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A05 - Damage modelling for the simulation of mechanical joining processes

The subproject deals with the modelling and simulation of plasticity and damage during mechanical joining processes in the context of the intended versatility of the processes. On the one hand, material models are developed, numerically implemented robustly and efficiently, and validated experimentally. On the other hand, a numerical method, the so-called Parametric Finite Element Method (PFEM), is developed further, which enables the efficient computation of a large number of material-geometry-combinations and thus represents the ideal solution method for the simulation of versatile processes.

The mechanical joining processes considered in the TRR are accompanied by very large local inelastic deformations. In particular, this results in a strong coupling of finite plasticity and damage. For a better understanding of these mesostructural phenomena occurring during the joining process, suitable material models and their use in the process simulation are indispensable. The development of these material models and their application to a virtual optimisation of the process control are key aspects of this project.

Moreover, the considered mechanical joining processes, such as clinching or self-pierce riveting, are characterised by high demands on the versatility. Targeted changes within the process chain – concerning the semi-finished product, the joint or the joining process – should be possible. The process simulation ought to consider these requirements for versatility and ideally consider such variations from the start. This results in the second focus of the present subproject. The PFEM is extended to include plasticity and damage and is used to simulate mechanical joining processes. In addition to the usual physical coordinates discretising the space, the PFEM introduces additional parametric coordinates that represent one or more geometric or material parameters of the joint. The solution of this higher-dimensional problem simultaneously and very efficiently provides the solutions for continuous variations (instead of a very time-consuming discrete sampling) of these parameters and thus allows a direct comparison of different process variants in light of their versatility.

In summary, the goal of subproject A05 are (i) the physically non-linear and geometrically exact material modelling of coupled plasticity and damage, (ii) their physically motivated regularisation and (iii) the numerical implementation of the models for process simulation within the PFEM to account for the desired versatility of the joining processes.

Publications


Open list in Research Information System

Inverse parameter identification of an anisotropic plasticity model for sheet metal

J. Friedlein, S. Wituschek, M. Lechner, J. Mergheim, P. Steinmann, IOP Conference Series: Materials Science and Engineering (2021), 1157, pp. 012004

The increasing economic and ecological demands on the mobility sector require efforts to reduce resource consumption in both the production and utilization phases. The use of lightweight construction technologies can save material and increase energy efficiency during operation. Multi-material systems consisting of different materials and geometries are used to achieve weight reduction. Since conventional joining processes reach their limits in the connection of these components, new methods and technologies are necessary in order to be able to react versatilely to varying process and disturbance variables. For fundamental investigations of new possibilities in joining technology, numerical investigations are helpful to identify process parameters. To generate valid results, robust and efficient material models are developed which are adapted to the requirements of versatile joining technologies, for instance to the high plastic strains associated with self-piercing riveting. To describe the inherent strain-induced plastic orthotropy of sheet metal an anisotropic Hill-plasticity model is formulated. Tensile tests for different sheet orientations are conducted both experimentally and numerically to adjust the anisotropic material parameters by inverse parameter identification for aluminium EN AW-6014 and steel HCT590X. Then, the layer compression test is used to validate the model and the previously identified parameters.


Stochastic local FEM for computational homogenization of heterogeneous materials exhibiting large plastic deformations

D. Pivovarov, J. Mergheim, K. Willner, P. Steinmann, Computational Mechanics (2021)

Computational homogenization is a powerful tool allowing to obtain homogenized properties of materials on the macroscale from simulations of the underlying microstructure. The response of the microstructure is, however, strongly affected by variations in the microstructure geometry. In particular, we consider heterogeneous materials with randomly distributed non-overlapping inclusions, which radii are also random. In this work we extend the earlier proposed non-deterministic computational homogenization framework to plastic materials, thereby increasing the model versatility and overall realism. We apply novel soft periodic boundary conditions and estimate their effect in case of non-periodic material microstructures. We study macroscopic plasticity signatures like the macroscopic von-Mises stress and make useful conclusions for further constitutive modeling. Simulations demonstrate the effect of the novel boundary conditions, which significantly differ from the standard periodic boundary conditions, and the large influence of parameter variations and hence the importance of the stochastic modeling.


Anisotropic plasticity‐damage material model for sheet metal — Regularised single surface formulation

J. Friedlein, J. Mergheim, P. Steinmann, PAMM (2021), 21

Sheet metal forming as well as mechanical joining demand increasingly accurate and efficient material modelling to capture large deformations, the inherent sheet orthotropy and even process-induced damage, which is expected to be influential. To account for large strains the additive logarithmic strain space is utilised that enables a straightforward incorporation of plastic anisotropy, herein modelled by a Hill48 yield function. A gradient-enhancement is used to equip the ductile damage model with an internal length scale curing the damage-induced localisation. An affine combination of the local and non-local softening variable is derived enabling a more efficient single surface formulation for the regularised plasticity-damage material model.


A finite plasticity gradient-damage model for sheet metals during forming and clinching

J. Friedlein, J. Mergheim, P. Steinmann, Key Engineering Materials (2021), 883 KEM, pp. 57

In recent years, clinching has gathered popularity to join sheets of different materials in industrial applications. The manufacturing process has some advantages, as reduced joining time, reduced costs, and the joints show good fatigue properties. To ensure the joint strength, reliable simulations of the material behaviour accounting for process-induced damage are expected to be beneficial to obtain credible values for the ultimate joint strength and its fatigue limit. A finite plasticity gradient-damage material model is outlined to describe the plastic and damage evolutions during the forming of sheet metals, later applied to clinching. The utilised gradient-enhancement cures the damage-induced localisation by introducing a global damage variable as an additional finite element field. Both, plasticity and damage are strongly coupled, but can, due to a dual-surface approach, evolve independently. The ability of the material model to predict damage in strongly deformed sheets, its flexibility and its regularization properties are illustrated by numerical examples.


Observations on additive plasticity in the logarithmic strain space at excessive strains

J. Friedlein, J. Mergheim, P. Steinmann, International Journal of Solids and Structures (2022), 239-240, pp. 111416

Additive plasticity in the logarithmic strain space is compared to multiplicative plasticity for various loading cases including coaxial and non-coaxial plastic deformations. Even though both finite plasticity approaches are based on total Lagrangian descriptions, the former is popular due to its inherent similarity to the infinitesimal theory and its easy extensibility. However, since its introduction several limitations of additive plasticity in the logarithmic strain space have been discovered. In this study, these problems such as stress rotation and softening are considered, revealing that fundamental differences compared to multiplicative plasticity occur for non-coaxial plastic deformations. We focus in particular on the observed softer response of the additive based approach, which is analysed in depth using three numerical examples including two well-known benchmarks for finite plasticity. By means of these finite element simulations the softer and possibly even localising response of additive plasticity in the logarithmic strain space is confirmed.


Open list in Research Information System

Contact

Prof. Dr.-Ing Paul Steinmann

Transregional Collaborative Research Centre 285

Teilprojekt A05

Paul Steinmann
Phone:
+49 9131 8528501

Contact

Prof. Dr.-Ing. Julia Mergheim

Transregional Collaborative Research Centre 285

Teilprojekt A05

Julia Mergheim
Phone:
+49 9131 8528505

Contact

M. Sc. Johannes Friedlein

Transregional Collaborative Research Centre 285

Teilprojekt A05

Johannes Friedlein
Phone:
+49 9131 85 64400