Stepped Isothermal Method Research Articles

Application of the non-linear three-component model for simulating accelerated creep behavior of polymer-alloy geocell sheets

The polymer-alloy geocell sheets (PAGS) represent a novel geocell material developed to replace conventional geocell materials. Accelerated creep testing, a convenient and precise performance evaluation method, presents a viable alternative to traditional creep testing for obtaining long-term creep strains. Nonetheless, there is a lack of prediction and in-depth exploration of accelerated creep testing. This paper aims to assess the efficacy of using the non-linear three-component (NLTC) model to simulate the accelerated creep behavior of PAGS. The predictive accuracy of the NLTC model has undergone evaluation through a comparison between stepped isothermal method (SIM) accelerated creep experimental tests and numerical simulations. Subsequently, the validated NLTC model was employed to simulate the time-temperature superposition method (TTSM), time-stress superposition method (TSSM), and stepped isostress method (SSM) accelerated creep tests, thereby verifying its effectiveness in predicting all accelerated creep tests. The results indicate that the NLTC model can effectively simulate creep deformation induced by temperature increases, particularly the temperatures below 41 °C. Although some errors are observed at elevated temperatures, it is within the acceptable range of 17.4%. Numerical simulation results of TTSM, TSSM, and SSM tests also suggest the model's proficiency in simulating the accelerated creep behavior by temperature and creep load increasing.

Creep behavior and viscoelastic-plastic models for polymer-blend HDPE geocell sheets based on the stepped isothermal method

Polymer-blend High Density Polyethylene (PHDPE) sheet materials have been developed in China as an alternative to the High Density Polyethylene (HDPE) materials for the manufacture of geocells. It is necessary to evaluate the creep properties of the geocell-reinforced structures to provide a reference for design of their service life and improve their application potential. In this paper, the Stepwise Isothermal Method (SIM) based accelerated creep testing, in which the test temperature was gradually increased but the load was kept constant, was employed obtain the long-term creep behavior of PHDPE and HDPE geocell sheets. Conventional creep tests were also carried out to assess the accuracy of the creep prediction. Furthermore, the Modified Burgers (MB) model and the Mon-linear Three-component (NLTC) model were proposed to predict long-term creep deformations of the PHDPE geocell sheets. The experimental results indicated that SIM creep tests could predict long-term creep deformations. The creep resistance of all three PHDPE, PHDPE-A, PHDPE-B, and PHDPE-C, was better than that of HDPE due to the addition of nylon 6 for the polymer-blending modification in HDPE. Comparison of the experimental data with the predicted data also confirmed that the MB and NLTC models could accurately model the viscoelastic-plastic creep behavior of PHDPE geocell sheets.

Use of Time–Temperature Superposition and Stepped Isothermal Method for Estimating Long-Term Properties of Recycled Aggregate Roller-Compacted Concrete

Abstract Although the use of recycled materials in civil engineering construction is a desirable option from a sustainability standpoint, the questionable long-term performance of these materials often hinders their widespread use in practice. The primary focus of this study was to perform accelerated aging and testing for estimating long-term properties of a roller-compacted concrete composed of crushed recycled aggregate, Type-I portland cement, and ASTM Class-F fly ash replacing up to 50 % of cement by weight. Accelerated aging was accomplished by curing cylindrical specimens at three different elevated temperature regimes for specific time durations. At the end of each time–temperature regime, the residual stiffness of the specimen was measured in a nondestructive fashion. Series of stiffness–time master curves were then constructed for each mix using the time–temperature superposition (TTS) technique and the stepped isothermal method (SIM). While the TTS method uses different sets of specimens for each elevated time–temperature regime, SIM uses a single set of specimens that are stepped up in temperature and held at each regime for a specified duration. Results indicated that for all mixes, the material stiffness degraded with time. Based on the Arrhenius equation, stiffness–equivalent age master curves were developed. Stiffness prediction was accomplished up to an equivalent age of almost 600 days, although the actual short-term test lasted only up to 6 days. It was also found that SIM and TTS provided comparable results, thus implying that the testing time and the number of specimens can be significantly reduced by using the SIM.

