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This paper presents an upgraded nonlinear creep consolidation model for VDI soft ground, incorporating a modified UH relation to capture soil creep deformation. Key novelties also include considering linear construction loads, TDP boundary conditions, and Swartzendruber's flow in the small strain consolidation domain. The system was solved using the implicit finite difference method, and numerical solutions were rigorously validated. A parametric analysis reveals that soil viscosity causes abnormal EPP increases under poor drainage conditions during early consolidation. Meanwhile, neglecting the time effect of the secondary consolidation coefficient delayed the overall EPP dissipation process and overestimated the settlement during the middle and late consolidation stages. Furthermore, TDP boundaries, Swartzendruber's flow, and construction processes significantly influence the creep consolidation process but not the final settlement. These findings offer fresh insights into the nonlinear creep consolidation of VDI soft ground, advancing the field. • An upgraded nonlinear creep consolidation system of vertical drain-improved soft ground is established. • The time-dependent permeable boundaries, Swartzendruber's flow, and self-weight stress were all included in this system. • The proposed solutions were validated with reported theoretical solutions and practical engineering cases. • The influence law of the related model parameters was revealed. [ABSTRACT FROM AUTHOR]

This paper presents a numerical model, called consolidation settlement - elastic-viscoplastic (CS-EVP), for the consolidation of layered soft soils, including soil self-weight and time-dependent compressibility effects. CS-EVP was developed using the piecewise-linear method for large-strain consolidation and elastic-viscoplastic model for time-dependent soil compressibility. The model accounts for vertical strain, soil self-weight, nonlinear hydraulic conductivity and compressibility, nonlinear creep with limited creep strain, and time-dependent surcharge and/or vacuum loading for layered heterogeneous soils. The accuracy of CS-EVP is verified by comparing calculated values with the results from finite-element simulations and a large-scale laboratory vacuum consolidation test of soft soil slurry. Lastly, simulated settlements and excess pore pressure profiles are compared with field measurements for embankment loading in Väsby, Sweden. The results indicate that CS-EVP provides good estimates of long-term large-strain consolidation under both laboratory and field conditions. [ABSTRACT FROM AUTHOR]

The consolidation of Hong Kong marine deposits (HKMDs), a typical soft clayey soil in Hong Kong, is a serious concern in engineering practices, such as coastal embankment construction and marine reclamations. Previous research works illustrate that high temperatures could accelerate the rate of consolidation of soft clayey soils, which has a great potential in future applications. Therefore, studies on the consolidation and stress–strain behaviors of clayey soils under various thermal conditions are necessary. In this paper, a modified temperature-controlled oedometer has been developed and employed to investigate the effects of vertical stress and temperature on the consolidation and creep behavior of remolded HKMD with a temperature range of 20°C–60°C. Scanning electrical microscope (SEM) tests were performed to observe the microstructure of HKMD specimens after oedometer tests under different temperatures. The results show that compression index Cc is nearly independent of temperature, while swelling index Cs is slightly affected by the thermal and stress paths. As temperature increases, both permeability and coefficient of consolidation increase, and the duration to the end of primary consolidation is shortened. It has also been found that with an increase of temperature, the preconsolidation pressure decreases, and there is a reduction in linear creep rate and creep strain limit. Both linear and nonlinear creep functions are adopted to analyze the creep behavior. [ABSTRACT FROM AUTHOR]

To predict the long-term storage capability (LTSC) of composite directors for multiple launch rocket system (MLRS) in the actual environment, a creep constitutive model was established in a three-dimensional nonlinear form, which considered the moisture effect by introducing a moisture shift factor that related to moisture absorption (MA) into the nonlinear Schapery equation. The constitutive model in integral form was transformed into the incremental-iterative form to realize the numerical finite element solution. Quasi-static experiments and short-term creep-recovery tests for E-glass/6509 epoxy resin laminate were performed to obtain the moisture and stress dependent parameters. A coupled numerical model in multi-physical field of moisture diffusion-nonlinear creep was established, and the MA process and creep deformation of composite directors during storage were predicted. The influence law of creep strain on rocket launching was further investigated by establishing the coupled rocket-directors launching dynamics model. The results show that the water diffuses to the inner surface of directors after storage one year in the environment of temperature of 30°C and relative humidity of 70% RH, while a saturated MA state was reached after 8 years storage. Maximum residual strain (RS) in 20 directors locates between the 2nd and 3rd positioning mounting rings, whose distribution likes a saddle-shape. The residual strains of the middle position in the upper and lower rows are the greatest, with a value of 0.38 mm; and the middle position in the left and right columns are the smallest, with a value of 0.33 mm. The creep strain increases the angular displacement and rate in the pitch and yaw directions of the rocket axis. [ABSTRACT FROM AUTHOR]

