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The thermoelastic time-dependent creep progress of a rotating annular plate with nonuniform thickness made of functionally graded piezoelectric material considering convection heat transfer and solar radiation was investigated. The nonlinear differential equation of heat transfer considering conduction, convection, and solar radiation was solved using the differential transformation method. Then, the differential equation of equilibrium for the annular plate containing creep strains was derived. Firstly, the differential equation was solved analytically for the zero time. Then, the stress and strain rates were achieved using Norton's law with Prandtl–Reuss equations. Finally, the time-dependent creep stresses and deformation redistributions were obtained employing an iterative method. [ABSTRACT FROM AUTHOR]

Creep failure is one of the dominate failure mode for high temperature turbine blade during service. Under the tensile stress caused by large centrifugal force of the blade and the thermal stress and thermal softening of the material, the displacement as well as the creep strain and damage increase gradually. The present paper firstly gives the temperature field of the blade under typical service condition with numerical modelling. Then Norton’s creep constitutive relation and Lemaitre-Chaboche damage model are introduced into the finite element model, and three different working conditions are considered here to investigate its effect on creep service life of the turbine blade. Numerical results show that, the proposed numerical approach can predict the evolution of the creep process and the damage. Meanwhile, the introduction of cyclic factor is capable of reflecting the fatigue effect of cyclic load. [ABSTRACT FROM AUTHOR]

The time-dependent creep behavior of a rotating disc composed of functionally graded material (FGM) with varying thickness was analyzed. The convection heat transfer along with internal heat generation was considered in thermoelastic analysis. The material properties were assumed to change radially as a power-law function. Also, the heat convection and heat conduction coefficients were taken as functions of temperature and radius. The nonlinear heat transfer equation was solved using the differential transformation method (DTM). Then, the equilibrium equation considering creep strains was derived. The derived differential equation was solved analytically for zero time. Considering the creep strains, the stress and strain rates were determined using Norton's law with Prandtl–Reuss equations for steady-state thermal boundary conditions. Finally, the time-dependent creep stress redistributions at any time were evaluated using an iterative method. The effects of the heat source, convection heat transfer, temperature dependency, inhomogeneity index, and angular velocity on the behavior of the disc were explored in numerical examples. [ABSTRACT FROM AUTHOR]

In this analysis, the creep responses of a non-constant thickness annular plate was presented. The material of disc is assumed functionally graded magneto-electro-elastic (FGMEE) in which the material properties change through the radius. Also, the heat transfer coefficients for convection and conduction are functions of radius and temperature. At first, the equation of heat transfer accounting for thermal gradient, convection boundary conditions, internal heat generation, and solar radiation effects was derived. The differential transformation method (DTM) was used to solve the resulting nonlinear differential equation. The equilibrium equation for the annular plate including creep strain effects was then obtained. Ignoring creep strains, an analytical solution was obtained for the zero-time of this equation. Then, creep strains were introduced using Norton's law and the Prandtl-Reuss relations to find the stress and strain rates under fixed temperature boundary conditions. Next, the equation of strain rates including creep strains was solved analytically. Finally, an iterative approach was used to evaluate the time-dependent redistribution of creep stresses at any time point. Numerical examples highlighted the influences of key parameters like internal heat generation, convective heat transfer, grading index, solar radiation, thickness profile, and angular speed on the stresses, deformations and electric and magnetic potentials.

This paper presents the parametric study of a simple aluminium frame made of EN 6082 T6 aluminium alloy exposed to transient heating. The heating rates up to 5 ∘ C/min were used in numerical calculations to simulate fire protection essential in modern structures. The main focus of the study was to determine the influence of creep strain on the reduction of the load-bearing capacity of the main columns in a typical structure. A significant impact of creep strain on the critical temperature and failure time is given with a total of 60 numerical simulations with the comparison between creep and the creep-free model. The results have shown that for the applied creep model, the failure of the column due to buckling was accelerated with the developed creep strain for 28% than without taking creep into account for the middle column. For faster heating rates and other specific columns, the given failure time gradually decreases but still with significant impact on the overall load-bearing capacity. Numerical calculations were carried out in ANSYS 16.2 software with the use of the Modified Time Hardening creep model whose parameters were obtained within the previous studies. [ABSTRACT FROM AUTHOR]

This paper aims to examine the thermomechanical response of sintered silver die attachments in power electronics using finite element analysis (FEA). In this work, several material parameters of the sintered silver bonds are investigated. Additionally, two common solder creep constitutive laws including Anand and Garofalo models are also studied. To ensure the fidelity of the simulation procedures, the finite element (FE) models are first correlated with digital image correlation data. Afterward, the FE models are utilized to examine the influence of the material and creep models on the die attach stresses, strains, and plastic works. The expected fatigue and lifetime predictions of the sintered silver layer are thoroughly discussed, accordingly. The results proved that the die attach layer mechanical response is highly driven by the material parameters and creep modeling procedures considered throughout the simulations. Thus, the resulting fatigue life is evaluated. Finally, a general modeling guideline for simulating thermomechanical response of sintered silver die attachments in power electronics are provided in great detail. [ABSTRACT FROM AUTHOR]

