Prediction of nonlinear viscoelastic behavior of polymeric composites using an artificial neural network

Creep tests at constant stress are performed for the carbon-fiber reinforced epoxy composite at various temperatures and initial stresses. A nonlinear viscoelastic constitutive model is developed, and its material parameters are determined by fitting it to creep test data. Model results are found to agree very well with the experimental data at low temperature and low stress conditions. However, the agreement deteriorates at high temperatures, particularly in the vicinity of the glass transition temperature.An alternative model based on an artificial neural network (ANN) is developed to predict the stress relaxation of the polymer matrix composite. The ANN model is trained and validated with 9000 experimental data sets obtained from stress relaxation tests performed at various constant strain (initial stress) and constant temperature conditions. Training of the ANN employs a scaled conjugate gradient method. The optimal brain surgeon algorithm is employed to optimize the topology. The optimal ANN configuration has 88 processing elements (3 in the input layer, 45 in the first hidden layer, 39 in the second hidden layer, and 1 in the output layer) and 410 links. The predictions of the ANN model are found to be more accurate over a wider range of stress and temperature conditions than those of the explicit nonlinear viscoelastic model, in particular near the glass transition temperature.     

图像相关法在高分子材料拉伸性能研究中的应用

本文对聚碳酸酯(PC)和丙烯腈丁二烯苯乙烯(ABS)的合金(PC/ABS)高分子材料不同环境温度下的拉伸性能进行了试验研究。根据图像相关分析法编制了图像法位移测量分析软件,并对该分析软件的适用性进行了分析。系统研究了PC/ABS高分子材料拉伸试验时三个方向的位移场和应变场。根据测得的位移场研究了该高分子材料拉伸过程中应变和应变率的变化以及应力应变变化规律,并对试验结果进行了详细分析。结果表明,文中采用的图像法位移测量系统具有较高的测试精度;拉伸过程中,试件厚度方向的收缩变形大于宽度方向的收缩变形;颈缩过程区具有非常高的应变率,颈缩后的平直颈缩区的应变率快速下降到一个很低的应变率继续缓慢变形;尽管载荷位移曲线出现了较大的载荷下降现象,PC/ABS拉伸时的真应力应变曲线没有明显的应力下降现象出现,因此,载荷下降现象主要由颈缩时的截面减小引起;高分子材料PC/ABS的屈服应力随环境温度的升高而降低。     

Cryogenic prestressed concrete: Fracture aspects

This paper attempts to make a review of the available information about fracture at low temperatures of reinforced concrete materials, as well as incipient standards related with this subject.Data on the fracture behaviour at low temperature of the steel for concrete reinforcement is scarce and sometimes controversial. The effect of ribs as stress raisers and the influence of surface defects on brittle fracture is analyzed. In prestressed concrete, tendon anchorages are sensitive zones where fracture may be produced and this risk may be increased at low temparatures. No international standards have yet been developed to prevent brittle fracture under cryogenic conditions. This paper suggests some requirements as well as related testing procedures.Modelling fracture of concrete is still in its infancy, even at room temperature; nevertheless, some known behaviour at low temperatures will be reviewed. Cryogenic concrete is a novel composite material that deserves further research. One of the major assets of a prestressed concrete structure is its fracture behaviour in comparison with a metal tank. Due to the multiplicity of individual tendons and reinforcing bars, the sudden break of a bar or prestressing wire, or even a tendon, should not cause a sudden collapse of the structure.     

Composite viscoelasticity in glassy,transitional and molten states

As part of a study of viscous and elastic behaviors, over a range of temperatures from below the glass transition up to the hot melt, we here report steady-shear viscosities at 0.007 to 13 s^?1 and at 160 to 220 °C of polystyrene containing 0 to 60% by mass of 0.18-micron diameter titanium dioxide particles. The materials were shearthinning without a yield stress, with a constant activation energy at constant stress, but having a shear-dependent activation energy at constant shear rate — proportional to the volume fraction of the polymer matrix. Superposition of the flow curves at different temperatures for the unfilled and filled systems was possible. All the data were represented by one equation with four parameters: 1) a shear stress coefficient (units Pa · s²); 2) a characteristic stress level for non-Newtonian behavior, independent of temperature and composition; 3) an activation energy at constant stress; and 4) an Einstein coefficient (or intrinsic viscosity of the filler). Other equations also fitted the data, but the others diverged widely when extrapolated.     

