ICAMS / Interdisciplinary Centre for Advanced Materials Simulation


Fatigue crack initiation: discrete dislocation interactions

Date: 06.07.2010
Place: Euromech 505, Paris, France

Steffen Brinckmann

Fatigue Crack Initiation: A Discrete Dislocation Dynamics and Cohesive Zone Simulation S. Brinckmann (ICAMS, Ruhr-University Bochum) E. van der Giessen (University of Groningen)

Dislocations have been found experimentally to play a fundamental role for fatigue crack initiation. They cluster and form structures. By their motion, dislocations shield the crack tip while they carry a stress singularity which elevates local stress fields.
This contribution presents a fatigue crack initiation study of a two-dimensional model of a grain near a stress-free surface, with edge dislocations moving on multiple slip systems. Their movement is controlled by the Peach-Koehler force, incorporating the singular long-range effects of the other dislocations, the image stress as well as the applied load. Dislocation pairs can be generated from Frank-Read sources. Annihilation is taken into account, as well as disappearance of dislocations at the free surface, which leads to a surface roughening. Finally, dislocations can become pinned at point obstacles, which represent, for instance, forest dislocations and small precipitates.
Additionally, cohesive surfaces account for both mode I and mode II opening. The stress leading to the crack opening can arise from the dislocations surrounding the cohesive surface as well as from the surface roughness. The latter is accounted for through the singular fields of wedges. We study the time evolution of the dislocation structures and dislocation density. We investigate how these developments influence the opening of any crack and whether the collective behavior of the dislocation structure or the surface roughness is more important in their contribution for fatigue crack initiation. Finally, molecular dynamics simulations of spontaneous annihilation of dislocation pairs on separate glide planes are presented, which investigate the asymmetry commonly observed in experiments: fatigued crystals grow in volume.

Supporting information:

Brinckmann_van der Giessen.pdf
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