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     

Effects of prior aging at 288°C in argon environment on time‐dependent deformation behavior of a thermoset polymer at elevated temperature,part 1: Experiments

The inelastic deformation behavior of PMR‐15 neat resin, a high‐temperature thermoset polymer, aged at 288°C in argon environment for up to 2000 h was investigated. The experimental program was designed to explore the in?uence of prior isothermal aging on monotonic loading and unloading at various strain rates. In addition, the relaxation response and the creep behavior of specimens subjected to prior aging of various durations were evaluated. All tests were performed at 288°C. The time‐dependent mechanical behavior of the PMR‐15 polymer is strongly influenced by prior isothermal aging. The elastic modulus increased and the departure from quasi‐linear behavior was delayed with prior aging time. Stress levels in the region of inelastic flow increased with prior aging time. Furthermore, prior aging significantly decreased the polymer's capacity for inelastic straining, including the material's capacity to accumulate creep strain. Conversely, the relaxation response was not affected by the prior aging. © 2009 Wiley Periodicals, Inc.? J Appl Polym Sci, 2009     

A constitutive model for non‐linear behavior of SMC accounting for linear viscoelasticity and micro‐damage

An approach for modeling sheet molding compound (SMC) composites as viscoelastic damageable material is presented. Continuum damage mechanics theory by Chow and Wang (Int. J. Fract., 33, 3 (1987)) was used in combination with linear viscoelasticity. The model was applied to a modern SMC composite material containing both hollow glass spheres for low density and toughening additive for improved impact resistance. Tensile tests and uniaxial creep test were employed to build the constitutive model. Validation was done by comparing test data with simulations of uniaxial creep on material with different degrees of damage. The model has good accuracy at moderate damage levels under controlled time‐dependent crack propagation. Tensile testing at two different fixed strain rates was simulated using quasi‐elastic method to calculate relaxation modulus. The model predicts the stress‐strain curve with good accuracy until the region is close to failure, where new mechanisms not accounted for are taking place. Finally, a simulation of a cyclic tensile test with increasing maximum strain per cycle was performed, and since both damage and viscoelasticity are included in the model, the slope change, accumulation of residual strain, and hysteresis in the stress‐strain, loading‐unloading curve are predicted. POLYM. COMPOS., 26:84–97, 2005. © 2004 Society of Plastics Engineers     

The effect of strain rate on the deformation and relaxation behavior of 6/6 nylon at room temperature

The influence of strain rate changes in the range from 10^?3 to 10^?6 1/s on the zero-to-tension loading and unloading behavior as well as short term relaxation properties is investigated using cylindrical specimens of circular cross section. A clip-on extensometer measures and controls the axial strain in an MTS servohydraulic, computer-controlled mechanical testing machine. Strains do not exceed twenty percent and all deformation is macroscopically homogeneous. An increase in strain rate causes an increase in stress level. Surprisingly, the total stress drop in a 20 min relaxation period increases with prior strain rate. When the relaxation test is started in the inelastic region with low tangent modulus the total stress drop is nearly independent of the stress and strain at which relaxation commences. Unloading to zero load is not linear but curved and the strain recovery at zero stress is significant. It occurs at an ever decreasing rate and does not exceed three percent in a 12 h period. Like the relaxation behavior the recovery rate increases with prior strain rate. Repeated relaxation periods during zero-to-tension cycling can show a stress magnitude decrease during loading but a stress magnitude increase during unloading. The results suggest that a unified model with an overstress dependence of the inelastic rate of deformation could be useful in modeling.     

Uniaxial time‐dependent ratcheting behavior of bronze powder filled polytetrafluoroethylene at room and high temperature

A series of tensile and ratcheting experiments for cold compaction polytetrafluoroethylene (PTFE) and bronze filled PTFE (PTFE/bronze) were conducted with Dynamic Mechanical Analyzer (DMA‐Q800) at room and high temperature (473 K). The effects of peak stress‐holding time, creep, recovery, mean stress history, stress‐rate history, and pretension on the ratcheting behavior of PTFE/bronze were investigated. It is found that longer peak stress‐holding time leads to larger ratcheting strain accumulation. In the meantime, the ratcheting strain accumulates more rapidly at high temperature and the influence of temperature is more obvious than that of the additive fraction of bronze. Creep strain produced during the uploading and the stress‐holding time only partially recovers in the unloading process. Moreover, prior lower stress rate enhances the deformation resistance and restrains the ratcheting of subsequent cycling at higher stress rate. The ratcheting strain in the subsequent cyclic loading at lower mean stress is also restrained by previous cyclic loading at higher mean stress. Finally, the elastic modulus increases and the ratcheting strain is restrained apparently after the pretension. In addition, the elastic modulus and ratcheting strain of the PTFE/bronze with both pretension and recovery are smaller than those with pretension but without recovery. POLYM. ENG. SCI., 54:1571–1578, 2014. © 2013 Society of Plastics Engineers     