Selection of ethylene-vinyl-acetate properties for modified bitumen with enhanced end-performance

Bitumen modification with ethylene-vinyl acetate (EVA), in a wide range temperatures (between ??30 and 100 °C), has been studied as a function of polymer concentration and EVA characteristics (vinyl acetate (VA) content and melt flow index (MFI)). Viscous flow, dynamic shear (DSR) temperature sweep, and technological tests were conducted to assess binder performance at medium-to-high in-service temperatures. Evaluation of binder low-temperature viscoelastic behavior has been performed using a solid rectangular fixture (SRF) in torsional mode, either in the linear viscoelastic region or under non-linear conditions (by strain breakage tests between ??30 and 0 °C). Further microstructural analysis based on modulated differential scanning calorimetry (MDSC) and optical microscopy was conducted to support rheological and technological results. Hence, total crystalline fraction (related to the VA content and polymer concentration) turned out to be a key parameter to achieve a suitable binder modification at medium-high temperatures. In addition, MFI appears to be an important EVA parameter at low temperatures, as it was found that lower MFI values enhanced resistance to low-temperature cracking.     

Effect of the rate of deformation on the crystallization behavior of polymers

Deformation has a significant influence on the crystallization process in a number of polymers. In this paper, the response of a recently developed model for crystallizing polymers is investigated when subject to uni-, bi-axial and constant width extensions for a range of strain rates. Both the loading and unloading behavior are examined for these deformations. The particular model studied here was developed to capture the effect of strain induced crystallization in polymers and has been applied to model crystallization in polyethylene terephthalate at temperatures just above its glass transition temperature. The model has been formulated using the notion of multiple natural configurations within a full thermodynamic framework. The connection between micro-structural changes taking place in the polymer and the form of the model are elucidated. The interplay between the relaxation processes, the rate of deformation and their combined effect on crystallization is illustrated. The results show an earlier onset of crystallization for high strain rates due to stretching of the polymer network. At low strain rates however, crystallization is not observed as the polymer network is able to relax during the deformation. A sharp upturn in the stress is observed after the onset of crystallization due to the formation of a rigid crystalline phase. The unloading curves clearly show a hysteric behavior with the amount of dissipation increasing for increasing values of strain rate. These results compare favorably with experimental observations available in literature.     

Thermal contraction and volume relaxation of amorphous polymers

Two experimental techniques are described for the determination of the change of specific volume of polymers with temperature and aging time, which allow measurements between – 160 °C and + 200 °C. Four technical amorphous polymers, PS, PVC, PMMA and PC have been investigated. Volume-temperature curves under constant rate of cooling are presented and interpreted with respect to relaxation processes known from other physical investigations. The rate dependence of dilatometric glass transition temperatures is compared with the time dependence of rheometric glass transition temperatures from shear creep data. Volume relaxation data at constant aging temperature are presented. Aging is found to proceed until very low temperatures in the glassy state for e.g. PMMA.For polystyrene, a comparison is made between the predictions of a very simple theory of volume relaxation due to Kovacs with experimental data, using additional information from volume temperature curves and the time temperature shift of the shear creep transition. The theory predicts a rate of volume relaxation which is much lower than that found by experiment.     

Effect of nanofillers on the phase separation and rheological properties of poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) blends

The viscoelastic characteristics of the blends of poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) (PMMA/SAN) were investigated at various temperatures below, near, and above the phase separation temperature. The investigated polymer system is characterized by a lower critical solution temperature. Rheological behavior of the blends in the region of a phase separation was compared with change of the light scattering intensity. The presence of nanofillers in the blend results in that the phase separation occurs at a higher temperature. At the isothermal conditions, the phase separation begins earlier and proceeds with a higher rate as compared with the same blend without filler. The results of the study show the considerable change of the viscoelastic characteristics of PMMA/SAN when the polymer system passes from the homogeneous state to the heterogeneous one. Such characteristics as the dependence of the storage modulus (G ^′) on the loss modulus (G ^″), the dependence of the loss viscosity (η ^″) on the dynamic viscosity (η ^′), the dependences of the complex viscosity (η*), and the free volume fraction (f) on the blend composition are the most sensitive to the phase separation. The phase separation affects the characteristics G ^′(ω), where ω is the frequency only in a low-frequency range. Temperatures of phase separation were estimated using dependence G ^′(T) at ω, which is the constant in the range of low frequencies.     

Crystallization of an ethylene-based butene plastomer: the effect of uniaxial extension

In this paper, the effect of uniaxial extension on the crystallization of an ethylene-based butane plastomer is examined by using rheometry coupled with differential scanning calorimetry (DSC). Uniaxial extension experiments were performed at temperatures below and above the peak melting point of the polyethylene in order to characterize its flow-induced crystallization behavior at extensional rates relevant to processing. The degree of crystallinity of the stretched samples was quantified by DSC, i.e., by analyzing the thermal behavior of samples after stretching. Analysis of the tensile strain-hardening behavior very near the peak melt temperature revealed that crystallization depends on temperature, strain, and strain rate. In addition, it was revealed that a very small window of temperatures spanning just 1–2°C can have a dramatic effect on polymer crystallization. Finally, flow-induced crystallization experiments at temperatures close to the peak melting point have shown the recrystallization of multiple crystalline structures within a polymer matrix, witnessed by double peaks within a narrow window of 89–93°C in the DSC thermographs, with the most demonstrable double peak behavior occurring at a temperature of 91°C, a temperature that is just 1°C cooler than the peak melt temperature of the polymer.     