Experimental characterization of creep behavior of laminated bamboo composites based on stepped isothermal method

AbstractIn the present research, the accelerated test using the stepped isothermal method (SIM) was applied to characterize the long‐term compressive behavior of laminated bamboo. Generating a creep master curve using the SIM data is somewhat empirical. In particular, the selection of the end and beginning segments in the rescaling process is not standardized, which will affect the determination of virtual time. The variability of the virtual time influences the construction of master curves, thus leading to errors in predicting long‐term behavior. The difference in selecting the end and beginning segments was studied in terms of the virtual time, rescaled isothermal curves, and master curves. A 40% end segment and a 30% beginning segment were recommended to build a more accurate master curve. The generated master curves showed repeatability and matched reasonably well with the conventional creep results. It indicates that SIM is a reliable tool for the accelerated test of the long‐term behavior of laminated bamboo. In addition, isochronous stress‐creep strain curves were constructed based on the master curves at different loadings to provide evidence to determine stress levels for structural design.

Short aramid fiber reinforced thermotropic liquid crystalline polyester composites: Fabrication, thermal, and mechanical properties

We conducted a study to analyze the impact of short aramid fibers (AFs) on the melt-rheological behavior, thermal transition, thermal stability, and mechanical durability of thermotropic liquid crystal polyesters (TLCPs). By using different AF loading contents ranging from 3–15 wt%, we produced TLCP matrix composites through masterbatch-based melt-compounding and injection-molding. The SEM images and FT-IR spectra demonstrate that the AFs are dispersed in the TLCP matrix with a microfibrillar structure through good interfacial adhesion caused by specific intermolecular interactions between the TLCP and AFs. As a result, the complex viscosity, shear storage/loss moduli, and thermal transition (melt-crystallization, glass transition, and melting) temperatures of the composites increase with increasing AF filler content. However, the melt-crystallization and melting enthalpies increase only at low AF loading contents of 3–5 wt%. At high AF contents of 7–15wt%, the enthalpies decrease owing to the partial aggregation of AF fillers. The thermogravimetric analysis proves that the thermal stability of TLCP/AF composites improves when the AF filler is introduced. The dynamic mechanical analysis using the stepped isothermal method shows that the addition of 5 wt% AF to the TLCP leads to an approximately 150% improvement in elastic moduli and long-term mechanical durability at elevated temperatures.

Prediction of Compressive Creep Behavior of Three-Dimensional Geomat Using Stepped Isothermal Method

Prediction of Compressive Creep Behavior of Three-Dimensional Geomat Using Stepped Isothermal Method

Accelerated and long-time creep testing of extruded polystyrene using isothermal and stepped isothermal method

Seasonal Sensible Thermal Energy Storages based on existing inside structures are cost effective and require insulation materials with high performance. The insulation must endure high temperatures and high pressure for a long period. In this study, the creep effects of the insulation material XPS (extruded polystyrene foam) at high temperatures and pressure are investigated to determine whether XPS is a suitable insulation material to be applied on the inside of STES. Two studies were conducted: an isothermal long-term creep experiment and a SIM (Stepped Isothermal Method) experiment. The results of the first study were fitted using the Findley approach and extrapolated to 50 years. The SIM study is a method to accelerate creep effects using thermal effects and the Boltzmann superposition principle. Creep strain after 50 years is directly calculated by extrapolation. Both studies indicate that creep strain in XPS remains well below 6% with an applied load of 61.1 kPa at 60 °C after 50 years. Additionally, a material model that predicts creep strain at different or varying temperatures and stress was developed using the SIM data. • Creep behavior of XPS at elevated temperatures of 60 °C for 50 years. • Stepped Isothermal Method (SIM) and Findley Fit with thermal insulation material. • SIM with samples whose ratio of thickness to cross-sectional area is lower than one. • SIM with the duration and extent of the temperature steps individual for each step.

Accelerated and long-time creep testing of extruded polystyrene using isothermal and stepped isothermal method

Accelerated and long-time creep testing of extruded polystyrene using isothermal and stepped isothermal method

Creep behaviour of rotomouldable grade materials: A comparative study

Rotational moulding industry is looking to increase its utility spectrum from simple hollow products for storage purposes to complex engineered products used in structural and automobile applications. As such applications require satisfactory performance of the products for several decades, consideration of their creep performance in design is extremely important. Off late, Polypropylene (PP) is looked upon as a preferred choice of material for such products apart from the conventional high density and linear low-density polyethylene (LLDPE) grade materials, mainly due to its excellent static mechanical properties. This study presents a comparative analysis of the creep behaviour of PP with two commercially used hexene (C6) and butene (C4) grades of LLDPE to understand the long-term performance of products made out of these materials. For this purpose, accelerated creep testing using stepped isothermal method was carried out at three commonly used stress levels of 3.5, 4, 4.5 MPa and the creep behaviour was predicted at reference temperatures of 25 °C, 40 °C, and 60 °C. To compare and validate these findings, conventional creep experiments were performed up to one year, and the creep behaviour was extrapolated using Findley's equation. Rheological studies and microstructural characterisation involving wide-angle X-ray scattering, Raman mapping, solid-state NMR, and FT-IR spectroscopy were done to correlate the above findings. The study was further extended to develop a creep model using Norton Bailey law to predict the primary and secondary creep stages. This model can be efficiently used to predict the creep behaviour in a shorter time and thus can be used in the design of engineered products where creep failure may be a concern.

