Plastic Flow Properties Research Articles

Exploring the influence of loading geometry on the plastic flow properties of geological materials: Results from combined torsion + axial compression tests on calcite rocks

For technical reasons, virtually all plastic deformation experiments on geological materials have been performed in either pure shear or simple shear. These special case loading geometries are rather restrictive for those seeking insight into how microstructure evolves under the more general loading geometries that occur during natural deformation. Moreover, they are insufficient to establish how plastic flow properties might vary with the 3rd invariant of the deviatoric stress tensor (J3) which describes the stress configuration, and so applications that use those flow properties (e.g. glaciological and geodynamical modelling) may be correspondingly compromised. We describe an inexpensive and relatively straightforward modification to the widely used Paterson rock deformation apparatus that allows torsion experiments to be performed under simultaneously applied axial loads. We illustrate the performance of this modification with the results of combined stress experiments performed on Carrara marble and Solnhofen limestone at 500°–600 °C and confining pressures of 300 MPa. The flow stresses are best described by the Drucker yield function which includes J3-dependence. However, that J3-dependence is small. Hence for these initially approximately isotropic calcite rocks, flow stresses are adequately described by the J3-independent von Mises yield criterion that is widely used in deformation modelling. Loading geometry does, however, have a profound influence on the type and rate of development of crystallographic preferred orientation, and hence of mechanical anisotropy. The apparatus modification extends the range of loading geometries that can be used to investigate microstructural evolution, as well as providing greater scope for determining the shape of the yield surface in plastically anisotropic materials.

Influence of martensite volume fraction and hardness on the plastic behavior of dual-phase steels: Experiments and micromechanical modeling

Model dual-phase microstructures were developed to decouple the effect of martensite volume fraction and martensite hardness on the plastic behavior of dual-phase steels. The martensite volume fraction ranges from 11% to 37%, involving two levels of martensite hardness. The yield strength and tensile strength increase with increasing martensite volume fraction, while the uniform elongation decreases. The martensite hardness has a weak impact on the initial yield strength, but it significantly affects the flow behavior for sufficiently large martensite volume fraction. Increasing the hardness of the martensite leads to higher tensile strength combined with only a limited impact on uniform elongation, resulting in an improved strength/ductility balance. The experimental results are successfully captured using finite element based micromechanical analysis. Among others, periodic cell calculations show very good predictive capabilities of the overall plastic response when the stage-IV hardening of the ferrite is taken into account. Our numerical analysis reveals that an accurate description of the elasto-plastic behavior of the martensite is a key element to rationalize the mechanical behavior of DP steels. This modeling approach provides a framework for designing dual-phase steels with optimized plastic flow properties.

On the notch ductility of a magnesium-rare earth alloy

The room-temperature notch ductility of magnesium-rare earth alloy WE43 is investigated for two loading orientations. This material is endowed with quasi-isotropic plastic flow properties, higher strength and similar uniaxial ductility in comparison with other commercially available Mg alloys. The authors have recently shown that the notch ductility of a Mg–Al–Zn alloy is greater than its uniaxial ductility over a wide range of notch geometries. This paper investigates whether the same trends hold for WE43, discusses the orientation dependence of ductility and the propensity for intergranular fracture at high levels of hydrostatic tension. The latter mode of fracture is analyzed by means of detailed fractography in order to elucidate the role of grain-boundary particles and precipitates in the fracture process.

Temperature and stress state influence on void evolution in a high-strength dual-phase steel

The formability of dual-phase steels strongly depends on void nucleation and growth resulting from strain heterogeneities at the microscale between soft ferrite and hard martensitic phases. These mechanisms are dependent on the thermomechanical loading paths undergone by the alloy during the forming process. This work provides new experimental results concerning void evolution in high-strength dual-phase steel at different temperatures. Traditional and notched tensile samples were used with a geometry that provides stress triaxiality values similar to those attained in bending processes. Mechanical tests were performed for temperatures in the range of 293 to 723K, and the quantification of voids (i.e., size and number) was performed for different levels of equivalent plastic strain, temperature and stress triaxiality. The results show that the mechanical properties of the dual-phase steels are maintained up to a temperature of 573K. For higher temperatures, a degradation of plastic flow properties occurs in relation to the decrease of the martensite phase ratio and the softening of the ferritic phase; triaxiality also has less influence on the mechanical properties at warm test conditions. These results were discussed in terms of diffusional processes and local evolution of the microstructure with TEM observations. This work could be used to optimise the bending forming conditions of these alloys in industrial processes.

