Prof. Marc FIVEL (SIMAP-GPM2, Universite Grenoble Alpes, France)

Crack Initiation and Propagation in Fatigues 216L Stainless Steels: A 3D Dislocation Dynamics Investigation
When Nov 07, 2016
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283446
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 Since in most of metals, crack nucleation and crack growth are intimately related to dislocation plasticity, discrete dislocation dynamics simulation is a very promising numerical tool to address damage mechanics. The recent improvement of 3D discrete dislocation dynamics codes makes it now possible to perform realistic simulations of the intragranular crack propagations. In this presentation, three types of 3D discrete dislocation dynamics investigations are presented with the goal to better understand fatigue damage in 316L stainless steels. First, crack initiation mechanisms are investigated in a surface grain cyclically loaded under symmetric plastic strain amplitude. After few cycles, dislocations organize into a complex 3D microstructure made of persistent slip bands. Extrusions are evidenced at the surface, precisely where the bands are located. Calculations of the elastic energy stored within the simulated grain and the stress tensor inside the simulation box show that the first crack will initiate at the surface, most probably at the bottom of the extrusion. Secondly, the propagation of a fatigue crack is investigated using the same modeling technics in which a crack is now introduced. The role of the pre-existing slip band on the crack path is analyzed. The magnitude of the crack tip slip displacement is evaluated quantitatively for various distances between the tip and the grain boundary. This shows that grain boundaries systematically amplify the slip dispersion ahead of the crack tip and consequently, slow down the crack growth rate. Finally, the model is used to study crack transmission from one grain to the next one. Assuming the first grain is cracked, we investigate indirect crack transmission to the next grain i.e. a crack nucleation from the formation of a persistant slip band in the second grain. It is found that crack transmission strongly depends on grain disorientation. In some situation the pre-existing crack can accelerate the crack formation in the next grain. For other orientations, the crack can impose its own persistent slip band, different from the one that would develop without the presence of the cracked grain. For both cases, the crack imposes the extrusion growth rate in the second grain.