Creep Compliance Master Curves Research Articles

Effect of Chemical Treatment of Flax Fiber and Resin Manipulation on Service Life of Their Composites Using Time-Temperature Superposition

In recent years there has been a resurgence of interest in the usage of natural fiber reinforced composites in more advanced structural applications. As a result, the need for improving their mechanical properties, as well as service life modeling and predictions have arisen. In this study effect of alkaline treatment of flax fiber as well as addition of 1% acrylic resin to vinyl ester on mechanical properties and long-term creep behavior of flax/vinyl ester composites was investigated. To perform the alkaline treatment, fibers were immersed into 1500 mL of 10 g/L sodium hydroxide/ethanol solution at 78 °C for 2 h. Findings revealed that alkaline treatment was successful in increasing interlaminar shear, tensile and flexural strength of the composite but decreased the tensile and flexural modulus by 10%. Addition of acrylic resin to the vinyl ester resin improved all mechanical properties except the flexural modulus which was decreased by 5%. In order to evaluate the long-term behavior, creep compliance master curves were generated using the time-temperature superposition principle. Results suggests that fiber and matrix treatments delay the creep response and slows the process of creep in flax/vinyl ester composites in the steady state region, respectively.

Service life assessment and moisture influence on bio-based thermosetting resins

In this study, three different types of bio-based resins are compared to a conventional oil-based epoxy in terms of moisture uptake, long-term properties and its influence of moisture and glass transition temperature, Tg. Moisture uptake is determined by means of gravimetric method, time temperature superposition (TTSP), and Tg data obtained in dynamic mechanical thermal analysis (DMTA). Moisture uptake show Fickian diffuison behavour for all resins, saturation level and diffusion coefficient however differ. The long-term properties is characterised by creep compliance master curves created by means of TTSP. The examined bio-based resins are compatible to the reference epoxy in term of stability up to 3–10 years. Comparison between master curves for virgin, wet, and dried material show that moisture present in the specimen increases creep rate, and that some of this increase remains after drying of samples. Tg measurements show that moisture inside the specimen decreases Tg; this is anticipated because of the plasticizing effect of water. The overall conclusions are that the bio-based resins of polyester, and epoxy type are comparable in performance with oil-based epoxy, LY556 and they can be used to develop high-performance composites.

Prediction of Bending Creep Behavior of Rice Hull Flour/Polypropylene Composite

The creep behaviors of rice hull flour/PP composite under different stress levels were studied through bending test. The results show that the bending creep behaviors of rice hull flour/PP composite have obviously correlation to the stress levels. Based on the time-temperature-stress equivalence principle and 15% stress level as a reference, the creep compliance master curve of 15% stress level was constructed by horizontally shift the creep compliance curve of other stress levels, which can predict the creep behavior of rice hull flour/PP composite at the 15% stress level.

Characterization of the viscoelastic behavior of bismaleimide resin before and after exposure to high temperatures

Creep and high strain rate mechanical properties, shrinkage strain, and thermal properties of a bismaleimide neat resin after exposure to a high temperature in air were evaluated and compared with the corresponding properties for a pristine resin. Under tension at a strain rate of 6×10−4 s−1, the Young’s modulus decreases and Poisson’s ratio increases with temperature, measured up to 310 °C. The tensile creep behavior was determined at stress levels of 12, 24, and 33 MPa at elevated temperatures. At each stress level, the creep compliance curves at different temperatures were shifted horizontally to form a master curve. These creep compliance master curves are nearly identical, indicating a linearly viscoelastic behavior up to 33 MPa. The bismaleimide resin was also exposed to air at other temperatures of 245, 260, and 280 °C for 1500 hours. After exposure to a high temperature, three regimes were observed in the resin through optical micrographs: an outer layer showing darker color, an interior that nearly maintained its original color, and a transition (or reacting) region in between. The average shrinkage on surface was determined as 3.4 % strain after 1500 hours of exposure to 260 °C in air. Compression at a high strain rate using a long split Hopkinson pressure bar shows that the bulk bismaleimide resin is rather insensitive to the exposure to a high temperature, exhibiting only a slight reduction in mechanical properties after 1500 hours of exposure to 245 °C. The uniaxial creep compliance of the neat resin was converted into the Young’s relaxation modulus, which was then used to calculate the Young’s modulus under tension at the strain rate and temperatures involved, and a good agreement was achieved between the calculated results and the experimental data, indicating that the rate-dependent Young’s modulus is the representation of viscoelastic properties.

