An experimental investigation of normal and shear stress interaction of an epoxy resin and model predictions

The axial‐torsional interaction of an epoxy resin was investigated by subjecting thin‐walled tubular specimens to combined normal and shear stress components. It is shown that a superimposed normal stress (tensile or compressive) or hydrostatic pressure will influence shear creep behavior. Similarly, a superposed shear stress affects the normal stress response of the resin. The axial‐torsional stress interaction is also observed in transient stress responses under different strain paths, and in the creep deformation with non‐proportional stress histories. Urear viscoelastic constitutive models are unable to predict the aforementioned behaviors. Two typical nonlinear viscoelastic constitutive models are examined with respect to their capabilities to predict the observed response. It Is shown that the predictions of these two models agree only qualitatively but not quantitatively with the experimental results.     

Some aspects of the mechanical response of BMI 5250‐4 neat resin at 191°C: Experiment and modeling

The inelastic deformation behavior of BMI‐5250‐4 neat resin, a high‐temperature polymer, was investigated at 191°C. The effects of loading rate on monotonic stress–strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. Effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods during both loading and unloading. The strain‐time behavior was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 191°C. The model was employed to predict the response of the material under monotonic loading/unloading and multi‐step load histories. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Failure analysis of carbon nanotube wires

A failure analysis of four carbon nanotube (CNT) wires comprised of 1-, 30-, 60-, and 100-yarns was conducted when subjected to constant tension and cyclic tension–tension loading conditions. Each wire had different controlling mechanisms of failure. Tensile and cyclic load-induced failures were related to the movement within yarns and/or among yarns in the CNT wires. The 1-yarn CNT wire exhibited a ductile fracture when constant tensile load was applied; recoverable deformation bands were observed on bending and straightening. The 30-yarn CNT wire showed a variant/independent fibrillar failure under constant tensile loading condition, while it failed by biaxial rotation, bend and twist under cyclic loading condition. The 60-yarn CNT wire resulted in a stake and socket fibre fracture when loaded to failure in constant tension; however, in the cyclic loading condition, the wire failed by kink band process. The 100-yarn wire failure mechanism was controlled by the surface wear in both constant tension and fatigue loading conditions. This failure analysis study presents detailed fracture surface features that can be used to diagnose the cause of failure, develop failure mechanisms, and improve the properties of CNT wires when used in real-life applications.     

Temperature-dependent mechanical properties and constitutive equation of cellulose nitrate

The strain-time response under tensile loading (creep tests) and the stress strain response under constant tensile stress rate (proportional loading tests) have been evaluated at 4 temperatures 20, 45, 55, and 65°C, for samples of cellulose nitrate. A time-dependent constitutive equation (or stress-strain relation) for the nonlinear visco-elastic material is deduced from invariant theory with a hypothesis of a creep potential. The procedure for determining the seven material constants involved in the deduced constitutive equation is described for the creep and proportional loading tests and the variation of these constants with temperature is presented. The deduced constitutive equation gives good agreement with the actual observations for the creep and proportional loading tests, independent of the values of temperature, creep stress, or stress rate.     

Effects of loading rate on viscoplastic properties of polymer geosynthetics and its constitutive modeling

On the basis of the special tensile test results under various loading histories, the rate‐dependent behaviors of three polymer geosynthetics due to their viscous properties have been investigated. All the investigated polymer geosynthetics show significant loading rate effects, creep deformation, and stress relaxation. Except for the polyester geogrid showing the combined viscosity, all the investigated polymer geosynthetics exhibit the isotach viscosity. An elasto‐viscoplastic constitutive model described in a nonlinear three‐component model framework is developed to simulate the rate‐dependent behaviors of polymer geosynthetics. The developed constitutive model is verified by comparing its simulated results with the experimental data of polymer geosynthetics presented in this study and those available from the literature. The comparison indicates that the developed model can reasonably interpret the rate‐dependent behaviors of polymer geosynthetics under arbitrary loading histories, including the step‐changed strain rate loading, creep, and stress relaxation applied during otherwise monotonic loading (ML). POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Tensile Creep Behavior of a Vitreous-Bonded Aluminum Oxide under Static and Cyclic Loading

