Effects of temperature and stress on creep properties of ethylene tetrafluoroethylene (ETFE) foils for transparent buildings

Creep properties of ethylene tetrafluoroethylene (ETFE) foils are indispensable for evaluating serviceability limit state, especially under high temperature and high stress. This paper concerned temperature and stress effects on creep properties of ETFE foils with experimental and theoretical studies. Experimental results showed that dimensionless stress effect on creep properties could be higher than that of temperature effect. A unified equation incorporating temperature, stress and time based on experimental results was determined and could be utilized to calculate the stress limits and long-term creep strains. The stress limits in response to creep strain of 10% were less than 5 MPa, 4 MPa and 3 MPa for temperature ranges of 40–50 °C, 50–60 °C and 70–80 °C, respectively. The long-term creep strain of ETFE foils under 40 °C was 5.96% concerning 50-year working time.Master curves of ETFE foils were evaluated considering time-temperature superposition (TTSP) and time-stress superposition (TSSP). Long-term creep strains with these master curves were identified and compared with experimental creep strains. It is found that TTSP could be a little underestimation of creep strains while TSSP could overestimate creep strains to some extent. Moreover, the maximum creep strain difference was only 0.48%, which justified the feasibility and suitability of using the unified equation to predict creep strains of ETFE foils.     

Transient behavior of entangled polymers at high shear rates

A new model is presented for describing the time-dependent flow of entangled polymer liquids at high shear rates. The results were obtained by extending the Doi and Edwards theory to include the effect of chain stretching. This nonlinear phenomenon is predicted to occur when the product of the shear rate and longitudinal relaxation time of the polymer exceeds one. If a constant-shear-rate flow is started under these conditions, it is shown that the shear stress and the normal stress are considerably larger than that predicted by the original reptation model. We also find that both of these stresses can pass through maxima before reaching a steady state and that the times required to reach these maxima are constants independent of the shear rate. In general the new model requires the numerical solution of coupled partial differential equations. However, at the highest shear rates where reptative relaxation is no longer important, an analytical solution for the stresses is found. The results obtained here are shown to agree well with experimental data and to be an improvement over a simpler model recently proposed.     

Study of Rheological Properties of Liquid and Solidlike Materials Using Burgers' Model

It was shown that the use of Burgers' model and introduction of the effective viscosity function make it possible to account for general trends in rheologic behavior of different isotropic liquid and solidlike materials: stress relaxation and strain delay, influence of loading or shear rate on a strain curve, curve hysteresis on loading and unloading, and the presence of a stress maximum and its shift at different shear rates for nonlinear-viscous systems. Generally, the rheological model consists of the binomial rheological equation and the modified Maxwell and Kelvin–Voigt differential equations. The model was tested for the case of tangential stresses when studying the polyacrylamide–aluminium acetate solutions and for the normal stresses, when studying the strain of the rock samples saturated with oil.     

Fabrication and microstructural characterization of functionally graded porous acrylonitrile butadiene styrene and the effect of cellular morphology on creep behavior

The ability to control material properties in space and time for functionally graded viscoelastic materials makes them an asset where they can be adapted to different design requirements. The continuous microstructure makes them advantageous over conventional composite materials. Functionally graded porous structures have the added advantage over conventional functionally graded materials of offering a significant weight reduction compared to a minor drop in strength. Functionally graded porous structures of acrylonitrile butadiene styrene (ABS) had been fabricated with a solid‐state constrained foaming process. Correlating the microstructure to material properties requires a deterministic analysis of the cellular structure. This is accomplished by analyzing the scanning electron microscopy images with a locally adaptive image threshold technique based on variational energy minimization. This characterization technique of the cellular morphology is analyst independent and works very well for porous structures. Inferences are drawn from the effect of processing on microstructure and then correlated to creep strain and creep compliance. Creep is strongly correlated to porosity and pore sizes but more associated to the size than to porosity. The results show the potential of controlling the cellular morphology and hence tailoring creep strain/compliance of ABS to some desired values. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 795–803     

Stress-dependent dynamic compliance spectra approach to the nonlinear viscoelastic response of polymers