Synergistic effect of polyurethane‐coated carbon fiber and electron beam irradiation on the thermal/mechanical properties and long‐term durability of polyamide‐based thermoplastic composites

AbstractTo attain thermoplastic polymer composites with enhanced thermal and mechanical properties as well as long‐term durability, in this study, polyurethane‐coated carbon fiber (CF) and electron beam (EB) irradiation are adopted as an effective reinforcing filler and efficient crosslinking process, respectively. For this purpose, polyamide 6 (PA)‐based composites with different CF contents of 1–10 wt% were fabricated through melt‐compounding and compression molding, and then irradiated with various EB doses of 50–200 kGy. The SEM and FT‐IR data reveal that CFs are well dispersed in the PA matrix with excellent interfacial adhesion via specific intermolecular interactions, which are even enhanced for the composites with crosslinked PA matrices after the EB irradiation. As the result, the thermal stability (initial decomposition temperature and residue at 800°C) and dynamic mechanical properties of PA/CF composites increased noticeably with increasing the CF content and EB irradiation dose. The initial storage modulus of 1.90 GPa for neat PA at 30°C was improved significantly to 2.94 GPa by 10 wt% CF addition and to 4.67 GPa by 200 kGy EB irradiation. In particular, the long‐term mechanical properties of PA/CF composites, which were evaluated using a stepped isothermal method based on the time–temperature superposition principle, were found to be highly enhanced by the synergistic effect of CF filler reinforcement, EB‐induced PA matrix crosslinking, and improved interfacial adhesion.

Polyamide 6‐based thermoplastic vulcanizate for thermostability: An experimental and theoretical investigation

AbstractIn engineering cases, nonuniform temperature and pressure loads will cause greater thermal stress and deformation of the structure. The creep of materials will also accelerate with the increase of temperature, which greatly reduces the service life of parts. This study improves the vulcanization effect and two‐phase compatibility of polyamide 6‐based thermoplastic vulcanizates (TPV), and the elastic modulus of the material was increased to resist deformation and creep. The compatibility and mechanical properties of the materials were analyzed by dynamic thermodynamic analysis. The temperature and stress of charge air pipe are numerically calculated by fluid‐thermal‐solid coupling. The results show that at 120°C, the strain of TPV is less than 3%, and no plastic deformation occurs. Based on the principle of time–temperature superposition, the long‐term creep behavior at reference temperature is predicted by stepped isothermal method through short‐term temperature rise creep experiment. The creep strain after 105 h at 60°C of TPV grafted by maleic anhydride (TPV‐g‐MAH) is only 2.8%, indicating that it has good durability in the application of charge air pipes.

Long-term stiffness prediction of particle filled polymers by dynamic mechanical analysis: Frequency sweep versus creep method

The long-term service life of polymers can be estimated with much shorter experiments by applying the time temperature superposition principle (TTS). In this approach, data is obtained at different temperatures, usually through a stepped isothermal method (SIM) on the same sample. Dynamic mechanical analysis (DMA) instruments offer two different measurement methods to obtain SIM data: (i) static creep tests and (ii) dynamic frequency sweeps. This paper compares both methods on highly graphite filled polypropylene. Our studies on reproducibility of each method show that the uncertainty for 20 year prediction can be lower than 6% for both methods. While creep-based tests require a shorter experimental time, frequency sweep based tests show a lower scatter on the final result. The two main factors introducing uncertainty on the end results are related to (i) the reproducibility of the experimental raw data and (ii) the TTS optimisation using shift factors. The optimisation of the shift factors by a numerical method improves the accuracy of the master curve. By comparing creep and frequency sweep SIM, it shows that for predictions of one decade, the methods deliver very comparable results (less than 10% difference). For longer predictions, the methods differ and are not interchangeable. Furthermore, DMA was also effectively used as a three-point bending setup, providing information about strain rate sensitivity and the linear visco-elastic region using the same test setup and same sample dimensions as for TTS.