Analysis of the nonlinear creep behavior of concrete/FRP-bonded assemblies

This paper investigates the creep behavior of adhesively bonded concrete/fiber-reinforced polymer (FRP) joints, through experimental and modeling approaches. The first part proposes a methodology for predicting the long-term creep response of the bulk epoxy adhesive; such a procedure consists of (1) performing short-term tensile creep experiments at various temperatures and stress levels, (2) building the creep compliance master curves according to the time–temperature superposition principle in order to assess the long-term evolution for each stress level, and (3) developing a rheological model whose parameters are identified by fitting the previous master curves. In our case, it was found that master curves (and, consequently, parameters of the rheological model) are dependent on the applied stress level, highlighting the nonlinear creep behavior of the bulk epoxy adhesive. Therefore, evolution laws of the model parameters were established to account for this stress dependence. The second part focuses on the creep response of the concrete/FRP assembly in the framework of a double lap joint shear test configuration. Experiments showed that creep of the adhesive layer leads to a progressive evolution of the strain profile along the lap joint, after only one month of sustained load at 30% of the ultimate strength. Besides, a finite element approach involving the abovementioned rheological model was used to predict the nonlinear creep behavior of the bonded assembly. It confirmed that creep modifies the stress distribution along the lap joint, especially the stress value at the loaded end, and leads to a slight increase in the effective load transfer length. This result is of paramount interest since the transfer length is a key parameter in the design of FRP-bonded strengthening systems. Moreover, instantaneous and long-term calculated strain profiles were found in fair agreement with experimental data, validating the modeling approach.

Modeling and Prediction of Creep Behavior of Polypropylene Packaging Belt

The short term tensile creep behaviors of polypropylene (PP) packaging belt under different stresses levels were studied through tensile creep test. The four-element model was applied to simulate the creep behaviors of the PP packaging belt. The results show that four-element model can be used to simulate the short time creep of PP packaging belt. The tensile creep behaviors of PP packaging belt have obviously correlation to the stress levels. The instantaneous elastic coefficient, delayed elastic coefficient and glutinous coefficient in Maxell model show a decreasing tendency with the increase of stress level. Based on the time-temperature-stress equivalence principle and take 15% stress level as a reference, the creep compliance master curve of 15% stress level was constructed by horizontal shift of the creep compliance curve of other stress levels, which can predict the creep behavior of PP packaging belt at the 15% stress level.

Prediction of long-term viscoelastic behavior of amorphous resin based on the time-temperature superposition principle

This paper deals with the prediction of long-term viscoelastic behavior of amorphous resin at a temperature below the glass transition temperature T g from measuring the short-term viscoelastic behavior at elevated temperatures based on the time-temperature superposition principle (TTSP) with vertical shift as well as horizontal shift. The long-term creep compliance as well as short-term and medium-term creep compliances were measured at elevated temperatures. The master curves of creep compliance can be constructed from measured data by shifting vertically as well as horizontally. The master curves of creep compliance constructed from measured data by short-term and medium-term creep tests agree well with those measured by long-term creep tests. Furthermore, the horizontal and vertical shift factors obtained from constructing the master curve are independent of the time period of creep tests. Therefore, the long-term viscoelastic behavior at a temperature below T g can be predicted accurately from measuring the short-term viscoelastic behavior at elevated temperatures based on the TTSP with vertical shift as well as horizontal shift.

Characterization of Ambient Temperature Cure Epoxies Used in Adhesive Anchor Applications

Thermo-viscoelastic properties of two commercial, ambient temperature-cure epoxy structural adhesives were analyzed and compared. The adhesives were formulated by the same manufacturer and appeared to have the same base chemistry; however, one system contained accelerators for shorter cure times. In the laboratory, dynamic mechanical temperature/frequency sweeps were performed on both systems to generate dynamic mechanical data and predict creep compliance master curves using frequency-temperature superposition principles. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and moisture sorption analysis were also used to assess the thermal and hygroscopic properties of the materials. Differences were observed in the thermal, hydrolytic, and dynamic mechanical properties of the two adhesive systems as well as in their estimated creep compliance behavior, which were attributed to differences in the curing agent(s) and accelerator(s) used in the adhesive systems. In most cases the differences in the properties of the epoxies were small, but a few properties, particularly the predicted creep behavior, exhibited very large differences. Results from laboratory creep testing confirmed the predicted difference in creep behavior. The data also suggest that dynamic mechanical testing combined with frequency-temperature superposition may be a useful metrology for predicting trends for in-service creep behavior from short term tests.