Deformation Property of TiC-Mo Solid Solution Single Crystal at High Temperature by Compression Test

To investigate the deformation properties of TiC-(5-20) mol% Mo solid solution single crystals at high temperature by compression testing, single crystals of various compositions were grown by the radio frequency floating zone technique and were deformed by compression at temperature from 1250K to 2270K at strain rates from <TEX>$5.1{\times}10^{-5}$</TEX> to <TEX>$5.9{\times}10^{-3}/s$</TEX>. The plastic flow property of solid solution single crystals was found to be clearly different among a three-temperature range (low, intermediate and high temperature ranges) whose boundaries were dependent on the strain rate. From the observed property, we conclude that the deformation in the low temperature range is controlled by the Peierls mechanism, in the intermediate temperature range by the dynamic strain aging and in the high temperature range by the solute atmosphere dragging mechanism. The work softening tends to become less evident with an increasing experimental temperature and with a decreasing strain rate. The temperature and strain rate dependence of the critical resolved shear stress is the strongest in the high temperature range. The curves are divided into three parts with different slopes by a transition temperature. The critical resolved shear stress (<TEX>${\tau}_{0.2}$</TEX>) at the high temperature range showed that Mo content dependence of <TEX>${\tau}_{0.2}$</TEX> with temperature and the dependence is very marked at lower temperature. In the higher temperature range, <TEX>${\tau}_{0.2}$</TEX> increases monotonously with an increasing Mo content.

Intermittent plastic flow of single crystals: central problems in plasticity: a review

The relationships between microstructure and plastic flow properties of copper single crystals deformed at low temperatures are reviewed based on the analysis of experimental results of mechanical, structural and electrical resistivity characterisation. Electrical resistivity measurements suggest that the flow stress correlates with the average density of defects accumulated in the microstructure. The characteristic microstructural length scale that determines the flow stress corresponds closely to the size of dislocation free channels and/or cells produced during plastic flow. The correspondence between micro- and macroscopic approaches to the plastic flow has been analysed based on the behaviour of dislocation mean free path and mean slip distance and their relationship to the microstructure. Production and annihilation of nanoscale debris and point defects and their role in plastic flow and questions pertinent to the influence of microstructural instabilities on the flow stress, workhardening, storage and annihilation of lattice defects have been discussed.

The influence of microstructure and composition on the plastic behaviour of dual-phase steels

This paper presents a systematic experimental study of the relative contributions of the martensite volume fraction, morphology and carbon content to the overall strength and ductility of dual-phase steels. To this end, model dual-phase steels combining three volume fractions of martensite, three martensitic carbon contents, and three arrangements of the phases were generated by suitable control of the heat treatments. The microstructures include long and short elongated, as well as equiaxed martensitic islands. The yield and tensile strengths increase with both the amount of martensite and its carbon content. The ductility in general increases with the volume fraction of martensite at constant martensitic carbon content, and conversely increases with the martensitic carbon content for a fixed volume fraction of martensite. Departure from the general trend is observed for combinations of a large volume fraction of martensite with a large martensitic carbon content, and is attributed to damage-induced softening or fracture. Specimens with long elongated martensite demonstrate an improved ductility compared to other microstructures. The plastic flow properties were further investigated by relying on a non-linear homogenization model which explicitly accounts for the microscopic parameters. The micromechanical model captures reasonably well the tensile behaviour for the complete range of martensite carbon contents and volume fractions.

