Search

We study the dynamics of flow instability and shear localization in cutting using direct high-speed imaging and low melting point alloy as a model material system. The onset of instability and departure from steady laminar flow is triggered by nucleation of shear band at the tool tip and subsequent propagation towards the free surface. The stress at the onset of shear band formation is found to be constant and a physical characteristic of the material. The shear band velocity and inhomogeneous strain field arising from the banding are quantitatively characterized using an image correlation method. [ABSTRACT FROM AUTHOR]

Shear banding is a type of plastic flow instability with often adverse implications for cutting and deformation processing of metals. Here, we study the mechanics of plastic flow evolution within single shear bands in Ti- and Ni-based alloy systems. The local shear band displacement profiles are quantitatively mapped at high resolution using a special micromarker technique. The results show that shear bands, once nucleated, evolve by a universal viscous sliding mechanism that is independent of microstructural details. The evolution of local deformation around the band is accurately captured by a momentum diffusion equation based on a Bingham-type flow rule. The predicted band viscosity is very small, compared to those of liquid metals. A plausible explanation for this small viscosity and fluid-like behavior at the band, based on phonon drag, is presented. [ABSTRACT FROM AUTHOR]

We examine large-strain deformation and unsteady flow modes in cutting using high-speed imaging. For metals which exhibit large workability and strain hardening, the commonly assumed laminar flow is inherently unstable. Instead, the cutting is characterized by sinuous flow, with large-amplitude folding, that is triggered by a plastic buckling instability linked to the material microstructure. A microstructure basis is also suggested for shear band flow in high-speed cutting, with the band region showing a fluid-like characteristic with very small viscosity. Mechanochemical Rehbinder effects, long reported in cutting of metals, are found to be closely linked to the unsteady flow modes. [ABSTRACT FROM AUTHOR]

Large strain plastic flow in cutting of metals is studied at multiple length scales using high-speed imaging and marker techniques, complemented by particle image velocimetry and electron microscopy. Quantitative analysis of streak-lines, strain fields and microstructure, shows the flow to be often unsteady. Instabilities such as segmentation driven by ductile fracture, vortex-like flow in ductile metals, and shear banding in low-thermal diffusivity systems are elucidated using direct observations. A constrained-cutting process is demonstrated for suppressing the instabilities and unsteady flow. [ABSTRACT FROM AUTHOR]