Showing papers on "Deformation (engineering) published in 2000"

Load transfer and deformation mechanisms in carbon nanotube-polystyrene composites

Abstract: Multiwall carbon nanotubes have been dispersed homogeneously throughout polystyrene matrices by a simple solution-evaporation method without destroying the integrity of the nanotubes. Tensile tests on composite films show that 1 wt % nanotube additions result in 36%–42% and ∼25% increases in elastic modulus and break stress, respectively, indicating significant load transfer across the nanotube-matrix interface. In situ transmission electron microscopy studies provided information regarding composite deformation mechanisms and interfacial bonding between the multiwall nanotubes and polymer matrix.

Microstructure controlled shear band pattern formation and enhanced plasticity of bulk metallic glasses containing in situ formed ductile phase dendrite dispersions

Abstract: Results are presented for a ductile metal reinforced bulk metallic glass matrix composite based on glass forming compositions in the Zr-Ti-Cu-Ni-Be system. Primary dendrite growth and solute partitioning in the molten state yields a microstructure consisting of a ductile crystalline Ti-Zr-Nb b phase, with bcc structure, in a Zr-Ti-Nb-Cu-Ni-Be bulk metallic glass matrix. Under unconstrained mechanical loading organized shear band patterns develop throughout the sample. This results in a dramatic increase in the plastic strain to failure, impact resistance, and toughness of the metallic glass. PACS numbers: 81.40. – z, 81.05.Kf Zr41.2Ti13.8Cu12.5Ni10Be22.5 (V1) exhibits an exceptional bulk metallic glass (BMG) forming ability that has motivated investigations of its mechanical behavior [1– 3]. This alloy exhibits a 1.9 GPa tensile yield strength, and a 2% elastic strain prior to failure under tensile or compressive loading. However, as in all metallic glasses, V1 specimens loaded in a state of uniaxial or plane stress fail catastrophically on one dominant shear band and show little global plasticity. Specimens loaded under constrained geometries (plane strain) fail in an elastic, perfectly plastic manner by the generation of multiple shear bands. Multiple shear bands are observed when the catastrophic instability is avoided via mechanical constraint, e.g., in uniaxial compression, bending, rolling, and under localized indentation. This behavior under deformation has limited the application of bulk metallic glasses as an engineering material. This Letter presents results for a new class of ductile metal reinforced BMG matrix composites prepared via in situ processing. Under loading, the two-phase microstructure leads to spatial variations in elastic properties as well as the conditions for yielding, the ductile phase having a lower yield strain. The initiation and propagation of shear bands is controlled by the scale and geometry of the ductile phase dispersion with the result that deformation occurs through the development of highly organized patterns of regularly spaced shear bands distributed uniformly throughout the sample. The compositions in the Zr-Ti-Cu-Ni-Be system are compactly written in terms of a pseudoternary Zr-Ti-X phase diagram, where X represents the moiety Be9Cu5Ni4, characteristic of Zr41.2Ti13.8Cu12.5Ni10Be22.5. Results presented here are for alloys of the form Zr1002x2zTixMz1002yXy, where M is an element that stabilizes the crystalline b phase in Ti- or Zr-based alloys. The inset in Fig. 1 shows the x-ray diffraction pattern for the nominal composition Zr75Ti18.34Nb6.6675X25; i.e., an alloy with M Nb, z 6.66, x 18.34, and y 25. The diffraction pattern was obtained with an INEL diffractometer (Co-Ka radiation) on the cross sectioned surface of a 25 g arc melted rod of roughly cylindrical diameter, f 1 cm. The peaks shown [with (hkl) values labeled] are due to the bcc phase. A Nelson-Riley extrapolation yields a lattice parameter a 3.496 A [4]. Upon cooling from the high temperature melt, the alloy undergoes partial crystallization by nucleation and subsequent dendritic growth of the b phase in the remaining liquid. The remaining liquid subsequently freezes to the glassy state producing a twophase microstructure containing b-phase dendrites in a glass matrix. The final microstructure of a chemically etched specimen is shown in the scanning electron microscopy (SEM) image of Fig. 1. SEM electron microprobe analysis gives the average composition for the b-phase dendrites (light phase in Fig. 1) to be Zr 71Ti16.3Nb10Cu1.8Ni0.9. Under the assumption that all of the Be in the alloy is partitioned into the matrix we estimate that the average composition of the amorphous matrix (dark phase) is Zr47Ti12.9Nb2.8Cu11Ni9.6Be16.7. Both are quoted