The effect of temperature gradients on the sharkskin surface instability in polymer extrusion through a slit die

The sharkskin surface instability is commonly observed in the extrusion of polymer melts. We present a series of experiments in which a specifically designed rectangular slit die with insulated and independently heated sides and is used to induce precise temperature gradients across a flowing polyethylene melt. Our previous experiments demonstrated that the character of the surface distortions produced by the sharkskin instability was a function of the die wall temperature and therefore the extrudate had viscoelastic properties at the surface. In this paper, we explore the role of temperature and viscoelastic property gradients near the capillary wall. The amplitude of the sharkskin instability is quantified and plotted against apparent shear and extension rates. Analysis of the data demonstrates that the amplitude and frequency of the instability is independent of bulk temperature and temperature gradient and is dependent only on wall temperature. The data are normalized using a dimensionless Weissenberg number based on the extension rate to collapse the data collected over all temperatures and gradients onto a single master curve. We conclude with an example of a rectangular extrudate exhibiting varying surface roughness due to differential die heating and discuss the implications of our observations on the sharkskin surface instability mechanism and on commercial applications.     

High-rate Characterization of 304L Stainless Steel at Elevated Temperatures for Recrystallization Investigation

An integrated experimental technique was developed for high-rate mechanical characterization of 304L stainless steel at elevated temperatures by using a modified split Hopkinson pressure bar (SHPB). A sandwich structure consisting of two platens and the specimen in between was heated before mechanical loading while the bars were maintained at room temperature to eliminate the temperature gradient effect on the wave propagation in the bars. Upon contacting the cold bars, temperature gradients form in the platens, leaving the temperature in specimen constant and uniform. Pulse shaping techniques were employed to maintain constant strain-rate deformation and dynamic stress equilibrium in the specimen. Dynamic compressive stress-strain curves at elevated temperatures for the 304L stainless steel were obtained. To relate recrystallization to impact loading, a momentum trapping system was employed to apply a single loading on the specimen during one dynamic experiment. We also controlled the quenching time to study its effect on recrystallization.     

A study on the cycle characteristics of an auto-cascade refrigeration system

This study presents experimental results on the cycle characteristics of an auto-cascade refrigeration system. The auto-cascade refrigeration (ACR) system is driven by a single compressor, using zeotropic mixture as refrigerant, which achieves cascade between high and low boiling point components by an evaporative condenser for the purpose of obtaining a lower evaporation temperature comparing with single refrigerant system. The coefficient of performance, cooling capacity, evaporation temperature, and pressures and temperatures of refrigerant at the inlet and outlet were measured and parameter analyses were conducted with different charging concentrations, different temperatures of cooling water and different matches of cycle flux between high and low boiling point components under the same evaporating pressure. The systematic theoretical analyses and experimental results depict the relations between the above parameters in an auto-cascade refrigeration system, which constitute a useful source for the auto-cascade refrigeration system’s design and analysis.     

Surface tension measurement of aqueous polymer solutions

The surface tension of aqueous polymer solutions of polyacrylamide (PAM), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), and hydroxyethyl cellulose (HEC) was studied over a range of polymer concentrations by using the maximum bubble pressure method at temperatures ranging from 20 to 65°C. The surface tension of water was also measured by the maximum bubble pressure method as well as by the DuNoüy ring method over the same temperature range. The experimental water data are in excellent agreement with the well-established tabulated data in the literature.

For a fixed concentration, all of the polymer solutions exhibited a decrease in surface tension with increasing temperature level. When compared with water at a fixed temperature level, the PAM and CMC solutions showed slightly higher surface tension values, whereas the PAA solutions yielded values equal to those found for water. In the case of the HEC solutions, the measured surface tensions decreased with concentration at a fixed temperature level and were lower than the values found for water. For a concentration of 2000 wppm the surface tension values for the hydroxyethyl cellulose were of the order of 10% lower than those for water at a fixed temperature level.

A comparison of the new measurements with the relatively limited previously published studies showed good agreement.     

An experimental analysis of fluctuating temperature measurements using hot-wires at different overheats

Adjacent parallel hot-wire anemometers at different temperatures have sometimes been used to measure fluctuating temperatures in turbulent flows. This work presents an extensive experimental comparison of temperatures measured with a parallel-wire probe to temperatures simultaneously measured with a standard cold-wire probe. The results show the parallel-wire probe to work well in low intensity flows with temperature signals which are not too small. However, the parallel-wire probe temperature measurements are not accurate for high turbulence intensities or for small temperature signals, and in general the cold-wire system is probably to be preferred.     

