Observation of plastic deformation wave in a tensile-loaded aluminum-alloy (original) (raw)

Physical concept of shear fracture mesomechanism and its applications

2011

The key objective of the present paper is an attempt to create an interface between the existing inconsistent views on the microscopic and macroscopic aspects of the mechanism of plastic deformation and shear fracture. This will be enabled by a focus on the course and effects of the evolution of dislocation structure, and will consist in considering an indirect, i.e. a mesoscopic scale of the discussed phenomena. Thanks to this, a synergy between the mechanisms of deformation and fracture of materials will be proven, which will provide an opportunity for a smooth transfer from the microscopic, through mesoscopic, to macroscopic scale of the analysed phenomena. This in turn will offer an opportunity to define and use the new criteria for controlling the mechanism of shear fracture. These criteria will be applicable to the complete range of temperatures and strain rates which are typical of metal working processes. Some examples of how these criteria may be applied in order to optimise the parameters of metal working will also be provided. These examples have made it possible to prove that the physical approach to shear fracture mesomechanism offers much broader cognitive and utilitarian opportunities than the existing purely mathematical methods. This is due to the fact that the physical approach allows for a deeper understanding of shear fracture meso- and macromechanism, and generates new criteria controlling this mechanism.

The physics of plastic deformation

International Journal of Plasticity, 1987

A simplified physical picture is extracted from the many complicated processes occurring during plastic deformation. It is based upon a set of continuously distributed straight edge dislocations, the carriers of plastic deformation, moving along their slip plane, interacting with each other and the lattice, multiplying and annihilating. The principles of continuum physics, that is the conservation laws of mass and momentum, and results from discrete dislocation modelling are then employed to analyze the situation and deduce a dosed set of relations describing the evolution of deformation and the associated forces that bring it about. A simple method is suggested for extending these relations to macroscales. This way, current phenomenological models of plasticity are physically substantiated. Moreover, a framework is provided for rigorously constructing small and large deformation theories of plasticity. Finally, a new possibility is made available for capturing the salient features of the heterogeneity of plastic flow including the wavelength of persistent slip bands, the width of shear bands, and the velocity of Portevin-Le Chatelier bands. I. INTRODUCTION Plastic deformation, as any other physical process, can be best understood by considering and properly analyzing the underlying mechanisms responsible for it. While several such mechanisms, including twinning, void growth, grain boundary sliding and phase transformations, may be envisioned, we single out the most important and simplest of them all: dislocation motion and evolution. Dislocations, however, are complex geometric objects and they reveal themselves indirectly through the electron microscope as "edges," "screws," "loops," "dipoles," "tangles" or "forests." Moreover, they do not just travel carrying deformation along, but they can also stop, multiply and annihilate. Their spatial distribution evolves neither isotropically nor uniformly. Instead, they move along specific slip planes in preferred slip directions, and they exhibit an ability to organize themselves in periodic layers, hexagonal and other ordered structures, in analogy to living systems. Such a trend to self-organization or symmetry breaking is a result of the competition between spatial gradients modelling dislocation motion/interaction, and nonlinearities modelling dislocation generation/annihilation. It is thus evident that the development of a physical theory of plastic deformation is *It is an honor and pleasure to have been given the opportunity to dedicate this article to Aris Phillips, not only because through his own research he set a permanent example for young scientists, in general) but also because he was a constant supporter of my earlier work on the mechanics and physics of diffusion in solids) in particular. In line with the great Greek tradition) he was an advocate of geometry) but he did not fail to recognize the importance of the analysis of the physical processes that bring geometric changes about. In fact, at the time that the majority of the mechanics journals showed hesitation towards new approaches to stress-assisted diffusion, environmental fracture and dislocation-based plasticity theories, Phillips' "Acta Mechanlca" became a vehicle for the dissemination of such ideas and helped their growth and maturity.

Physical mesomechanical criteria of plastic deformation and fracture

Physical Mesomechanics, 2009

A method is proposed to identify the near crack-tip region in a deforming object as the plastic zone and to diagnose its status as to whether or not the crack is about to open. The in-plane displacement field is visualized as a two-dimensional, full-field optical interferometric fringe pattern, and the diagnosis is made based on the plastic deformation and critical fracture criteria derived from a recent theory of deformation and fracture called physical mesomechanics. The proposed method is demonstrated for tensile experiments conducted for tin and steel specimens.

Physical Concept of Shear Fracture Mesomechanism and its Applications Vision article

The key objective of the present paper is an attempt to create an interface between the existing inconsistent views on the microscopic and macroscopic aspects of the mechanism of plastic deformation and shear fracture. This will be enabled by a focus on the course and effects of the evolution of dislocation structure, and will consist in considering an indirect, i.e. a mesoscopic scale of the discussed phenomena. Thanks to this, a synergy between the mechanisms of deformation and fracture of materials will be proven, which will provide an opportunity for a smooth transfer from the microscopic, through mesoscopic, to macroscopic scale of the analysed phenomena. This in turn will offer an opportunity to define and use the new criteria for controlling the mechanism of shear fracture. These criteria will be applicable to the complete range of temperatures and strain rates which are typical of metal working processes. Some examples of how these criteria may be applied in order to optimise the parameters of metal working will also be provided. These examples have made it possible to prove that the physical approach to shear fracture mesomechanism offers much broader cognitive and utilitarian opportunities than the existing purely mathematical methods. This is due to the fact that the physical approach allows for a deeper understanding of shear fracture meso-and macromechanism, and generates new criteria controlling this mechanism.

Propagative modes of plastic deformation

Le Journal de Physique IV, 1993

A major objective of the theory of defects is to relate the mechanical behaviour of macroscopic materials to the spatio-temporal evolution of the microstructure. The present paper deals with the dislocation-dynamical foundations of plastic instabilities and the propagation of slip in coherent plastic deformation modes (solitary plastic waves). Solitary waves arise from a proper balance between nonlinear localization effects and dispersion. By the dislocation dynamical approach, both nonlinear interactions and spatietemporal couplings are accessible in a quantitative way. Particularly, intrinsic length scales may be identified, in order to address the problem of propagation velocity selection. This is illustrated by means of various models of propagative plastic instabilities observed in tensile tests. The model assumptions are as follows: 1.) The microscopic cause of the repeated non-uniform yielding of the Portevin-Le Chdtelier (PLC) effect is dynamic strain-ageing, while the macroscopic propagation of PLC bands is controlled by intergranular incompatibility stresses. 2.) Llders bands in polycrystals result from a dislocation dynamics which is diffusion-like (owing to the random grain orientation) and bistable (owing to the stabilizing effect of the grain boundaries). 3.) This is to be compared with Llders bands in single crystals where the dislocation-poor initial state is unstable and band propagation is traced back to an interplay of cross-slip and non-axial stresses. 4.) Thermomechanical fronts arise from the interplay between heat generation during plastic deformation, heat conduction, and strain hardening.

Dynamics of plastic deformation based on restoring and energy dissipative mechanisms in plasticity

Physical Mesomechanics, 2008

The dynamics of plasticity is considered based on the field theoretical approach developed by physical mesomechanics. The equation of motion governing mesoscopic volume elements in plastically deforming media is derived from the mesomechanical field equation. Theoretical analysis on this equation of motion indicates that in the plastic regime solid-state media exert two types of forces; the restoring force and energy dissipating force. The former is associated with the shear modulus, and causes the displacement field to be oscillatory. The latter is associated with a quantity analogous to the electric charge, and causes the displacement field to be decaying. Experimental observations that support these theoretical considerations are presented.