Tensile Creep and Creep-Recovery Behavior of a SiC-Fiber─Si3N4-Matrix Composite

The tensile creep and creep-recovery behavior of a unidirectional SiC-fiber/Si₃N₄-matrix composite was investigated at 1200°C in air. A primary objective of the study was to determine how various sustained and cyclic creep loading histories would influence the creep rate, accumulated creep strain, and the amount of strain recovered upon specimen unloading. The key results obtained from the investigation can be summarized as follows: (1) A threshold stress of 60 MPa was identified, below which the creep rate of the composite was exceedingly low (∼10⁻¹² s⁻¹). (2) Periodic fiber fracture was identified as a fundamental damage mode for sustained tensile creep at stresses of 200 and 250 MPa. (3) Because of transient stress redistribution between the fibers and matrix, the creep life and failure mode at 250 MPa. were strongly influenced by the rate at which the initial creep stress was applied. (4) Very dramatic creep-strain recovery occurred during cyclic creep; for cyclic loading between stress limits of 200 and 2 MPa, 80% of the prior creep strain was recovered during 50-h-creep/ 50-h-unloading cycles and over 90% during 300-s-creep/ 300-s-unloading cycles. (5) Cyclic loading significantly lowered the duration of primary creep and overall creep-strain accumulation. The implications of the results for microstructural and component design are discussed.     

Cyclic interaction between normal and shear stresses of an epoxy polymer—Experiments and model predictions

This investigation focuses on the axial‐torsional loading interaction of an epoxy polymer, Epon 826/Epi‐Cure Curing Agent 9551. Thin‐walled tubular specimens were subjected to combined constant tensile (or shear) stress and cyclic shear (or tension) loading schemes. Pure tensile creep and shear creep tests were also performed to compare the creep deformation to that with superimposed cyclic shear or cyclic tension. Test data clearly showed that cyclic shear (or cyclic tension) have a readily discernible effect on the tensile (or shear) creep deformation. Similarly, a superimposed constant tensile (or shear) load affects the hysteresis responses in cyclic shear (or cyclic tension). A nonlinear constitutive model developed by the authors was used to simulate the observed normal‐shear stress interaction. Due to the inclusion of an effective stress parameter in its nonlinear function, this model was able to account for the normal‐shear coupling effect. However, the incorporation of a general loading/unloading rule led to inaccurate simulation of the observed oscillatory creep response. A modification of the general rule was proposed and better predictions on both the cyclic and the creep responses could be obtained. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

Applying modified hyperbranched polyester in hydroxyl‐terminated polyether/ammonium perchlorate/aluminium/cyclotrimethylenetrinitramine (HTPE/AP/Al/RDX) composite solid propellant

Composite solid propellants demand fine and stable mechanical properties, creep resistance and stress relaxation performance during their long storage and usage time. In this study, modified hyperbranched polyester (MHBPE) was prepared and introduced into HTPE/AP/Al/RDX (HTPE, hydroxyl‐terminated polyether; AP, ammonium perchlorate; RDX, cyclotrimethylenetrinitramine) solid propellant as an effective additive. The static tensile and dynamic mechanical properties of this propellant before and after the introduction of MHBPE were evaluated. The elevated interfacial interaction by using MHBPE between the binder and RDX fillers improved the toughness and elasticity of the propellant. The enhancement mechanisms were also confirmed by the influence on the fracture surface morphology of the binder which was investigated by SEM. In addition, some influence on the dynamic mechanical properties of HTPE/AP/Al/RDX propellant caused by MHBPE was investigated by dynamic mechanical analysis. The creep behaviors of the HTPE/AP/Al/RDX propellants with and without MHBPE were also investigated at different stresses and temperatures. Reduced creep strain rate and strain were obtained for the modified propellant, implying enhanced creep resistance performance. The creep properties were quantitatively evaluated using a six‐element model and the long‐term creep performance of the propellant was predicted using the time–temperature superposition principle. A creep behavior of nearly 10⁶ s at 30 °C could be acquired in a short‐term experiment (800 s) at 30–70 °C. Moreover, the stress relaxation investigation of the propellants with and without MHBPE at ?40 °C, 20 °C and 70 °C suggested that MHBPE/HTPE/AP/Al/RDX propellant possessed better response ability to deformation. Thus, the application of MHBPE provides an efficient route of reinforcement to enhance the creep resistance and stress relaxation properties. © 2020 Society of Chemical Industry     

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.     

