Simple Shear Geometry Research Articles

Transition in deformation mechanism of aluminosilicate glass at high pressure and room temperature

AbstractDeformation experiments for 20(MgO or Na2O)‐20Al2O3‐60SiO2 glasses were performed in simple shear geometry at 1.5‐5 GPa and room temperature. An abrupt change in the thinning rate and the turning of the birefringence azimuth at a shear strain of γ = 1‐2 indicate a transition of deformation mechanism from uniaxial compression aided by densification to shear flow in the glasses. The high‐dense magnesium aluminosilicate glass showed strain softening controlled by the rearrangement of the tetrahedral network. On the other hand, low‐dense sodium aluminosilicate glass deformed by packing‐induced flow associated with densification and via the rearrangement of the tetrahedral network at lower and higher strains, respectively. The transition of the deformation mechanism was triggered by the limitations of the densification of the tetrahedral network. The difference of deformation mechanism brought about higher strain in magnesium aluminosilicate glass than sodium aluminosilicate glass at the same stress condition. Easiness of remarkable deformation, which relaxed residual stress, and high deformability contributed to the high ductility of the MgO‐aluminosilicate glass.

Glass formation by severe plastic deformation of crystalline Cu|Zr nano-layers

We use molecular dynamics simulation to study the formation of glasses during severe plastic deformation of crystalline Cu|Zr nano-layers in a simple shear geometry. The early stage of plasticity is governed by dislocation plasticity. Dislocation-interface interaction induces interface roughening and slow atomic-level intermixing around interfaces. A solid-state crystal-to-amorphous transition starts near the interfaces. At high strain, the interfaces are refined to merge, leading to the formation of an intermixed glassy zone that eventually spans our simulation cell. Dislocation emission is then suppressed and strain is fully accommodated by this glassy phase. We characterize the crystal-to-amorphous transition using local structural measures, such as the pair distribution function and Voronoi analysis. The onset of glass formation occurs at a critical shear strain that increases with layer thickness. Our simulations indicate that the parameter controlling this onset is the theoretical affine interface distance, i.e. the amorphization occurs at a critical value of around 3–4 nm that increases slightly with the increase of layer thickness. Our work provides insights on mechanically driven glass formation in multicomponent systems, as e.g. realized in ball-mills or high-pressure torsion experiments.

Substructural Alignment during ECAE Processing of an Al-0.1Mg Aluminium Alloy

An investigation has been carried out into the microstructures developed during the early stages of equal channel angular extrusion (ECAE) in a polycrystalline single-phase Al-0.13Mg alloy, with emphasis on the substructural alignment with respect to the die geometry and the crystallographic slip systems, which is essentially related to the grain refinement and texture development during deformation. The material was processed by ECAE at room temperature to three passes, via a 90° die. Microstructures were examined and characterized by EBSD. It was found that dislocation cell bands and microshear bands were respectively the most characteristic deformation structures of the first and second pass ECAE. Both formed across the whole specimen and to align approximately with the die shear plane, regardless of the orientation of individual grains. This confirmed that substructural alignment was in response to the direction of the maximum resolved shear stress rather than to the crystallographic slip systems. However, a significant fraction of material developed preferred orientations during deformation that allowed the coincidence between the crystallographic slip systems and the simple shear geometry to occur, which governed texture development in the material. The third pass deformation was characterized with the formation of a fibre structure with a significant fraction of high angle boundaries, being aligned at an angle to the extrusion direction, which was determined by the total shear strain applied.

Temperature dependence of [100](010) and [001](010) dislocation mobility in natural olivine

Abstract Dislocation recovery experiments were conducted on pre-deformed olivine single crystals at 1450 to 1760 K, room pressure, and oxygen partial pressures near the Ni–NiO buffer to determine the annihilation rates for [100] and [001] dislocations on the (010) plane. Olivine single crystals were first deformed to activate the desired slip systems under simple shear geometry and then annealed at target conditions. The edge and screw dislocations with Burgers vectors, b, of [100] and [001], respectively, both elongated in the [001] direction were produced by the deformation. The dislocation annihilation rate constants of both types of dislocations are identical within 0.3 log unit. The activation energies for both dislocations are also identical, i.e., ∼400 kJ/mol, which is also identical to that of the Si self-diffusion coefficient. This correspondence suggests that olivine dislocation creep controlled by a diffusion-controlled process under low-stress and high-temperature conditions. This study offers a potential insight into the formation of AG-type fabric in olivine.

Shearrate diffusion and constitutive relations during transients in simple shear

Granular matter, consisting of hard, frictional, cohesionless spheres, sheared in a simple shear geometry with smooth walls undergoes a velocity driven transition from a jammed or creeping state (low wall velocity) to a flow state with a finite shear rate in the bulk (high wall velocity). In the flow state, the state variables volume fraction \(\nu \), inertial number I and the macroscopic friction \(\mu \) of the bulk follow an exponential transient. The characteristic time of this progression grows with the wall velocity and the system size and is typically large compared to the inverse shear rate. It is shown that I, first being stationary in the shear zones, spreads diffusively into the bulk. The other state variables follow according to the constitutive laws, well known from the steady state.

