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A01 - Joinability prognosis

The application of mechanical joining technology offers the possibility of joining mixed structures with a wide range of specifications and material-geometry combinations. Due to the limited transparency of the processes externally, the high number of combinable tool variants as well as variable force- and stroke-based process parameters, a comprehensive validation of the joinability in versatile process chains along the process steps is not possible with a conventional, experiment-based approach.

The vision of the subproject is therefore the development of methods for the holistic prediction of the joining safety, joining suitability and joining possibility along the versatile process chain. For the necessary comprehensiveness with regard to the joining part materials, joining processes and operating conditions, in addition to the development of methods, already developed methods of other subprojects of the TRR are integrated in TP A01. Combined methods from experimental test methods and simulation models, which describe the joint behavior on the basis of the material behavior, will be developed in order to open up fundamental operating principles and to enable comprehensive prediction on the basis of simulation. Finally, the methods will be used to develop targeted flexibilization and robustification measures for versatile process chains against the varying input variables.

In TP A01, a simulation strategy is being developed that virtually maps the process steps of joining part production, joining process and loading phase and allows the relevant state variables, such as geometry, material hardening and damage, to be transferred for a continuous simulation of mechanical joinability. The simulation model is to be used in an iterative process to investigate the usefulness and sensitivity of the transferred state variables with regard to the overall process and to the respective subsequent process steps. For the validation of the simulation models, the entire process chain will be reproduced experimentally in detail, with geometry and material condition being determined as locally as possible after each process step, as well as the process variables. This requires the development of new test methods for characterizing the frictional properties and damage behavior of joining part materials under the complex boundary conditions of the process chain. In addition, suitable specimen forms must be developed which can run through the entire process chain and allow targeted adjustment of the material condition during joining part production in the area of the joining zone as well as combined loading conditions.

Publications


Open list in Research Information System

Numerical analysis of the robustness of clinching process considering the pre-forming of the parts

C.R. Bielak, M. Böhnke, R. Beck, M. Bobbert, G. Meschut, Journal of Advanced Joining Processes. (2020)

DOI



Development of a Method for the Identification of Friction Coefficients in Sheet Metal Materials for the Numerical Simulation of Clinching Processes

M.S. Rossel, M. Böhnke, C.R. Bielak, M. Bobbert, G. Meschut, in: Sheet Metal 2021, Trans Tech Publications Ltd, 2021, pp. 81-88

In order to reduce the fuel consumption and consequently the greenhouse emissions, the automotive industry is implementing lightweight constructions in the body in white production. As a result, the use of aluminum alloys is continuously increasing. Due to poor weldability of aluminum in combination with other materials, mechanical joining technologies like clinching are increasingly used. In order to predict relevant characteristics of clinched joints and to ensure the reliability of the process, it is simulated numerically during product development processes. In this regard the predictive accuracy of the simulated process highly depends on the implemented friction model. In particular, the frictional behavior between the sheet metals affects the geometrical formation of the clinched joint significantly. This paper presents a testing method, which enables to determine the frictional coefficients between sheet metal materials for the simulation of clinching processes. For this purpose, the correlation of interface pressure and the relative velocity between aluminum sheets in clinching processes is investigated using numerical simulation. Furthermore, the developed testing method focuses on the specimen geometry as well as the reproduction of the occurring friction conditions between two sheet metal materials in clinching processes. Based on a methodical approach the test setup is explained and the functionality of the method is proven by experimental tests using sheet metal material EN AW6014.


Concept development of a method for identifying friction coefficients for the numerical simulation of clinching processes

M. Böhnke, M.S. Rossel, C.R. Bielak, M. Bobbert, G. Meschut, The International Journal of Advanced Manufacturing Technology (2021)

