David H. Zeuch

Modeling and mechanistic interpretation of creep of rock salt below 200°C

Abstract Upper and lower bound estimates of the steady-state strain rates of rock salt from five locations are analyzed. The measurements were obtained in triaxial creep tests including systematic changes in stress and temperature. The various data sets are correlated in activation analyses and fitted by models, and combinations of models, for power-law creep, cross-slip, and dislocation glide to show that a cross-slip model yields the best, mechanistically most credible fit. Good fits for cross-slip and high power-law stress exponents rule out the possibility that pressure solution affected the data in a major way to the lowest recorded strain rates of order 10−10s−1.

Mechanical properties and shear failure surfaces for two alumina powders in triaxial compression

In the manufacture of ceramic components, near-net-shape parts are commonly formed by uniaxially pressing granulated powders in rigid dies. Density gradients that are introduced into a powder compact during press-forming often increase the cost of manufacturing, and can degrade the performance and reliability of the finished part. Finite element method (FEM) modeling can be used to predict powder compaction response, and can provide insight into the causes of density gradients in green powder compacts; however, accurate numerical simulations require accurate material properties and realistic constitutive laws. To support an effort to implement an advanced cap plasticity model within the finite element framework to realistically simulate powder compaction, we have undertaken a project to directly measure as many of the requisite powder properties for modeling as possible. A soil mechanics approach has been refined and used to measure the pressure dependent properties of ceramic powders up to 68.9 MPa (10,000 psi). Due to the large strains associated with compacting low bulk density ceramic powders, a two-stage process was developed to accurately determine the pressure-density relationship of a ceramic powder in hydrostatic compression, and the properties of that same powder compact under deviatoric loading at the same specific pressures. Using this approach, the seven parameters that are required for application of a modified Drucker-Prager cap plasticity model were determined directly. The details of the experimental techniques used to obtain the modeling parameters and the results for two different granulated alumina powders are presented.

On the inter-relationship between grain size sensitive creep and dynamic recrystallization of olivine☆

Abstract The recent theoretical results of Twiss (1976) and Goetze (1978) suggest that at high temperatures and sufficiently high stresses the creep behavior of dry olivine should be dominated by either nonlinear diffusion accommodated grain-boundary sliding or nonlinear Coble creep mechanisms. This would result following the production of a fine grain-size by dynamic recrystallization. For the high-temperature experimental work performed by Karato et al. (1982) dry single crystals of olivine were almost totally recrystallized during creep, and temperature changing experiments were performed on the resulting dynamically recrystallizing polycrystalline aggregates. However, the activation energy for creep determined by Karato et al. (1982) was far higher than that predicted by the models of Twiss (1976) or Goetze (1978), although the conditions required for operation of at least the model of Twiss (1976) apparently were satisfied. The data for the highly recrystallized specimens from the higher stress, lower temperature experiments of Zeuch and Green (1979) and Zeuch (1980) are in good agreement with the results of Karato et al. (1982). These latter experiments were conducted under conditions where either the model of Twiss (1976) or Goetze (1978) should have been applicable. I tentatively conclude that although fine grain sizes were produced in the experiments of Karato et al. (1982), Zeuch and Green (1979) and Zeuch (1980) by dynamic recrystallization, there is no evidence for a transition to grain-boundary diffusional creep mechanisms at high or low stresses despite the predictions of Twiss (1976) or Goetze (1978). Instead, deformation is dominated by dislocation movement with recovery by dynamic recrystallization.

The effects of nonhydrostatic compression and applied electric field on the electromechanical behavior of poled lead zirconate titanate 95/5-2Nb ceramic during the ferroelectric to antiferroelectric polymorphic transformation

We conducted hydrostatic compression and constant-stress-difference experiments, with and without an applied electric field, on poled, niobium-doped lead zirconate titanate ceramic. The objective was to quantify the effects of nonhydrostatic stress and electric field bias on electromechanical behavior of the ceramic during the ferroelectric, rhombohedral {r_arrow} antiferroelectric, orthorhombic phase transformation. Increasing stress difference (shear stress) decreases the mean stress at which the transformation occurs. Increasing shear stress also retards the rate of transformation, causing reductions in both the rate of charge release and peak voltage attained during depoling. Application of the electric field bias slightly increases the transformation pressure for poled ceramic. Previously, we showed that under nonhydrostatic stress, the transformation took place in {ital unpoled} ceramic when the maximum compressive stress equalled the hydrostatic pressure at which the transformation would otherwise occur. This simple stress criterion does not apply to poled ceramic. However, poled material has a preferred crystallographic orientation and mechanical anisotropy, whereas unpoled ceramic is isotropic. We present a qualitative model for the transformation under nonhydrostatic stress-related to that anisotropy, which resolves these seemingly disparate observations. {copyright} {ital 1999 Materials Research Society.}

Uniaxial compression experiments on lead zirconate titanate 95/5-2Nb ceramic: Evidence for an orientation-dependent, ''maximum compressive stress'' criterion for onset of the ferroelectric to antiferroelectric polymorphic transformation

Some time ago we presented evidence that, under nonhydrostatic loading, the F{sub R1} {r_arrow} A{sub O} polymorphic transformation of unpoled PZT 95/5-2Nb (PNZT) ceramic began when the maximum compressive stress equaled the hydro-static pressure at which the transformation otherwise took place. Recently we showed that this simple criterion did not apply to nonhydrostatically compressed, poled ceramic. However, unpoled ceramic is isotropic, whereas poled ceramic has a preferred crystallographic orientation and is mechanically anisotropic. If we further assume that the transformation depends not only on the magnitude of the compressive stress, but also its orientation relative to some feature(s) of PNZTs crystallography, then these disparate results can be qualitatively resolved. It has long been known that this transformation can be triggered in uniaxial compression. Our modified hypothesis makes two predictions for transformation of unpoled polycrystals under uniaxial stress: (i) the transformation should begin when the maximum compressive stress, {sigma}{sub 1}, equals the hydrostatic pressure for transformation, and (ii) a steadily increasing axial stress should be required to drive the transformation.