A Particle Tracking Velocimetric Study of Interfacial Slip at Polymer–Polymer Interfaces

Particle tracking velocimetry (PTV) was used in conjunction with rheological measurements to investigate slip at polymer–polymer interfaces during and after startup shear using simple shear geometry, i.e., a sliding plate rheometer. Polymer pairs include styrene butadiene rubber (SBR) and polyisoprene (PI) of narrow molecular weight distribution, as well as a PI and polydimethylsiloxane (PDMS), with Flory interaction parameter χ ≈ 0.001 and χ ≈ 0.08 respectively. During startup shear, the SBR-PI pair was able to deform without interfacial failure up to a critical strain γ = 2, revealing a level of interfacial strength consistent with thermodynamic considerations. This pair also exhibited arrested interfacial slip after shear cessation for high Weissenberg numbers when the step strain exceeds unity. In contrast, the PDMS–PI interface is much weaker, and slip occurs at γ as low as 0.1. These results shed light on the nature of possible mechanical failure at interphases in multicomponent polymer systems during shear and after shear cessation.

A portable high-pressure stress cell based on the V7 Paris–Edinburgh apparatus

We describe a new device, based on a V7 Paris–Edinburgh press, for torsional testing of material at pressures up to 7 GPa (extendable to 15 GPa). Samples are deformed using a simple shear geometry between opposed anvils by rotating the lower anvil, via a rotational actuator, with respect to an upper, stationary, anvil. Use of conical anvil profiles greatly increases sample dimensions more than other high-pressure torsional apparatus did. Samples of polycrystalline Zr (2 mm thick, 3.5 mm diameter) have been sheared at strains exceeding γ ∼1.5 at constant strain rate and at pressures from 1.8 to 5 GPa, and textural development has been studied by electron microscopy. Use of amorphous-boron-epoxy gaskets means that nearly simple shear of samples can be routinely achieved. This apparatus allows study of the plastic and anelastic behaviour of materials under high pressure, and is particularly suited for performing in situ investigations using synchrotron or neutron radiation.

Nonlinear elasticity of stiff filament networks: strain stiffening, negative normal stress, and filament alignment in fibrin gels.

Many biomaterials formed by cross-linked semiflexible or rigid filaments exhibit nonlinear theology in the form of strain-stiffening and negative normal stress when samples are deformed in simple shear geometry. Two different classes of theoretical models have been developed to explain this nonlinear elastic response, which is neither predicted by rubber elasticity theory nor observed in elastomers or gels formed by flexible polymers. One model considers the response of isotropic networks of semiflexible polymers that have nonlinear force-elongation relations arising from their thermal fluctuations. The other considers networks of rigid filaments with linear force-elongation relations in which nonlinearity arises from nonaffine deformation and a shift from filament bending to stretching at increasing strains. Fibrin gels are a good experimental system to test these theories because the fibrin monomer assembles under different conditions to form either thermally fluctuating protofibrils with persistence length on the order of the network mesh size, or thicker rigid fibers. Comparison of rheologic and optical measurements shows that strain stiffening and negative normal stress appear at smaller strains than those at which filament orientation is evident from birefringence. Comparisons of shear to normal stresses and the strain-dependence of shear moduli and birefringence suggest methods to evaluate the applicability of different theories of rod-like polymer networks. The strain-dependence of the ratio of normal stress to shear stress is one parameter that distinguishes semiflexible and rigid filament models, and comparisons with experiments reveal conditions under which specific theories may be applicable.

Effects of water and iron content on the rheological contrast between garnet and olivine

The effects of water and iron content on the relative creep strengths of garnet and olivine were investigated by shear deformation experiments. Garnet and olivine samples were sandwiched together between alumina pistons in a simple shear geometry and were deformed at P = 1–2 GPa, T = 1473 K and strain rates ranging from 10 −5 to 10 −3 s −1 using a Griggs-type solid-medium apparatus. The stress- and strain-rate relation, as well as the deformation microstructures including lattice-preferred orientation and dynamic recrystallization, indicates that the deformation by dislocation creep. The creep tests show that the Fe-rich garnet (Alm 67Prp 29Grs 3) was slightly weaker than olivine (Fo90), whereas the Mg-rich garnet (Alm 19Prp 68Grs 12) was significantly stronger than olivine under dry conditions. The wet experiments show that the creep rate of the Mg-rich garnet is more sensitive to water than olivine; the water fugacity exponent on strain rate was estimated to be ∼2.4 for garnet and ∼1.2 for olivine, and the Mg-rich garnet becomes weaker than olivine in a water-rich environment. The experimental results show that the rheological contrast between garnet and olivine depends strongly on water content and to a lesser degree on Fe content. Consequently, the geodynamic behavior of geochemical reservoirs can be sensitive to their chemical environments in the upper mantle.