This work studies the creep characteristics of concrete sustaining initial damage by considering both the initial damage to concrete and the damage caused by crack growth in the creep process. The damage state of concrete is characterized by the characteristics of the amplitude of ultrasonic waves. Creep tests on concrete prisms with different degrees of initial damage were carried out at loads of 40%, 50%, and 60% compressive strength. The results show that the initial damage to concrete significantly impacts its creep behavior. The modified creep curve of concrete, including the initial damage variable, is consistent with the creep curve of undamaged concrete at different stress levels, proving the applicability of the damage variable. [ABSTRACT FROM AUTHOR]

In order to research the displacement characteristics and stability of a soil slope reinforced with carbon-fiber-reinforced plastic (CFRP) and anti-slide piles, the displacement composition, aging deformation and failure mode of a soil mass were analyzed. According to the Mohr–Coulomb strength criterion, a new nonlinear, accelerated creep model of soil mass was founded with the addition of a self-building M-C plastic element. Furthermore, a viscoplastic strain analytical formula of an M-C plastic element was obtained, and the tensile deformation characteristics of a CFRP sheet were also discovered under a landslide thrust creep load. According to the environmental conditions of the anti-slide pile, the CFRP was arranged along the load-bearing side of the pile to control deformation. Combining the calculation example, it is shown that the horizontal displacement of the soil slope's composite structure decreases by approximately 40% with CFRP reinforcement. Furthermore, for the first two calculation conditions, after one year, the maximum horizontal displacement decreased by 50% and increased by 10%, respectively. Simultaneously, the overall safety factor increased by 31.3% without soil creep properties. On the contrary, the overall safety factor was reduced, and the slope has a tendency toward unstable failure. Moreover, there is no through plastic zone in the slope. The stability of the reinforced slope and the bearing capacity of the pile are related to the CFRP method. Simultaneously, the structure can reduce the costs and construction difficulty of anti-slide piles in a complex environment surrounded by the soil creep effect. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of the shear creep behavior of the adhesive joint in concrete structures strengthened by externally bonded fibre-reinforced polymers (FRP) composites. Based on experimental data collected in a previous study, creep constitutive equations were developed for the adhesive layer and implemented into a finite element code. The proposed model extends the classical one-dimensional formulation of Burgers creep model to a fully 3D model and introduces the nonlinearity of the model parameters. This numerical approach was first used to simulate the nonlinear creep behavior of bulk epoxy samples; it was then extended to predict the nonlinear creep response of the FRP-concrete interface in double lap shear specimens. Globally, a fair agreement was obtained between numerical results and experimental evidences. As a main result, it was found that creep induces a redistribution of the interfacial shear stress along the FRP-concrete lap joint, leading both to a stress relaxation near the loaded end of the adhesive joint and to an increase in the effective transfer length. [ABSTRACT FROM AUTHOR]

The moisture content of rock significantly influences its time-dependent deformation behavior. This paper studied the creep behavior of sandstone under saturated and dry conditions under different driving stress ratio (not the same creep stress level).Their impacts on the time-to-failure (TTF) of sandstone were also investigated. The results show that under an identical driving stress ratio, the dry specimens' creep strain and steady-state creep rate are relatively higher than those of the saturated ones. Nevertheless, the water-saturated specimen has a shorter creep strain path and TTF due to the softening effect of water. It is evident that in terms of creep deformation and creep rate, the impact of creep stress is much more significant than that of moisture content; however, in terms of creep TTF, the moisture content of sandstone plays a dominant role. The rationality of the generalized Kelvin, Burgers, and power-law creep models were evaluated based on the experimental data. The power-law model was found to be capable of fitting the test data, and better predicting sandstone creep deformation than other two models. Finally, based on the assumption of a critical strain value εcri, a method for predicting creep TTF from limited laboratory data was proposed, providing a foundation for incorporating TTF prediction into continuum numerical simulations. [ABSTRACT FROM AUTHOR]

This paper is focused on deriving a common solution to the strain energy release rate for delamination cracks in multilayered inhomogeneous beams under nonlinear creep. The layers of the beams exhibit material inhomogeneity in the thickness direction. The solution is obtained for the nonlinear stress–strain-time relation. The J-integral approach is applied to verify the solution. The variation of the strain energy release rate with time as a result of the nonlinear creep behaviour is analyzed. [ABSTRACT FROM AUTHOR]

