Shear localization in dynamic deformation of materials: microstructural evolution and self-organization

The plastic deformation of crystalline and non-crystalline solids incorpora tes microscopically localized deformation modes that can be precursors to s hear localization. Shear localization has been found to be an important and sometimes dominant deformation and fracture mode in metals, fractured and granular ceramics. polymers, and metallic glasses at high strains and strai n rates. Experiments involving the collapse of a thick walled cylinder enab le controlled and reproducible application of plastic deformation at very h igh strain rates to specimens. These experiments were supplemented by hat-s haped specimens tested in a compression Hopkinson bar. The initiation and p ropagation of shear bands has been studied in metals (Ti, Ta, Ti-6Al-4V, an d stainless steel), granular and prefractured ceramics (Al2O3 and SiQ, a po lymer (teflon) and a metallic glass (Co58Ni10Fe5Si11B16). The first aspect that was investigated is the microstructural evolution inside the shear ban ds. A fine recrystallized structure is observed in Ti. Cu, Al-Li, and Ta. a nd it is becoming clear that a recrystallization mechanism is operating. Th e fast deformation and short cooling times inhibit grain-boundary migration ; it is shown, for the first time, that a rotational mechanism, presented i n terms of dislocation energetics and grain-boundary reorientation, can ope rate within the time of the deformation process. In pre-fractured and granu lar ceramics, a process of comminution takes place when the particles are g geater than a critical size a(e). When they are smaller than a(e), particle deformation takes place. For the granular SiC, a novel mechanism of shear- induced bonding was experimentally identified inside the shear bands. For a ll materials, shear bands exhibit a clear self-organization, with a charact eristic spacing that is a function of a number of parameters. This self-org anization is analyzed in terms of fundamental material parameters in the fr ame of Grady-Kipp (momentum diffusion), Wright-Ockendon, and Molinari (pert urbation) models. (C) 2001 Elsevier Science B.V. All rights reserved.