Thermoviscoelastic Analysis and Creep Testing of Ambient Temperature Cure Epoxies Used in Adhesive Anchor Applications

Thermoviscoelastic properties and creep response of two commercial ambient temperature cure epoxy structural adhesives were analyzed and compared. The adhesives were formulated by the same manufacturer and appeared to be chemically similar; however, one system contained accelerators to shorten its cure time. In the laboratory, dynamic mechanical temperature/frequency sweeps were performed on both systems to generate dynamic mechanical and creep compliance master curves using time-temperature superposition principles. Differences were observed in the dynamic mechanical properties of the two adhesive systems as well as in their calculated creep compliance, which have been attributed to differences in their curing agent(s) and accelerator(s). Full-scale creep tests were carried out on anchors installed in concrete slabs and subjected to sustained loads for 82 days. These results were in good agreement with the creep compliance estimated using time-temperature superposition, suggesting that dynamic mechanical...

Viscoelastic Stress Analysis of Constrained Proton Exchange Membranes Under Humidity Cycling

Many premature failures in proton exchange membrane (PEM) fuel cells are attributed to crossover of the reactant gas from microcracks in the membranes. The formation of these microcracks is believed to result from chemical and/or mechanical degradation of the constrained membrane during fuel cell operation. By characterizing the through-membrane leakage, we report failures resulting from crack formation in several PEMs mounted in 50cm2 fuel cell fixtures and mechanically stressed as the environment was cycled between wet and dry conditions in the absence of chemical potential. The humidity cycling tests also show that the failure from crossover leaks is delayed if membranes are subjected to smaller humidity swings. To understand the mechanical response of PEMs constrained by bipolar plates and subjected to changing humidity levels, we use Nafion® NR-111 as a model membrane and conduct numerical stress analyses to simulate the humidity cycling test. We also report the measurement of material properties required for the stress analysis—water content, coefficient of hygral expansion, and creep compliance. From the creep test results, we have found that the principle of time-temperature-humidity superposition can be applied to Nafion® NR-111 to construct a creep compliance master curve by shifting individual compliance curves with respect to temperature and water content. The stress prediction obtained using the commercial finite element program ABAQUS® agrees well with the stress measurement of Nafion® NR-111 from both tensile and relaxation tests for strains up to 8%. The stress analysis used to model the humidity cycling test shows that the membrane can develop significant residual tensile stress after humidity cycling. The result shows that the larger the humidity swing and/or the faster the hydration/dehydration rate, the higher the residual tensile stress. This result is confirmed experimentally as PEM failure is significantly delayed by decreasing the magnitude of the relative humidity cycle. Based on the current study, we also discuss potential improvements for material characterization, material state diagnostics, and a stress model for PEMs.

Temperature Dependence of Creep Behavior of PP–MWNT Nanocomposites

AbstractThe creep behavior of poly(propylene)–multi‐walled carbon nanotube (MWNT) composites has been studied with short term tensile creep tests at different temperatures and is discussed in relation to the structural characteristics determined by AFM, DSC, and polarized light microscopy. Master curves of creep compliance have been constructed using a time–temperature superposition (TTS) concept based on the William–Landel–Ferry (WLF) equation. The nanocomposites have shown an increase in creep compliance with increasing temperature as a consequence of temperature‐activated motion of the polymer chains. This is critically discussed in the light of activation enthalpy and other influencing factors such as polymer–nanotube interaction and thermal expansion coefficients following semi‐empirical approximations.magnified image

