Investigation of Pinning Force Density Fp and Critical Current Density Jc of Nd0.33Eu0.33Gd0.33Ba2Cu3Oy(NEG-123) Superconductors with Addition of 211 Phase Particles and Analysis with a Theoretical Evaluation Based on Flux Creep-Flow Model

The pinning force density Fp and the critical current density Jc were investigated in Nd0.33Eu0.33Gd0.33Ba2Cu3O(NEG-123) superconductors with addition of NEG-211 and EG-211 secondary phase particles of the volume fractions up to 10 mol% by analyzing experimental data of DC magnetization and compared with theoretical calculation based on flux creep-flow model taking the pinning parameter into account. The pinning parameters such that the number of flux lines in the flux bundle (g2), the most probable value of pinning strength (Am), distribution width (σ2), upper critical field (Bc2) were determined so that a good fit was obtained between theoretical and experimental results. The Chittagong Univ. J. Sci. 42(1): 24-38, 2020

THIXOTROPIC FLOCCULATION EFFECTS IN CARBON BLACK–REINFORCED RUBBER: KINETICS AND THERMAL ACTIVATION

ABSTRACT A new rheological methodology is used to quantify the kinetics and thermal activation of thixotropic recovery (flocculation) of uncrosslinked carbon black–reinforced emulsion SBR following high shears and over a range of annealing temperatures. A wide range of carbon black types are examined to determine the influence of aggregate morphology and surface area on compound flocculation. Several kinetic parameters are correlated with the carbon black aggregate structure and surface area, the results of which imply a transition in mechanisms controlling modulus recovery between shorter and longer recovery time scales. Thermal activation of flocculation is found to scale to the surface area and to the mean aggregate diameter of the carbon blacks following power law relationships. The thermal activation data for a subset of compounds with different carbon blacks prepared at different loadings collapses onto a single master line by rescaling the data to a parameter that is proportional to the theoretical interparticle force calculated for the idealized situation of two spherical particles in proximity. Three different van der Waals force models are evaluated, and in each case, an effective superposition of the thermal activation data is achieved. This indicates that the attractive force between aggregates plays a key role in the flocculation of carbon black in rubber, and this force can be traced back to the aggregate and primary particle sizes, interaggregate distances, and effective volume fractions. The activation energy for the viscosity of the unfilled, uncrosslinked SBR is similar to analogous values calculated for the thermal activation of flocculation. This coupling of energetics may be the result of creep/flow of rubber out of gaps between aggregates resulting from interaggregate attractive forces and any potential diffusive motion of the aggregates. Bound rubber data appear to contain information relating to aggregate packing, which could be exploited in future work to further explore the mechanism of flocculation.

Revealing the Plastic Mode of Time-Dependent Deformation of a LiTaO3 Single Crystal by Nanoindentation

Recently, instrumental nanoindentation has been widely applied to detect time-dependent plastic deformation or creep behavior in numerous materials, particularly thin films and heterogeneous materials. However, deformation mechanism at nanoindentation holding stage has not been well revealed hitherto. In the current work, nanoindentation holding tests with high loads were performed on a brittle LiTaO3 single crystal. The surface morphologies of residual impressions with various holding times were investigated. It was indicated that generation of secondary cracks and propagation of both main and secondary cracks were the dominating mechanism for time-dependent plastic deformation at the initial holding stage, and the density and length of cracks were invariable at the steady-state holding stage, which suggested a nonlocalized plastic deformation beneath the indenter. It could be concluded that time-dependent plastic deformation of brittle ceramic under nanoindentation is composed of instant cracking as the continuation of loading sequence and homogeneous creep flow by high shear-compression stress at room temperature.

Tectonic inheritance controls nappe detachment, transport and stacking in the Helvetic nappe system, Switzerland: insights from thermomechanical simulations