Laminar free-convective heat transfer from a vertical isothermal plate to water at low temperatures with variable physical properties

All studies concerning laminar free convection along a vertical isothermal plate in water at low temperatures have been conducted assuming constant dynamic viscosity and thermal conductivity both taken at ambient or film temperature. In this study the problem has been treated taking into account the temperature dependence of all water physical properties. The results are obtained with the numerical solution of the boundary layer equations. The variation of μ and k with temperature has a small influence on wall heat transfer but a strong influence on wall shear stress. These quantities show a significant reduction at density extremum.     

Gelation kinetics in elastomer/thermoset polymer blends

The chemical gelation of polymer blends involving a thermoset component has been investigated utilizing multiple waveform dynamic rheology, complemented by differential scanning calorimetry and small angle x-ray scattering measurements. Three epoxy/rubber mixtures, with different degrees of pre-reacted material and catalyst, were used. The curing temperatures were determined with simple temperature ramps, whereas the characteristic gel times were quantified using the well-known self-similarity of the gel structure, which yielded a very low gel exponent (0.05), indicative of the high molecular weights and additives used in the samples; this exponent was unchanged with temperature, suggesting the same gelation mechanism. These times decreased with increasing temperature. Gelation was not affecting the phase-separated morphology. The presence of pre-reacted material and catalyst were found to reduce the gel times (for different temperature steps). Finally, the shear history of the samples did not affet the curing temperatures and kinetics. These results can serve as a useful guide for appropriately blending thermosetting and thermoplastic compounds in order to obtain structural materials for different applications.     

Creep performance of PVC aged at temperature relatively close to glass transition temperature

In order to predict the mechanical performance of the polyvinyl chloride(PVC) at a high operating temperature,a series of short-term tensile creep tests(onetenth of the physical aging time) of the PVC are carried out at 63 C with a small constant stress by a dynamic mechanical analyzer(DMA).The Struik-Kohlrausch(SK) formula and Struik shifting methods are used to describe these creep data for various physical aging time.A new phenomenological model based on the multiple relaxation mechanisms of an amorphous polymer is developed to quantitatively characterize the SK parameters(the initial creep compliance,the characteristic retardation time,and the shape factor) determined by the aging time.It is shown that the momentary creep compliance curve of the PVC at 63 C can be very well fitted by the SK formula for each aging time.However,the SK parameters for the creep curves are not constant during the aging process at the elevated temperatures,and the evolution of these parameters and the creep rate versus aging time curves at the double logarithmic coordinates have shown a nonlinear phenomenon.Moreover,the creep master curves obtained by the superposition with the Struik shifting methods are unsatisfactory in such a case.Finally,the predicted results calculated from the present model incorporating with the SK formula are in excellent agreement with the creep experimental data for the PVC isothermally aged at the temperature relatively close to the glass transition temperature.     

From melt flow index to rheogram

A knowledge of the complete flow curve or rheogram of a polymeric melt depicting the variation of the melt viscosity over industrially relevant range of shear rate and temperature is essential in the design of polymer processing equipment, process optimization and trouble-shooting. These data are generated on sophisticated rheometers that are beyond the financial and technical means of most plastics processors. The only flow parameter available to the processor is the melt flow index of the material. In the present work, a method has been proposed to estimate the rheograms of a melt at temperatures relevant to its processing conditions with the use of a master curve, knowing the melt flow index and glass transition temperature of the material. Master curves that coalesce rheograms of different grades at various temperatures have been generated and presented for low density polyethylene, high density polyethylene, polypropylene, polystyrene and styrene-acrylonitrile copolymer.     

Temperature measurement of a curved surface using thermographic phosphors

An optical technique for surface temperature measurement based on the fluorescent emission of rare earth ion-doped phosphors was demonstrated in an experiment with a heated cylinder in crossflow. In this experiment, a uniform heat flux was imposed by applying a constant voltage across the thin stainless steel cylinder surface to produce surface temperatures between 24°C and 55°C. The fluorescent emission of a thermographic phosphor, lanthanum oxysulfide doped with europium (La₂0₂S:Eu³⁺) deposited on the surface, was recorded to determine the temperature distribution at the curved surface. When excited by ultraviolet radiation, the phosphor emits a spectrum containing certain emission lines, the intensities of which vary with temperature. For a single temperature-sensitive line, ratios of the intensity at a reference temperature to the intensity at different temperatures were correlated as a function of surface temperature. The use of intensity ratio correlations avoids complications due to geometric (viewing angle) effects. Digitized images of the cylinder permitted calculation of surface temperatures and local Nusselt numbers. Differences between surface temperatures measured by calibrated thermocouples and temperatures determined from the phosphor technique were at most 1.2°C.