Flexural properties of surface reinforced wood/plastic deck board

AbstractDecking and railing is the largest and fastest growing market for wood–plastic composites (WPCs). Despite WPC's advantages in comparison to lumber, its modulus and creep resistance need to be further improved for demanding structural applications. In this study, WPC deck boards were reinforced by the composite sheets made of commingled glass and polypropylene fiber. Various reinforcement arrangements were carried out to identify the optimal one. Scanning electron microscopy revealed good bonding at the reinforcement/WPC interface. All reinforced samples exhibited considerably increased modulus of rupture, modulus of elasticity, and strain at break. The creep resistance of the reinforced WPC boards was also greatly improved. Creep strain was simulated with Findley's model. Master curves of creep compliance were generated by time–temperature–stress superposition principle. The Prony series was found to be the analytical expression of the master curves with acceptable accuracy. With improved mechanical properties, the reinforced WPC board can be used in more demanding applications. POLYM. ENG. SCI., 47:281–288, 2007. © 2007 Society of Plastics Engineers.

Effect of Physical Aging on Creep Behavior of Glass Fiber Reinforced Polycarbonate

An important thing of FRPs to use in structural member is to understand the visco-elastic behavior of them. In this study, creep behavior of glass fiber reinforced Polycarbonate (GFRPC) was researched. Before the creep test, GFRPCs were heated for various times at test temperature for aging treatment to research the effect of physical aging. The result shows that creep behavior of GFRPCs with various aging treatment could be able to make master curves of creep compliance on each aging conditions. And time-temperature shift factors with various aging conditions represented good agreement to time-temperature superposition principle of Arrhenius type plotting. To research the effect of physical aging quantitatively, it was compared with the master curves of various aging conditions. Therefore, the effect that the physical aging exerted on the visco-elasticity behavior was able to be understood including the existence of the fiber content.

Estimating creep deformation of glass-fiber-reinforced polycarbonate

Thermoplastic resin and fiber-reinforced thermo-plastics (FRTPs) were used without post-cure treatment as “molded material.” For such materials, creep behavior and physical aging occur simultaneously. This study examined the creep behavior of polycarbonate (PC) and glass-fiber-reinforced polycarbonate (GFRPC) injection moldings, including the effect of physical aging and fiber content, and determined that the time–temperature superposition principle could be applied to the creep behavior for different fiber contents. The effects of physical aging on creep behavior were evaluated quantitatively on pure resin and with various fiber contents without heat treatment. We found that the effect of physical aging could be evaluated with the proposed factor, “aging shift rate.” To discuss the linearity of viscoelasticity in FRTPs, this study used two shift factors: time and modulus shift factors. The fiber content affected creep behavior by both retarding and restraining it through changing the elastic modulus. This was shown by generating a grand master curve of creep compliance, which included the effects of time, temperature, and fiber content. Using the grand master curve of creep compliance and shift factors, it was possible to estimate the creep deformation of molded materials under varying conditions and fiber contents. The estimated creep deformation gave a very good fit to the experimental creep deformation.

Mechanistic and Volumetric Properties of Asphalt Mixtures with Recycled Asphalt Pavement

This research examines how the addition of recycled asphalt pavement (RAP) changes the volumetric and mechanistic properties of asphalt mixtures. A Superpave® 19-mm mixture containing 0% RAP was the control for evaluating properties of mixes containing 15%, 25%, and 40% RAP. Two types of RAP were evaluated: a processed RAP and an unprocessed RAP (grindings). Testing included dynamic modulus in tension and compression, creep compliance in compression, and creep flow in compression. Dynamic modulus and creep compliance master curves were constructed with the use of the time-temperature superposition principle to describe the behavior of each mix over a range of temperatures. The voids in mineral aggregate (VMA) and voids filled with asphalt (VFA) of the RAP mixtures increased at the 25% and 40% levels, and there was also an influence of preheating time on the volumetric properties. The dynamic modulus of the processed RAP mixtures increased from the control to 15% RAP level, but the 25% and 40% RAP mixtures...