Simulation of the Effects of Shock Stress and Electrical Field Strength on Shock‐Induced Depoling of Normally Poled PZT 95/5

Shock‐induced depoling of the ferroelectric ceramic PZT 95/5 is utilized in pulsed power applications. Experiments to examine the shock response of normally poled PZT 95/5 under uniaxial strain conditions show that depoling kinetics, as reflected in current generation through an external circuit, is inhibited by both decreasing the shock pressure and increasing the electric field within the ceramic. A model to describe the response of the ferroelectric ceramic has been developed and implemented into simulation codes. Measured currents in the external circuit and transmitted waveforms at a window interface have been compared with results from simulations for experiments with shock pressures varying from 0.6 to 4.6 GPa and electric fields varying from 0.3 to 37 kV/cm.

Further observations on the effects of nonhydrostatic compression on the [ital F][sub [ital R]1][r arrow][ital A][sub [ital O]] polymorphic phase transformation in niobium-doped, lead-zirconate-titanate ceramic

We recently performed a series of experiments on Nb-doped lead-zirconate-titanate ceramic to investigate the influence of constant shear stresses on the displacive, first-order rhombohedral/ferroelectric[r arrow]orthorhombic/antiferroelectric polymorphic transformation. In a previous paper and report we demonstrated that increasing shear stresses lowers the mean stress and confining pressure at which the transformation occurs, but we did not identify a criterion by which the transformation could be predicted to take place under nonhydrostatic stress. In this paper we use the dielectric anomaly which accompanies the transformation as an indicator of onset of the transition, and correct for the effects of high-pressure-seal friction on measurement of the maximum compressive stress applied to the test specimens during deviatoric loading. We show that a convincing case can be made that the transformation occurs when the maximum compressive stress equals the hydrostatic pressure at which the transformation would otherwise occur.

Application of a model for grain boundary sliding to high temperature flow of carrara marble

Abstract It has been demonstrated that grain boundary sliding may contribute up to 50 percent of the total strain during experimental, high temperature deformation of Carrara Marble (Schmid, Paterson and Boland, 1980), yet the creep behavior was characterized by a high stress exponent and an apparent thermal dependence related to volume diffusion of carbon in calcite. By adopting the model of Gifkins (1976, 1977) for dislocation accommodated grain boundary sliding, incorporating Nabarros model of creep by climbing edge dislocations (Weertman, 1975) and using the experimentally determined relationship between stress and subgrain (recrystallized grain) size, a model is developed which fits the high temperature creep data very well. In effect, the model assumes that deformation occurs by a combination of climb of edge dislocations and dislocation accommodated grain boundary sliding. It is shown that the model can be easily and reasonably extended to include creep by climb-controlled dislocation glide.

Pulsed Dielectric Breakdown of Aluminum Oxide (ALOX) Filled Epoxy Encapsulants: Effects of Formulation and Electric Stress Concentration

Aluminum oxide (ALOX) filled epoxy is the dielectric encapsulant in shock driven high-voltage power supplies. ALOX encapsulants display a high dielectric strength under purely electrical stress, but minimal information is available on the combined effects of high voltage and mechanical shock. We report breakdown results from applying electrical stress in the form of a unipolar high-voltage pulse of the order of 10-{micro}s duration, and our findings may establish a basis for understanding the results from proposed combined-stress experiments. A test specimen geometry giving approximately uniform fields is used to compare three ALOX encapsulant formulations, which include the new-baseline 459 epoxy resin encapsulant and a variant in which the Alcoa T-64 alumina filler is replaced with Sumitomo AA-10 alumina. None of these encapsulants show a sensitivity to ionizing radiation. We also report results from specimens with sharp-edged electrodes that cause strong, localized field enhancement as might be present near electrically-discharged mechanical fractures in an encapsulant. Under these conditions the 459-epoxy ALOX encapsulant displays approximately 40% lower dielectric strength than the older Z-cured Epon 828 formulation. An investigation of several processing variables did not reveal an explanation for this reduced performance. The 459-epoxy encapsulant appears to suffer electrical breakdown if the peak field anywhere reaches a critical level. The stress-strain characteristics of Z-cured ALOX encapsulant are measured under high triaxial pressure and we find that this stress causes permanent deformation and a network of microscopic fractures. Recommendations are made for future experimental work.

Raman study of lead zirconate titanate under uniaxial stress

The authors used micro-Raman spectroscopy to monitor the ferroelectric (FE) to antiferroelectric (AFE) phase transition in PZT ceramic bars during the application of uniaxial stress. They designed and constructed a simple loading device, which can apply sufficient uniaxial force to transform reasonably large ceramic bars while being small enough to fit on the mechanical stage of the microscope used for Raman analysis. Raman spectra of individual grains in ceramic PZT bars were obtained as the stress on the bar was increased in increments. At the same time gauges attached to the PZT bar recorded axial and lateral strains induced by the applied stress. The Raman spectra were used to calculate an FE coordinate, which is related to the fraction of FE phase present. The authors present data showing changes in the FE coordinates of individual PZT grains and correlate these changes to stress-strain data, which plot the macroscopic evolution of the FE-to-AFE transformation. Their data indicates that the FE-to-AFE transformation does not occur simultaneously for all PZT grains but that grains react individually to local conditions.