Browsing by Author "Mlikota, Marijo"

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Open Access
Multiscale modelling and simulation of metal fatigue and its applications
(2019) Mlikota, Marijo; Schmauder, Siegfried (Prof. Dr. rer. nat. Dr. h. c.)
Multiscale materials modelling (MMM) has been recently growing and simultaneously becoming a significant tool for understanding complexities of contemporary materials as well as a valuable driver for their development. In view of that, this work presents a MMM approach based on the application of different numerical techniques for predicting the fatigue life of metallic materials. The work contains several fatigue problems of metals where the modelling approach has been successfully applied, including its highlight, which is the virtual determination of the fatigue life (S-N or Wöhler) curve. The approach is realized by coupling the analysis of microscopic (crack initiation on the basis of the physically-based Tanaka-Mura model) and macroscopic (crack growth on the basis of classical Fracture Mechanics) fatigue behaviour, together with the molecular dynamics (MD) and experimentally-based input determination. Particular emphasis has been placed on the application of the modelling approach to demonstrate the importance of the parameter critical resolved shear stress (CRSS) for the fatigue performance of several metals. The discovered relation between endurance limit and the CRSS provides a facet of fatigue theory that is numerically predictive and which allows the selection of those types of materials, which are more fatigue resistant. In addition to the CRSS, factors such as grain size, mean stress, plasticity, residual stresses and others have been also investigated with the aim to identify their influence on the S-N curves as well as endurance limits of various metallic materials. Aside of that, it is now possible to estimate the length of the short crack at the initiation end by application of the physically-based micromechanical simulations what opens new doors to an easier detection of the critical crack lengths in practical applications, e.g. for component and plant inspection. The experimental determination of the Paris law constants is typically tedious and time-consuming. The successful determination of these constants by using the present physically-based multiscale materials simulation approach provides, on the contrary, an efficient method to equip engineers with these highly relevant fatigue data. As the results of this work demonstrate, the introduced MMM procedure for metal fatigue characterization plays an important role in the understanding of present days’ complex and advanced materials. Apart from that, this physically-based MMM approach represents a breakthrough in the field of fatigue research and opens the door for fast and cost-effective development of virtual metallic materials for present and future fatigue applications, such as, e.g., for additive manufactured materials.
Open Access
A newly discovered relation between the critical resolved shear stress and the fatigue endurance limit for metallic materials
(2020) Mlikota, Marijo; Schmauder, Siegfried
Open Access
Simulation of the fatigue crack initiation in SAE 52100 martensitic hardened bearing steel during rolling contact
(2022) Dogahe, Kiarash Jamali; Guski, Vinzenz; Mlikota, Marijo; Schmauder, Siegfried; Holweger, Walter; Spille, Joshua; Mayer, Joachim; Schwedt, Alexander; Görlach, Bernd; Wranik, Jürgen
An investigation on the White Etching Crack (WEC) phenomenon as a severe damage mode in bearing applications led to the observation that in a latent pre-damage state period, visible alterations appear on the surface of the raceway. A detailed inspection of the microstructure underneath the alterations reveals the existence of plenty of nano-sized pores in a depth range of 80 µm to 200 µm. The depth of the maximum Hertzian stress is calculated to be at 127 µm subsurface. The present study investigates the effect of these nanopores on the fatigue crack initiation in SAE 52100 martensitic hardened bearing steel. In this sense, two micro-models by means of the Finite Element Method (FEM) are developed for both a sample with and a sample without pores. The number of cycles required for the crack initiation for both samples is calculated, using the physical-based Tanaka-Mura model. It is shown that pores reduce the number of cycles in bearing application to come to an earlier transition from microstructural short cracks (MSC) to long crack (LC) propagation significantly.