Simple model for heterogeneous flows of yield stress fluids

Various experiments evidence spatial heterogeneities in sheared yield stress fluids. To account for heterogeneities in the velocity gradient direction, we use a simple model corresponding to a nonmonotonic local flow curve and study a simple shear geometry. Different types of boundary conditions are considered. Under controlled macroscopic shear stress Sigma, we find homogeneous flow in the bulk and a hysteretic macroscopic stress-shear-rate curve. Under controlled macroscopic shear rate Gamma;, shear banding is predicted within a range of values of Gamma;. For small shear rates, stick-slip can also be observed. These qualitative behaviors are robust to changes in the boundary conditions.

A scanning electron microscope study of the effects of dynamic recrystallization on lattice preferred orientation in olivine

Effects of dynamic recrystallization on lattice preferred orientation (LPO) in olivine were investigated through the combination of two SEM-based techniques, electron backscattered diffraction (EBSD) technique for crystallographic orientation measurement and backscattered electron imaging (BEI) for dislocation observation. Samples are experimentally deformed olivine aggregates in simple shear geometry. In the sample deformed at T=1473 K and high stresses (∼480 MPa), only incipient dynamic recrystallization is observed along grain-boundaries. Orientations of these small recrystallized grains are more random than that of relict grains, suggesting an important role of grain-boundary sliding at this stage of recrystallization. In the sample deformed at T=1573 K and low stress (∼160 MPa), dynamic recrystallization is nearly complete and the LPO is characterized by two [100] peaks. One peak is located at the orientation subparallel to the shear direction and is dominated by grains with high Schmid factor. The other occurs at high angles to the shear direction and is due to the contribution from grains with low Schmid factor. Grains with high Schmid factor tend to have higher dislocation densities than those with low Schmid factor. Based on these observations, we identify two mechanisms by which dynamic recrystallization affects LPO: (1) enhancement of grain-boundary sliding due to grain-size reduction, leading to the modification of LPO caused by the relaxation of constraint for deformation; (2) grain-boundary migration by which grains with lower dislocation densities grow at the expense of grains with higher dislocation densities. Based on the deformation mechanism maps and stress versus recrystallized grain-size relation, we suggest that the first mechanism always plays an important role whereas the second mechanism has an important effect only under limited conditions.

Dynamic Viscoelastic Behavior of Smectic Liquid Crystalline Side-Chain Polymers

Abstract A series of cholesterol-containing liquid crystalline side-chain polymers (pChMO-n, n = 5 and 11, the carbon number of the side alkyl chain) and a butoxybiphenyl containing liquid crystalline side-chain polymer (pBBMO-n, n = 6) have been prepared. The dynamic viscoelastic properties of these liquid crystalline polymers in the smectic A and isotropic phases were investigated with parallel-plate simple shear geometry. The slope of G′ against frequency in the smectic A phase is relatively small due to the influence of the smectic structure of the side chains. In the isotropic phase, the slope of G′ curves for these polymers is smaller than that in the flow region observed on isotropic amorphous polymers. Furthermore, the G′ curve for these three polymers in the isotropic phase has various slopes. This difference in the slope in the isotropic phase is accounted for by the difference in the molecular structure such as length of the side flexible spacer and the type of mesogenic groups.

Dynamic Viscoelastic Behavior of the Liquid Crystalline Side-Chain Polymers

Abstract Dynamic viscoelastic properties of three liquid crystalline polymers with different spacer length (n) were investigated with parallel-plate simple shear geometry and under uniaxial extension geometry. Temperature dependence curves of G' for the polymers were obtained at various frequencies. It was found that the slope of G' above T cl was smaller than that below T cl. In particular, G' of the polymer with the shorter spacer length (n = 5) varied very slowly with the temperature above T cl. The values (103-105 dyne/cm2 varied with the frequency) of this quasi-plateau modulus indicate that the plateau region is not due to the entanglements between the main-chain. The existence of loose entanglements between the large, rigid side chains is suggested.

Specimen Cage Modification for Tem in-Situ Shearing

ABSTRACTIn-situ strain experiments with the electron microscope have contributed substantially to our understanding of deformation in metals and ceramics. However, physical characteristics of rock-forming minerals limit the direct application of experimental procedures developed for metals. The minerals muscovite (KAl2(Si3Al)O10(OH,F)2) and biotite (K(Mg0.6–1.8 Fe2.4–1.2) (Si3Al)O10 (OH,F)2) are chosen to develop techniques for preliminary deformation experiments. The present communication describes modifications of sample preparation techniques for micas. The standard tensile straining configuration is altered to a simple shear geometry by means of an intermediary specimen cage to allow the use of a more favorable sample orientation. The extension of the shearing geometry to the in-situ deformation of other rock-forming minerals is discussed.