Creep affects the long-term deformation of concrete structures. Nonlinear creep further overestimates the safety factor of structures and affects the safety service performance. The coupling of creep and a damage model considering the rate effect is conducive to accurate prediction of nonlinear creep, but the iterative process of strain makes the calculation method more complex. The purpose of this study is to propose a nonlinear creep explicit method that considers the damage evolution of concrete under compression. Two groups of axial compression members with compressive stresses of 0.2 fc and 0.4 fc were made. Considering the law of concrete damage evolution under uniaxial compression, coupled with elastic creep and damage incremental strain, the lower limit of the medium stress level that gives rise to nonlinear creep is analyzed. The concrete nonlinear creep amplification coefficient with a loading age of 28 days and loading duration of 360 days is studied with consideration for the uncertainty of relative humidity and the theoretical thickness of the component. On this basis, the explicit calculation formula of the nonlinear creep amplification coefficient related to the concrete axial compressive strength and stress level is given. The results indicate that the nonlinear creep amplification coefficient increases nonlinearly with an increase in the stress level, and, when the compressive stress level ratio is higher than 0.6, the nonlinear creep amplification coefficient increases significantly; when the stress level is determined, the creep amplification coefficient decreases gradually with an increase in the compressive strength of the concrete. It is suggested that a stress level range of 0.35~0.75 should be used for the study of a nonlinear creep amplification factor under the medium stress state. [ABSTRACT FROM AUTHOR]

The present paper is concerned with fracture analysis of an arch structure with three lengthwise cracks under nonlinear creep. The time-dependent fracture is studied in terms of the strain energy release rate with considering the creep. Time-dependent solutions to the strain energy release rate are derived for the three cracks by using the complementary strain energy. For this purpose, time-dependent expressions for the complementary strain energy densities are obtained. The creep-induced time-dependent behavior is treated by using a nonlinear stress–strain–time relationship. Time-dependent solutions for the changes of the curvatures of the crack arms and the uncracked portion of the arch are developed. The bending moments which are used to derive the strain energy release rate are determined by treating the arch as a statically undetermined structure with one degree of internal redundancy. Time-dependent solutions to the strain energy release rate are obtained also by considering the balance of the energy for verification. The variations of the strain energy release rates with the time for the three cracks are investigated. [ABSTRACT FROM AUTHOR]

Time-dependent responses of cracked concrete structures are complex, due to the intertwined effects between creep, shrinkage, and cracking. There still lacks an effective numerical model to accurately predict their nonlinear long-term deflections. To this end, a computational framework is constructed, of which the aforementioned intertwined effects are properly treated. The model inherits merits of gradient-enhanced damage (GED) model and microprestress-solidification (MPS) theory. By incorporating higher order deformation gradient, the proposed GED-MPS model circumvents damage localization and mesh-sensitive problems encountered in classical continuum damage theory. Moreover, the model reflects creep and shrinkage of concrete with respect to underlying moisture transport and heat transfer. Residing on the Kelvin chain model, rate-type creep formulation works fully compatible with the gradient nonlocal damage model. 1-D illustration of the model reveals that the model could regularize mesh-sensitivity of nonlinear concrete creep affected by cracking. Furthermore, the model depicts long-term deflections and cracking evolutions of simply-supported reinforced concrete beams in an agreed manner. It is noteworthy that the gradient nonlocal enhanced microprestress-solidification theory is implemented in the general finite element software Abaqus/Standard with the implicit solver, which renders the model suitable for large-scale creep-sensitive structures.

Prefabricated vertical drains (PVDs) combined with vacuum and/or surcharge loading have been widely adopted to improve the strength of soft soils. Precise consolidation analysis is the theoretical basis for the design of preloading method with PVD. Current consolidation theories for layered soils with PVD seldom consider the influence of large strain, nonlinear creep, and self-weight loading simultaneously. This paper, thus, presents a finite strain elastic visco-plastic consolidation model, called RCS-EVP, for radial consolidation of layered soils with PVD. RCS-EVP is developed based on the piecewise-linear method. It takes into account nonlinear creep with limit creep strain, variable boundary conditions, anisotropy of soil hydraulic conductivity, and variable compressibility and hydraulic conductivity during the consolidation under self-weight, time-dependent surcharge and/or vacuum loading. The performance of RCS-EVP is evaluated by comparing with the results from finite element simulations and a laboratory physical model test. The variations of settlement and pore pressure of a soft soil ground improved by vacuum preloading with PVD are estimated using RCS-EVP. The results indicate that RCS-EVP provides good estimates of long-term consolidation of layered soils with PVD under both laboratory and in-situ conditions. [ABSTRACT FROM AUTHOR]