<jats:title>Abstract</jats:title><jats:p>In order to reduce fuel consumption and thus pollutant emissions, the automotive industry is increasingly developing lightweight construction concepts that are accompanied by an increasing usage of aluminum materials. Due to poor weldability of aluminum in combination with other materials, mechanical joining methods such as clinching were developed and established in series production. In order to predict the relevant characteristics of clinched joints and to ensure the reliability of the process, it is simulated numerically during product development processes. In this regard, the predictive accuracy of the simulated process highly depends on the implemented friction model. In particular, the frictional behavior between the sheet metals as well as between the sheet metal and clinching tools has a significant impact on the geometrical formation of the clinched joint. No testing methods exist that can sufficiently investigate the frictional behavior in sheet materials, especially under high interface pressures, different relative velocities, and long friction paths, while allowing a decoupled consideration of the test parameters. This paper describes the development of further testing concepts based on a proven tribo-torsion test method for determining friction coefficients between sheet metal materials for the simulation of clinching processes. For this purpose, the correlation of interface pressure and the relative velocity between aluminum and steel sheet material in clinching processes is investigated using numerical simulation. Based on these findings, the developed concepts focus on determining friction coefficients at interface pressures of the above materials, yield stress, as well as the reproduction of the occurring friction conditions between sheet metal materials and tool surfaces in clinching processes using tool substitutes. Furthermore, wear investigations between sheet metal material and tool surface were carried out in the friction tests with subsequent EDX analyses of the frictioned tool surfaces. The developed method also allows an optical deformation measurement of the sheet metal material specimen by means of digital image correlation (DIC). Based on a methodological approach, the test setups and the test systems used are explained, and the functionality of the concepts is proven by experimental tests using different sheet metal materials.</jats:p>


Influence of various procedures for the determination of flow curves on the predictive accuracy of numerical simulations for mechanical joining processes

M. Böhnke, F. Kappe, M. Bobbert, G. Meschut, Materials Testing (2021), 63(6), pp. 493-500

The predictive quality of numerical simulations for mechanical joining processes depends on the implemented material model, especially regarding the plasticity of the joining parts. Therefore, experimental material characterization processes are conducted to determine the material properties of sheet metal and generate flow curves. In this regard, there are a number of procedures which are accompanied by varying experimental efforts. This paper presents various methods of determining flow curves for HCT590X as well as EN AW-6014, including varying specimen geometries and diverse hardening laws for extrapolation procedures. The flow curves thus generated are compared considering the variety of plastic strains occurring in mechanical joining processes. The material data generated are implemented in simulation models for the joining technologies, clinching and self-piercing riveting. The influence of the varied methods on the predictive accuracy of the simulation model is analysed. The evaluation of the differing flow curves is achieved by comparing the geometric formation of the joints and the required joining forces of the processes with experimentally investigated joints.




Numerical investigation of the clinched joint loadings considering the initial pre-strain in the joining area

S. Martin, C.R. Bielak, M. Bobbert, T. Tröster, G. Meschut, Production Engineering (2022)

<jats:title>Abstract</jats:title><jats:p>The components of a body in white consist of many individual thin-walled sheet metal parts, which usually are manufactured in deep-drawing processes. In general, the conditions in a deep-drawing process change due to changing tribology conditions, varying degrees of spring back, or scattering material properties in the sheet blanks, which affects the resulting pre-strain. Mechanical joining processes, especially clinching, are influenced by these process-related pre-strains. The final geometric shape of a clinched joint is affected to a significant level by the prior material deformation when joining with constant process parameters. That leads to a change in the stiffness and force transmission in the clinched joint due to the different geometric dimensions, such as interlock, neck thickness and bottom thickness, which directly affect the load bearing capacity. Here, the influence of the pre-straining in the deep drawing process on the force distribution in clinch points in an automotive assembly is investigated by finite-element models numerically. In further studies, the results are implemented in an optimization tool for designing clinched components. The methodology starts with a pre-straining of metal sheets. This step is followed by 2D rotationally symmetric forming simulations of the joining process. The resulting mesh of each forming simulation is rotated and 3D models are obtained. The clinched joint solid model with pre-strains is used further to determine the joint stiffnesses. With the simulation of the same test set-up with an equivalent point-connector model, the equivalent stiffness for each pre-strain combination is determined. Simulations are performed on a clinched component to assess the influence of pre-strain and sheet thinning on the clinched joint loadings by using the equivalent stiffnesses. The investigations clearly show that for the selected component, the loadings at the clinch points are dependent on the sheet thinning and the stiffnesses due to pre-strain. The magnitude of the influence varies depending on the quantity considered. For example, the shear force is more sensitive to the joint stiffness than to the sheet thinning.</jats:p>