Critical stress of high‐density polyethylene during stress and photo‐oxidative aging

The long‐term photo‐oxidative aging behavior of high‐density polyethylene (HDPE) under different tensile stress was studied using a stress‐aging apparatus. The aging behavior was investigated through the methods of the surface morphology observation, gel content measurement, Fourier transform infrared spectroscopy, and creep behavior. It was found that stress has influence on the development of cracks and stress induces cracking through creep deformation. With increasing stress, the cracking time decreases in a reversed S‐shape curve way, and there is a critical stress near 7 MPa where the cracking time has a maximum decreasing rate. Meanwhile, the creep deformation increases rapidly when the stress exceeds the critical stress. The critical stress of HDPE is about 20–25% of breaking strength, and HDPE with low comonomer content has good dimensional stability when the stress is less than the critical stress, while HDPE with high comonomer content has a good performance when the stress exceeds the critical stress. This study may be useful for the rational selection of HDPE for the sheath material of bridge cable. POLYM. ENG. SCI., 55:2277–2284, 2015. © 2015 Society of Plastics Engineers     

Creep and relaxation of cement paste caused by stress‐induced dissolution of hydrated solid components

The creep and relaxation of cement paste caused by dissolving solid hydration products is evaluated in this work. According to the second law of thermodynamics, dissolution or precipitation of solid constituents may be altered by the change in stress/strain fields inside cement paste via alteration of the stress power or strain energy. Thus, it is hypothesized that stress‐induced dissolution can affect the overall creep/relaxation behavior of cement composites. A novel, fully coupled thermodynamic, mechanical, and microstructural model (TM²) that uses the finite element method was developed to predict the time‐evolving properties of cement paste under prescribed strains and to test the hypothesis. In the model, the strain energy was incorporated to accurately predict the effect of stress and strain fields on cement microstructure change. From the simulation results, depending on the stress/strain levels and the choice of the domain (over which the thermodynamic equilibrium is enforced), stress‐induced dissolution of solid constituents can lead to significant creep/relaxation.     

Uniaxial compressive creep of polytetrafluoroethylene

A machine to measure the creep deformation of plastics under uniaxial compressive loads is described. The problems associated with accurate creep testing in compression, primarily the application of a uniform stress to the specimen and the measurement of the resultant strain, receive particular attention. For the specimen geometries used, the effect on the measured strain of frictional restraints at the specimen ends is negligible provided the strain measurement is made with an extensometer attached to the specimen. The effect of fabrication techniques on the deformation behavior of polytetrafluoroethylene (PTFE) has been examined. Sintering time and temperature are found to be the most significant variables in the processing of PTFE. A comparison of uniaxial tensile and compressive creep data has shown that the non-linear viscoelastic behavior of the material extends into the low strain region.     

Influence of temperature on viscoelastic–viscoplastic behavior of poly(lactic acid) under loading–unloading

Experimental data are reported on poly(lactic acid) (PLA) in tensile loading–unloading tests and relaxation tests under stretching and retraction at temperatures ranging from room temperature up to 50°C. Two characteristic features of the time‐dependent response of PLA are revealed: (i) with a decrease in minimum stress under retraction at a fixed temperature, relaxation curves change their shape from monotonically decaying with time (simple relaxation), to non‐monotonic (mixed relaxation) to monotonically increasing (inverse relaxation) and (ii) with an increase in temperature, inverse relaxation after unloading down to the zero stress evolves into mixed relaxation with a pronounced shift of the peak position to smaller relaxation times. Constitutive equations are derived for the mechanical behavior of PLA, and adjustable parameters in the stress–strain relations are found by fitting the observations. Ability of the model to predict the time‐dependent response under cyclic deformation is confirmed by numerical simulation. POLYM. ENG. SCI., 57:239–247, 2017. © 2016 Society of Plastics Engineers     

The influence of the stress relaxation and creep recovery times on the viscoelastic properties of open cell foams

The aim of this work is to study the mechanical behavior of flexible polyurethane foams used in cushioning applications. In particular, the differences between slow recovery (SR) and fast recovery (FR) foams are highlighted. To characterize the flexible polyurethane foams, creep and hysteresis tests were performed at different strain rate, stress levels, and temperatures. Significant differences were observed between the SR and FR foams, particularly in terms of residual deformation after unloading, hysteresis area, and creep behavior at different stress levels. Creep compliance at different stress levels was compared with a Voigt‐Kelvin model. Stress–strain loading curves were compared with a phenomenological model originally modified to account for the strain rate dependence. In both cases, it is possible to show that the main differences observed in the behavior of the foams are due to the different relaxation and recovery times of the foams. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Compression behavior of woven glass/epoxy specimens using a new end‐loaded hybrid specimen

