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This research project aims at the prediction of the operation behavior and lifetime of compo­nents under service loads. The investigations are conducted on round specimens of the workpiece material 16MnCrS5 in the initial and deformed state. Cold forging as a representative of the bulk forming process is examined as manufacturing technique.

To realize the primary objective, the ductile initial damage is characterized, the failure-inducing microstructural defects are quantified and described on the basis of thermomechanically coupled material and defect models. Eventually, project C01 aims at the reduction of material resources and costs in accordance with the overall aim of the TRR 188 – the increase of part efficiency and especially the reduction of safety factors. In the first funding period, the combination of experimental studies and material-mechanic modeling focuses on the macroscopic scale. On this scale, the influence of ductile pre-damage of the material induced by the forming process on the evolution of deformation and failure mechanisms under multi-axial cyclic load is to be investigated. The cyclic loading tests and the modeling focuses on Low Cycle Fatigue (LCF).

To examine complex cyclic load paths, a main emphasis is put on the torsional and axial-tor­sional material characterization. Therefore, material in the initial state as well as extruded components with diffe­rent ductile pre-damage are investigated. The material behavior during loading is characterized continuously by stress-strain hysteresis and temperature detection. The former is quantified by an axial-torsional extensometer, the latter by thermography. Based on temperature field rises being detected throughout the loading, conclusions can be drawn with regard to local dissipation effects. Following the intention of analyzing the sensitivity of material to the pre-damage, axial-torsional tests with proportional (in phase) and 90° phase-shifted (out of phase) axial and torsional loadings are included in the experimental research conception.

LCF can be accompanied by accumulated microstructural material alterations like persistent slip-bands, the re-organization of shear bands, the re-assembly of dislocations to cell struc­tures as well as the nucleation and growth of pores. To identify these effects, distinct damage levels and fatigue states are characterized by means of light and electron microscopy, respectively. Additionally, fractographic analyses are carried out in order to draw conclusions as to the underlying mechanisms of damage. The experimental analyses described above will be continuously incorporated into the development of a new macroscopic material model. The main focus of this model is the realistic description of the initial anisotropy of the material as well as the one of the anisotropic damage evolution under different service loads. For this purpose, an already existing local isotropic material model, which reproduces both ductile damage (relevant for LCF) as well as quasi-brittle damage (relevant for High Cycle Fatigue, HCF), will be fundamentally extended.

170721 Figure TRR C01 en

Proposed approach for the development of a thermomechanically coupled material model under service loads

In addition to the extension of the model towards anisotropic material degradation, an extension to the thermomechanically coupled framework will also be elaborated. Thus, experimental data measured by means of thermography can also be used in order to identify the model parameters. Since locally constitutive models lead to mesh-dependent results as far as the finite element method is concerned, the final coupled constitutive model will be regularized by means of the gradient theory.

While in the first funding period the focus is on LCF, the HCF material behavior will be addressed in the second funding period. Additionally, digital image correlation will be applied and integrated into the parameter identification procedure. In the third funding period, the influence of increased operational temperatures will be analyzed.

Project leaders
Prof. Dr.-Ing. Jörn Mosler
Institute of Mechanics (IM), TU Dortmund University

Prof. Dr.-Ing. habil. Frank Walther
Department of Materials Test Engineering (WPT), TU Dortmund University

Project coordinators
Klas Feike, M. Sc.
Institute of Mechanics (IM), TU Dortmund University

Lars Lingnau M. Sc.
Department of Materials Test Engineering (WPT), TU Dortmund University