This paper presents an in-depth investigation of the creep behavior of a thick-walled cylinder subjected to thermomechanical loads with internal heat generation under various boundary conditions. The cylinder is subjected to internal pressure with the incoming heat flux in the inner layer and the outgoing heat flux from the outer layer accompanied by heat generation. The displacement field follows the kinematics of the first-order shear deformation theory (FSDT). Simultaneously, the temperature field is treated as two-dimensional, exhibiting variations both along the thickness and the length of the cylinder, with a linear temperature gradient across the cylinder's thickness. Using the energy method, the equilibrium equations and general boundary conditions are derived for the cylinder. Norton's model is incorporated into rate forms of the above-mentioned equations to obtain time-dependent stress and strain results using an iterative method. The redistribution, displacements, strains and stresses over time have been obtained by the semi-analytical iteration method. Moreover, the effectiveness of the proposed method in addressing axisymmetric cylindrical shells under various boundary conditions and thermo-mechanical loading is demonstrated. A parametric study on the creep behavior has also been carried out which reveals critical insights. Notably, the study demonstrates that effective stress and radial displacement during creep can be effectively managed by optimizing the external cooling profile or the internal heating profile. Furthermore, the investigation reveals that the presence of a heat source markedly influences the effective stress and displacement within structure, highlighting the interplay between thermal and mechanical factors in determining the structural integrity. To validate the findings of this study, the finite element method was employed, with the results indicating good agreement between the two approaches.

Polyurea is an elastomeric polymer with segregated segmental microstructure and superior mechanical properties that are sensitive to loading time and temperature. In the research leading to this report, the primary goal was to elucidate the contributions of different segmental structures to the time-dependent creep deformation and material properties, such as Poisson's ratio and the creep modulus. The approach consisted of recording high-resolution digital images and resolving the full-field strain components of creep-loaded polyurea using digital image correlation (DIC) analysis. The resulting normal axial and lateral strains exemplified a typical creep response of incompressible elastomeric polymers, denoting the primary and secondary creep regions. Despite the uniaxial loading conditions, the presence of detectable shear strains demonstrated the occurrence of creep-activated shear softening behavior. The resolved strain contour maps, unique to DIC strain analysis, revealed noteworthy strain localization (demarked as striations on the strain contour plots) in selected regions within the observed regions of interest. The localized strains were separated and organized as functions of elapsed loading time. The high-strain regions were attributed to the soft segments of polyurea with inferior mechanical properties to their hard counterparts. The extracted time history of striations revealed three distinct molecular relaxation processes: one within primary creep attributed to the convoluted contributions of the hard/soft segmental microstructure and two processes within secondary creep associated independently with the hard and soft domains, respectively. The outcomes of this research are important for the development of effective and versatile polyurea-based impact-mitigating structures. [ABSTRACT FROM AUTHOR]

With the aim of describing and explaining the creep tensile property of shape memory polymers (SMPs) better, a variable--order fractional creep constitutive model with fewer parameters and good simulation performance was constructed on the basis of existing constitutive models. The consists of a Hookean spring connected in series with a Newtonian dashpot was employed to propose a new variable--order fractional Maxwell model for SMPs. Drawing support from the Laplace transform, we derive the analytical solution of this novel model. The verification of the obtained theoretical results with the creep test data for different fiber weight fractions and stress levels available from the literature reveals their satisfactory agreement. It can be concluded that the developed variable--order fractional constitutive model can better describe the complex viscoelastic relationship of shape memory polymers which is difficult to be described by the classical integer order derivative, and obtain a more accurate constitutive model of shape memory polymers with fewer parameters.

The present work is performed on the main steam pipe system in Heleneholmsverket, a CHP in Sweden and consists of the following parts (i) numerical analysis of the in-service creep behaviour of the pipe system, (ii) creep testing of new and service exposed materials from welded components, (iii) characterisation of the creep damage distribution in creep tested welds of the actual piping. The entire system has been modelled for creep evaluation to make it possible to compare the simulated creep stress and strain distributions in selected welds with observed amounts of creep cavitation, which can be correlated to the accumulated creep strain. Creep data for the analyses were produced by creep testing of service exposed base and weld metals from a pipe weld and a T-piece branch weld from the system. In addition, the creep tested welds were studied metallographically to map the creep damage and make it possible to compare the damage development with the resulting creep stress and strain distributions in the weld. In the previous project also a T-piece branch weld was investigated in a similar way and those results were used for verification of the re-analyses in present project with the updated system model. The following results were achieved: The model of the entire steam pipe system was created in Abaqus and the strain distributions were verified in comparison to a corresponding elastic Caepipe model. The Norton creep law was used for the simulations. In addition, also primary creep was analysed. The effects of primary creep on the long-term creep behaviour was significant and the results shows the importance of including primary creep into the model. There is no effect of starts and stops on the stress and strain distributions in the system during creep. The system analysis results showed enhanced strains up to 2.1% at one bend and 0.5–1.0% in some parts of the system. Although replica testing had not been conducted directly at the bend the high strains indirectly agreed with the observations of small creep cracks had been observed in replica testing of in a weld at one of the ends of the actual bend. Furthermore, several components in the system have been exchanged due to creep crack formation. Moderate levels creep damage was observed in the pipe weld. The analysis of this pipe weld gave somewhat lower creep strains than expected. The stress and strain distributions matched with the maximum principal stress criterion but not with the von Mises stress that Abaqus uses for creep analyses by default. The analysis of the branch weld matched well with observed creep damage distributions whereas the maximum strain level of 0.4% appears to be rather low in comparison to the quite extensive creep damage. However, local constraint and multiaxiality in welds lead to significantly lower creep ductility compared to uniaxial creep and contribute to a reasonable agreement between the strain and the damage levels. The creep tests of service exposed material resulted in relatively high Norton creep law exponents and no shift to lower values at the lowest tested stresses. It is hardly possible to perform tests at even lower stresses and therefore the simulations at service conditions resulted in unreasonably low creep strains. [ABSTRACT FROM AUTHOR]

