Solution of the thermoelastoplastic problem for a thin disk of plastically compressible material subjected to thermal loading

Elastoplastic solutions for thin plates and disks are sensitive to loading and plasticity conditions [1–5]. The plasticity condition for a number of metal materials depends on the mean stress [6–8]. In this case, when using the associated flow rule, plastic deformations do not satisfy the incompressibility condition, which is commonly accepted in statements of boundary-value problems for thin elastoplastic plates and disks [9–13]. It is of interest to determine the effect of plastic compressibility on the behavior of solutions for such structures. In this paper, a hollow disk in a rigid container subjected to a uniform temperature field is considered. The plasticity condition proposed in [14] is accepted. A general study of the set of equations including this plasticity condition and the associated flow rule was performed in [15]. The solution under the Mises plasticity condition was obtained in [1].     

Dislocation Plasticity Effects on Interfacial Fracture

Analyses are reviewed where plastic flow in the vicinity of an interfacial crack is represented in terms of the nucleation and glide of discrete dislocations. Attention is confined to cracks along a metal-ceramic interface, with the ceramic idealized as being rigid. Both monotonic and fatigue loading are considered. The main focus is on the stress and deformation fields near the crack tip predicted by discrete dislocation plasticity, in comparison with those obtained from conventional continuum plasticity theory. The role that discrete dislocation plasticity can play in interpreting interface fracture properties in the presence of plastic flow is discussed.     

Evaluating the effects of loading parameters on single-crystal slip in tantalum using molecular mechanics

This study is aimed at developing a physics-based crystal plasticity finite element model for body-centred cubic (BCC) metals, through the introduction of atomic-level deformation information from molecular dynamics (MD) investigations of dislocation motion at the onset of plastic flow. In this study, three critical variables governing crystal plasticity mediated by dislocation motion are considered. MD simulations are first performed across a range of finite temperatures up to 600K to quantify the temperature dependence of critical stress required for slip initiation. An important feature of slip in BCC metals is that it is not solely dependent on the Schmid law measure of resolved shear stress, commonly employed in crystal plasticity models. The configuration of a screw dislocation and its subsequent motion is studied under different load orientations to quantify these non-Schmid effects. Finally, the influence of strain rates on thermal activation is studied by inducing higher stresses during activation at higher applied strain rates. Functional dependence of the critical resolved shear stress on temperature, loading orientation and strain rate is determined from the MD simulation results. The functional forms are derived from the thermal activation mechanisms that govern the plastic behaviour and quantification of relevant deformation variables. The resulting physics-based rate-dependent crystal plasticity model is implemented in a crystal plasticity finite element code. Uniaxial simulations reveal orientation-dependent tension–compression asymmetry of yield that more accurately represents single-crystal experimental results than standard models.     

非晶力学流变的自组织临界行为

非晶材料是由液体快冷冻结而成的结构无序的亚稳态固体.在受力条件下,非晶材料表现出独特和复杂的流变行为,具有跨尺度的高度时空不均匀特征,并在一定条件下表现出自组织临界行为,和自然界以及物理系统中许多复杂体系的动力学行为相似.本文结合作者近年来在非晶合金流变行为方面的研究结果,对非晶材料流变的研究进展和物理机制的认识进行介绍,包括非晶材料流变的跨尺度特征、表征和微观结构机制,以及近年来发现的非晶力学流变的自组织临界行为、物理机制等.最后,对非晶材料流变行为研究中亟需解决的问题进行了总结和展望.     

Wave phenomena during creep in macrocrystalline aluminum

A wave model of plastic flow, which has been theoretically substantiated and experimentally verified under the conditions of active quasistatic loading of diverse materials, is being developed on the basis of concepts of the autocatalytic nature of elementary acts of plastic deformation. Data from the study of the evolution of distortion fields during low-temperature creep of macrocrystalline aluminum are given in order to explain the tighter relation between the parameters of plastic-deformation waves and the characteristics of the elementary processes of plastic shear. The wave nature of this evolution is emphasized and a linear correlation is found between the creep rate and the velocity of the plasticity waves. The activation volumes of the processes controlling the velocity of the plastic waves and the creep rate are shown to be correlated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 5–9, April, 1991.     

Kinetics of localized plasticity macrodomains at the prefracture stage in metals

The behavior of localized plasticity macrodomains is experimentally studied at the final stage of the plastic flow in going to necking and ductile fracture in fcc, bcc, and hcp materials. General features of the localization process at the stage of prefracture are found. They are a constant velocity of domains and their tendency to consistently move toward the focus of a bundle of straight lines in space-time diagrams. A correlation between the type of fracture and the kinetics of localized plasticity domains is established.     

