Rapoport-Samoilenko test for cathode carbon materials—II. Swelling with external pressure and effect of creep

The creep strain of an anthracitic commercial cathode material used for aluminium production has been measured on solid cylinder samples. Experimental results have been obtained using a Rapoport-Samoilenko-type apparatus for virgin material at room and high temperatures as well as for electrolyzed material under loading. When an external pressure is applied to the material, the reduction of sodium expansion is explained by the effect of compressive creep deformation related to the sodium penetration into the binder phase of carbon. The creep strain is much larger when sodium is absorbed under pressure compared with the creep strain when an external pressure is applied to the material after binder phase sodium saturation. A constitutive model for creep deformation in the binder phase of cathode carbon materials which is able to reproduce the relationship between the creep strain, external pressure and time during the Rapoport-Samoilenko-type test has been developed. The model has been extended to a three-dimensional stress-strain state. The calculations demonstrate that creep has to be considered in order to obtain realistic stress-strain results.     

A comprehensive study on frictional dependence and predictive accuracy of viscoelastic model for optical glass using compression creep test

Compression creep tests (CCTs) have been widely used in phenomenological characterization of viscoelastic materials such as glasses. However, disturbed by specimen-tool interface friction, the real stress-strain data regarding the pure viscoelastic deformation are frequently misestimated in conventional CCTs, causing decreased accuracies of the derived viscoelastic parameters. This study proposes a comprehensive CCT-based approach to develop a viscoelastic model with weakened frictional disturbance and enhanced predictive accuracy. An integrated calculation procedure is first built to mathematically characterize the frictional and viscoelastic behaviors of glass during compression. Uniaxial CCTs of a typical borosilicate glass (L-BAL42) are then performed at varied frictional conditions. The quantified coefficients of interface friction indicate that a minor frictional disturbance is achieved when Nickel foils are used as interfacial layers, whereby a more realistic viscoelastic constitutive relation of the glass is derived. The obtained frictional and viscoelastic constants are further incorporated into computational modeling of the CCT and precision molding processes. The demonstrated consistencies between the simulated and measured results (creep displacement and molding force) suggest that, by technically slashing the interface friction and theoretically correcting the friction-involved stress in CCTs, the frictional disturbance to experimental stress-strain data can be effectively weakened, and a viscoelastic model of enhanced predictive accuracy can be thus developed.     

High-Temperature Mechanical Properties of Mullite Under Compression

The high strength potential of single-phase mullite was investigated under compressive stress-strain and creep testing conditions at 1400° and 1500°C. In single-crystal experiments, no plastic deformation and, hence, no dislocation glide was observed. Polycrystalline mullite was deformed via the Nabarro-Herring mechanism accompanied by grain-boundary sliding and some cavitation. In stress-strain tests, deformation was enhanced by intergranular separation.     

Stress Relaxation, Creep Recovery, and Newtonian Viscous Flow in Silicon Nitride

A new approach to tensile creep testing and analysis based on stress relaxation is described for sintered silicon nitride. Creep rate data covering up to 5 orders of magnitude are generated in tests lasting less than 1 day. Tests from various initial stresses at temperatures up to 1300°C are analyzed and compared with creep rates measured during conventional constant load testing. It is shown that at least 40% of the creep strain accumulated under all test conditions is recoverable, and that the deformation may properly be described as viscoelastic. A regime which approximated as Newtonian viscous behavior (creep rate directly proportional to stress) was observed during decreasing stress at temperatures between 1200° and 1300°C. This resulted in anomalous behavior at low strains in pseudo stress-strain curves generated from the stress relaxation data. However, the otherwise systematic rate dependence provides a possible basis for design in terms of a secant modulus analysis. The anelastic, recoverable component of creep may lead to complex deformation history-dependent phenomena.     