Creep deformation and rupture behavior of a vitreousbonded aluminum oxide was investigated under uniaxial static and cyclic tensile loadings at 1000°, 1100°, and 1175°C. The material was more creep resistant, i.e., having lower creep strain rates, under cyclic loading compared to that under static loading. For the same maximum applied stress, the ratio of steady-state creep rate under static loading to that under cyclic loading at 1100°C was approximately 100. However, the value of this ratio decreased to about 10 when the testing temperature was raised to 1175°C or lowered to 1000°C. Under static loading the material had more propensity to develop creep damage in the form of micro- and macrocracks, leading to early failure, whereas under cyclic loading the creep damage was more uniformly distributed in the form of cavities confined to the multigrain junctions. Viscous bridging by the grain boundary second phase may be the primary contributor to the lower creep deformation rate and improved lifetime under cyclic loading.     

Micromechanical model of time-dependent damage and deformation behavior for an orthogonal 3D-woven SiC/SiC composite at elevated temperature in vacuum

This paper presents a micromechanical model to predict the time-dependent damage and deformation behavior of an orthogonal 3-D woven SiC fiber/BN interface/SiC matrix composite under constant tensile loading at elevated temperature in vacuum. In-situ observation under monotonic tensile loading at room temperature, load–unload tensile testing at 1200 °C in argon, and constant load tensile testing at 1200 °C in vacuum were conducted to investigate the effects of microscopic damage on deformation behavior. The experimentally obtained results led to production of a time-dependent nonlinear stress–strain response model for the orthogonal 3-D woven SiC/SiC. It was established using the linear viscoelastic model, micro-damage propagation model, and a shear-lag model. The predicted creep deformation was found to agree well with the experimentally obtained results.     

Strain‐rate and temperature effects on the deformation of polypropylene and its simulation under monotonic compression and bending

Monotonic compressive loading and bending tests are conducted for solid polypropylene (PP) under constant or time‐varying strain‐rates and temperatures of 10, 25, 40°C. The observed compressive stress‐strain responses under constant conditions have revealed that the inelastic deformation behavior is remarkably dependent on loading rates and temperatures of normal use. The examination of such inelastic behavior has indicated that the strain‐rate effects correspond with the temperature effects based on the concept of time‐temperature equivalence. The viscoplastic constitutive theory based on overstress (VBO) has successfully reproduced the experimental responses with stress‐jumping phenomena using the equivalent time. Four‐point bending tests are performed under monotonic loading and holding for PP beams at three different temperatures. The observed deformation behavior has shown that the Bernoulli‐Euler hypothesis is valid. The VBO model and beam bending theory has generated the basic equations for PP beams, showing an analogy with the uniaxial one. In the numerical analysis, the equations are transformed into nonlinear ordinary differential equations with use of Gaussian quadrature for the spatial integrals. The comparison of numerical and experimental results has suggested some modifications for actually loaded moment taking the effect of deflection and friction into consideration. Finally, the numerical calculation has simulated the experimental time‐histories of curvatures fairly well.     

Cyclic deformation behavior of an epoxy polymer. Part I: Experimental investigation

A series of uniaxial cyclic tests were carried out on solid cylindrical specimens of an epoxy resin, Epon 826/Epi‐Cure Curing Agent 9551. The focus of the study was to investigate time‐dependent viscoelastic behavior of this thermosetting polymer material under cyclic loading and to develop a constitutive model with the capabilities to simulate the observed deformation response. The tests include stress‐controlled or strain‐controlled cyclic loading with/without mean stress or mean strain at various amplitudes and loading rates. It was found that the cyclic stress‐strain response of this material is amplitude‐dependent and rate‐dependent, and the response to axial tension is different from that in compression. The stress‐strain loops exhibit more pronounced nonlinearity with high amplitudes or low loading rates. For stress‐controlled cyclic loading with mean stress, ratcheting strain is accumulated, which is of viscoelastic nature, and this is confirmed by its full recovery after load removal. For strain‐controlled cyclic loading with mean strain, the mean stress relaxation occurs, which contributes to the observed longer life in comparison to the stress‐controlled cyclic loading with mean stress. Polym. Eng. Sci. 44:2240–2246, 2004. © 2004 Society of Plastics Engineers.     