The present work reports a discrete, stress-dependent dynamic compliance spectra method which may be used to predict the mechanical response of nonlinear viscoelastic polymers during strain-defined processes. The method is based on the observation that the real and complex parts of the discrete dynamic compliance frequency components obtained from creep measurements are smooth, easily fit functions of stress. Comparisons between experimental measurements and model calculations show that the model exhibits excellent quantitative agreement with the basis creep measurements at all experimental stress levels. The model exhibits good quantitative agreement with stress relaxation measurements at moderate levels of applied strain. However, the model underestimates the experimental stress relaxation at an applied strain of 3.26%. The stress relaxation error appears to be a real material effect resulting from the different strain character of creep and stress relaxation tests. The model provides a good quantitative agreement with experimental constant strain rate measurements up to approximately 4% strain, after which the model underestimates the experimental flow stress. This effect is explained by the time dependence of the stress-activated configurational changes necessary for large strains in glassy polymers. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2301–2309, 1998     

A new explicit Bessel and Neumann fitted eighth algebraic order method for the numerical solution of the Schrödinger equation

An explicit eighth algebraic order Bessel and Neumann fitted method is developed in this paper for the numerical solution of the Schrödinger equation. The new method has free parameters which are defined in order the method is fitted to spherical Bessel and Neumann functions. A variable-step procedure is obtained based on the newly developed method and the method of Simos [17]. Numerical illustrations based on the numerical solution of the radial Schrödinger equation and of coupled differential equations arising from the Schrödinger equation indicate that this new approach is more efficient than other well known methods.     

Rate-dependent nonlinear constitutive equation of polypropylene

In order to investigate the viscoelastic-plastic properties of polymer solids and construct a constitutive equation for them, torsional tests were performed on polypropylene (PP) hollow cylinders. Testing was conducted under constant strain rate, abrupt change of strain rate, stress relaxation, creep, and cyclic loading conditions. The experimental data were compared with the numerical results derived from an overstress model. It is shown that the overstress theory explains the viscoelastic-plastic properties of PP well, provided that the current strain is not below the maximum previous strain.     

Flow-alignment and rheology of polymer melts: Computation of the single-link orientational distribution function

The single-link orientational distribution function and the space-averaged stresses in the fluid are computed for the case of steady shear flow of polymer melts. The computation is achieved with Galerkin's method with spherical harmonics and Euler polynomials as trial functions. The stress components become power functions of shear rate when the latter is large. The single-link orientational distribution function f solves the Fokker-Planck equation subject to a boundary condition for f at the chain ends. A solution is obtained for every shear rate and ratio of the orientational and one-dimensional diffusion coefficient. It is demonstrated that the Fokker-Planck equation with appropriate boundary condition is useful in order to predict the flow-alignment and stresses in good agreement with experimental data as well as with recent results of a nonequilibrium molecular dynamics computer simulation on polymer melts.     

On the Strength Calculation of the Rotating parts

The existing solutions of differential equations of equilibrium of an infinitesimal element of the rotating parts of an isotropic elastic solid known as the Navier equilibrium equations are considered. Examples of the flat disk calculation by solving the differential equilibrium equations by the sweep method and the finite element method in the modern program “Autodesk Simulation Multiphysics” are represented; paradoxical changes of radial and hoop stresses are revealed. An original method of derivation formulas based only on the principle of d’Alembert to calculate radial and hoop stresses in parts that operate under centrifugal (inertial) forces is proposed. The solution for rotating disks of any profile that corrects unnatural classical solutions is obtained. Analysis of the obtained new formulas for calculating stresses shows that it is necessary to reject the concept of “equal-strength disk” because of the inability to provide the equality of the hoop and radial stress in all sections of the disk. A new method of the optimum strength disk profile calculation, which requires a restriction of outer radius disk, is suggested. In designing of optimum strength rotating parts is recommended to limit outer disk radius of

where [σ] − the allowable stress, ρ − density of the disk material; ω − angular velocity of disk rotation.     

Characterization of tension set behavior of a silicone rubber at different loads and temperatures via digital image correlation

This work aims to determine the tensile set behavior of a silicone rubber under different stress magnitudes and temperatures through digital image correlation implemented in an improved creep experimental set-up. Creep-recovery strains were measured with time at 20, 40 and 60 °C under tensile strengths of 98.1, 196.2 and 394.3 kPa, respectively. The behavior of creep and recovery strain with time at the different stress magnitudes and temperatures was successfully obtained by the experiments. The corresponding elastic and viscous components of the material for each condition were determined from the results. Overall, all obtained creep behaviors matched with the behavior of a four-element model of creep-recovery. The increase of temperature generated an increase of creep compliance at the three loads, but the increase of tensile load produced a decrease of creep compliance for the three temperatures. The strain was not recovered entirely in any case for the test time stated.     