Role of Intergranular Damage-Induced Decrease in Young's Modulus in the Nonlinear Deformation and Fracture of an Alumina at Elevated Temperatures

The effect of the time-dependent decrease in Young's modulus due to damage accumulation by pore growth and intergranular cracking on the stress-strain behavior of a coarse-grained polycrystalline alumina deformed under conditions of displacement control at elevated temperatures was investigated. Considerable nonlinearity in stress-strain behavior, which increased with decreasing strain rate, was noted. At the higher strain rates, the failure stress was found to be independent of strain rate, thought to be due to a strain-rate-dependent fracture toughness due to the growth of microcracks at the tip of the failure-initiating macrocrack, which offsets the expected strain-rate sensitivity due to the growth of a single macrocrack only. Pore growth and intergranular cracking, accompanied by major reduction in Young's modulus by as much as a factor of 5, was most pronounced at the intermediate values of strain rate. This decrease in Young's modulus, under conditions of displacement-controlled loading, results in a decrease in stress, referred to as strain softening, which contributed to the observed nonlinear deformation. This conclusion was confirmed by a theoretical analysis, which showed that in addition to diffusional creep, time-dependent decreases in Young's modulus (elastic creep) by crack growth can make significant contributions to nonlinear deformation.     

The effect of temperature on compressive fatigue of polystyrene

Compressive fatigue experiments have been designed to compare this long term mechanical life property with shorter term stress-strain behavior. Fatigue lifetime curves can be represented by three distinct regions. The fatigue failure curves shift to shorter lifetimes and lower stresses as temperature is increased. The results are discussed in terms of the Zhurkov model of static fatigue failure. Using a Coulomb-Navier yield criterion modified rate expression, it is clear that activation energy and activation volume are functions of temperature. A change in temperature dependence of activation energy and endurance limiting stress occurs near the β transition suggesting that this molecular process is related to the fatigue process. The nearly identical dependence of fatigue and stress-strain activation energies and activation volumes with temperature suggest that both deformation processes may be controlled by a similar mechanism, i.e., localized plastic deformation. Utilizing these concepts, a simple model of fatigue allows correlation of the endurance limiting stress and the number of stress cycles at the endurance limiting stress with measures of resistance to plastic flow as determined from stress-strain data for this polystyrene. These data are used to project the longest and shortest mechanical fatigue lifetimes for these deformation conditions.     

Creep rate variability in gel-spun polyethylene fibers

The phenomenon of stepwise creep has been discovered and studied earlier for unoriented polymers by a new laser interferometer method. This same technique has been used for studying long-term menhanical properties of UHMWPE gel-spun fibers of various draw ratios. A variability of creep rate has been observed. The creep rate variability has been characterized by two parameters: the typical period of deformation increment, with enhanced deformation rate (step), and the variability (inhomogeneity) of the deformation rate over a deformation interval. Despite a large scatter of data, at least three levels of deformation steps could be derived: (i) microns, (ii) tens of microns, (iii) hundred and more microns. The larger levels of deformation steps decrease with increasing draw ratio. Their dependence on strain is not observed. The creep rate variability significantly depends on the creep strain. First, it increases with strain and then decreases just prior to rupture. The effect of draw ratio on creep rate variability is not significant. It is assumed that the creep rate variability is a cooperative deformation. It is suggested that interfibrillar slip and slip between fibrillar layers occurs via a slip-stick motion making a significant contribution to total deformation. The mechanism does not change with draw ratio.     

Creep and recovery behaviour of ultra-high molecular weight polyethylene in the region of small uniaxial deformations

The creep and recovery behaviour of an ultra-high molecular weight polyethylene (UHMWPE) has been studied in the region of small uniaxial deformations. At deformations as small as 5 × 10^?4 the stress-strain behaviour is non-rectilinear and the recovery cannot be described by a theory of fading memory. A new one-dimensional constitutive relation is presented which describes quantitatively the multistep creep and recovery behaviour of this material in the case where the specimens are not mechanically preconditioned. The multistep in strain-stress relaxation behaviour of the UHMWPE has also been investigated for the case in which the second step in strain is approximately half the magnitude of the first step. Calculations of the strain necessary to give the observed stress in a two-step stress-relaxation experiment have been made assuming that the stress-relaxation experiment can be represented by a series of multistep creep experiments where in each step the stress is adjusted so as to maintain a constant deformation. The agreement between the experimental values and the calculated values are very good. The proposed equation, which describes plasto-viscoelastic behaviour, appears to be able to describe quantitatively the creep and recovery behaviour of a wide range of semicrystalline polymers.     