Molecular mechanisms involved in creep phenomena of paper

The phenomenon of mechanosorptive creep (i.e., the increasing creep occurring in some hygroscopic materials subjected to moisture cycling) was studied for paper from a molecular point of view. Paper was tested in creep at different loading levels in a constant high humidity of 90% relative humidity (RH) and in a cyclic climate between 30 and 90% RH. Throughout the creep tests, spectra from the mid‐ and near‐IR, as well as dynamic mechanical data, were recorded to determine molecular changes occurring with time. In tensile stress scans the instantaneous, dynamic elastic modulus was found to increase. It is suggested that this increase was due to orientation of the cellulose molecules, which was detected as changes in the mid‐IR spectra at 1160 cm⁻¹ assigned to the C₁ O C₄ stretching. During creep in constant and cyclic humidity, the modulus was found to increase with time, more so for the cyclic humidity. Changes in the mid‐IR spectra at 1184 and 1030 cm⁻¹, which is assigned to CH₂, CH, and C O, may indicate sliding between the cellulose chains. The near‐IR measurements mainly showed differences in the moisture content. In stress scans the moisture content increased with increasing tensile load. In creep at constant 90% RH, the moisture content was also found to increase in a manner similar to the stress scan. In the cyclic humidity with a conditioning time of 70 min at 90% RH the moisture content decreased successively with increasing numbers of cycles. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1590–1595, 2001     

Pre‐necking and post‐necking relaxation and creep behavior of polycarbonate: A phenomenological study

Experiments have been performed to investigate the mechanical response of unfilled polycarbonate vis‐à‐vis the influence of prior deformation on stress relaxation and creep. Piecewise linear deformation histories, which involve strain‐controlled tensile loading of a specimen to a maximum load and partial unloading to a target strain/stress point as prologue to a relaxation test, have been shown to qualitatively influence the recorded stress‐time behavior. In particular, the stress magnitude during relaxation first increases and is then followed by a decrease. Analogously, in creep tests during unloading, the strain might decrease and then increase. Time characteristics for this U‐turn in the deformation response are influenced by the placement of the test. The influence of prior specimen conditioning on this phenomenon is investigated by comparing test data from virgin samples to that of specimens having high (～85%) inelastic strain from prior tensile elongation. Findings suggest that the observed persistence in the occurrence of this reversal effect for both types of specimens is evidence of the need to incorporate this behavior into the fold of material modeling. Additionally, this novel relaxation and creep behavior has been observed in other amorphous (poly(phenylene oxide)) and crystalline (high‐density polyethylene) polymers. Polym. Eng. Sci. 44:1783–1791, 2004. © 2004 Society of Plastics Engineers.     

Effect of external loads on damage detection of rubber‐toughened nanocomposites using carbon nanotubes sensory network

Multiwalled carbon nanotubes of 0.2% weight fraction are used as a sensory network for detecting and characterizing the damage of particulate epoxy composites under shear loading conditions. Three different weight fractions of carboxyl‐terminated butadiene acrylonitrile copolymer rubber [10 parts per hundred of epoxy resin (phr), 20 phr, and 30 phr] are used for toughening a thermoset epoxy composite. The electrical response of the specimens is measured, nearest the central shearing plane, using a four‐circumferential ring probe technique in conjunction with a high‐resolution data acquisition system. A collection of the electromechanical response results are reported with respect to the shear strain. The resistance changes observed under shear loading are related to nonlinear deformation mechanisms, void initiation, and growth around rubber particulate. With increasing rubber content, the strength of the material decreases and a greater drop in resistance is recorded as a result of decreased distance between neighboring carbon nanotubes (CNTs) due to declustering and straightening of molecular chains of host matrix. In the end, a comparison for 30 phr composites under shear loading with that of tensile and compression loading conditions is presented. For initial deformation, there is no change in resistance under shear loading condition; however, the significant resistance change can be noticed under both tension and compression. The specimen under shear loading conditions experiences smaller decrease in resistance when compared with both tension and compression. However, the decrease in resistance is higher for compression due to higher decrease in distance between neighboring CNTs. POLYM. COMPOS., 37:360–369, 2016. © 2014 Society of Plastics Engineers     

