Evaluation of common tests for fracture characterisation of advanced high-strength sheet steels with the help of the FEA

The paper presents results of evaluation of common tests for fracture characterization of advanced high-strength sheet steels with the help of the FEA. The tests include three in-plane shear tests, two uniaxial tension tests, two plane strain tension tests and two equibiaxial tension tests. Three high-strength steels with different yield loci, strain hardening rates and strengths in three different thicknesses each were used. The evaluation was performed based on the spatial distribution of the equivalent plastic strain and damage variable in the specimen at the moment of crack initiation as well as on the time variation of the stress state at the crack initiation location. For in-plane shear, uniaxial tension and plane strain tension, no test can be unconditionally recommended as disadvantages of all studied tests in these groups cannot be neglected. However, in each of these groups, a test can be chosen, which represents an acceptable compromise between its advantages and disadvantages: the shear test on an IFUM butterfly specimen for in-plane shear, the tensile test on a holed specimen for uniaxial tension and the tensile test on a waisted specimen for plane strain tension. On the contrary, the bulge test on a circular specimen with a punch of Ø 100 mm can be unconditionally recommended for equibiaxial tension. In the future, optimisation of the studied tests for in-plane shear, uniaxial tension and plane strain tension appears to be necessary.FOST

Advanced Wear Simulation for Bulk Metal Forming Processes

In the recent decades the finite element method has become an essential tool for the cost-efficient virtual process design in the metal forming sector in order to counter the constantly increasing quality standards, particularly from the automotive industry as well as intensified international competition in the forging industry. An optimized process design taking precise tool wear prediction into account is a way to increase the cost-efficiency of the bulk metal forming processes. The main objective of the work presented in this paper is a modelling algorithm, which allows predicting die wear with respect to a geometry update during the forming simulation. Changes in the contact area caused by geometry update lead to the different die wear distribution. It primarily concerns the die areas, which undergo high thermal and mechanical loads

Numerical analysis of a deep drawing process with additional force transmission for an extension of the process limits

By sheet metal forming processes the forming limits and part characteristics are defined through the process specific loads. In deep drawing processes the maximum deep draw ratios as well as the springback behaviour of the metal parts are depending on the stress distribution in the part material during the forming process. While exceeding the load limits, a failure in the material occurs, which can be avoided by additional force transmission activated in the deep drawing process before the forming limit of material is achieved. This contribution deals with numerical investigation of process effect caused by additional force transmission regarding the extension of the process limits. Here, the steel material HCT 600X+Z (1.0941) in thickness s 0 = 1.0 mm is analyzed numerically using the anisotropic model Hill48. This model is validated by the means of cup test by Swift. Both, the FEA of conventional and forming process with additional force transmission are carried out. The numerical results are compared with reference geometry of rectangle cup.DFG/BE169/139-

Implementation of the Bai and Wierzbicki fracture criterion in QForm and its application for cold metal forming and deep drawing technology

The paper presents implementation of fracture prediction algorithm for a cold metal forming process simulation in the software QForm. Authors programmed the function for calculation of the criterion proposed by Bai and Wierzbicki. Obtained results of the simulation in QForm for a deep drawing process are compared with results of experiments. A good agreement between the simulation and experiment was achieved.Ministry of Education and Science of the Russian FederationDAA

Experimental-numerical evaluation of a new butterfly specimen for fracture characterisation of AHSS in a wide range of stress states

Results of an experimental-numerical evaluation of a new butterfly specimen for fracture characterisation of AHHS sheets in a wide range of stress states are presented. The test on the new butterfly specimen is performed in a uniaxial tensile machine and provides sufficient data for calibration of common fracture models. In the first part, results of a numerical specimen evaluation are presented, which was performed with a material model of a dual-phase steel DP600 taken from literature with plastic flow and fracture descriptions. In the second part, results of an experimental-numerical specimen evaluation are shown, which was conducted on another dual-phase steel DP600, which was available with a description of plastic flow only and whose fracture behaviour was characterised in the frame of this work. The overall performance of the new butterfly specimen at different load cases with regard to characterisation of the fracture behaviour of AHSS was investigated. The dependency of the fracture strain on the stress triaxiality and Lode angle as well as space resolution is quantified. A parametrised CrachFEM ductile shear fracture model and modified Mohr-Coloumb ductile shear fracture model are presented as a result of this quantification. The test procedure and results analysis are believed to contribute to current discussions on requirements to AHSS fracture characterisation

