The flexural creep of refractory materials

Conclusions An equation was derived for determining the maximum stresses in a rectangular-section specimen subjected to creep in pure bending when the rates of creep of the material in compression and tension are unequal. It was shown that the redistribution of the stresses in this case may result in a significant decrease of the maximum tension stresses in the specimen.Translated from Ogneupory, No. 5, pp. 42–45, May, 1977.     

Yielding of polycarbonate and polysulfone in creep and stress-strain

A modified Eyring rate expression based on an asymmetrical potential energy barrier is used to predict the onset of plastic yield in glassy polymers. Equations for the prediction of plastic yield delay times in creep and yield stresses in constant rate of strain testing are developed and compared to experimental data for samples of polycarbonate and polysulfone with various thermal histories. The two equations accurately fit the experimental results for a suitable choice of parameters. Parameters from the creep data can be used to predict yield stress values in constant rate of strain testing. Variation in simple thermal history changes the model parameters in a way that is qualitatively accounted for from a physical interpretation of the constants.     

The creep behavior of solid-filled rubber composites

This paper is using an experimental method to study the creep behavior of solid-filled silicone-rubber composites (GFSC). The specimens made of silcone-rubber as the matrix and 0–40% volume fraction (V_f) of glassbeads, have been through uniazial tensile tests and constant-loading creep tests. The results show that the material behavior is ovviously nonlinear and the Eyring's reaction rate principle is suitable to describe the viscoelastic behavior of this rubber composite. Thus, to solve the creep problems of rubber composites, the Boltzmann's superposition technique can not be applied. Although Findley et. al (1976), have proposed the mutiple integrals form, but solving the multiple integrals form is quite complicated and does not be guarantee convergence. We have employed SoChen's form of the nonlinear stress-strain relationship of rubber elasticity. This can avoid the above mentioned problems and the analytic solution is obtained. The resulting solution is in very good agreement with the experimental results.     

Nonlinear stress-strain behavior of glassy polymers

We have attempted to provide a unified account of the structural relaxation and mechanical properties of glassy polymers by reviewing recent results of a predictive model. One would like to understand how the effects of structural relaxation influence the performance of amorphous solids. The physics of glassy polymers is still evolving and the functional relationships between relaxation and deformation have not been firmly established. However, significant progress has already been made that can be utilized in solving problems in the large deformation of polymers. Equations have been derived that can be applied to provide quantitative prediction of the nonlinear stress-strain relationships as a function of physical aging, strain rate, temperature, external stress field, and the filler concentration in composites.     

The anisotropy of creep behavior in oriented thermoplastics

Detailed studies have been carried out on the anisotropy of creep and creep rupture behavior of thermoplastics oriented by the imposition of a large permanent deformation. This deformation is usually such as to produce simple fiber symmetry within the specimen. Experimental techniques have been devised for the accurate measurement of all three principal strains during tensile creep on small samples which are cut from the oriented specimens at various angles to the symmetry axis. In this way a full characterization of the creep behavior up to strains of 5 percent has been obtained at room temperature. Results are presented for work on rigid poly (vinyl chloride), poly(methyl methacrylate), and low density polyethylene. The results are discussed in terms of the time dependence and nonlinearity of the anisotropy. Creep rupture results on similar specimens are also presented and discussed. Anisotropy due to orientation is shown to be important in determining engineering properties and in understanding structure-properties relationships.     

Large deformation rate-dependent stress-strain behavior of polyurea and polyurethanes

The thermoplastic elastomer polyurethane and the elastomeric thermoset polyurea are finding new applications in increasing the survivability of structures under impact loading, including those encountered in blast and ballistic events. However, the mechanical behavior of polyurea and polyurethane materials under these high rate conditions is relatively unknown. Here, the rate-dependent stress-strain behavior of one polyurea and three representative polyurethane materials is studied by dynamic mechanical analysis, quasi-static compression testing and split Hopkinson pressure bar (SHPB) testing. The polyurethane chemistries were chosen to probe the influence of the hard segment content on the mechanical behavior, where the volume fraction and the amorphous vs. crystalline structure of the hard segment domains were varied. The large strain stress-strain behavior of polyurea and polyurethane shows strong hysteresis, cyclic softening, and strong rate-dependence. The polyurethane with a non-crystalline well-dispersed hard segment morphology did not exhibit cyclic softening. The materials are observed to transition from a rubbery-like behavior under low strain rate (∼10⁻³-10⁰ s⁻¹) loading conditions to either a leathery or glassy-like behavior under high strain rate (∼10⁻³ s⁻¹) loading conditions.     