Fatigue behavior and life prediction of PI/SiO2 nanocomposite films

In this report, the fatigue behavior and lifetime of Polyimide/silica (PI/SiO₂) hybrid films are investigated. To evaluate the fatigue property of this class of hybrid films, the stress‐life cyclic experiments under tension–tension fatigue loading with 10 Hz of the frequency are performed, and the stress ratio is 0.1. Dynamic creep and cyclic softening/hardening are analyzed based on the change of hysteresis loops during the fatigue process. The structure‐property relations are discussed to further understand their phenomenon and deformation mechanisms. To predict the fatigue life of this class of hybrid films, a semiempirical model is proposed based on fatigue modulus concept. The simulated results are well agreeable with the testing values. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Thermo‐mechanical Coupling Properties of Proton Exchange Membrane in Liquid Water

In this study, the effects of the combination and interaction of thermal and mechanical loading on mechanical behaviors of proton exchange membranes (PEMs) under immersed condition were investigated. Experiments under two kinds of loading path were performed: the quasi‐simultaneous loading path where mechanical loads and temperature occurred simultaneously and the rectangular loading path where thermal and mechanical loads were inserted in turn. The quasi‐simultaneous loading path is composed of in‐phase, 90° and 180° out‐of‐phase loading paths. Comparison between in‐phase and 180° out‐of‐phase loading paths showed that strain accumulation under in‐phase loading path mainly resulted from creep strain while that under 180° out‐of‐phase loading path were mostly induced by cyclic stress. Accumulation of strain was comprised of tensile strain and creep strain, and these two strain components showed different evolution trends with thermo‐mechanical cycles. In addition, the modulus of the membrane was history‐dependent as indicated from results of rectangular loading path. Through comparing creep strain with strain energy density which is an indicator of damage in the membrane, we came to the conclusion that creep strain contributed to the damage of the membrane in the first three cycles.     

Plastic deformation of low‐density polyethylene reinforced with biodegradable polylactide,Part 2: Creep characterization and modeling

The behavior of low‐density polyethylene (LDPE) and two blends prepared with polylactide (PLA) was determined by means of a novel video‐controlled testing method under stretching at constant true strain rate, under creep at constant true stress, and under creep at constant nominal stress. Most tests were performed at 23°C and 50°C. In this second part, the experimental data are modeled with the G'Sell‐Jonas phenomenological law expressing the axial true stress versus axial true strain and axial true strain rate. This model describes correctly the various deformation stages: (i) initial viscoelasticity, (ii) plastic yielding, and (iii) strain hardening up to rupture. It shows clearly the reinforcing effect of the PLA particles that increases the yield stress in stretching experiments and slows down the deformation kinetics under creep. It is shown how the local stress/strain behavior is related to the standard force/extension curves. Consequently, it is proposed that tensile tests at constant true strain rates should be systematically preferred to creep tests for the characterization of constitutive relations because they take much less time to be performed. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers.     

Creep of Nextel™ 610 Fiber at 1100°C in Air and in Steam

Creep of Nextel^?610 fibers was investigated at 1100°C and 100–500 MPa in air and in steam. The effect of loading rate on fiber tensile strength was also explored. The presence of steam accelerated creep and reduced fiber lifetimes. Loading rate had a considerable effect on tensile strength in steam, but not in air. A linear elastic crack growth model was used to predict the creep lifetimes from the constant loading rate data. The dependence of tensile strength on loading rate and the predictability of creep lifetimes suggest that the failure mechanism in steam was environmentally assisted subcritical crack growth. The creep‐rupture data were analyzed in terms of a Monkman‐Grant (MG) relationship. Monkman‐Grant parameters for creep‐rupture data were the same in steam and air, and predicted creep‐rupture at 1100°C in both environments. A grain‐size increase of about 25% was observed by TEM after 100 h at 1100°C in steam, which was about two times that observed in air.     