Assessment of the stepped isothermal method for accelerated creep testing of high-density polyethylene

Thermoplastic materials are increasingly used in demanding structural applications under, in some cases, long-term static loading over several decades. In this regard, the stepped isothermal method (SIM) with creep testing at stepwise increased temperature levels in combination with time-temperature superposition (TTSP) provides a very time efficient procedure for long-term creep characterization. In the present study, the creep behavior of an injection molded high-density polyethylene material (HDPE) was investigated by SIM in the thermally untreated state as well as after annealing.Due to experimental issues regarding the heating behavior of the specimens and non-linear viscoelastic behavior, particularly at elevated temperatures, bi-directional curve shifting was required in order to generate meaningful master curves for creep compliance. In a first step, an Arrhenius equation was used for the horizontal curve shifting, based on activation energies, determined in additional multi-frequency dynamic mechanical analysis (DMA). Continuous master curves were then obtained by empirical vertical shifting of the individual creep curve segments for the different temperature levels. In general, good agreement was observed between the resulting SIM master curves and the corresponding conventionally measured creep compliance curves at least for a time range up to 300 hours. Furthermore, significant differences in the creep tendency of the annealed material state compared to the thermally untreated condition revealed the distinct influence of the thermal history on the resulting creep behavior.

Enhanced thermal stability and long-term mechanical durability at elevated temperatures of thermotropic liquid crystal polyester/glass fiber composites

We herein report the influences of glass fibers (GFs) on the thermal stability and long-term mechanical performance at elevated temperatures of thermotropic liquid crystal polyester (TLCP) composites for advanced applications in the industrial sectors of automotive, electric/electronics, aerospace, and construction. For this purpose, TLCP-based composites with 10–40 wt% GF loadings are fabricated by facile melt-mixing and injection-molding. The scanning electron microscopy (SEM) images and X-ray diffraction patterns demonstrate that the GFs are dispersed in the TLCP matrix with a microfibrillar structure. The specific interactions between TLCP and GFs in the composites are identified by ATR FT-IR spectroscopic analyses. The thermal degradation temperatures of the composites increase slightly with the increment of GFs with high thermal stability and heat capacity. The long-term mechanical durability of TLCP/GF composites at 150 °C is analyzed by obtaining time-dependent storage modulus master curves based on the stepped isothermal method and time–temperature superposition principle. The dynamic mechanical moduli, activation energy, and long-term mechanical durability are found to be highly enhanced for TLCP composites with higher GF loadings, which is owing to the efficient stress transfer from the TLCP matrix to reinforcing GF fillers via specific interactions at the composite interface.

Creep Behaviour of Recycled Poly(ethylene) Terephthalate Non-Woven Geotextiles.

At the beginning of this century, due to well-established Brazilian recycling processes, geosynthetics’ manufacturers started to use recycled poly(ethylene) terephthalate (PET) yarns/filaments (from PET bottles) in geotextile production. Despite the fact that recycled products cannot act as reinforcement functions, geosynthetics are constantly under sustained tensile load and experiences evolutions of the axial strain (creep behaviour). Thus, this study aims to assess the influence of the structure of (needle-punched) non-woven geotextiles manufactured using recycled PET yarns on their creep behaviour. Two geotextiles with different fibre/filament production processes were investigated (short-staple fibres—GTXnwS—and continuous filaments—GTXnwC). Unconfined in-isolated conventional and accelerated (using the stepped isothermal method) creep tests were performed at 5%, 10%, 20%, 40% and 60% of geotextiles’ ultimate tensile strength. The geotextiles investigated provided similar creep behaviour to geotextiles manufactured with virgin PET material. The standard deviation of the axial strain tends to increase as the load level applied increase. The structure of the GTXnwS harms its tensile –strain behaviour, promoting axial deformation under sustained loads, at least 50% higher than GTXnwC for the same load level applied. The influence of the load level and geotextile structure in the initial axial strain is pointed out. Long-term predictions based on creep tests performed using the stepped isothermal method have proven to be conservative and they must be restricted for quality control of the investigated geotextiles.

