PSI - Issue 2_A

Solveig Melin et al. / Procedia Structural Integrity 2 (2016) 1351–1358 S Melin, P Hansson, A Ahadi/ Structural Integrity Procedia 00 (2016) 000–000

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remaining ligaments are of the same size, the geometry influences the behavior. As regards rupture strains, the [100]-orientation gains in comparison with the [110]-orientation; 0.84 < ε fv [110] / ε f [110] < 0.98 for the largest beams. Thus, the beam with s = 18 a 0 , holding a relatively speaking smaller defect, reaches 98% of the strength of a solid beam. This is not the case for the beam of the same size holding an edge for which ε fe [110] / ε f [110] ≈ 0.46, only. Again a geometry influence is at hand. For the [100]-orientation only one comparison can be made between the two defect beams due to void closure and influence from the clamed ends; the one for s = 12 a 0 . For that case ε fe [100] is higher than ε fv [100] . 4. Conclusions Beams of single-crystal Cu, of dimensions 100 a 0 × s × s , s = 6 a 0 , 12 a 0 and 18 a 0 along the ( x,y,z ) directions, with a 0 denoting the lattice parameter, have been loaded in displacement controlled tension along x until rupture. Two crystallographic orientations have been considered; the [100]-orientation for which we designate crystallographic orientations according to ( x , y , z ) = ([100], [010], [001]), and the [110]-orientation for which ( x , y , z ) = ([110], [-110], [001]). Solid beams have been compared to beams with edge crack-like defects or centrally placed through-the thickness voids. The defect sizes were of width a 0 along x and height 2 a 0 along y . The introduction of a defect markedly lowers the strain at plastic initiation; in general to about one half of what applies to solid beams. The strain at initiation was, in practice, found to be independent of cross section size. Also a loss in rupture strain is at hand due to a defect. Even if the introduced defect is small, still the defect corners acts as stress raisers, triggering slip along {111}-planes. Generally, a beam with a through void is seen to initiate plasticity later than a beam with an edge defect. It turns out that the crystallographic orientation plays a crucial role. For the two orientations considered here, the [110]-orientation always initiates plasticity before the [100]-orientation. This can be explained by the rougher surfaces for the [110]-orientation, making it easier to initiate slip along preferred slip directions. For beams holding through-the thickness voids, void closure provoked by different slip events occurred if the cross section size was small enough. These beams thereafter fail through necking of the now healed cross section. Comparison between when the void closure was completed indicated that void closure delays the final rupture and, thus, strengthens the beam. For larger cross section sizes no void closure occurred. Instead the beams split in two ligaments, each necking independently. References Ahadi, A. Melin, S., 2016. Size dependence of the Poisson’s ratio in single crystal fcc copper nanobeams. Computational Materials Science 111, 322-327. Ellad, B. T., Miller, R. E., 2011. Modeling Materials Continuum, Atomistic and Multiscale Techniques. Cambridge University press. Foiles, S. M, Baskes, M. I., Daw, M. S., 1986. Embedded-atom-method functions for the fcc metals Cu, Ag, Au, Ni, Pd, Pt, and their alloys Physical review B 33(12). Hansson, Jansson, 2013. Nanoindentation of thin copper coatings Materials Structure & Micromechanics of Fracture VII, 417-429. Hansson (2015), Influence of the crystallographic orientation and thickness of thin copper coatings during Nanoindentation, Eng. Frac. Mech. 150, 143-152. Holian, B.L., Ravelo, R. 1995. Fracture simulations using large-scale molecular-dynamics. Phys. Rev. B 51 17 11275-11288. Hommel, M., Kraft, O., 2001. Deformation behavior of thin cupper films on deformable substrates. Acta Mater. 49 3935-47. Kelchner, CL, Plimpton, SJ, Hamilton JC, 1998. Dislocation nucleation and defect structure during surface indentation Phys. Rev. B 58:11085-8 LAMMPS; http://lammps.sandia.gov Liang. H. Y, Liu, G R, Han, X, 2006, Atomistic simulation on the stiffening and softening mechanism of nanowires Computational Methods, 1667-1672. Olsson, PAT, Melin, S, Persson, C, 2007, Atomistic simulations of tensile and bending properties of single-chrystal bcc iron nanobeams. Phys. Rev. B 76 224112. Schweiger, R, Dehm, G, Kraft, O, 2003a. Cyclic deformation of polycrystalline Cu films. Phil. Mag. 83, 693-710. Schweiger, R, Kraft, O, 2003b. Size effects in the fatigue behavior in thin Ag films. Acta Mater. 51, 195-206. Stukowski, A, 2010. Visualization and analysis of atomistic simulation data with OVITO–the Open Visualization Tool. Modelling Simul. Mater. Sci. Eng. 18. Zhou, LG, Huang, HC, 2004. Are surfaces elastically softer or stiffer? Appl. Phys. Lett. 84, 1940 .