Current research shows that high-entropy alloys can offer superior oxidation resistance compared to more traditional austenitic stainless steels with equivalent tensile mechanical performance. While many reports have been published on creep of CoCrFeNiMn, few focus on the CoCrFeNi family of low- to medium-entropy alloys. In this manuscript, uniaxial tensile creep test results for more than 46,000 accumulated hours are presented on the equiatomic CoCrFeNi alloy. The alloy was manufactured at NETL using a vacuum induction melt solidification approach with a computationally designed homogenization cycle. After completing the homogenization cycle, the ingot was first hot-forged and then hot-rolled into 10-mm-thick plate, a product form that is relevant to many industries. Creep tests were performed in the temperature range of 600-650 °C using applied stresses from 120 to 189 MPa. This study builds upon previous results on the CoCrFeNiMn family of high-entropy alloys and offers a modicum of clarification about performance compared to small button melts by using ASTM standard test procedures and mechanical test specimens. The apparent creep activation energy and creep stress exponent were similar to those for CoCrFe medium-entropy and CoCrFeNiMn high-entropy alloys. The post-test creep microstructure showed some Cr segregation, which has also been noted for other CoCrFe alloys. The CoCrFeNi alloy also presents a double minimum or two minimum creep rate phenomena. [ABSTRACT FROM AUTHOR]

This research is aimed at investigating the mechanical behavior of the bitumen by the addition of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) obtained from waste plastic bottles and bags. Polymers (HDPE and LDPE) with percentages of 0%, 2%, 4%, and 6% in shredded form by weight of bitumen were used to evaluate the spectroscopic, structural, morphological, and rheological properties of polymer-modified binders. The rheological properties for different factors; viscosity (ἠ) from Rotational Viscometer (RV), rutting factor G*/Sin (δ), fatigue characteristics G*. Sin (δ), for the modified binder from dynamic shear rheometer (DSR), Short and long-term aging from rolling thin film oven (RTFO), and pressure aging vessel (PAV) was determined. The thermal characteristics, grain size, and texture of polymers for both LDPE and HDPE were found using bending beam rheometer (BBR) and X-ray diffraction (XRD), respectively. Fourier transform infrared (FTIR) analysis revealed the presence of polymer contents in the modified binder. Scanning electron microscopy (SEM) images revealed the presence of HDPE and LDPE particles on the surface of the binder. Creep Rate (m) and Stiffness (S) analysis in relationship with temperature showed a deduction in stress rate relaxation. Results have revealed the best rutting resistance for 6% HDPE. It also showed an improvement of 95.27% in G*/Sin (δ) which increased the performance of the bituminous mix. Similarly, the addition of 4% LDPE resulted in maximum dynamic viscosity irrespective of the temperatures. Moreover, fatigue resistance has shown a significant change with the HDPE and LDPE. The festinating features of waste plastic modified binder make it important to be used in the new construction of roads to address the high viscosity and mixing problems produced by plastic waste and to improve the performance of flexible pavements all over the world. [ABSTRACT FROM AUTHOR]

Creep is a common important physical phenomenon in rubber material, which induces the instability of geometrical dimension and deteriorates the mechanical performances. The present work develops an optimal design approach of a commercial rubber isolator based on creep analysis. First and foremost, a nonlinear creep constitutive model of rubber material is established, which can capture the hyper-elastic and time-dependent creep behaviors. Complete mechanical and creep tests of rubber materials are conducted, and material parameters are identified according to the experimental data. Then, the parametric finite element model of a rubber isolator is established, with which the time-dependent creep analysis based on the proposed creep constitutive model is conducted. The accuracy of the numerical creep analysis is validated at material level and structural component level. For engineering application, a sensitivity analysis and optimization design for creep-resistance of the rubber isolator is developed by combing finite element simulation and optimization method. The results show that creep-resistance characteristics of the optimal rubber isolator is largely improved, which provides a long-term stable behavior in vibration attenuation. The proposed method may provide an efficient tool for predicting the creep performance and optimal analysis of other commercial rubber-base products. [ABSTRACT FROM AUTHOR]

Wood and composites cantilever beam structure has gained attention among researchers in the current years due to its universal structural applications, such as bridges, aeroplane wings, buildings, and transmission towers. However, when the structure is exposed to a constant loading for a very long time, it induces a structural collapse due to creep deformation. Therefore, it is essential to understand and identify the initial creep that can lead to structural collapse. In this study, wood and composite materials exhibit the same structural material morphology which performs as anisotropic material as it majorly contributes to failure. In this study, a state-of-the-art review of creep analysis and engineering design is carried out, with particular emphasis on the creep methodology of a cantilever beam. The existing theories and creep design approaches are grouped into two analysis methods, namely experimental and numerical approaches. To be more specific, the experimental works involved two main methods, namely load-based (conventional) and temperature-based (accelerated) techniques. Selected creep test on cantilever beam structure and coupon scale of wood and composite were highlighted and proposed as the building blocks for a prospective structural creep methodology. These aids build confidence in the underlying methods while guiding future work and areas, especially for long-term service of full-scale structure. At the end, the challenges of creep behaviour description accuracy and improvement on the strength criteria in engineering design were presented.