Tectonic nappes have been investigated for more than a hundred years. Although geological studies often refer to a “nappe theory”, the physical mechanisms of nappe formation are still disputed. We apply two-dimensional numerical simulations of shortening of a passive margin to investigate the thermomechanical processes of detachment (or shearing off), transport and stacking of nappes. We use a visco-elasto-plastic model with standard creep flow laws, Drucker–Prager and von Mises yield criteria. We consider tectonic inheritance with two initial mechanical heterogeneities: (1) lateral heterogeneity of the basement–cover interface due to half-grabens and horsts and (2) vertical heterogeneities due to layering of mechanically strong and weak sedimentary units. The model shows detachment and horizontal transport of a thrust nappe that gets stacked on a fold nappe. The detachment of the thrust sheet is triggered by stress concentrations around the sediment–basement contact and the resulting brittle–plastic shear band that shears off the sedimentary units from the sediment–basement contact. Horizontal transport is facilitated by a basal shear zone just above the basement–cover contact, composed of thin, weak sediments that act as a décollement. Fold nappe formation occurs by a dominantly ductile closure of a half-graben and the associated extrusion of the half-graben fill. We apply our model to the Helvetic nappe system in western Switzerland, which is characterized by stacking of the Wildhorn thrust nappe above the Morcles fold nappe. The modeled structures, the deformation rates and the temperature field agree with data from the Helvetic nappe system. Mechanical heterogeneities must locally generate effective viscosity (i.e., ratio of stress to viscoplastic strain rate) contrast of about 3 orders of magnitude to model nappe structures similar to the ones of the Helvetic nappe system. Our results indicate that the structural evolution of the Helvetic nappe system was controlled by tectonic inheritance and that material softening mechanisms are not essential to reproduce the first-order nappe structures.

Towards a nappe theory: Thermo-mechanical simulations of nappe detachment, transport and stacking in the Helvetic Nappe System, Switzerland

Tectonic nappes are observed for more than a hundred years. Although geological studies often refer to a nappe theory, the physical mechanisms of nappe formation are still incompletely understood. We apply two-dimensional numerical simulations of shortening of a passive margin, to investigate the thermo-mechanical processes of detachment, transport and stacking of nappes. We use a visco-elasto-plastic model with standard creep flow laws and Drucker-Prager yield criterion. We consider tectonic inheritance with two initial mechanical heterogeneities: (1) lateral heterogeneity of the basement-cover interface due to half-grabens and horsts and (2) vertical heterogeneities due to layering of mechanically strong and weak sedimentary units. The model shows detachment and horizontal transport of a thrust nappe and stacking of this thrust nappe above a fold nappe. The detachment of the thrust sheet is triggered by stress concentrations around the sediment-basement contact and the resulting brittle-plastic shear band formation. The horizontal transport is facilitated by a basal shear zone just above the basement-cover contact, composed of thin, weak sediments. Fold nappe formation occurs by a dominantly ductile closure of a half-graben and the associated extrusion of the half-graben fill. We apply our model to the Helvetic nappe system in Western Switzerland, which is characterized by stacking of the Wildhorn thrust nappe above the Morcles fold nappe. The modeled structures and temperature field agree with data from the Helvetic nappe system. The mechanical heterogeneities must generate contrasts in effective viscosity (i.e. ratio of stress to strain rate) of four orders of magnitude to model nappe structures similar to the ones of the Helvetic nappe system.

Nanoindentation Studies of Plasticity and Dislocation Creep in Halite

Previous deformation experiments on halite have collectively explored different creep mechanisms, including dislocation creep and pressure solution. Here, we use an alternative to conventional uniaxial or triaxial deformation experiments—nanoindentation tests—to measure the hardness and creep behavior of single crystals of halite at room temperature. The hardness tests reveal two key phenomena: (1) strain rate-dependent hardness characterized by a value of the stress exponent of ~25, and (2) an indentation size effect, whereby hardness decreases with increasing size of the indents. Indentation creep tests were performed for hold times ranging from 3600 to 106 s, with a constant load of 100 mN. For hold times longer than 3 × 104 s, a transition from plasticity to power-law creep is observed as the stress decreases during the hold, with the latter characterized by a value of the stress exponent of 4.87 ± 0.91. An existing theoretical analysis allows us to directly compare our indentation creep data with dislocation creep flow laws for halite derived from triaxial experiments on polycrystalline samples. Using this analysis, we show an excellent agreement between our data and the flow laws, with the strain rate at a given stress varying by less than 5% for a commonly used flow law. Our results underscore the utility of using nanoindentation as an alternative to more conventional methods to measure the creep behavior of geological materials.

Testing Effects on Shear Transformation Zone Size of Metallic Glassy Films Under Nanoindentation

Room-temperature creep tests are performed at the plastic regions of two different metallic glassy films under Berkovich nanoindetation. Relying on the strain rate sensitivity of the steady-state creep curve, shear transformation zone (STZ) size is estimated based on the cooperative shear model (CSM). By applying various indentation depths, loading rates, and holding times, the testing effects on the STZ size of metallic glasses are systematically studied. Experimental results indicate that STZ size is greatly increased with increased indentation depth and shortened holding time. Meanwhile, STZ size is weakly dependent on the loading history. Both the intrinsic and extrinsic reasons are discussed, to reveal the testing effects on the nanoindentation creep flow and STZ size.