Functionality Study of an Optical Measurement Concept for Local Force Signal Determination in High Strain Rate Tensile Tests

M. Böhnke, E. Unruh, S. Sell, M. Bobbert, D. Hein, G. Meschut, Key Engineering Materials (2022), 926, pp. 1564-1572

<jats:p>Many mechanical material properties show a dependence on the strain rate, e.g. yield stress or elongation at fracture. The quantitative description of the material behavior under dynamic loading is of major importance for the evaluation of crash safety. This is carried out using numerical methods and requires characteristic values for the materials used. For the standardized determination of dynamic characteristic values in sheet metal materials, tensile tests performed according to the guideline from [1]. A particular challenge in dynamic tensile tests is the force measurement during the test. For this purpose, strain gauges are attached on each specimen, wired to the measuring equipment and calibrated. This is a common way to determine a force signal that is as low in vibration and as free of bending moments as possible. The preparation effort for the used strain gauges are enormous. For these reasons, an optical method to determine the force by strain measurement using DIC is presented. The experiments are carried out on a high speed tensile testing system. In combioantion with a 3D DIC high speed system for optical strain measurement. The elastic deformation of the specimen in the dynamometric section is measured using strain gauges and the optical method. The measured signals are then compared to validate the presented method. The investigations are conducted using the dual phase steel material HCT590X and the aluminum material EN AW-6014 T4. Strain rates of up to 240 s-1 are investigated.</jats:p>


Numerical investigation of a friction test to determine the friction coefficients for the clinching process

C.R. Bielak, M. Böhnke, M. Bobbert, G. Meschut, Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications (2022), 146442072210934

<jats:p> Clinching as a mechanical joining process has become established in many areas of car body. In order to predict relevant properties of clinched joints and to ensure the reliability of the process, it is numerically simulated during the product development process. The prediction accuracy of the simulated process depends on the implemented friction model. Therefore, a new method for determining friction coefficients in sheet metal materials was developed and tested. The aim of this study is the numerical investigation of this experimental method by means of FE simulation. The experimental setup is modelled in a 3D numerical simulation taking into account the process parameters varying in the experiment, such as geometric properties, contact pressure and contact velocity. Furthermore, the contact description of the model is calibrated via the experimentally determined friction coefficients according to clinch-relevant parameter space. It is shown that the assumptions made in the determination of the experimental data in preliminary work are valid. In addition, it is investigated to what extent the standard Coulomb friction model in the FEM can reproduce the results of the experimental method. </jats:p>


Development of a Modified Punch Test for Investigating the Failure Behavior in Sheet Metal Materials

M. Böhnke, C.R. Bielak, M. Bobbert, G. Meschut, in: The Minerals, Metals &amp; Materials Series, Springer International Publishing, 2022

DOI


Influence of heat treatment on the suitability for clinching of the aluminium casting alloy AlSi9

M. Neuser, M. Böhnke, O. Grydin, M. Bobbert, M. Schaper, G. Meschut, Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications (2022), 146442072210758

<jats:p> In many manufacturing areas, multi-material designs are implemented in which individual components are joined together to form complex structures with numerous joints. For example, in the automotive sector, cast components are used at the junctions of the body and joined with different types of sheet metal and extruded profiles. To be able to join structures consisting of different materials, alternative joining technologies have emerged in recent years. This includes clinching, which allows assembling of two or more thin sheet metal and casting parts by solely cold forming the material. Clinching the brittle and usually less ductile cast aluminium alloys remains a challenge because the brittle character of the cast aluminium alloys can cause cracks during the forming of the clinched joint. In this study, the influence of the heat treatment time of an aluminium casting alloy AlSi9 on the joinability in the clinching process is investigated. Specific heat treatment of the naturally hard AlSi9 leads to a modification of the eutectic microstructure, which can increase ductility. Based on this, it will be examined if specific clinching die geometries can be used, which achieve an optimized geometrical formation of the clinched joint. The load-bearing capacities of the clinched joints are determined and compared by shear tensile and head tensile tests. Furthermore, the joints are examined microscopically to investigate the influence of the heat treatment on the failure behaviour during the load-bearing tests as well as crack initiation within the joining process. </jats:p>