An Asperity Microcontact Model Incorporating the Transition From Elastic Deformation to Fully Plastic Flow

Abstract: This paper presents an elastic-plastic asperity microcontact model for contact between two nominally flat surfaces. The transition from elastic deformation to fully plastic flow of the contacting asperity is modeled based on contact-mechanics theories in conjunction with the continuity and smoothness of variables across different modes of deformation. The relations of the mean contact pressure and contact area of the asperity to its contact interference in the elastoplastic regime of deformation are respectively modeled by logarithmic and fourth-order polynomial functions. These asperity-scale equations are then used to develop the elastic-plastic contact model between two rough surfaces, allowing the mean surface separation and the real area of contact to be calculated as functions of the contact load and surface plasticity index. Results are presented for a wide range of contact load and plasticity index, showing the importance of accurately modeling the deformation in the elastoplastic transitional regime of the asperity contacts. The results are also compared with those calculated by the GW and CEB models, showing that the present model is more complete in describing the contact of rough surfaces.

Investigations and applications of severe plastic deformation

Abstract: Preface Introduction I: Innovations in Severe Plastic Deformation Processing and Process Modeling Severe Plastic Deformation of Materials by Equal Channel Angular Extrusion (ECAE) RE Goforth, et al Severe Plastic Deformation of Steels: Structure, Properties and Techniques SV Dobatkin Application of ECAP - Technology for Producing Nano- and Microcrystalline Materials VI Kopylov Severe Deformation Based Process for Grain Subdivision and Resulting Microstructures AK Ghosh, W Huang Modeling of Continual Flows in Angular Domains BV Koutcheryaev Synthesis and Characterization of Nanocrystalline Tial Based Alloys ON Senkov, FH Froes Formation of Submicrocrystalline Structure in TiAl and Ti3Al Intermetallics via Hot Working G Salishchev, et al Severe Plastic Deformation Processes Modeling and Workability SL Semiatin, et al The Effect of Strain Path on the Rate of Formation of High Angle Grain Boundaries During ECAE PB Prangnell, et al Thermomechanical Conditions for Submicrocrystalline Structure Formation by Severe Plastic Deformation FZ Utyashev, et al II: Microstructural Characterization and Modeling of Severe Plastic Deformation Materials Strengthening Processes of Metals by Severe Plastic Deformation Analyses with Electron and Synchrotron Radiation MJ Zehetbauer Size Distribution of Grains or Subgrains, Dislocation Density and Dislocation Character by Using the Dislocation Model of Strain Anisotropy in X-Ray Line Profile Analysis T Ungar X Ray-Studies and Computer Simulation of Nanostructured SPD Metals IV Alexandrov An Analysis of Heterophase Structures of Ti3Al, TiAl, Ni3Al Intermetallics Synthesized by the Method of the SphericalShock Wave Action BA Greenberg, et al Structural Changes Induced by Severe Plastic Deformation of Fe- and Co-Based Amorphous Alloys N Noskova, et al Structure of Grains and Internal Stress Fields in Ultrafine Grained NI Produced by Severe Plastic Deformation NA Koneva, et al Crystal Lattice Distorsions in Ultrafine-Grained Metals Produced by Severe Plastic Deformation AN Tyumentsev, et al Grain and Subgrain Size-Distribution and Dislocation Densities in Severely Deformed Copper Determined by a New Procedure of X-Ray Line Profile Analysis T Ungar, et al Calculation of Energy Intensity and Temperature of Mechanoactivation Process in Planetary Ball Mill by Computer Simulation EV Shelekhov, et al III: Microstructure Evolution During Severe Plastic Deformation Processing Microstructural Evolution During Processing by Severe Plastic Deformation TG Langdon, et al Characterization of Ultrafine-Grained Structures Produced by Severe Plastic Deformation Z Horita, et al Fragmentation in Large Strain Cold Rolled Aluminium as Observed by Synchrotron X-Ray Bragg Peak Profile Analysis (SXPA), Electron Back Scatter Patterning (EBSP) and Transmission Electron Microscopy (TEM) E Schafler, et al Influence of Thermal Treatment and Cyclic Plastic Deformation on the Defect Structure in Ultrafine-Grained Nickel E Thiele, et al Nanostructure State as Nonequilibrium Transition in Grain Boundary Defects in SPD Condition OB Naimark Texture, Structural Evolution and Mechanical Properties in AA5083 Processed by ECAE L Dupuy, et al A TEM-Based Disclination Model for the Substructure Evolution under Severe Plastic Deformation M Seefeldt, et al Physical Mesomechanics of Ultrafine-Grained Metals VE Panin Microstructure Evo