Cavitation Contributes Substantially to Tensile Creep in Silicon Nitride

During tensile creep of a hot isostatically pressed (HIPed) silicon nitride, the volume fraction of cavities increases linearly with strain; these cavities produce nearly all of the measured strain. In contrast, compressive creep in the same stress and temperature range produces very little cavitation. A stress exponent that increases with stress (ε∞σ ⁿ , 2 < n < 7) characterizes the tensile creep response, while the compressive creep response exhibits a stress dependence of unity. Furthermore, under the same stress and temperature, the material creeps nearly 100 times faster in tension than in compression. Transmission electron microscopy (TEM) indicates that the cavities formed during tensile creep occur in pockets of residual crystalline silicate phase located at silicon nitride multigrain junctions. Small-angle X-ray scattering (SAXS) from crept material quantifies the size distribution of cavities observed in TEM and demonstrates that cavity addition, rather than cavity growth, dominates the cavitation process. These observations are in accord with a model for creep based on the deformation of granular materials in which the microstructure must dilate for individual grains to slide past one another. During tensile creep the silicon nitride grains remain rigid; cavitation in the multigrain junctions allows the silicate to flow from cavities to surrounding silicate pockets, allowing the dilatation of the microstructure and deformation of the material. Silicon nitride grain boundary sliding accommodates this expansion and leads to extension of the specimen. In compression, where cavitation is suppressed, deformation occurs by solution—reprecipitation of silicon nitride.     

热力耦合作用下浇注式沥青混凝土三轴蠕变特性研究

为了深入研究浇注式沥青混凝土沥青混合料在温度、应力耦合作用下的三轴蠕变特性,采用MTS815试验机开展了不同温度、不同应力水平和不同围压条件下的三轴蠕变试验,获取了相应的蠕变曲线,并将改进的Burgers模型推广到三维形式,用来描述浇注式沥青混凝土的蠕变本构关系。研究结果表明：浇注式沥青混凝土蠕变曲线的第一阶段不明显,这反映出其内部空隙率较低;蠕变变形值随温度升高和偏应力水平增大而明显增大,这反映出高温重载共同作用是浇注式沥青混凝土路面出现车辙的决定性因素;蠕变变形随围压增大略有减小,围压对蠕变变形有抑制作用,三轴蠕变试验获得的蠕变规律较之单轴蠕变试验更能反映实际受力过程中的变形规律,三维的改进Burgers模型能够准确反映热力耦合作用下浇注式沥青混凝土的蠕变特性。     

Creep enhanced adsorbtion of water or aqueous zinc chloride solution increases the creep rate of nylon 6,6

The creep behavior of nylon 6,6 at 21°C was significantly altered when the local “dry” environment was changed to water mist or an aqueous zinc chloride mist. Nylon 6,6 was found to exhibit logarithmic creep because the relation between the log of the strain rate and the creep strain was linear with a negative slope. The effect of changing the creep environment from dry to wet, with the addition of moisture from an ultrasonic humidifier was to decrease the negative slope by 50–70% within 5–10 min. This effect could be interpreted as a decrease in modulus, which allowed for easier creep deformation. Based on the stress‐free diffusivity of water in nylon and the dimensions of the test sample the time to saturate the sample was estimated to be about 100 h. Therefore, there appeared to be synergism between the creep deformation and the environment that dramatically enhanced the rate of saturation and slowed the decrease in the creep rate. The tentative explanation provided is that the aqueous solutions, by binding to the hydrogen bonds in nylon, are dragged into the sample during creep deformation, and the dragged‐in aqueous solution then plasticizes nylon. This is analogous to the conclusion in another recent study that showed that deformation, during a hardness test, in the presence of aqueous zinc chloride, transported the solution species deeper into the sample than could be reasonably explained by ordinary diffusion processes. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 494–497, 2001     

Creep Anisotropy of a Continuous-Fiber-Reinforced Silicon Carbide/Calcium Aluminosilicate Composite

Creep studies conducted on a unidirectional silicon carbide calcium aluminosilicate composite indicate that the Nicalon fibers provide longitudinal creep strengthening at 1200°C. The deformation is transient in nature because grain growth in the fibers enhances their creep resistance. The' transverse creep strength is considerably smaller, being dominated by the matrix, resulting in appreciable creep anisotropy. This anisotropy leads to severe distortion when off-axis loadings are imposed. Residual stresses develop upon unloading alter creep, and cause superficial matrix cracking.     