Numerical and experimental determination of cut-edge after blanking of thin steel sheet of DP1000 within use of stress based damage model

The proposed study focuses on blanking of thin steel sheets of Dogal1000DP +Z100MBO. Numerical and experimental investigations of the influence of clearance and punch speed on the cutting force and the geometry of the sheared edge were done. Tensile and stack compression test at elevated temperatures has been chosen to determine the flow and fracture behavior of Dogal1000DP +Z100MBO at different stress states. It is shown that the flow curve determined by stack compression test leads to better results in force - displacement prediction of a blanking process compared to determination of flow curve by tensile test. Stress based fracture criterion were chosen to describe damage behaviour. Moreover significant influence of fracture locus for negative stress triaxialities on the geometry of the numerically predicted sheared edge is shown

Material Modelling of Short Fiber Reinforced Thermoplastic for the FEA of a Clinching Test

In modern car body construction, multi-material and hybrid design is used, whereby short fibre reinforced plastics combined with light metals represent an interesting class of work-piece materials. In order to realize modern hybrid construction, suitable joining techniques are therefore required. Clinching represents a cost-effective and easy to implement joining method. In this paper the material modelling of the short fibre reinforced thermoplastic sheets considering the fibre orientation tensor for the FEA of the clinching process is presented

FEA-based optimisation of a clinching process with an open multiple-part die aimed at damage minimisation in CR240BH-AlSi10MnMg joints

The paper presents results of a FEA-based tool design optimisation for a clinching process with an open multiple-part die. The studied materials are the bake-hardening steel CR240BH (1.5 mm) on the punch side and the die-cast aluminium alloy AlSi10MnMg (2.95 mm) on the die side. The objective of the optimisation was to minimise probability of cracks that appear at the outer circumference of the bottom of the clinch joint in AlSi10MnMg. In the framework of the optimisation, it was possible to minimise crack probability by varying geometrical parameters of the tools though at the cost of slightly worse but still tolerable geometrical parameters of the clinch joint.German Federation of Industrial Research Associations (AiF), CologneGerman Federal Ministry of Economic Affairs and Energy (BMWi)Volkswagen AGBTM Blechverbindungstechnik Gmb

Properties and Application of High-manganese TWIP-steels in Sheet Metal Forming

AbstractWithin this work uniaxial tensile tests have been performed with high-manganese TWIP-steel and different dual-phase steels to determine mechanical properties. The transfer of the results form uniaxial tensile tests to multi-axial stresses has been made with deep drawing experiments to describe and assess deep and stretch formability of the analysed materials. Forming limits of materials are demonstrated by forming limit diagrams. FE-simulation systems have been applied to predict deep drawing and spring-back behaviour of high-manganese TWIP in comparison to dual-phase steels. The simulation results are discussed between the different materials

Advanced Wear Simulation for Bulk Metal Forming Processes

In the recent decades the finite element method has become an essential tool for the cost-efficient virtual process design in the metal forming sector in order to counter the constantly increasing quality standards, particularly from the automotive industry as well as intensified international competition in the forging industry. An optimized process design taking precise tool wear prediction into account is a way to increase the cost-efficiency of the bulk metal forming processes. The main objective of the work presented in this paper is a modelling algorithm, which allows predicting die wear with respect to a geometry update during the forming simulation. Changes in the contact area caused by geometry update lead to the different die wear distribution. It primarily concerns the die areas, which undergo high thermal and mechanical loads