The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
Aalto

Computational Modelling of Fracture and Dislocations

Peter Szelestey

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium S4 at Helsinki University of Technology (Espoo, Finland) on the 4th of February, 2005, at 12 o'clock noon.

Overview in PDF format (ISBN 951-22-7500-7)   [736 KB]
Dissertation is also available in print (ISBN 951-22-7499-X)

Abstract

Mechanical properties of solids bear great significance because of their importance in various fields of engineering and materials science. Fracture and plasticity are the two characteristic mechanisms by which materials permanently deform under external loading. Beside experiments and theoretical model calculation computational modelling greatly contributes to the understanding of these phenomena. This dissertation consists of various studies of topics related to these fields.

First, the branching instability of dynamic fracture is studied in a simple lattice model which describes a brittle material at mesoscopic length-scales. It is shown that the presence of anisotropy leads to a variation in the fracture pattern and crack tip velocity oscillations.

The second part of the thesis consists of atomic level computational modelling of dislocations using molecular dynamics method. Here, the interatomic potential plays a definite and relevant role. For that reason a semi-empirical, many-body embedded-atom potential is developed which turns out to be especially suitable for dislocation studies in fcc crystals, because of the realistic stacking-fault energies it predicts. Dislocation properties at the atomic level determine the micro-structure and in turn the plastic properties of materials. The static dislocation core structure is determined for dissociated dislocations in nickel and compared to analytical calculations. Furthermore, the effective Peierls stress, characterizing the dislocation mobility, and the variation in the dislocation structure through its motion is investigated for the screw orientation as a function of the separation distance of partials. Finally, the interaction of a dissociated screw dislocation and a vacancy type stacking-fault tetrahedron is studied. A wide variety of dislocation processes are found including bending and jog line formation, depending on the internal structure of the dislocation, the orientation and position of the defect.

This thesis consists of an overview and of the following 6 publications:

  1. Szelestey P., Heino P., Kertész J. and Kaski K., 2000. Effect of anisotropy on the instability of crack propagation. Physical Review E 61, number 4, pages 3378-3383.
  2. Perondi L., Szelestey P. and Kaski K., 2000. Structure of a dissociated edge dislocation in copper. Materials Research Society Symposium Proceedings 578, pages 223-228.
  3. Szelestey P., Patriarca M., Perondi L. and Kaski K., 2002. Modified EAM potentials for modelling stacking-fault behavior in Cu, Al, Au, and Ni. International Journal of Modern Physics B 16, number 19, pages 2823-2835.
  4. Szelestey P., Patriarca M. and Kaski K., 2003. Computational study of core structure and Peierls stress of dissociated dislocations in nickel. Modelling and Simulation in Materials Science and Engineering 11, number 6, pages 883-895.
  5. Szelestey P., Patriarca M. and Kaski K., 2005. Dissociated dislocations in Ni: a computational study. Materials Science and Engineering A 390, numbers 1-2, pages 393-399.
  6. Szelestey P., Patriarca M. and Kaski K., Computational study of a screw dislocation interacting with a stacking-fault tetrahedron. Modelling and Simulation in Materials Science and Engineering, submitted for publication.

Keywords: branching instability, molecular dynamics, EAM potential, dislocation core structure, Peierls stress, dislocation-defect interaction

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© 2005 Helsinki University of Technology

Last update 2011-05-26