The increased cross section of overhead conductors can obviously improve the power transmission capacity. Whereas its large creep elongation may make the design works be more difficult. In this paper, time hardening model was proposed to analyze the creep properties of the large cross section conductor. To perform such analysis, the creep tests of the typical large cross section conductor JL1/G2A-1250/100-84/19 under different temperature conditions were carried out. Then the time hardening models excluding and including temperature coefficient were deduced respectively. The results showed that the calculation based on the time hardening model were well agreed with the results of laboratory tests and field observation. What is more, the accuracy of the time hardening model in this paper is higher than the fitting model. Finally, the section integration model was discussed based on the nonsynchronous stringing process so as to provide a new method for calculating the creep value of large cross section conductors. [ABSTRACT FROM AUTHOR]

This research is aimed at investigating the mechanical behavior of the bitumen by the addition of high-density polyethylene (HDPE) and low-density polyethylene (LDPE) obtained from waste plastic bottles and bags. Polymers (HDPE and LDPE) with percentages of 0%, 2%, 4%, and 6% in shredded form by weight of bitumen were used to evaluate the spectroscopic, structural, morphological, and rheological properties of polymer-modified binders. The rheological properties for different factors; viscosity (ἠ) from Rotational Viscometer (RV), rutting factor G*/Sin (δ), fatigue characteristics G*. Sin (δ), for the modified binder from dynamic shear rheometer (DSR), Short and long-term aging from rolling thin film oven (RTFO), and pressure aging vessel (PAV) was determined. The thermal characteristics, grain size, and texture of polymers for both LDPE and HDPE were found using bending beam rheometer (BBR) and X-ray diffraction (XRD), respectively. Fourier transform infrared (FTIR) analysis revealed the presence of polymer contents in the modified binder. Scanning electron microscopy (SEM) images revealed the presence of HDPE and LDPE particles on the surface of the binder. Creep Rate (m) and Stiffness (S) analysis in relationship with temperature showed a deduction in stress rate relaxation. Results have revealed the best rutting resistance for 6% HDPE. It also showed an improvement of 95.27% in G*/Sin (δ) which increased the performance of the bituminous mix. Similarly, the addition of 4% LDPE resulted in maximum dynamic viscosity irrespective of the temperatures. Moreover, fatigue resistance has shown a significant change with the HDPE and LDPE. The festinating features of waste plastic modified binder make it important to be used in the new construction of roads to address the high viscosity and mixing problems produced by plastic waste and to improve the performance of flexible pavements all over the world.

In recent years, composites consisting of polymers and cellulosic materials have attracted increasing research attention. Polypropylene (PP) is among the most common polymer types found in excavated waste from landfills. Moreover, wood waste generated from wood products manufacturing such as sawdust (SD) offers a good potential for the fabrication of composite materials, and it is readily available in the environment. In this paper, wood polymer composites (WPC) consisting of recycled PP (rPP) and (SD) were prepared and characterised. A range of mechanical properties, including tensile strength, flexural properties, creep and hardness were studied, along with morphology, thermal properties, water degradation and contact angle. The results showed that the mechanical and thermal properties of rPP increased with an increase in 40 wt% of the SD content. Furthermore, the SD content significantly influenced the water uptake of the composites. Time–temperature superposition (TTS) was applied to predict the long-term mechanical performance from short-term accelerated creep tests at a range of elevated temperatures. The short-term creep test showed efficient homogeneity between the fillers and matrix with increasing temperature. The produced wood polymer composites displayed a comparable physical property to virgin polymer and wood and could potentially be used for various structural materials.

The potential of high-entropy alloys (HEAs) to meet or exceed austenitic stainless steel performance with the additional benefit of improved hot corrosion/oxidation resistance makes FCC HEAs attractive for use in energy applications. While shorter-term creep tests have been reported in the literature on HEAs, not all methodologies utilize repeatable techniques. This manuscript reports on over 23,500 accumulated hours of tensile creep testing with adherence to ASTM standards on a melt-solidified ingot of CoCrFeNiMn HEA converted to wrought plate using conventional thermo-mechanical processing techniques. The typical standard creep analyses are reported, i.e., Larson–Miller parameter, Monkman–Grant relationship, activation energy for creep, and creep stress exponents were calculated and compared to previously reported short-term creep tests. Additionally, characteristics of creep fracture and microstructural evolution are reported with cursory dislocation mechanisms investigated. [ABSTRACT FROM AUTHOR]

Summary: Some studies suggest that creep parameters should be determined using a greater quantity of creep test data to provide more reliable prediction regarding the deformation of soft soils. This study aims to investigate the effect of loading duration on model updating. One‐dimensional consolidation data of intact Vanttila clay under different loading durations collected from the literature is used for demonstration. The Bayesian probabilistic method is used to identify all unknown parameters based on the consolidation data during the entire consolidation process, and their uncertainty can be quantified through the obtained posterior probability density functions. Additionally, the optimal models are also determined from among 9 model candidates. The analyses indicate that the optimal models can describe the creep behavior of intact soft soils under different loading durations, and the adopted method can evaluate the effect of loading duration on uncertainty in the creep analysis. The uncertainty of a specific model and its model parameters decreases as more creep data are involved in the updating process, and the updated models that use more creep data can better capture the deformation behavior of an intact sample. The proposed method can provide quantified uncertainty in the process of model updating and assist engineers to decide whether the creep test data are sufficient for the creep analysis. [ABSTRACT FROM AUTHOR]