Experimental analysis of deformation mechanisms in a closed-cell aluminum alloy foam

Abstract: The evolution of plastic deformation in a cellular Al alloy upon axial compression is monitored through a digital image correlation procedure. Three stages in the deformation response have been identified. The first involves localized plastic straining at cell nodes. It occurs uniformly and leads to a nominal loading modulus appreciably lower than the stiffness. The second comprises discrete bands of concentrated strain containing cell membranes that experience plastic buckling, elastically constrained by surrounding cells. In this phase, as the loading increases, previously formed bands harden, giving rise to new bands in neighboring regions. The localized bands exhibit a long-range correlation with neighboring bands separated by 3–4 cells along the loading direction. This length scale characterizes the continuum limit. Thirdly, coincident with a stress peak, σo, one of the bands exhibits complete plastic collapse. As the strain increases, this process repeats, subject to small stress oscillations around σo.

Gradient-dependent deformation of two-phase single crystals

Abstract: In this work, a gradient- and rate-dependent crystallographic formulation is proposed to investigate the macroscopic behaviour of two-phase single crystals. The slip-system-based constitutive formulation relies on strain-gradient concepts to account for the additional strengthening mechanism associated with the deformation gradients within a single crystal with a high volume fraction of dispersed inclusions. The resulting total slip resistance in each active system is assumed to be due to a mixed population of forest obstacles arising from both statistically stored and geometrically necessary dislocations. The non-local theory is implemented numerically into the finite element method and used to investigate the effect of the relevant microstructural (i.e., size and volume fraction of precipitated inclusions) and deformation-gradient-related length scales on the macroscopic behaviour of a typical nickel-based superalloy single crystal. An analytical framework to link the strain-gradient effects at the microscopic level with the macroscopic behaviour of an equivalent homogeneous single crystal is also proposed.

Microsample tensile testing of nanocrystalline metals

Abstract: A novel non-contact strain measurement technique has been employed to measure the tensile properties of extremely small ‘microsamples’ of pure high-density ultrafine-grained Al (ufg-Al) nanocrystalline Cu (n-Cu) and nanocrystalline Ni (n-Ni). These microsample tests confirmed the absence of Young's modulus variations for metals with grain sizes approaching 25 nm. Significant strength enhancements were associated with the nanocrystalline specimens; the tensile stresses achieved in these microsample tests were measured to be an appreciable fraction of the theoretical shear strength for these metals. The ufg-Al samples (diameter, 250 nm) exhibited extensive plasticity while deformation in the n-Ni (diameter, 28 nm) remained almost entirely elastic up to failure at 1500MPa. The n-Cu samples were found to have a multiscale grain structure that produced an attractive balance of strength and ductility. Transmission electron microscopy investigations of deformed n-Ni failed to produce any evidence of dis...

On the die corner gap formation in equal channel angular pressing

Abstract: A die internal corner gap is usually found during equal channel angular pressing (ECAP) of materials. Finite element analysis using the DEFORM2D code is carried out in order to investigate the corner gap formation between the die and workpiece during the plane strain ECAP process. The comparison of the deformation and the corner gap formation behaviour between the strain hardening material and the quasi-perfect plastic material was made. The mechanism of the corner gap formation is described in conjunction with the strain hardening behaviour and local flow velocity of the workpiece in the deforming zone. The adjustment of the corner angle from the die corner angle to the arc curvature of the workpiece is necessary for a better prediction of the strain during ECAP.

Carbon Fiber Reinforced Shape Memory Polymer Composites

Abstract: In this paper we present results on the deformation of carbon fiber reinforced shape memory polymer matrix composites for deployable space structure applications. The composites were processed using resin transfer molding or a pre-impregnated (pre-preg) laminate press, with both satin and plain weave fiber architectures. The polymer matrix glass transition temperature, Tg, was approximately 95°C. Composite specimens were bent to specific radii at T = 120°C, and cooled while constrained to a temperature of 25°C, which left them frozen in the bent state. Heating the specimens above Tg caused the composites to return to their original unbent shape with up to 95% recovery based on bend angle. The effect of constraint hold times up to 350 hours on the recoverability was found to be negligible. Microscopic investigations revealed that the dominant local deformation mode of the composites was buckling of the carbon fibers on the inner surface of the bend. Localized buckling out of the material plane lead to detr...

