PhD defence by Hamidreza Mahdavi "Micromechanical Modeling of Rolling Contact Fatigue"

This thesis investigates the micromechanics of crack initiation and propagation from subsurface inclusions in relation to rolling contact fatigue (RCF) in bearings, using various models with different levels of complexity. First, the problem is examined using a purely elastic model based on Eshelby’s method where it is assumed that crack initiation occurs at the location of the maximum micro-scale von Mises stress. Then, a 3D elasto-plastic finite element model is developed to evaluate crack initiation due to the accumulated cyclic plastic strain at the micro-scale. Based on these models, the effect of superimposed compressive macro-residual stresses on fatigue crack initiation is investigated. 

Finally, it is assumed that a crack, aligned parallel to the bearing raceway, has already initiated around a fully debonded inclusion or void, and its criticality is studied using Linear Elastic Fracture Mechanics (LEFM). It is shown that for describing crack propagation parallel to the bearing raceway under realistic Hertzian pressures, the yielding behavior of the bearing steel below the conventional 0.2% plastic strain needs to be modeled. Therefore, in order to model a smooth transition between the initially linear response and the conventional yield limit, a non-linear kinematic hardening (NLKH) constitutive law is used in relation to the fracture mechanics-based evaluation of RCF in this thesis. The parameters of the model are estimated by fitting the constitutive curve to a conventional stress-strain curve, which is obtained based on nanoindentation. In this regard, nanoindentation tests are performed on a wide variety of high strength steels. Then, a combination of the finite element method and a developed semi-analytical model is employed to determine the mechanical properties and consequently the stress-strain curves of the steels from nanoindentation measurements.

Main supervisor:
Professor Christian F. Niordson, DTU Mechanical Engineering

Co-supervisors:
Associate Professor Kim Lau Nielsen, DTU Mechanical Engineering
Senior Researcher Konstantinos Poulios, DTU Mechanical Engineering

Examiners:
Associate Professor Casper Schousboe Andreasen, DTU Mechanical Engineering
Associate Professor Ramin Aghababaei, Aarhus University, Denmark
Principal Scientisk Guillermo Enrique Morales Espejel, SKF Research & Technology Development, The Netherlands

Chairman:
Professor Christian Berggreen, DTU Mechanical Engineering

Tidspunkt

fre 29 okt 21
13:00 - 17:00

Arrangør

DTU Mekanik

Kontaktperson

Hvor

Building 421, Auditorium 074
The Technical University of Denmark