The fabric clip is an important component in textile machine. The creep deformation on the clip body hole at high temperature acted on different pulling forces during a period of 6 240 h was researched by using ABAQUS software. The Norton-Bailey's with time hardening constitutive equation was used to simulate the creep deformation on the clip body hole made of A380 aluminum alloy at high temperature, and the simulation results showed that for different pulling forces, the creep deformation on the clip body hole increased with an increase in the pulling force. Then, the experimental dimensional changes test was performed on three different specimens of the clip body hole at different service time(l 200 h, 4 800 h, 6 240 h) by using a Global Performance test machine. A comparison between the finite element method (FEM) and the experimental results show a good correlation. The simulation results also indicate that the creep deformation on the clip body hole has an important effect on the relaxation of the blade clamping fastness which leads to fabric clip failure. [ABSTRACT FROM AUTHOR]

In this paper, creep properties estimation of Incoloy800H under various loads and temperatures were performed experimentally and analytically. Compared to standard creep test, a similar creep character could be obtained by small punch creep test. On the basis of geometrical shape of deformed specimen, we proposed a modified geometrical model. The relatively simple yet accurate correlation between strain and small punch deflection was established. The reference stress under multi-axial deformation state was discussed and proved to be as constant at the identical condition. Additionally, to analyze the tertiary deformation and fracture mechanism of small punch creep test, creep damage tolerance factor was introduced and modified. The results indicated that necking and precipitate coarsening was the domain model in this phase. Meanwhile, the physical significance of three stages of deflection-time (D-T) curve was proved to be different with that of uniaxial creep curve. [ABSTRACT FROM AUTHOR]

This work focuses on the effect of electron beam irradiation on the physico-mechanical, dynamic mechanical and dielectric properties of blends based on ethylene octene copolymer (EOC) and poly dimethyl siloxane (PDMS) rubber. It is found that electron beam irradiation caused considerable improvement in the physico-mechanical properties; the tensile strength was enhanced by nearly 35% for 70:30 EOC:PDMS blend. Phase morphology of the blends analyzed before irradiation by scanning electron microscopy (SEM) exhibited droplet/matrix morphology with sizes of the PDMS rubber domain varying from 0.55 µm to 0.47 µm as the amount of PDMS rubber decreased from 30 wt% to 10 wt%. This reduction in the PDMS rubber domain has been correlated with the physico-mechanical properties of the blends. Further, the dynamic mechanical properties and creep behavior of these EOC:PDMS blends before and after irradiation has been studied. It is inferred that the 70:30 blend after radiation crosslinking shows a 17% decrease in the creep compliance, i.e. higher creep resistance compared to neat blends. All the radiation crosslinked blends exhibited lower dielectric constant, lower dielectric loss and higher electrical resistivity as compared to the virgin blends which makes it suitable for cable insulating application. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A computational modeling procedure is developed for simulating long-term creep behavior of Glass Fiber Reinforced Plastic (GFRP) mortar pipes subjected to internal pressure. The modeling procedure includes creep evaluation, stress analysis, failure evaluation and degradation of mechanical properties. Different levels of internal pressure are examined to obtain sufficient data point on failure-pressure versus time-to-failure graph up to 100,000 min. At each specific pressure level, the modeling procedure continues till the functional failure is distinguished implying on weepage phenomenon. Obtained data are extended to 50 years using extrapolation technique for predicting the remaining strength of investigated GFRP pipe after 50 years. [ABSTRACT FROM AUTHOR]

The main objective of this paper is to simulate long-term hydrostatic tests on Glass Fiber Reinforced Plastic (GFRP) pipes. An experimental procedure for obtaining pressure class of GFRP pipes on the basis of long-term behavior is very time consuming and costly that sometimes take about 2 years for collecting required data. Then, obtained results are extrapolated to 50 years. In this work, a theoretical modeling procedure is developed to obtain residual strength of pipes after 50 years taking into account creep phenomenon. Developed progressive modeling consists of creep modeling, stress analysis and failure evaluation. An integrated modeling procedure is developed reporting time-to-failure at any desired internal pressure. As a case study and also validation purpose, the developed modeling procedure is conducted for predicting long-term behavior of a specific GFRP pipe subjected to internal pressure. A comparison between real experimental data and theoretical modeling is presented. A very good agreement between predicted 50-year hydrostatic pressure and experimental data implies on the proficiency of the developed modeling. Since the developed modeling is just in need of short-term experimental data on pure resin, it could be used as an appropriate engineering tool for industrial centers. [ABSTRACT FROM AUTHOR]