Mechanistic and Volumetric Properties of Asphalt Mixtures with Recycled Asphalt Pavement

This research examines how the addition of recycled asphalt pavement (RAP) changes the volumetric and mechanistic properties of asphalt mixtures. A Superpave® 19-mm mixture containing 0% RAP was the control for evaluating properties of mixes containing 15%, 25%, and 40% RAP. Two types of RAP were evaluated: a processed RAP and an unprocessed RAP (grindings). Testing included dynamic modulus in tension and compression, creep compliance in compression, and creep flow in compression. Dynamic modulus and creep compliance master curves were constructed with the use of the time–temperature superposition principle to describe the behavior of each mix over a range of temperatures. The voids in mineral aggregate (VMA) and voids filled with asphalt (VFA) of the RAP mixtures increased at the 25% and 40% levels, and there was also an influence of preheating time on the volumetric properties. The dynamic modulus of the processed RAP mixtures increased from the control to 15% RAP level, but the 25% and 40% RAP mixtures had dynamic modulus curves similar to that of the control mixture in both tension and compression. The creep compliance curves showed similar trends. A combination of gradation, asphalt content, and volumetric properties is likely the cause of these trends.

ビニルエステル樹脂をマトリックスとするガラス短繊維強化プラスチックの動的粘弾性および曲げクリープ挙動

This paper is concerned with the prediction of dynamic viscoelastic and flexural creep behavior for GFRP consisting of short fiberglass and vinyl ester resin. The flexural creep tests were carried out under the various conditions of stress and temperatures. On the other hand, the dynamic viscoelastic tests were carried out under the various temperatures at 25 μm vibration amplitude. The master curves of creep compliance, storage elastic modulus, loss elastic modulus and loss tangent of GFRP were obtained by applying the time-temperature superposition principle to these experimental results. Furthermore, the temperature dependence of the time-temperature shift factors calculated from the flexural creep data was in agreement with that calculated from the dynamic viscoelastic data. On the basis of these results, the creep compliance of GFRP is predicted by using the experimental results of dynamic viscoelastic tests in this paper.

Study on creep behavior of glass fiber reinforced polycarbonate

In this study, tests on creep behavior of glass fiber reinforced polycarbonate were carried out at elevated temperatures. The result showed that creep behavior of GFRPC composites represented good agreement with Arrhenius reciprocation law of time-temperature. The effect of fiber volume fraction on creep behavior was also been studied. Here two shift factors—a modulus shift factor and a time shift factor—were introduced to draw the grand master curve of the creep compliance curves of various fiber contents. It became possible to estimate the creep behavior of GFRPC composites with any fiber contents using the grand master curve of creep compliance master curves and time and modulus shift factors. So it became possible to design materials for creep by reinforcing quantity of fibers to obtain the required objectives.

Investigating time-temperature superpositioning in crosslinked polymers using the tube-junction model

This article examines the application of time–temperature superpositioning (TTS) in certain thermorheologically complex polymers using a recently developed phenomenological model that describes crosslinked polymer viscoelasticity based on fundamental physical considerations. The model's capability to calculate both isochronal temperature sweeps and isothermal frequency sweeps of storage and loss moduli allows us to simulate conditions typical of certain thermorheologically complex polymers. We use the model to generate modulus frequency sweeps over the limited range of frequencies that are typically accessible to experiments. We apply TTS to shift these sweeps along the frequency axis to construct master curves. The model master curves are then compared with the model's “true” moduli curves over the full frequency domain at the reference temperature. This comparison suggests that nonsuperposability may go unnoticed if we only rely on the smoothness of the storage modulus master curve. Superpositioning to achieve a smooth loss modulus master curve tends to be more reliable. This has serious implications for assessing the reliability of relaxation moduli and creep compliance master curves that have no associated loss component that can be used to assess the quality of superpositioning. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 127–142, 1999

Creep Behavior of Metal Fiber-PPE Composites and Effect of Test Surroundings

The effect of environment on creep behavior of Poly-Phenylene Ether (PPE) composites with stainless steel fiber was investigated in this research. The results of creep behavior of PPE composites, carried out both in air and oil surroundings at elevated temperatures, show very good agreement with the Arrhenius reciprocation law of time-temperature. It was, however, observed that there was comparatively greater departure from good superposition in the creep compliance curve for oil surroundings in long period creep. The minute changes in activation energy for creep phenomena in different surroundings were observed. The effect of fiber volume fraction on creep behavior was also studied. In addition, a brief investigation of the effect of physical aging was done, with the results clearly showing that smoothness in the creep compliance master curve depends on the degree of physical aging of the matrix resin.