Industrializable microstructure-sensitive fatigue simulation

• Industrialisierbare mikrostruktursensitive Ermüdungssimulation

Natkowski, Erik; Münstermann, Sebastian (Thesis advisor); Eberl, Christoph (Thesis advisor)

Aachen : RWTH Aachen University (2022, 2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

Mechanical fatigue in the early stages is heavily dependent on the material's microstructure. Large portions of the overall fatigue life is spent in these stages, making their accurate representation crucial in tailored dimensioning methods against fatigue. The integration of crystal plasticity material behavior in finite element simulations has distinguished itself from other methods by offering a reasonably accurate description of micromechanical phenomena while still being relatively computationally efficient. Typically applied crystal plasticity models in an industrial context use a phenomenological formulation for the slip rate. Still, only a few cycles can be simulated in a reasonable time span, forcing the use of mesoscopic surrogate measures as damage indicators. With their help, it is extrapolated to thousands or millions of cycles in order to determine the fatigue life. For simplicity, the fatigue life on the microscale is often assumed to equal the crack initiation lifetime but especially in regimes of low loading amplitudes, this approach falls short of lifetimes actually observed in experiments. This creates the necessity to consider at least also the early stages of fatigue life with short crack growth models. A particular challenge in short crack modeling consists of adopting the aforementioned simplifications for computational efficiency while still reproducing the relevant physical mechanisms and adding as less calculation expense as possible to the overall simulation model. Three research papers form the core part of this cumulative thesis. In the first paper, a short crack model is presented that fulfills the requirements imposed by industrialized fatigue life predictions. Therefore, this model is first validated against experimentally observed crack paths in microfatigue specimens to confirm its physical meaningfulness. For different steels and loading conditions, the model's fatigue life predictions are then compared to experimental data, by which its applicability across distinct scenarios is verified. The corresponding analyses are given in the second paper. An additional drawback of current crystal plasticity models is the consideration of only a bulk material state, thereby neglecting near-surface conditions. However, crack initiation occurs mostly at part surfaces, and effects like surface roughness, residual stresses, and stress concentrations caused by the macroscopic geometry significantly influence the overall fatigue life. Within this work, a framework is described to introduce these three features into micromechanical simulation in an efficient manner. The models are parameterized based on fatigue specimens by the known specimen geometry as well as measured surface profiles and residual stresses. While surface roughness and macro geometry models are described in the third research paper, a residual stress approach is added in this thesis. The coupling of the industrializable short crack and surface models enables micromechanical fatigue life prediction for technically relevant conditions. Therefore, this work presents a step toward the wide-scale industrial application of crystal plasticity simulations for accurate fatigue life dimensioning.

Institutions

• Division of Materials Science and Engineering [520000]
• Integrity of Materials and Structures Teaching and Research Area [522520]