This paper presents an experimental and numerical study on the creep behavior of glued laminated bamboo (glubam) and carbon-fiber-reinforced plastics (CFRP)-enhanced glubam beams. Short-term mechanical tests were first carried out to determine the tensile, compressive strengths, and modulus of elasticity of glubam. Then the Burger's creep coefficients of glubam were determined by the in-house tensile and compressive creep tests, which were carried out under different constant stress levels for 365 days. A numerical analysis procedure based on the strain-relaxation method is established to simulate the creep response of CFRP-enhanced glubam beams. Parametric studies show that increasing the amount of the CFRP laminate can effectively decrease the creep response as well as the creep stress in the glubam beam. Comparison between the numerical and the test result of a full-scale model CFRP enhanced glubam girder bridge subjected to 3.5 years creep loading also show that the numerical creep deflections can well capture the trend of creep test results. [ABSTRACT FROM AUTHOR]

Metal to Metal contact (MMC) bolted flange joint, which appears between gasket centering ring and flange facing, is widely used in nuclear and petrochemical industry for severe conditions. Compared to the floating type joints, the gasket outer metal ring is compressed after bolt-up and the stored energy can compensate the loss of residual gasket stress due to internal pressure, bending moment and creep, resulting in a low leakage. As it is difficult to assess the flanged joint integrity affected by creep analytically, a three-dimensional nonlinear finite element model is developed to highlight the creep behaviour of MMC flanged joint under different load cases at elevated temperature. The results show that MMC flanged joint can reduce the unloading effects due to creep and a suitable preload is recommended. [ABSTRACT FROM AUTHOR]

This paper aims to develop a new systematic creep analysis model to calculate the friction of a ball screw. In order to investigate the friction behavior better, a proper transformed coordinate system was established. Through the creep analysis and the principle of force balance, the creep parameters can be easily obtained, such as the vertical creep ratio, the horizontal creep ratio and the spin ratio acting on three contact areas. Based on roll contact theory, the ball screw friction can be predicted more accurately. The effectiveness of the creep analysis model is verified through experiments. Furthermore, the influence of the creep parameters on the ball screw friction and friction distribution will be discussed. The study not only provides a new perspective and approach for the study of ball screw friction, but also provides the theory basis for reasonably reducing the friction and improving the mechanical efficiency of a ball screw. [ABSTRACT FROM AUTHOR]

This study focuses on the viscoelastic analysis of laminated composite shell structures under long-term creep mechanical loading. An advanced finite element technique named as cell/element-based smoothed discrete shear gap method (CS-DSG3) is employed to obtain the numerical solutions for both elastic and viscoelastic problems owing to its accuracy and rapid convergence. In the CS-DSG3 shell element, each triangular shell element is divided into three DSG3 subtriangles to avoid transverse shear locking. Subsequently, the total element strain is computed from the three partial strains of the subtriangles using a smoothing technique. To overcome the difficulty in the constitutive equations in integral forms for a viscoelastic material, all formulations are transformed into the Laplace domain using the convolution theorem. Finally, time-dependent deformations are obtained and converted back to the real-time domain using inverse Laplace techniques. For validation, various numerical examples of a pinched cylinder, clamped hyperbolic paraboloid, and hemispherical shell formed of isotropic elastic, isotropic viscoelastic, and anisotropic composite materials are selected to investigate creep behavior under mechanical loading. The present study extends the finite element simulation of anisotropic viscoelastic shell structures to achieve high accuracy and efficiency based on the Laplace transform and CS-DSG3. • The long-term time-dependent mechanical response of viscoelastic composite shell structures are investigated. • The computation storage is significant reduced by employing Laplace transformation for viscoelastic analysis without using any time steps. • The cell-based smoothed discrete shear gap method based on the first-order shear deformation theory yields accurate numerical results, avoids shear locking phenomenon. • The numerical examples of pinched cylinder, clamped hyperbolic paraboloid and hemispherical shells are selected to investigate carefully creep behavior under mechanical loading. [ABSTRACT FROM AUTHOR]

Abstract: A semi-analytical solution for rotating axisymmetric disks made of functionally graded materials was previously proposed by Hosseini Kordkheili and Naghdabadi [1]. In the present work the solution is employed to study thermoelastic creep behavior of the functionally graded rotating disks with variable thickness in to the time domain. The rate type governing differential equations for the considered structure are derived and analytically solved in terms of rate of strain as a reduced to a set of linear algebraic equations. The advantage of this method is to avoid simplifications and restrictions which are normally associated with other creep solution techniques in the literature. The thermal and structural properties of the base metal are also considered as a function of temperature. It is noted that ignoring the temperature dependency of these properties caused up to 200% errors in the creep solution results. Also, results for the strain rates presented due to centrifugal force and thermal loadings for different disk cross section profiles as well as different boundary conditions. Results obtained within this solution are verified with those available in the literature for easier cases. [Copyright &y& Elsevier]

In recent years, composites consisting of polymers and cellulosic materials have attracted increasing research attention. Polypropylene (PP) is among the most common polymer types found in excavated waste from landfills. Moreover, wood waste generated from wood products manufacturing such as sawdust (SD) offers a good potential for the fabrication of composite materials, and it is readily available in the environment. In this paper, wood polymer composites (WPC) consisting of recycled PP (rPP) and (SD) were prepared and characterised. A range of mechanical properties, including tensile strength, flexural properties, creep and hardness were studied, along with morphology, thermal properties, water degradation and contact angle. The results showed that the mechanical and thermal properties of rPP increased with an increase in 40 wt% of the SD content. Furthermore, the SD content significantly influenced the water uptake of the composites. Time-temperature superposition (TTS) was applied to predict the long-term mechanical performance from short-term accelerated creep tests at a range of elevated temperatures. The short-term creep test showed efficient homogeneity between the fillers and matrix with increasing temperature. The produced wood polymer composites displayed a comparable physical property to virgin polymer and wood and could potentially be used for various structural materials.