New Joining Technology for Optimized Metal/Composite Assemblies

The development of a new joining technology, which is used to manufacture high strength hybrid constructions with thermoplastic composites (FRP) and metals, is introduced. Similar to natural regulation effects at trees, fibers around the FRP joint become aligned along the lines of force and will not be destroyed by the joining process. This is achieved by the local utilization of the specific plastic flow properties of the FRT and metal component. Compared with usual joining methods—such as flow drill screws, blind and self-piercing rivets—noticeably higher tensile properties can be realized through the novel process management. The load-bearing capability increasing effect could be proved on hybrid joints with hot-dip galvanized steel HX420LAD and orthotropic glass—as well as carbon—fiber reinforced plastics. The results, which were determined in tensile-shear and cross-shear tests according to DIN EN ISO 14273 and DIN EN ISO 14272, are compared with holding loads of established joining techniques with similar joining point diameter and material combinations.

Mechanical behavior and failure criterion of the gangue-based haydite concrete under triaxial loading

Triaxial loading tests were carried out on the gangue-based haydite concrete cube specimens (100 × 100 × 100 mm3) and plate specimens (100 × 100 × 50 mm3) to investigate the mechanical behavior of this kind of lightweight aggregate concrete (LWAC) under multi-axial stress state, and accordingly to develop the failure criterion for the finite element analysis of LWAC structures. Experimental results revealed that, under triaxial compression loading, most of the haydites within the specimen were crushed when the stress ratios σ 1/ σ 3 ≥ 0.3 and σ 2/ σ 3 ≥ 0.5. The gangue haydite concrete exhibited “plastic flow plateau” at this stage, which was conservatively regarded as the ultimate strength of the LWAC under triaxial compressive stress state. As a result, the tensile and compressive meridians on failure envelope surface are intersected with hydrostatic axial at two points, completely different from the characteristics of normal weight concrete for which the failure surface is unclosed and open-ended under compression. In terms of the test data, a four-parameter triaxial failure criterion for LWAC was developed. The strength envelope in the deviatoric plane adopted elliptic curve similar to that of Willam–Warnke model, and the tensile and compressive meridians were represented by quadratic functions with four parameters. Two alternative methods, i.e. the characteristic strength points method and the least square regression method, were used to determine the model parameters. After compared with the failure criterion recommended by fib Model Code 2010, methods proposed in this study are preferred especially when the plastic flow property is taken into account.

Tool Design Induced Anisotropic Flow Behavior of Hot Extruded Aluminum Profiles

The hot extrusion process may lead to frequently observed textures in the profiles, like fiber structures in longitudinal direction. Aluminum profiles (AA6060) were extruded with different tool types (flat-face dies and modified porthole dies). Compression tests of cylindrical specimens, which were machined out of these profiles likewise in extrusion direction, were conducted to examine possible effects of the in-plane anisotropy in lateral direction. A hardness distribution over the cross section of the specimens was measured. It was found that dependent on tool design and profile geometry, the specimens developed preferred lateral flow directions during upsetting. Simulations of the upsetting test, with assigned Hill parameters to consider anisotropy of the material, showed, that this anisotropy, not the local hardness nonuniformity, is the main reason for the detected plastic flow properties.

Mechanochemical degradation of the crosslinked and foamed EVA multicomponent and multiphase waste material for resource application

Effects of mechanochemical degradation of the crosslinked and foamed ethylene-vinyl acetate copolymer (EVA) multicomponent and multiphase waste material in a solid state are studied with a HAAKE rheometer. The ruptures of chemical bonds in its net and stereo-structures are caused by external mechanical energy input by the Roller rotors of the HAAKE rheometer. A resource application method of the degradation product is obtained. The optimum mechanochemical degradation conditions are at 120–130 °C in a Roller rotor rotation speed range of 40–60 rpm for 24 min. Smaller polymer molecules have been generated so that the interfacial morphology, phase compatibility and plastic flow properties of the degradation product are all improved. The sol extracted from the degradation product is characterized with thermogravimetric analysis, gel permeation chromatography, nuclear magnetic resonance and Fourier transform infrared spectroscopy. Free radical reaction mechanisms are demonstrated in the degradation process. The degradation product partly replaces pure EVA in preparing the crosslinked and foamed material. Results show that the degradation product could be in resource application.