A new compression specimen was applied to woven glass/epoxy laminates. The specimen consists of epoxy layers cast on the sides of the laminate to prevent buckling. Thin‐sheet aluminum ends enable alignment and avoid crushing under end loading, which does not require any special fixture. The compression stress–strain behavior of the laminate was obtained from the specimens by discounting the previously measured stress–strain curve of the epoxy backings. Despite the higher scatter in compression tests, the average modulus was practically identical to the tensile modulus. Moreover, failure occurred away from the ends in nearly all of the specimens tested. The average compressive strength was 84% of the tensile strength and consistent with the flexural strength measured in four‐point bending tests. The present compression specimen could, therefore, become an interesting alternative to the more elaborate standard test methods available. Nevertheless, this new compression testing approach needs further evaluation involving application to other materials. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Nonlinear viscoelastic model for the prediction of double yielding in a linear low‐density polyethylene film

Tensile testing and tensile creep experiments for linear low‐density polyethylene in a thin‐film form were examined and analyzed in terms of a nonlinear viscoelastic model. The proposed model, based on two distinct thermally activated rate processes (Eyring models), was proved to describe the double‐yield‐point tensile behavior of the material tested. The required model parameters were evaluated from the corresponding creep‐strain curves, and this revealed the relationship between the main aspects of the inelastic behavior of polymers, that is, the monotonic loading and creep response. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3519–3527, 2004     

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.     

Tensile behavior of PVC‐coated woven membrane materials under uni‐ and bi‐axial loads

Tensile characteristics are the most significant mechanical properties for coated woven fabrics as membrane materials used in lightweight constructions. Factors that might affect test results of the material under uni‐ and bi‐axial tensile loads are examined. After series of tensile tests on PVC‐coated membrane materials, it is demonstrated that (1) to measure the strains in the two perpendicular directions, the contact method by the needle extensometer does not interfere the correct data recording; (2) the positions where the strains are measured on specimens have a great influence on the test results of the stiffness and Poisson's ratio in warp direction under uni‐axial load; (3) to perform bi‐axial tensile tests the size of the cruciform specimen in bi‐axial tensile test can be much smaller than those suggested in the literature. The tensile behavior of coated membrane materials under bi‐axial loads are affected dramatically by the stress ratio in the warp and fill directions. Besides the residual strains of coated membrane materials are affected not only by the properties of the constituent yarns and woven structure but also by loading conditions during the coating process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Uniaxial tensile behavior and thermoforming characteristics of high barrier EVOH‐based blends of interest in food packaging

The uniaxial tensile characteristics of blends of an ethylene‐vinyl alcohol copolymer (EVOH‐32 mol% ethylene) with an amorphous PA and/or a nylon‐containing ionomer, used as barrier layer in multilayer food packaging structures, was assessed in this paper. The stress‐strain behavior of these materials at elevated temperatures and at different strain rates was examined. The stress‐strain curves obtained were used to understand the influence of temperature and strain rate on the uniaxial deformation process of the materials, these being of general importance during typical processing steps including thermoforming. A male mold for deep‐draw was used to assess the thermoforming (biaxial deformation in nature) behavior of extruded sheets at 100, 120, 140 and 150°C, and the results were broadly found to be in agreement with results from simple uniaxial tensile tests. From the preliminary thermoforming results, it was found that EVOH/aPA extruded blends did not improve the poor formability of EVOH alone. In contrast, significant improvement in thermoformability was achieved by blending EVOH with a compatibilized ionomer. Optimum forming capacity was achieved in a ternary blend by addition of a compatibilized ionomer to an EVOH/aPA blend in the range of 140°C–150°C. The ternary blend showed a lower reduction of thickness in the sidewalls, as well as a higher dimensional uniformity in the thermoformed part. Polym. Eng. Sci. 44:598–608, 2004. © 2004 Society of Plastics Engineers.     

Tensile stress relaxation of short‐coir‐fiber‐reinforced natural rubber composites

The stress relaxation behavior of natural rubber (NR) and its composites reinforced with short coir fibers under tension was analyzed. The rate of stress relaxation was a measure of the increase in the entropy of the compounds: the higher the rate was, the greater the entropy was. At lower strain levels, the relaxation mechanism of NR was independent of strain level. However, the rate of relaxation increased with the strain level. Also, the strain level influenced the rate of stress relaxation considerably in the coir‐reinforced NR composites. However, the relaxation mechanisms of both the unfilled compound and the composite were influenced by the strain rate. The rate of relaxation was influenced by fiber loading and fiber orientation. From the rate of stress relaxation, we found that fiber–rubber adhesion was best in the composite containing fibers subjected to a chemical treatment with alkali, toluene diisocyanate, and NR solutions along with a hexaresorcinol system as a bonding agent. In this study, the stress relaxation curves could not be viewed as segments with varying slopes; however, a multitude of inflection points were observed on the curves. Hence, we propose neither a two‐step nor three‐step mechanism for the coir‐fiber‐reinforced NR composites as reported for some other systems. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 94: 96–104, 2004