A synergistic model of stress and oxidation induced damage and failure in silicon carbide-based ceramic matrix composites

A micromechanics-based modeling approach that allows for the simultaneous consideration of deformation, damage, and oxidation associated with each constituent of silicon carbide (SiC)-based ceramic matrix composites (CMC), including the fiber, fiber coating, and matrix, is described. Chemical kinetics models from the literature are combined with a progressive damage model. Rupture predictions of unnotched and notched stress-hold (creep) specimens are compared with experimental measurements from a SiC/SiC CMC to assess the efficacy of the modeling approach. Techniques of improving creep rupture life are explored using the model.     

Version of mathematical modeling of deformation processes in synthetic fibres

A mathematical model of the deformation properties of synthetic fibres in the region of nondestructive mechanical effects is proposed based on a normalized arctangent function of the logarithm of the reduced time, which significantly increases the time, load, and deformation intervals in which calculation prediction of the viscoelastic processes of synthetic fibres is performed. Methods of determining the viscoelastic characteristics as parameters of this model based on the results of brief tests in simple relaxation and creep regimes were developed based on the proposed mathematical model. The methods of considering the irreversible pseudoplastic component of deformation in mathematical modeling of the viscoelastic properties of synthetic fibres increase the reliability of predicting complex deformation modes. __________ Translated from Khimicheskie Volokna, No. 6, pp. 49–51, November–December, 2007.     

耐火材料高温弯曲应力-应变关系测试方法及影响因素

介绍了耐火材料弯曲应力-应变测试仪的基本构成以及对试验炉、加荷装置、变形量测量装置和计算机控制系统等各构件的要求;重点介绍了使用该仪器研究耐火材料应力-应变关系的方法以及影响测试结果的相关的因素。该方法可以用来研究耐火材料在高温下承受弯曲应力时的变形问题,确定耐火材料在各温度下的应力-应变关系,测试和判断耐火材料的弹性、塑性和粘滞流动的温度范围,并根据应力-应变关系计算耐火材料在不同温度下的弹性模量。     

Tensile creep behavior of three-dimensional four-step braided SiC/SiC composite at elevated temperature

This article presents experimental results for tensile creep deformation and rupture behavior of three-dimensional four-step braided SiC/SiC composites at 1100 °C and 1300 °C in air. The creep behavior at 1300 °C exhibited a long transient creep regime and the creep rate decreased continuously with time. The creep behavior at 1100 °C exhibited an apparent steady-rate regime and the creep deformation was smaller than that at 1300 °C. However, the creep rupture time at both temperatures showed little difference. The mechanisms controlling creep deformation and rupture behavior were analyzed.     

Computer Simulation of Creep and Fracture of Highly Drawn Polymers

Abstract

A computer kinetic model was proposed describing creep and fracture of the microfibrilla in the drawn semicrystalline polymer and allowing for the concurrent and interrelated processes of slippage of polymer chains, redistribution of the polymer units between the amorphous regions and chains scissions. A self-consistent system of non-linear kinetic equations for tie chains length distribution function was written and its numerical analysis was made with a computer. As a result, a stress-strain curve for a single microfibrilla and an averaged curve for a high drawn polymer fiber were obtained. Also a portion of tie chains scissions and the concentration of defects in crystallites as a function of fiber deformation were obtained.     

Calculation of spinneret feeder elements for creep

A calculation scheme is proposed which allows establishing the effect of creep on the sag of the spinneret plate in a spinneret feeder and its lifetime. The sag increases and the feeder lifetime decreases with an increase in the stresses arising when the plate is loaded. Translated from Khimicheskie Volokna, No. 6, pp. 45–48, November–December, 2008.