A fast and precise methodology of creep master curve construction for geosynthetics based on stepped isothermal method (SIM)

The variability of the virtual time of stepped isothermal method (SIM) was observed during the creep master curves construction of a new high-performance geosynthetics material, which led to the significant errors in predicting the long-term creep. After analyzing the process of construction, the difference in selecting the beginning and end segments is the reason causing the errors. Therefore, a set of computer-based routine was programmed to eliminate the errors. Based on the amount of data of SIM and conventional creep tests, the virtual time of each corresponding step, the curves of scaled creep data, and the comparison between these two creep data were obtained in the present study by the routine. They were used to study the influence of different beginning segments and end segments on the master creep curve. Overall, The 5% of elevated temperature step and 50% of the previous step is suggested to regard as the beginning segment and end segment respectively to obtain the more well-founded virtual time and more accurate creep curve. This study not only provides a reference for research focusing on accelerated creep tests but proposed a fast and accurate methodology to construct the creep master of SIM tests.

An analytical model to predict the creep behaviour of linear low-density polyethylene (LLDPE) and polypropylene (PP) used in rotational moulding

Rotational Moulding (RM) is a versatile plastic processing method for the production of hollow products. Since the life expectancy of RM products are over several decades, prediction of mechanical properties like creep will be useful during the design phase of a product. In this research, an analytical model based on time hardening model was developed to predict and compare the creep behaviour of linear low-density polyethylene (LLDPE) and PP at 40 °C. The model uses some constants obtained from the experimental findings of a typical accelerated creep test using stepped isothermal method- time-temperature superposition (SIM-TTS). Based on the creep performance, the comparison of the two materials (LLDPE and PP) has been carried out and inferences have been made about their long term performance under constant stress.

Effects of maleated polypropylene content on the extended creep behavior of wood‒polypropylene composites using the stepped isothermal method and the stepped isostress method

This study investigated the effectiveness of maleated polypropylene (MAPP) as a coupling agent for improving the physico-mechanical properties and creep resistances of wood‒polypropylene composites (WPCs). The results revealed that the composites with MAPP showed significantly decreased water absorption and thickness swelling, while the flexural properties and the wood screw holding strength increased as the MAPP content increased up to 3 wt%. Additionally, flexural creep tests were conducted at a series of elevated temperatures and stresses using the stepped isothermal method (SIM) and the stepped isostress method (SSM), respectively. The SSM-predicted creep compliance curves fit better with the experimental data than the SIM-predicted curves. On the other hand, the creep master curves for all of the WPCs were constructed from different SSM test conditions and were highly consistent with the long-term experimental creep data. The creep resistance values of the composites with MAPP were greater than those without MAPP, especially for the WPC with 5 wt% MAPP (WPCM5), due to the improvement in the interfacial compatibility between the wood fibers and PP matrix. Furthermore, according to the SSM-predicted creep behavior, the improvement in creep resistance (ICR) of WPCM5 reached 61% over a 20-year period compared to the WPC without MAPP.

Time‐Lapsing Evaluation of the Retardation Behaviour of Polypropylene Homo‐ and Copolymers using the Stepped Isothermal Method based on Depth‐Sensing Macroindentation

SummaryThe Stepped Isothermal Method (SIM) is a relatively new method for accelerated determination of the creep behaviour of polymers. The need for such a new method arises from the fact that for modern plastic components such as pipe systems minimum lifetimes of 50 years and more are achieved. The analysis of creep applying conventional methods of testing over such long periods of time is not reasonable from an economic point of view. This forms the basis for the development of accelerated procedures such as SIM. Plastic components can exhibit creep characteristics to be a function of the position due to processing. Such position‐dependent creep behaviour cannot be imaged by volume‐sensitive creep tests such as SIM under uniaxial tensile load. Therefore, SIM was implemented by means of recording macroindentation to characterize the local long‐time creep behaviour and the expressiveness of this method for different polypropylene materials was shown in this study.

Depth‐Sensing Macroindentation Test and Stepped Iso­thermal Method – Accelerated Assessment of the Local Retardation Behaviour of Thermoplastic Polymers

SummaryThe prototype of a depth‐sensing macrohardness testing system with temperature chamber and temperature controlling unit for contact heating/cooling was upgraded in terms of control and software in such a way, that it allows application of the Stepped Isothermal Method (SIM) as for now. The SIM is a single‐specimen method, for which the temperature is raised stepwise at constant load and kept constant for a given time for each step, where the changes in indentation depth are measured as a function of time in this case. After correction of the thermally induced change in indentation depth and mathematical procedures a master curve construction is realised along a logarithmic time axis. The method was successfully applied to the assessment of the long‐term creep behaviour of different thermoplastic polymers such as PP, PE‐HD, PET and PVDF. The results obtained were compared with results of the SIM using tensile testing and the short‐term creep test using the depth‐sensing macrohardness testing system.