Asymmetric tensile and compressive creep deformation of hot-isostatically-pressed Y2O3-doped -Si3N4

The uniaxial tensile and compressive creep rates of an yttria-containing hot-isostatically-pressed silicon nitride were examined at several temperatures between 1316 and 1399°C and found to have different stress dependencies. Minimum creep rates were always faster in tension than compression for an equal magnitude of stress. An empirical model was formulated which represented the minimum creep rate as a function of temperature for both tensile and compressive stresses. The model also depicted the asymmetric creep deformation using exponential and linear dependence on tensile and compressive stress, respectively. Unlike other models which represent either tensile or compressive creep deformation as a respective function of tensile or compressive stress, the model in the present study predicted creep deformation rate for both tensile and compressive stresses without conditional or a priori knowledge of the sign of stress. A statistical weight function was introduced to improve the correlation of the model’s regressed fit to the experimental data. Post-testing TEM microstructural analysis revealed that differences in the amount of tensile- and compressive-stress-induced cavitation accounted for the creep strain asymmetry between them, and that cavitation initiated in tensile and compressively crept specimens for magnitudes of creep strain in excess of 0·1%.     

Tensile creep due to restraining stresses in high-strength concrete at early ages

This paper reports an experimental study on the early-age tensile creep behavior of high strength concrete (HSC) comprising of silica fume concrete, fly ash concrete and plain concrete under uniaxial restraining stresses. A series of restraint shrinkage tests were carried out adopting semi-adiabatic and isothermal conditions to determine the effects of temperature history on the tensile creep properties for young concretes. Furthermore, the effects of restraining stress history on creep were also discussed under three different degrees of restraint conditions. It was found that the initial thermal dilation deformation delayed the development of tensile creep and weakened the creep potential of early age concretes. It was also observed that the young concrete subjected to a lower restraining tensile stress history had a higher potential of visco-elastic response in tension at early ages.     

Viscoelastic constitutive model for uniaxial time‐dependent ratcheting of polyetherimide polymer

Based on the experimental observations, a cyclic nonlinear viscoelastic constitutive model was proposed to describe the uniaxial time‐dependent ratcheting of polyetherimide (PEI) polymer under tension–compression and tension–tension cyclic loading. The model was constructed by extending the nonlinear viscoelastic Schapery model (Schapery, Polym. Eng. Sci., 9, 295 (1969)). The extension emphasized the changes of parameter functions used in the original model, which enabled the model to describe the ratcheting of polymer material. Comparing the simulations with corresponding experimental results, the capability of the extended model to predict the uniaxial time‐dependent ratcheting of PEI was verified. It is shown that the extended model can reasonably describe the uniaxial time‐dependent ratcheting of the polymer under the tension–compression and tension–tension cyclic loading with different peak‐holdings, stress rates, and stress levels. POLYM. ENG. SCI., 52:1874–1881, 2012. © 2012 Society of Plastics Engineers     

Thermal and mechanical properties of polyisobutylene‐based thermoplastic polyurethanes

We investigated thermal and mechanical properties of thermoplastic polyurethanes (TPUs) with the soft segment comprising of both polyisobutylene (PIB) and poly(tetramethylene)oxide (PTMO) diols. Thermal analysis reveals that the hard segment in all the TPUs investigated is completely amorphous. Significant mixing between the hard and soft segments was also observed. By adjusting the ratio between the hard and soft segments, the mechanical properties of these TPUs were tuned over a wide range, which are comparable to conventional polyether‐based TPUs. Constant stress creep and cyclic stress hysteresis analysis suggested a strong dependence of permanent deformation on hard segment content. The melt viscosity correlation with shear rate and shear stress follows a typical non‐Newtonian behavior, showing decrease in shear viscosity with increase in shear rate. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 891‐897, 2013