A DCAMM seminar will be presented by
Researcher, PhD Carl Dahlberg
KTH Royal Institute of Technology
Stockholm, Sweden
Abstract:
A new experimental method, Kysar et al. (2010), to directly measure the deformed state of single and bi-crystal specimens will be presented. The experiment is performed on oriented FCC crystals deformed by wedge indentation parallel to the [1 1 0] direction such that the mid-section of the specimen remain in a state of plane deformation throughout the deformation. The lattice rotation field of a newly exposed material plane is measured with Electron Backscatter Diffraction (EBSD). Ambiguity due to the inherent crystal symmetry is handled by an algorithm employing quaternion algebra such that a full field map of lattice rotation about the plane normal is acquired. Due to the plane deformation state plastic slip is constrained to pairs of crystallographic slip planes that results in three effective in-plane slip systems. Via numerical differentiation of the lattice rotation field all non-zero components of Nye's tensor can be determined and a strict lower bound of geometrically necessary dislocation (GND) density can be obtained for the effective slip systems. Experimental results from indentation of Nickel single crystals will be presented to show the richness of the method. Crystal plasticity finite element (CPFE) simulations exhibit good agreement with experiments on the scale of a few tens of mm, but can not hope to capture the fine scale localization patterns in GND densities. Strain gradient crystal plasticity (SGCP) type of theories should be able to capture these localization features. The same experimental method have also been employed to examine the deformed state of an Aluminium bi-crystal.
Experimental results will be presented that highlight the activity across the grain boundary (GB) and the effects of it on lattice rotations and dislocation densities. Evidence of slip plane alignment and some indication of dislocation pile up against the GB is shown. Comparison to CPFE results show good agreement of the overall lattice rotation fields and GB deformation. Since no explicit constitutive law for plastic slip interaction at the GB is possible in a CPFE framework the numerical results are due only to slip system mismatch across the GB. Suggestions for handling of GB in a SGCP framework can be based on previous work on isotropic strain gradient plasticity interface models, e.g. Dahlberg and Faleskog (2013), Dahlberg et al.(2013)
Danish pastry, coffee and tea will be served 15 minutes before the seminar starts.
All interested persons are invited.