Study of plastic shear localization in aluminum alloys under dynamic loading

The paper presents an experimental and theoretical study of plastic shear localization mechanisms observed in AlMg6 alloy shear-compression specimens dynamically loaded during Hopkinson-Kolsky bar tests. The mechanisms of plastic shear instability are associated with collective effects in microshear ensembles in spatially localized areas. The lateral surface of specimens was studied in a real-time mode using a high-speed infra-red camera CEDIP Silver 450M. The temperature field distribution obtained at different time allowed us to trace the evolution of plastic strain localization. Based on the equations describing a relationship between non-equilibrium transitions and mechanisms of structural relaxation and plastic flow, numerical modeling of plastic shear localization was performed. A numerical experiment relevant to the loading scheme realized in our study was carried out using a system of constitutive equations constructed to take into account a relationship between the structural relaxation mechanisms caused by the collective behavior of microshears and the autowave modes of evolution of a localized plastic flow. Upon the experiment completion the microstructure of the saved specimens was analyzed using a New View-5010 microscope-interferometer. Constancy of the Hurst exponent is observed in a wide range of spatial scales after the dynamic deformation of samples. The Hurst exponent reflects the relationship between the behavior of defects and the surface roughness of different scale levels. For dynamically deformed specimens, constancy of the Hurst index was observed over a much wider range of spatial scales. This indicates more pronounced features of strain localization preceding adiabatic shear failure. These specific features can be caused by the collective multi-scale behavior of defects, which initiates a sharp decrease in stress relaxation time and consequently a localized plastic flow and generation of fracture nuclei by adiabatic shear. Infrared scanning in-situ of the strain localization zone and a consequent study of the defect structure confirmed our supposition that non-equilibrium transitions play a crucial role in defect ensembles during the evolution of a localized plastic flow.