Plastics and fiber-reinforced plastics (FRP) are used in the aerospace industry because of their mechanical properties. However, despite their excellent high-temperature mechanical properties, plastics and FRP eventually deform visco-elastically at high temperatures. Most of the research has focused on the creep behavior of FRPs, but few studies have investigated the linear visco-elastic behavior. Linear visco-elastic behavior and non-linear visco-elastic behavior occur with physical aging in these plastics. In this study, the non-linear visco-elastic behavior of plastics and FRP was investigated based on the bending creep deformation of polycarbonate (PC) and polyoxymethylene (POM). Moreover, the effects of the fiber volume fraction on the creep characteristics were investigated using glass fiber-reinforced polycarbonate (GFRPC). The creep deformation was calculated using the linear visco-elastic theory based on these effects, and comparison between experimental and estimated data showed that the creep analysis sufficiently predicted the creep behavior. [ABSTRACT FROM AUTHOR]

Industry requires validated high-resolution methods for the characterisation viscoelastic materials to obtain local (or small volume) polymer properties for input to part design (e.g. micro-mouldings, packaging, coatings, composite interfaces, etc.). This paper examines the capability of dynamic (oscillatory) indentation (DI) and simple force-controlled 'force-increase ramp and hold' indentation creep methods to deliver equivalent results compatible with those obtained on the same materials by dynamic mechanical analysis (DMA) and uniaxial tensile testing. We test three commercial polymers (two photo stress materials and polyoxymethylene (POM)). A creep analysis based on conical-pyramidal elastic-viscoelastic correspondence 1 and a three-element standard linear solid (SLS) model is used to give an output of two elastic moduli (E1 and E2) and a viscosity (η). Mathematically, E1, E2 and η can be used to calculate loss and storage modulus values at any frequency without further measurement. Indentation creep results obtained using various maximum forces (Pmax) and two indenter geometries (pyramidal (Berkovich) and conical (0.6 µm tip radius)) are compared with DI and DMA measurements at 40 Hz on the same materials and with tensile data from the POM sample fitted using the same SLS creep model. Between-method agreement of storage modulus values for all materials is sufficient to suggest that a route to validated measurement methods is available. However, simple models (as commonly used in DI and DMA) are inadequate to generate reproducible quantitative values for viscosity parameters. We show that more complex models are necessary to successfully produce loss/viscosity parameters that are equivalent. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents failure analysis on a super alloy Inconel® 800 superheater tube in Kapar Power Station Malaysia. Visual inspection, microscopic examinations and creep analysis utilizing available related data are carried out to evaluate the failure mechanism and its root cause. The failed high temperature superheater (HTSH) tube was found snapped into two parts, heavily distorted shape and bent at several points. Microstructures of the failed tube showed that creep crack initiated at both external and internal surfaces of the tube and propagated as grain boundary creep cavities coalesced to form intergranular cracks. The severe geometry of tube causing steam flow starvation is identified to have caused increasing tube metal temperature resulting in overheating of the failed tube. Creep rupture is revealed as the cause of failure of the superheater tube. [Copyright &y& Elsevier]

The scope and extent of mine subsidence caused by exploitation of metal ores are usually low. However, in some cases, owing to such factors as special geological conditions in ore body, backward mining technology and unsound management systems, exploitation of metal ores would also lead to a large area of mine subsidence, causing great damages to the mining environment and infrastructure. A serious mine subsidence happened in January 2007 at the Haolaigou Iron Ore Mine has caused a huge collapse pit on the earth’s surfaces. This provides a suitable place to study the influence of mine subsidence. Based on former research results, a 3D geological model is constructed using the ROCK MODEL software developed by authors’ group. Based on in situ geological conditions, the range of orebody mining and the physical and mechanical parameters of rock and soil mass were determined by inverse analysis. Creep analysis of the collapse pit was also performed. Furthermore, the elastoplastic and creep of the secondary collapse were computed, and the final range of the collapse pit was predicted. Finally, the influences of mining subsidence on the ecological environment and public infrastructure are discussed and control measures were proposed. The results indicate that the collapse pit would not affect the road even if creep is considered; however, the secondary collapse would influence the road. So the collapse pit should be dealt with as soon as possible. [ABSTRACT FROM AUTHOR]

Abstract: Soldering is extensively used to assemble electronic components to printed circuit boards or chips to a substrate in microelectronic devices. These solder joints serve as mechanical, thermal and electrical interconnections, therefore, their integrity is a key reliability concern. However, newly introduced lead-free solders do not have a long history of applications in the industry and there is a lack of established material models of their behaviour over the wide temperature range experienced by electronics systems. Therefore, an extensive reliability study is required before introducing a new lead-free solder material in the electronic industries. Moreover, most of the solder materials have low melting temperatures, and are prone to creep in service. The cyclic temperature operating condition (powercycling) of the solder joint can result in the creep fatigue failure. Thus, a computational technique is used to investigate creep damage in solder joints. The present paper, deals with creep damage of lead-free solder joints for powercycling using finite element analysis with the consideration of experimentally observed non-uniform temperature distributions in the 1206 surface mount chip resistor. In addition, a comparison is made for inelastic strain accumulation and fatigue life for creep damage study for spatially uniform and non-uniform temperature powercycling. [Copyright &y& Elsevier]