Mechanical reliability of alloy-based electrode materials for rechargeable Li-ion batteries

Lithium alloys with metallic or semi-metallic elements are attractive candidate materials for the next-generation high-capacity rechargeable Li-ion battery anodes, due to their large specific and volumetric capacities. The key challenge in the application of these materials has been the very large volume changes, and the associated stress buildup and failure during insertion and extraction of lithium. While such stress buildup bears resemblance to the process of thermo-stress development, a phenomenon relatively well-understood, the physics involved in these alloy-based electrodes is much more complex in nature, more challenging to address, and richer in the variety of influencing factors. The reasons not only lie in the fact that the mechanical deformations are much larger, but also arise from the fact that the processes entail interactions among mass diffusion, chemical reactions, non-linear plastic flow and material property evolutions. In this paper, we present a review of some of the fundamental issues and the latest research related to the mechanical reliability of such alloy-based anode materials, with a focus on Li/Si, a material with the highest known theoretical energy storage capacity. The review primarily concerns continuum-level analyses, with relevant experimental data and atomistic-level results as input.

Extending the contact regimes to single-crystal indentations

This article provides a fresh look into the concept of the contact regimes in mechanistic analyses of indentation experiments performed in single crystals. In this context, spherical microindentation experiments in fcc metals are examined through detailed continuum crystal plasticity finite element simulations in order to provide meaning to the onset of fully-plastic and elasto-plastic contact regimes, which are well-known to rule the behavior of polycrystals exhibiting isotropic uniaxial stress–strain curves. Attention is then given to evaluate the applicability of Taborʼs hardness relation in ruling fully-plastic single-crystal spherical indentations as well as the extraction of the uniaxial plastic flow properties from a series of microindentation tests performed at different penetrations. A discussion is finally provided on the applicability of self-similarity assumptions to the analysis of single-crystal fully-plastic indentations.

Relations between particle shape, particle interaction and some rheological properties of hectorite dispersions.

Abstract The relation between viscosity and concentration of dilute aqueous hectorite (&amp;lt;0·2% hectorite) follows the Schulz-Blaschke equation, from which the intrinsic viscosity and particle interaction index (K) was calculated. More concentrated (up to 5·5% hectorite) dispersions possessed plastic flow properties, and a modified Schulz-Blaschke equation was used to calculate an interaction factor (&amp;#x004b;̄). A correlation was established of (&amp;#x004b;̄) and the static yield of the gels assessed by a cone and plate viscometer. The static yield value of gels was increased by the addition of sodium chloride over the range 0·001–0·005m. In the presence of 0·01m uni-univalent electrolytes the gelling power of the cations was in the order NH4 &amp;gt; K &amp;gt; Na &amp;gt; Li. A relation between sodium chloride concentration, hectorite concentration and static yield value of the gel was derived.

Instantaneous Plastic Flow Properties of Thin Brass Sheets Under Uniaxial and Biaxial Testing

In the prediction of formability of sheet metals, it is necessary to determine material properties which are normally evaluated on the data obtained by performing simple tests. The most important parameters affecting the value of limit strains of a sheet metal are the strain hardening exponent (n) and plastic anisotropy ratio (r). It was established that especially the value of n-parameter of brass sheets strongly depend on the specimen elongation and stress/strain state. The effect of instantaneous (differential) nt - value and rt - value on the forming limit curve was computed theoretically.

Void growth and coalescence in anisotropic plastic solids

Large strain finite element calculations of unit cells subjected to triaxial axisymmetric loadings are presented for plastically orthotropic materials containing a periodic distribution of aligned spheroidal voids. The spatial distribution of voids and the plastic flow properties of the matrix are assumed to respect transverse isotropy about the axis of symmetry of the imposed loading so that a two-dimensional axisymmetric analysis is adequate. The parameters varied pertain to load triaxiality, matrix anisotropy, initial porosity and initial void shape so as to include the limiting case of penny-shaped cracks. Attention is focussed on comparing the individual and coupled effects of void shape and material anisotropy on the effective stress–strain response and on the evolution of microstructural variables. In addition, the effect of matrix anisotropy on the mode of plastic flow localization is discussed. From the results, two distinct regimes of behavior are identified: (i) at high triaxialities, the effect of material anisotropy is found to be persistent, unlike that of initial void shape and (ii) at moderate triaxialities the influence of void shape is found to depend strongly on matrix anisotropy. The findings are interpreted in light of recent, microscopically informed models of porous metal plasticity. Conversely, observations are made in relation to the relevance of these results in the development and calibration of a broader set of continuum damage mechanics models.