Effect of hydrostatic pressure on the plasticity of copper

A new technique is designed to plot stress-strain curves during torsion at hydrostatic pressure up to 250 MPa. It is used to study the effect of pressure on the torsional strain to failure of copper. The experimental results demonstrate that the plasticity of the material increases in the pressure range 0–150 MPa and remains constant in the range 150–250 MPa. This reaction of the material is likely to be related to the fact that pressure can increase the dislocation density required for plastic flow.     

Model of Plastic Deformation and Failure of Solids

Russian Physics Journal - A model of localized plastic flow development based on the idea about interaction between plasticity carriers and acoustic emission pulses generated by developing...     

Discrete rearranging disordered patterns, part II: 2D plasticity, elasticity and flow of a foam

The plastic flow of a foam results from bubble rearrangements. We study their occurrence in experiments where a foam is forced to flow in 2D: around an obstacle; through a narrow hole; or sheared between rotating disks. We describe their orientation and frequency using a topological matrix defined in the companion paper (F. Graner, B. Dollet, C. Raufaste, and P. Marmottant, this issue, 25 (2008) DOI 10.1140/epje/i2007-10298-8), which links them with continuous plasticity at large scale. We then suggest a phenomenological equation to predict the plastic strain rate: its orientation is determined from the foam's local elastic strain; and its rate is determined from the foam's local elongation rate. We obtain a good agreement with statistical measurements. This enables us to describe the foam as a continuous medium with fluid, elastic and plastic properties. We derive its constitutive equation, then test several of its terms and predictions.     

Structural states and specific features of the deformation behavior of metals and alloys with mixed nano-and micrograin structure

A classification of the structural states of materials with a mixed nano-and microcrystalline structure is proposed. Theoretical analysis of the structural mechanisms and peculiarities of plastic flow of singlephase and two-phase nanostructured metals and alloys with a bimodal size distribution of grains and phases is performed. The effect of grain-boundary and dislocation mechanisms of plastic flow on the specific features of the deformation behavior and plasticity of nanocrystalline materials is analyzed. A microstructural model of strain hardening of a material with two-scale nano-and micrograin structure is proposed and the condition for the loss of plastic flow stability of such a material is investigated. The dependence of the yield strength and uniform strain of nanocrystalline materials with a two-scale structure on the grain size and the ratio of the volume fractions of the nano-and microstructural components is calculated.     

Size effect in plastically deformed passivated thin films

The flow theory of mechanism-based strain gradient plasticity theory (MSG) developed by Qiu et al. (2003) is extended for incompressible material. The MSG flow theory is used to predict the increase of plastic work hardening for plane strain tension of surface-passivated Cu thin film. The theoretical predictions agree well with experiments for suitably chosen material parameters. Contributed by HWANG Keh-Chih     

A discrete dislocation plasticity analysis of a single-crystal semi-infinite medium indented by a rigid surface exhibiting multi-scale roughness

Discrete dislocation plasticity was used to analyse plane-strain indentation of a single-crystal elastic–plastic semi-infinite medium by a rigid surface exhibiting multi-scale roughness, characterised by self-affine (fractal) behaviour. Constitutive rules of dislocation emission, glide and annihilation were used to model short-range dislocation interactions. Dislocation multiplication and the development of subsurface shear stresses due to asperity microcontacts forming between a single-crystal medium and a rough surface were examined in terms of surface roughness and topography (fractal) parameters, slip-plane direction and spacing, dislocation source density, and contact load (surface interference). The effect of multi-scale interactions between asperity microcontacts on plasticity is elucidated in light of results showing the evolution of dislocation structures. Numerical solutions yield insight into plastic flow of crystalline materials in normal contact with surfaces exhibiting multi-scale roughness.     

Plastic distortion as a fundamental mechanism in nonlinear mesomechanics of plastic deformation and fracture

Any deformed solid represents two self-consistent functional subsystems: a 3D crystal subsystem and a 2D planar subsystem (surface layers and all internal interfaces). In the planar subsystem, which lacks thermodynamic equilibrium and translation invariance, a primary plastic flow develops as nonlinear waves of structural transformations. At the nanoscale, such planar nonlinear transformations create lattice curvature in the 3D subsystem, resulting in bifurcational interstitial states there. The bifurcational states give rise to a fundamentally new mechanism of plastic deformation and fracture—plastic distortion—which is allowed for neither in continuum mechanics nor in fracture mechanics. The paper substantiates that plastic distortion plays a leading role in dislocation generation and glide, plasticity and superplasticity, plastic strain localization and fracture.     