柔软的球状和长方体状氧化硅中空结构的制备研究

采用自组装形成的芘纳米结构作为模板,成功地制备了柔软的球状和长方体状氧化硅中空结构.当不同量的芘在十六烷基三甲基溴化铵(CTAB)溶液中自组装时,产生的自组装结构展现出明显的从球状到长方体状的形貌变化.这些结构被用作氧化硅前驱体溶胶-凝胶反应的模板,获得了球状和长方体状氧化硅/芘复合结构.通过乙醇除去模板后,生成了柔软的球状(直径约为400nm)和长方体状(长为0.5—2.5μm)的氧化硅中空结构.这些结果展现了采用有机纳米结构作为模板来合成无机中空结构的优势:合成简便、结构多样以及结构形貌的灵活可控.     

A Universal Reduced Rupture Creep Approach for Prediction of Long Term Failure Behavior of Aged Glass Polymers from the Short Term Test of Rupture Creep Compliance by the Unified Master Curved Extrapolation

The prediction of long term failure behaviors and lifetime of aged glass polymers from the short term tests of reduced rupture creep compliance (or strain) is one of difficult problems in polymer science and engineering. A new “universal reduced rupture creep approach” with exact theoretical analysis and computations is proposed in this work. Failure by creep for polymeric material is an important problem to be addressed in the engineering. A universal equation on reduced extensional failure creep compliance for PMMA has been derived. It is successful in relating the reduced extensional failure creep compliance with aging time, temperature, levels of stress, the average growth dimensional number and the parameter in K-W-W function. Based on the universal equation, a method for the prediction of failure behavior, failure strain criterion, failure time of PMMA has been developed which is named as a universal “reduced rupture creep approach”. The results show that the predicted failure strain and failure time of PMMA at di?erent aging times for different levels of stress are all in agreement with those obtained directly from experiments, and the proposed method is reliable and practical. The dependences of reduced extensional failure creep compliance on the conditions of aging time, failure creep stress, the structure of fluidized-domain constituent chains are discussed. The shifting factor, exponent for time-stress superposition at differentlevels of stress and the shifting factor, exponent for time-time aging superposition at different aging time are theoretically defined respectively.     

Physical and chemical aging in PMMA and their effects on creep and creep rupture behavior

Samples of PMMA were aged at 80°C for between 1 h and 254 days and tested in creep in uniaxial extension at large stresses and at room temperature. Both chemical and physical aging effects were found to be important in determining the failure behavior of the samples. This behavior was found to be describable within the context of a commonly used failure criterion that the time to failure multiplied by the strain rate at failure is constant. The creep behavior was found to follow classical time–aging time and time–stress correspondence principles.     

Effects of graded porous structure on local strain distribution under compression in silicone rubber

Silicone rubber samples with gradually changing pore sizes within the range of 70–610 μm are produced using an improved spacer method. The samples are scanned using an X‐ray computed tomography to evaluate their graded structure as compared to uniform rubber. A compressive test reveals that graded porous silicone rubber has characteristic stress–strain curves whose slope changes within a specific strain range depending on the porous structure. Analysis results of local strain based on a digital image correlation of the graded porous silicone rubber under compression demonstrate that the characteristic stress–strain properties are caused by shifts in the main deformation region in the graded structure. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1033–1042     

Viscoelastic properties of UHMW-PE fibers in simple elongation

Stress relaxation after a simple elongational step strain, creep under a constant simple elongational stress, and stress build-up under a constant Hencky strain rate have been measured for ultrahigh-molecular-weight polyethylene (UHMW-PE) fibers. The data from the various experiments are consistent with the Boltzman superposition principle in the experimental region of small strains or short times. This leads to a simple constitutive equation in which temperature can be incorporated via time-temperature superposition. The measured power-law relaxation of the UHMW-PE fiber leads to analytical expressions for the dynamic quantities in simple elongation. The constitutive equation is the one-dimensional equivalent of the gel equation derived for cross-linking gels at the gel point. The similarity between the rheological behavior of fibers and cross-linking gels at the transition point might lead to an enhanced understanding of the molecular processes occurring during deformation.     

