Indentation of a planar single crystal by a circular rigid indenter is analyzed using discrete dislocation plasticity. The crystal has three slip systems and is initially dislocation-free, but edge dislocations can nucleate from point sources inside the crystal. The lattice resistance to dislocation motion, the interaction with point obstacles and dislocation annihilation are incorporated through a set of constitutive rules. Indentation is displacement driven and at each stage of the loading history, a boundary-value problem is solved for the evolution of the indentation pressure and the contact size. Preliminary results are presented for various values of the density of dislocation sources.
Plane strain indentation of a single crystal by a rigid wedge is analyzed using discrete dislocation plasticity. We consider two wedge geometries having different sharpness, as specified by the half-angle of the indenter: α = 70° and 85°. The dislocations are all of edge character and modeled as line singularities in a linear elastic material. The crystal has initial sources and obstacles randomly distributed over three slip systems. The lattice resistance to dislocation motion, dislocation nucleation, dislocation interaction with obstacles, and dislocation annihilation are incorporated through a set of constitutive rules. Several definitions of the contact area (contact length in plane strain) are used to illustrate the sensitivity of the hardness value in the submicron indentation regime to the definition of contact area. The size dependence of the indentation hardness is found to be sensitive to the definition of contact area used and to depend on the wedge half-angle. For a relatively sharp indenter, with a half-angle of 70°, an indentation size effect is not obtained when the contact area is small and when the hardness is based on the actual contact length, while there does appear to be a size effect for some hardness values based on other measures of contact length.
The effect of indenter shape on sub-micron indentation according to discrete dislocation plasticity Widjaja, A.; Needleman, A.; Van der Giessen, E. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Abstract Two-dimensional discrete dislocation simulations of indentation in the submicron range are presented for wedge indenters with a sharp tip and for indenters with a circular tip. Plane strain calculations are carried out for single crystals that are initially free of mobile dislocations and with all dislocations nucleating from a specified distribution of internal sources. The hardness is expressed in terms of the indentation force divided by the actual contact area accounting for roughness of the surface in contact with the indenter. For wedge indenters the hardness is found to decrease with increasing indentation depth, while for indenters with a circular tip the hardness increases somewhat with increasing indentation depth. However, at a given indentation depth, the indentation hardness of circular indenters increases with decreasing tip radius. The difference in hardness evolution for the two tip shapes is mainly due to the manner in which the evolution of the contact area depends on indenter tip shape. The nominal hardness, i.e. that based on the geometric contact area neglecting material sinkin or pile-up and surface roughness, is found to follow the inverse square root size dependence predicted by Nix and Gao [1] and by Swadener et al [2], even though the plastic zone found in the simulations differs significantly in shape and size from that assumed in deriving the scaling laws.
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