Deformation and Failure Modes of Adhesively Bonded Elastic Layers

Abstract: Adhesively bonded elastic layers with thicknesses that are small relative to their lateral dimensions are used in a wide variety of applications. The mechanical response of the compliant layer when a normal stress is imposed across its thickness is determined by the effects of lateral constraints, which are characterized by the ratio of the lateral dimensions of the layer to its thickness. From this degree of confinement and from the material properties of the compliant layer, we predict three distinct deformation modes: (1) edge crack propagation, (2) internal crack propagation, and (3) cavitation. The conditions conductive for each mode are presented in the form of a deformation map developed from fracture mechanics and bulk instability criteria. We use experimental data from elastic and viscoelastic materials to illustrate the predictions of this deformation map. We also discuss the evolution of the deformation to large strains, where nonlinear effects such as fibrillation and yielding dominate the failure process.

The Influence of Intrinsic Strain Softening on Strain Localization in Polycarbonate: Modeling and Experimental Validation

Abstract: Intrinsic strain softening appears to be the main cause for the occurrence of plastic localization phenomena in deformation of glassy polymers. This is supported by the homogeneous plastic deformation behavior that is observed in polycarbonate samples that have been mechanically pretreated to remove (saturate) the strain softening effect. In this study, some experimental results are presented and a numerical analysis is performed simulating the effect of mechanical conditioning by cyclic torsion on the subsequent deformation of polycarbonate. To facilitate the numerical analysis of the mechanical rejuvenation effect, a previously developed model, the compressible Leonov model, is extended to describe the phenomenological aspects of the large strain mechanical behavior of glassy polymers. The model covers common observable features, like strain rate, temperature and pressure dependent yield, and the subsequent strain softening and strain-hardening phenomena. The model, as presented in this study, is purely single mode' (i.e., only one relaxation time is involved), and therefore it is not possible to capture the nonlinear viscoelastic pre-yield behavior accurately. The attention is particularly focused on the large strain phenomena. From the simulations it becomes clear that the preconditioning treatment removes the intrinsic softening effect, which leads to a more stable mode of deformation.

Analysis of the billet deformation behaviour in equal channel angular extrusion

Abstract: Equal channel angular extrusion is a relatively novel method for deforming materials to very high plastic strains, with no net change in the billet's shape. However, before the technique can be exploited it is important to understand the deformation behaviour within the die and its relationship to the tooling configuration and friction conditions. Billets containing scribed grids, simple finite element analysis, and microstructural evidence have been used to investigate this issue. It has been found that the strain achieved is sensitive to the die angle, friction conditions, and the application of a back-pressure, all of which can have a large effect on the microstructure and strain inhomogeneity within the processed billet. The distribution of strain is most uniform, and approximates most closely to a simple shear, if the deformation zone is constrained to be as narrow as possible. The best processing conditions appear to be obtained with a sharp die corner, low friction, and a constraining back-pressure. Embedded marker wire experiments have shown that on repeatedly extruding a billet with a constant strain path rotation of material occurs around the billet's ends. This results in the sheared billet wrapping around on itself during the process and thus maintaining the billet's shape, despite the increasing shear strain in each extrusion cycle.

Substrate curvature due to thin film mismatch strain in the nonlinear deformation range

Abstract: The physical system considered is a thin film bonded to the surface of an initially flat circular substrate, in the case when a residual stress exists due to an incompatible mismatch strain in the film. The magnitude of the mismatch strain is often inferred from a measurement of the curvature it induces in the substrate. This discussion is focused on the limit of the linear range of the relationship between the mismatch strain and the substrate curvature, on the degree to which the substrate curvature becomes spatially nonuniform in the range of geometrically nonlinear deformation, and on the bifurcation of deformation mode from axial symmetry to asymmetry with increasing mismatch strain. Results are obtained on the basis of both simple models and more detailed finite element simulations.

Transmission electron microscopy observation of deformation microstructure under spherical indentation in silicon

Abstract: Spherical indentation of crystalline silicon has been studied using cross-sectional transmission electron microscopy (XTEM). Indentation loads were chosen below and above the yield point for silicon to investigate the modes of plastic deformation. Slip planes are visible in the XTEM micrographs in both indentation loads studied. A thin layer of polycrystalline material has been identified (indexed as Si-XII from diffraction patterns) on the low-load indentation. The higher-load indentation revealed a large region of amorphous silicon. The sequence of structural deformation by indentation in silicon has been observed with the initial deformation mechanism being slip until phase transformations can take place.