相对分子质量变化对杜仲橡胶应力-应变行为的影响

研究了相对分子质量分布较宽的一组杜仲橡胶分级样品的应力 应变关系 ,揭示了杜仲橡胶随相对分子质量变化而出现脆性 塑性 韧性转变 ,确认了杜仲橡胶脆性 塑性转变的相对分子质量区域。试验结果表明 ,脆性 塑性转变过渡区内 ,杜仲橡胶力学性能不稳定 ;随着相对分子质量增大 ,拉伸强度和扯断伸长率表现为先增大 ,达到最大值后逐渐减小。     

Development of new refractory materials exhibiting increased barrier properties

OOO Kontakt together with Bogdanovich OAO Ogneupory has created for the first time in Russia compact barrier chamotte articles with high barrier properties. These refractories operate successfully in electrolyzers with a current strength of 300 kA. The quality indices of articles are not surpassed by overseas barrier refractories and they are half the price of imported refractories. __________ Translated from Novye Ogneupory, No. 10, pp. 33–35, October 2007.     

Viscoelastic analysis applied to the determination of long‐term creep behavior for magnetic tape materials

Creep‐compliance experiments were performed for three representative magnetic tapes. Two of these tapes used a magnetic particle (MP) coating, and one used a metal‐evaporated (ME) coating. The MP tapes used the following polyester substrates: semitensilized poly(ethylene naphthalate) (PEN) and supertensilized poly(ethylene terephthalate). The ME tape used an aromatic poly(amide) or aramid substrate. Time–temperature superposition was used to make creep‐compliance predictions at 30 and 50°C reference temperatures. Comparisons were made with dimensional stability requirements based on position error signal (PES) specifications for magnetic tape drives along with in‐cartridge creep specifications based on PES measurements. Circumferential and lateral creep strains were determined that account for storage of the tapes in a reel, and creep strains were predicted for future tapes with thinner, lower compliance coatings. A rule of mixtures method was also used to extract compliance information for individual layers of MP‐PEN tapes, and stress profiles through the thickness of the tapes were determined. Additional measurements and analyses were performed to determine the creep recovery and shrinkage characteristics for the magnetic tapes. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1106–1128, 2006     

Study on the creep behavior of polypropylene

The creep behavior and creep failure law of polypropylene (PP) were investigated by using a multifunctional stress‐aging testing machine under different aging environmental conditions (temperature, UV, and stress). Photoinduced changes in samples were studied using gel permeation chromatography and X‐ray photoelectron spectrometer. Surface morphologies were also observed by scanning electron microscopy. It was found that there is a critical failure strain (ε_crit) for PP during the creep course. Once the creep deformation exceeds the ε_crit, creep failure of PP takes place very rapidly. The value of ε_crit is independent of the tensile stress and UV irradiation, whereas it is only affected by the temperature and the nature of the PP, such as molecular weight and molecular structure. With increasing temperature, the value of ε_crit increases gradually. In addition, the creep rate of PP increases rapidly with increasing tensile stress and temperature as well as under irradiation with UV light. This study may provide a new way to predict the service lifetime of PP. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Volume and deviator creep of calcium-leached cement-based materials

This paper provides new experimental evidence of the specific volume creep and deviator creep of cement-based materials. Creep tests conducted on calcium-leached cement pastes and mortars under various triaxial loading conditions provide conclusive evidence of a specific short-term creep and a specific long-term creep. It is found that at least two competing dissipative mechanisms are at work in short-term creep of cement-based materials: (1) a creep relaxation mechanism in the calcium-silicate-hydrate (C-S-H) solid phase activated by microstress concentrations in the heterogeneous microstructure; and (2) stress relaxation by microcracking beyond a relatively small stress threshold. If the first dominates over the second, the short-term volume creep is contracting, and in the inverse case, a short-term dilating behavior is found. Furthermore, provided that microcracking stabilizes during the short-term creep, the long-term creep occurs at constant volume. Based on these results, we argue that the concept of a creep Poisson's ratio, which is commonly employed in engineering practice to extrapolate uniaxial creep tests to multiaxial stress conditions, should be abandoned when triaxial stress states affect the durability performance of concrete structures.     