Provision of cross-domain knowledge in mechanical joining using ontologies

C. Zirngibl, P. Kügler, J. Popp, C.R. Bielak, M. Bobbert, D. Drummer, G. Meschut, S. Wartzack, B. Schleich, Production Engineering (2022)

Since the application of mechanical joining methods, such as clinching or riveting, offers a robust solution for the generation of advanced multi-material connections, the use in the field of lightweight designs (e.g. automotive industry) is steadily increasing. Therefore, not only the design of an individual joint is required but also the dimensioning of the entire joining connection is crucial. However, in comparison to thermal joining techniques, such as spot welding, the evaluation of the joints’ resistance against defined requirements (e.g. types of load, minimal amount of load cycles) mainly relies on the consideration of expert knowledge, a few design principles and a small amount of experimental data. Since this generally implies the involvement of several domains, such as the material characterization or the part design, a tremendous amount of data and knowledge is separately generated for a certain dimensioning process. Nevertheless, the lack of formalization and standardization in representing the gained knowledge leads to a difficult and inconsistent reuse, sharing or searching of already existing information. Thus, this contribution presents a specific ontology for the provision of cross-domain knowledge about mechanical joining processes and highlights two potential use cases of this ontology in the design of clinched and pin joints.</jats:p>


Numerical and experimental identification of fatigue crack initiation sites in clinched joints

L. Ewenz, C.R. Bielak, M. Otroshi, M. Bobbert, G. Meschut, M. Zimmermann, Production Engineering (2022), 16(2-3), pp. 305-313

<jats:title>Abstract</jats:title><jats:p>In this paper, a study based on experimental and numerical simulations is performed to analyze fatigue cracks in clinched joints. An experimental investigation is conducted to determine the failure modes of clinched joints under cyclic loading at different load amplitudes with single-lap shear tests. In addition, numerical FEM simulations of clinching process and subsequent shear loading are performed to support the experimental investigations by analyzing the state of stresses at the location of failure. An attempt is made to explain the location of crack initiation in the experiments using evaluation variables such as contact shear stress and maximum principal stress.</jats:p>


A Review on the Modeling of the Clinching Process Chain - Part I: Design Phase

B. Schramm, S. Martin, C. Steinfelder, C.R. Bielak, A. Brosius, G. Meschut, T. Tröster, T. Wallmersperger, J. Mergheim, Journal of Advanced Joining Processes (2022), 6, 100133

DOI


A Review on the Modeling of the Clinching Process Chain - Part II: Joining Process

B. Schramm, J. Friedlein, B. Gröger, C.R. Bielak, M. Bobbert, M. Gude, G. Meschut, T. Wallmersperger, J. Mergheim, Journal of Advanced Joining Processes (2022), 100134

DOI


Open list in Research Information System

Contact

Prof. Dr.-Ing. Gerson Meschut

Transregional Collaborative Research Centre 285

Teilprojekte A01, C02, Z

Gerson Meschut
Phone:
+49 5251 60 3031
Fax:
+49 5251 60 3239

Contact

M. Sc. Christian Roman Bielak

Transregional Collaborative Research Centre 285

Teilprojekt A01

Christian Roman Bielak
Phone:
+49 5251 60 3811
Office:
P1.3.22.1

Contact

M. Sc. Max Böhnke

Transregional Collaborative Research Centre 285

Teilprojekt A01

Max Böhnke
Phone:
+49 5251 60 2661
Office:
P1.4.16.2

Contact

M. Sc. Julian Vorderbrüggen

Transregional Collaborative Research Centre 285

Teilprojekt A01

Julian Vorderbrüggen
Phone:
+49 5251 60 5283

Contact

M. Sc. Benedikt Uhe

Transregional Collaborative Research Centre 285

Teilprojekt A01

Benedikt Uhe
Phone:
+49 5251 60-3760
Office:
P1.4.16