This study examined the rheological/mucoadhesive properties of poly(acrylic acid) PAA organogels as platforms for drug delivery to the oral cavity. Organogels were prepared using PAA (3%, 5%, 10% w/w) dissolved in ethylene glycol (EG), propylene glycol (PG), 1,3-propylene glycol (1,3-PG), 1,5-propanediol (1,5-PD), polyethylene glycol 400 (PEG 400), or glycerol. All organogels exhibited pseudoplastic flow. The increase in storage (G′) and loss (G″) moduli of organogels as a function of frequency was minimal, G″ was greater than G″ (at all frequencies), and the loss tangent <1, indicative of gel behavior. Organogels prepared using EG, PG, and 1,3-propanediol (1,3-PD) exhibited similar flow/viscoelastic properties. Enhanced rheological structuring was associated with organogels prepared using glycerol (in particular) and PEG 400 due to their interaction with adjacent carboxylic acid groups on each chain and on adjacent chains. All organogels (with the exception of 1,5-PD) exhibited greater network structure than aqueous PAA gels. Organogel mucoadhesion increased with polymer concentration. Greatest mucoadhesion was associated with glycerol-based formulations, whereas aqueous PAA gels exhibited the lowest mucoadhesion. The enhanced network structure and the excellent mucoadhesive properties of these organogels, both of which may be engineered through choice of polymer concentration/solvent type, may be clinically useful for the delivery of drugs to the oral cavity. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 2632–2646, 2007 [ABSTRACT FROM AUTHOR]

In this paper, time-dependent deflections, stresses and internal forces in prestressed concrete box-girder bridges due to creep of concrete are investigated. Simple equations, correlated with a step-by-step numerical simulation analysis, are developed to calculate long-term behavior of segmentally erected prestressed concrete box-girder bridges built by the balanced-cantilever method. Three-dimensional finite-element models of the mentioned bridges, including effects of the load history, material nonlinearity, creep and aging of concrete, were developed using ABAQUS software. The three-dimensional shell elements are used for modeling box-girder walls, while Rebar elements are used for modeling prestressing tendons. The step-by-step procedure allows the simulation of the construction stages, effects of time-dependent deformations of materials and changes in the bridges' structural system. Different examples of bridges, built by the balanced-cantilever method, are studied over a 30-year duration. Practical equations are modified to calculate time-dependent deflections and redistribution of internal stresses and forces in bridges constructed by the balanced-cantilever method, and good agreements between the results of the proposed method and numerical analysis are found. Significant time-dependent effects on bridge deflections and redistribution of internal forces and stresses are observed. [ABSTRACT FROM AUTHOR]

Deflection tests have been used to estimate the stresses developed in the alumina layer formed during short-term oxidation of a Fe-22Cr-5Al-0.3Y Fecralloy steel at 1000°C. Elastic analysis of the deflecting specimen is inappropriate under these test conditions because of the low creep strength of the alloy. Accordingly, a recent creep analysis has been used in this work using currently determined creep properties of the alloy substrate. The results of the analysis show that for the thin oxides produced (<1 μm), the planar stress within the oxide layer is everywhere compressive. Average values are approximately 850 MPa after 0.5 hr oxidation but reduce to <200 MPa after 6.5 hr. These values are very much less than would be expected under conditions of elastic deformation. [ABSTRACT FROM AUTHOR]

• Three different analysis methodologies are compared for failure analysis of fir-tree joints. • Liu-Murakami creep damage parameters for Inconel 718 at 620 °C are determined for the first time. • FE efficient modeling strategy is suggested for predicting stress and creep in blade-disk attachment. • Simple FE models are efficient for damage tolerance analysis of blade-disk attachment. Due to several physical and computational difficulties in Finite Element (FE) analysis of contact in fir-tree blade-disk attachments of gas turbines, there has always been a great demand for stress, fatigue and creep life assessment of fir-tree joints without modeling physical contact. However, reliability of these models is not trivial. To meet these challenges, in the current paper, three different analysis methodologies were examined for creep life assessment in a turbine engine disk. This is a significant problem for designers and operators of gas turbines. Two simple models were developed with removal of the blade from the FE domain as well as a detailed 3D model containing a blade and it's contact details. Numerical results were compared with available experimental stress measurement. Creep material behavior models were also developed and parameters for the Liu-Murakami damage model together with the Norton creep law were determined for Inconel 718 at 620 °C. A creep subroutine was written and creep analyses of these joints were performed for typical service temperature of 620 °C and the maximum time period of 100,000 h. FE results were compared with fracture mode of service induced cracks. For the different load cases ranging from 3000 rpm to 9000 rpm, it is demonstrated that methodology based on simple models are very efficient. The equivalent pressure model always underestimates the value of peak stress whereas the virtual link model provided more realistic results. The results are discussed according to the different failure criteria for the fir-tree joints life assessment. Results presented in the paper are valuable for failure analysis of these critical components. Moreover, the stress distribution obtained from the simple realistic model can be used for the life assessment codes applicable to other types of failure mechanism. [ABSTRACT FROM AUTHOR]