Crystal plasticity approach of necking in stretching rods

The fragmentation of a ductile metal rod under dynamic extension simulating an expanding ring is initiated by multiple necking. This is a structural instability justified by the mechanics at the macroscopic scale. However, the computation of the plastic flow at the mesoscale, i.e. the one of the crystal aggregate, applying crystal plasticity modelling inside grains provides new insights on both the overall process and the plastic slip activity in the most developed necks, potentially controlling their subsequent failure. Full field simulations of FCC copper polycrystal rods show a large scatter of inter-neck spacing and neck intensity, in contrast with the macroscopic concept of a dominant periodic instability mode. Scatter of the neck number and positions for different grain and crystal orientation instances is also significant even though they have no clear connection with initial grain-scale strain map. The necking pattern is next found to evolve with subsequent neck competition and arrest of the less pronounced necks consistently with a Mott type obscuration mechanism. The number of active necks in the late stage is nevertheless weakly sensitive to grain structure. Furthermore, when strain concentrates in the most developed necks, plastic slip activity takes place in a narrow zone of elongated grains submitted to very high plastic deformations. These grains undergo significant lattice rotations and thus mobilize continuously evolving sets of slip systems. Owing to slip system interactions, critical shear stresses still increase and saturate in the very late localization stage, together with the development of a marked texture. In-grain strain localization, which is a potential seed of neck failure, is addressed by using a finite deformation linear stability analysis. Band type localization modes are displayed with time increasing growth rate. In the late stage, such modes involve both elastic and plastic deformation components and develop almost perpendicularly to the stretching direction. They also induce significant lattice rotation and may trigger the formation of laminated subgrain structures. • Simulation of necking instability incorporating polycrystalline microstructure • Single crystal behavior ruled by a physically motivated crystal plasticity law • Grain scale heterogeneity drives the variability of neck intensity and spacing • Strongest necks inhibit less developed ones • Neck regions undergo evolving multi-slip and tend to stable crystal orientations • In-grain band type localization modes exist with increasing growth rate