The influence of short-chain branching on the creep behavior of oriented polyethylene,and its effect on the efficiency of crosslinking by electron irradiation

Studies have been made of the creep behavior of oriented (15:1) polyethylenes containing 0.4 and 1.3 butyl branches per 1000 C atoms. Increasing the branch concentration reduces significantly the creep strain and the equilibrium strain rate. The data have been fitted to an established model comprising two thermally activated processes in parallel, relating to the amorphous network at low stress, and the crystal phase at high stress. Analysis based on this model indicates the similarity between branching, entanglements, and crosslinks on the creep response. The creep behavior of electron-beam-irradiated materials shows that increasing the branch concentration makes the polyethylene more susceptible to mainchain scission, indicated by increased creep flow rates at higher stress, consistent with previous rubber elasticity studies. Irradiation in an acetylene atmosphere with low (< 1 Mrad) doses is shown to reduce the creep rates at all accessible stresses, and this attributed to an increase in crosslinking compared with scission. © 1994 John Wiley & Sons, Inc.     

Extended Poisson–Nernst–Planck modeling of membrane blockage via insoluble reaction products

A generalized time-dependent mathematical model is developed for a diffusion–migration–reaction system incorporating a pore blockage effect due to generation of insoluble precipitates in a porous membrane. The system behavior is investigated via direct numerical solution of an extended, highly non-linear equation set based on the classical Poisson–Nernst–Planck equations for ion transport. In order to treat the buildup of solid reaction products in the membrane, this novel formulation incorporates both a reaction term and a space- and time-dependent diffusivity expression based on a simple precipitation model. The model is demonstrated for a generalized case and then extended to cover the well-known reaction of silver and chloride ions to form insoluble AgCl. Time-dependent concentration profiles of all ions in the membrane are obtained and the effects of precipitate buildup in the pore space are investigated. The role of counterions in the transient behavior of the system is also clarified.     

Thermodynamics of the surfaces of nonionic spherical micelles with relatively large extensions of the interfacial layer

The structural properties of nonionic spherical micelles with relatively large extensions of the interfacial layer are investigated, and the size dependences of their adsorption, interfacial tension, and chemical potential are obtained. Such familiar thermodynamic relationships as the Gibbs and Laplace equations, the differential equation for the chemical potential, and the concept of hydrophilic–lipophilic balance are used. The method is applied to micelles formed in surfactant solutions of a homologous series of tetraethylene glycol alkyl ethers. The region of the existence of micellar solutions and the structural characteristics of the interfacial layer of micelles are determined. The interfacial tension minimum corresponding to ideal hydrophilic–lipophilic balance in the micelle interfacial layer is detected. The chemical potential is negative over the range of the homologous series, and its derivative with respect to the tension radius is also negative.

     

Mechanical properties of fluorinated ethylene propylene (FEP) foils in use for neutrino detector project

The use of fluorinated ethylene propylene (FEP) foils as engineering materials for aerospace, solar thermal collector and neutrino detector applications has attracted considerable attention in recent decades. Mechanical properties are indispensable for analyzing corresponding structural behavior to meet the demands of safety and serviceability. In this paper, uniaxial tensile tests taking into account loading speeds, uniaxial tensile cyclic tests in terms of stress amplitude and loading cycles and creep tests considering loading stress and time were carried out to characterize mechanical properties. For uniaxial tensile properties, elastic modulus, yield stress, breaking strength and elongation were analyzed in detail. It is found that these mechanical properties except breaking elongation increased with loading speeds and that mechanical properties obtained in transverse direction were more sensitive than those obtained in machine direction. For cyclic properties, elastic modulus and ratcheting strain tended to be stable after certain cycles, demonstrating that cyclic elastic moduli were more suitable for analyzing structural behavior than those obtained in uniaxial tensile experiments. For creep properties, apparent strain at 6 MPa suggested that special attention was necessary for analyzing structural behavior if maximum stress was larger than 6 MPa. In general, this study could provide useful observations and values for understanding mechanical properties of FEP foils.