A thermodynamic discussion of the possibility of singular yield conditions in plasticity theory

A generalization is given of the author's theory for plasticity phenomena. The generalization leads to the possibility that the yield surface has singularities. From the theory a formula may be derived which is analogous to a formula proposed by Koiter for plastic flow in media with singular yield surfaces. The possibility of elastic relaxation phenomena in the preplastic range is included in the developed formalism.     

Bridging room-temperature and high-temperature plasticity in decagonal Al–Ni–Co quasicrystals by micro-thermomechanical testing

Ever since quasicrystals were first discovered, they have been found to possess many unusual and useful properties. A long-standing problem, however, significantly impedes their practical usage: steady-state plastic deformation has only been found at high temperatures or under confining hydrostatic pressures. At low and intermediate temperatures, they are very brittle, suffer from low ductility and formability and, consequently, their deformation mechanisms are still not clear. Here, we systematically study the deformation behaviour of decagonal Al–Ni–Co quasicrystals using a micro-thermomechanical technique over a range of temperatures (25–500 °C), strain rates and sample sizes accompanying microstructural analysis. We demonstrate three temperature regimes for the quasicrystal plasticity: at room temperature, cracking controls deformation; at 100–300 °C, dislocation activities control the plastic deformation exhibiting serrated flows and a constant flow stress; at 400–500 °C, diffusion enhances the plasticity showing homogenous deformation. The micrometer-sized quasicrystals exhibit both high strengths of ~2.5–3.5 GPa and enhanced ductility of over 15% strains between 100 and 500 °C. This study improves understanding of quasicrystal plasticity in their low- and intermediate-temperature regimes, which was poorly understood before, and sheds light on their applications as small-sized structural materials.     

On the homogeneous nucleation and propagation of dislocations under shock compression

The dynamic response of crystalline materials subjected to extreme shock compression is not well understood. The interaction between the propagating shock wave and the material’s defect occurs at the sub-nanosecond timescale which makes in situ experimental measurements very challenging. Therefore, computer simulation coupled with theoretical modelling and available experimental data is useful to determine the underlying physics behind shock-induced plasticity. In this work, multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper reported at ultra-high strain rates shock loading. We compare the value of threshold stress for homogeneous nucleation obtained from elastodynamic solution and standard nucleation theory with MDDP predictions for copper single crystals oriented in the [0 0 1]. MDDP homogeneous nucleation simulations are then carried out to investigate several aspects of shock-induced deformation such as; stress profile characteristics, plastic relaxation, dislocation microstructure evolution and temperature rise behind the wave front. The computation results show that the stresses exhibit an elastic overshoot followed by rapid relaxation such that the 1D state of strain is transformed into a 3D state of strain due to plastic flow. We demonstrate that MDDP computations of the dislocation density, peak pressure, dynamics yielding and flow stress are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models. MDDP-based models for dislocation density evolution, saturation dislocation density, temperature rise due to plastic work and strain rate hardening are proposed. Additionally, we demonstrated using MDDP computations along with recent experimental reports the breakdown of the fourth power law of Swegle and Grady in the homogeneous nucleation regime.     

Localization of twinning plastic strain in doped γ-Fe single crystals

The patterns of plastic flow localization in high-manganese γ-Fe fcc single crystals oriented for twinning upon stretching are obtained. Basic space-time features of strain localization at the stages of yield plateau, easy glide, and linear hardening are established. The velocity of strain localization sites during stretching is determined. Conditions under which plasticity autowaves appear in the strained medium are discussed. It is demonstrated that the local strain distributions in the case of twinning are similar to those due to dislocation glide.     

Plasticity determined by indentation and theoretical plasticity of materials

Characterization of the plasticity of materials by the part of plastic strain in the total elastic-plastic strain and application of this characteristic at indentation is considered. The dependence of the new plasticity characteristic on the structure and temperature is discussed. The concept of theoretical plasticity is introduced and the theoretical plasticity is calculated for a number of materials.     

On a New Type of Plastic Deformation Waves in Solids

Plastic-strain localization of fcc, bcc, and hcp single crystalline and polycrystalline materials is investigated in different stages in the loading curve under active uniaxial tension. Localization patterns are found to be of an ordered character and can be classified into four types, each corresponding to a certain stage in the loading curve. The regularities observed are interpreted in terms of self-organization in open systems. It is shown that all types of localization patterns can be treated as autowaves of plastic flow. The wave parameters are analyzed and the fundamental difference from the well-known waves of plasticity under shock loading is demonstrated.