain histories of the FEM simulations. Amongst these are Barlats nonquadratic yield locus Yld2000-2d yield criterion and the homogeneous anisotropic hardening model.

Numerical Tool Path Optimization for Conventional Sheet Metal Spinning Processes

To this day, conventional sheet metal spinning processes are designed with a very low degree of automation. They are usually executed by experienced personnel, who actively adjust the tool paths during production. The practically unlimited freedom in designing the tool paths enables the efficient manufacturing of complex geometries on one hand, but is challenging to translate into a standardized procedure on the other. The present study aims to propose a systematic methodology, based on a 3D FEM model combined with a numerical optimization strategy, in order to design tool paths. The accurate numerical modelling of the spinning process is firstly discussed, followed by an analysis of appropriate objective functions and constraints required to obtain a failure free tool path design.

Numerical Heat Treatment Modelling of Fine Blanked Sheet Metal and Experimental Validation

Heat treatment is one of the major sources of dimensional inaccuracy in the manufacturing of fine blanked parts. Tools and equipment often need to be iteratively corrected in order to achieve the desired quality. Numerical simulation of the heat treatment process can substantially reduce these efforts. The simulation accuracy on the other hand is strongly dependent on the accurate characterization of the thermo-mechanical boundary conditions as well as material properties. The present contribution aims to propose a novel approach in the calibration of numerical models by using a modified Jominy test as well as heat treatment experiments with parts having residual stresses from a bending process. The results are validated by comparing numerical phase content and hardness values with the corresponding experiments.