Plastic Flow Properties and Microstructural Evolution in an Ultrafine-Grained Al-Mg-Si Alloy at Elevated Temperatures

An AA6082 alloy was subjected to eight passes of equal channel angular pressing at 100 °C, resulting in an ultrafine grain size of 0.2 to 0.4 μm. The tensile deformation behavior of the material was studied over the temperature range of 100 °C to 350 °C and strain rate range of 10−4 to 10−1 s−1. The evolution of microstructure under tensile deformation was investigated by analyzing both the deformation relief on the specimen surface and the dislocation structure. While extensive microshear banding was found at the lower temperatures of 100 °C to 150 °C, deformation at higher temperatures was characterized by cooperative grain boundary sliding and the development of a bimodal microstructure. Dislocation glide was identified as the main deformation mechanism within coarse grains, whereas no dislocation activity was apparent in the ultrafine grains.

Atomic and dislocation dynamics simulations of plastic deformation in reactor pressure vessel steel

The collective behavior of dislocations in reactor pressure vessel (RPV) steel involves dislocation properties on different phenomenological scales. In the multiscale approach, adopted in this work, we use atomic simulations to provide input data for larger scale simulations. We show in this paper how first-principles calculations can be used to describe the Peierls potential of screw dislocations, allowing for the validation of the empirical interatomic potential used in molecular dynamics simulations. The latter are used to compute the velocity of dislocations as a function of the applied stress and the temperature. The mobility laws obtained in this way are employed in dislocation dynamics simulations in order to predict properties of plastic flow, namely dislocation–dislocation interactions and dislocation interactions with carbides at low and high temperature.

Anisotropy in the plastic flow properties of single-crystal α titanium determined from micro-cantilever beams

Single-crystal micro-cantilever beams were manufactured from a polycrystalline commercially pure Ti sample using a focused ion beam. The cantilevers were approximately 5 μm wide and 30 μm long. A nano-indenter was then used to conduct micro-bending tests. Slip systems are selectively activated in these α-Ti beams by varying the crystal orientation along the beam. Increasing end deflections, from 1 to 8 μm, were applied to a series of similarly oriented cantilevers to show the progressive development of deformation. Load drops associated with strain bursts were seen in the mechanical response of some of the plastically deforming cantilevers. These appear to correlate with the formation of intense slip bands in the cantilevers. A crystal plasticity-based finite element model was applied to simulate the bending behaviour of single-crystal beams in the tested crystal orientations. Critical resolved shear stresses of 181 MPa for 〈 a〉 on the prismatic, 474 MPa for 〈 c + a 〉 on the pyramidal and 209 MPa for 〈 a〉 on the basal planes were determined via the reverse process of fitting the model load–displacement curves to experimental ones.

Dislocation dynamics modelling of the ductile-brittle-transition

Many materials like silicon, tungsten or ferritic steels show a transition between high temperature ductile fracture with stable crack grow and high deformation energy absorption and low temperature brittle fracture in an unstable and low deformation mode, the ductile-brittle-transition. Especially in steels, the temperature transition is accompanied by a strong increase of the measured fracture toughness over a certain temperature range and strong scatter in the toughness data in this transition regime. The change in fracture modes is affected by dynamic interactions between dislocations and the inhomogeneous stress fields of notches and small cracks. In the present work a dislocation dynamics model for the ductile-brittle-transition is proposed, which takes those interactions into account. The model can explain an increase with temperature of apparent toughness in the quasi-brittle regime and different levels of scatter in the different temperature regimes. Furthermore it can predict changing failure sites in materials with heterogeneous microstructure. Based on the model, the effects of crack tip blunting, stress state, external strain rate and irradiation-induced changes in the plastic flow properties can be discussed.