氟橡胶/碳纳米管复合材料的拉伸应力-应变行为

以氟橡胶/炭黑复合材料作对比研究了氟橡胶/碳纳米管复合材料的拉伸应力-应变、拉伸应力弛豫和拉伸应力软化行为。结果表明,碳纳米管对氟橡胶有良好的增强效果,相同用量碳纳米管的增强效果要高于炭黑,低应变时碳纳米管的增强效果更明显;与氟橡胶/炭黑复合材料相比,氟橡胶/碳纳米管复合材料的应力弛豫现象更为明显,应力弛豫速率更快;随着碳纳米管和炭黑用量的增加,复合材料的应力软化效应增大,氟橡胶/碳纳米管复合材料的应力软化效应明显高于氟橡胶/炭黑复合材料。     

Analytical approach of creep behavior of carpet yarn

Based on mechanical models, the creep behavior of carpet yarns after dynamic loading was investigated. For prediction the creep elongation, the frequently used mechanical models reported in the literature were analyzed. The mechanical models which were used in this article were: standard linear model, four‐element model, two‐component Kelvin's model, and Eyring's model. The obtained creep formulas were fitted to experimental creep data, and the parameters of the model can be obtained using the Marquardt algorithm for nonlinear regression. When comparing the experimental creep curve with the fitted curve from the mechanical model, it is clear that the four‐element model explain the experimental creep curve better. During tufting machine stops, the carpet yarns were undergone constant load. The confirmed viscoelastic model will be used to calculate total creep elongation during carpet machine stoppage. Thus, the start‐up marks which occurred at carpet machine restarts can be exactly eliminated by adjusting the feeding length according to the creep elongation. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Prediction of the creep behavior of reinforced plastics

The time-dependent deformation of orthotropic and transversely isotropic viscoelastic materials under biaxial constant load is given in the range of linear and reversible stress-strain behavior for isothermal processes. This allows one to calculate the deformation of plastics on the basis of isochronous stress-strain diagrams. In addition, a method is presented which allows a calculation of the creep moduli of mat-reinforced unsaturated polyesters and their dependence on glass content, temperature, and time. This calculation requires only specific creep data of matrix material and the elastic modulus of the reinforcement.     

Modeling creep behavior in ceramic matrix composites

In this work, a three-dimensional viscoplasticity formulation with progressive damage is developed and used to investigate the complex time-dependent constituent load transfer and progressive damage behavior in ceramic matrix composites (CMCs) subjected to creep. The viscoplasticity formulation is based on Hill's orthotropic plastic potential, an associative flow rule, and the Norton-Bailey creep power law with Arrhenius temperature dependence. A fracture mechanics-informed isotropic matrix damage model is used to account for CMC brittle matrix damage initiation and propagation, in which two scalar damage variables capture the effects of matrix porosity as well as matrix property degradation due to matrix crack initiation and propagation. The Curtin progressive fiber damage model is utilized to simulate progressive fiber failure. The creep-damage formulation is subsequently implemented as a constitutive model in the generalized method of cells (GMC) micromechanics formulation to simulate time-dependent deformation and material damage under creep loading conditions. The developed framework is used to simulate creep of single fiber SiC/SiC microcomposites. Simulation results are in excellent agreement with experimental and numerical data available in the literature.     

A tetranuclear holmium compound exhibiting single molecule magnet behavior

The synthesis, structure, and magnetic properties of a new tetranuclear holmium cluster [Ho^III₄(μ₃-OH)₂(L)₄(piv)₂(DMF)₂]·2DMF (1) is reported, where the Schiff base H₂L ligand is 2-hydroxy-3-methoxy-phenylsalicylaldimine and piv is pivalate. The cluster has a planar “butterfly” Ho₄ core, which is bridged by two μ₃-hydroxide and six phenoxide oxygen atoms. Magnetically, compound 1 displays field-induced slow relaxation of magnetization. Fitting the dynamic magnetic data to the Arrhenius law gives energy barrier ΔE/k_B = 21.49 K and τ₀ = 6.97 × 10^− 7 s.