The available models for estimating the creep strains of concrete generally assume concrete as a homogeneous material. Since concrete is a composite, such models cannot take into account incompatible strains between cement paste and aggregates during creep loading. The main objective of this paper is to demonstrate that initial defects could increase the creep strain level and justify the application of a coupling between creep and damage at the macroscopic scale. To do so, different contents of air-entraining agent were used to simulate the internal defects of concrete and high-stress continuous load was applied to concrete samples with different initial defects. By analyzing the stress-strain curves of concretes containing different initial defects after compression, it was concluded that the existence of internal defects had a significant effect on the basic mechanical properties of concrete samples. Acoustic emission technology was used to obtain the internal acoustic emission response of concrete samples under high stress. Creep development rate is nonlinear to load holding level based on the analysis of experimental data and internal structure of concrete samples. [ABSTRACT FROM AUTHOR]

A damage-plasticity model is extended to account for the effects of simultaneously occurring stiffness degradation, residual deformation, and creep. Assuming the additivity of small strains, the model combines damage mechanics, plasticity theory, and an improved version of the ACI model to characterize creep at low stress levels. The coupling between damage and creep produced by medium and high stress levels is accounted for by introducing a damage-dependent influence function. An explicit numerical algorithm is developed to implement the proposed model in the simulations of structural response. The proposed model is systematically validated by comparing its results with experimental data, suggesting that it offers promise for capturing the long-term mechanical behavior of reinforced concrete structures. [ABSTRACT FROM AUTHOR]

In bridges, UHPC can bear significant sustained tensile stress without cracking under service loading. Its significant impact on the long-term behavior of large-span bridges due to tensile creep characteristics becomes evident. In this paper, both linear and nonlinear tensile creep behavior of steam-cured UHPC was experimentally investigated by using dog-bone-shaped specimens under sustained direct tension for a duration of 360 days. The tensile creep test was conducted under varied tensile stress levels ranging from 0. 3 f t to 0. 9 f t . Tensile creep coefficients were calculated by using the experimental results. Nonlinear tensile creep occurs in the steam-cured UHPC when the sustained tensile stress is larger than 0.3 f t. The threshold of nonlinear tensile creep is lower than that of normal-strength concrete under compression. When the sustained tensile stress is larger than 0.8 f t , tensile creep failure occurs a short time after loading. The nonlinear creep impact under tension is less pronounced than that under compression due to the distinct function of steel fibers within UHPC when subjected to compression and tension force. The findings in the scanning electron microscopy (SEM) test reveal that the mechanism driving the nonlinear tensile creep of UHPC is the damage growth at the fibers-matrix interface under the high sustained tensile stress. A design-oriented model for the creep coefficient of UHPC is developed, which incorporates both linear and nonlinear creep effects. The model's credibility is verified using the experimental results. • Nonlinear tensile creep occurs in the steam-cured UHPC when σ c > 0. 3 f t . • Damage growth at the fibers-matrix interface drives the nonlinear tensile creep behavior. • Nonlinear creep impact under tension is less pronounced than that under compression. • A creep coefficient model is proposed for steam-cured UHPC and was verified. [ABSTRACT FROM AUTHOR]

Many prestressed concrete bridges are reported to suffer from excessive vertical deflections and cracking during their service life. Creep softens the structure significantly, and therefore an accurate prediction of creep is necessary to determine long-term deflections in elements under eccentric axial compression such as prestressed concrete girders. This study proposes a modification to the creep damage model of Model Code 2010 to account for the effect of load eccentricity. The modified creep model considers damage due to differential drying shrinkage. Initially, the creep behaviour of small scale concrete specimens under eccentric compression load is investigated experimentally. Twelve small-scale concrete prisms were subjected to eccentric axial loading to assess their shrinkage and creep behaviour. The main parameters investigated include the load eccentricity and exposure conditions. Based on the experimental results, an inverse analysis is conducted to determine the main parameters of the modified creep model. Subsequently, a numerical hygro-mechanical simulation is carried out to examine the effect of load eccentricity on the development of shrinkage and creep, and on the interaction between drying, damage and creep. The results indicate that eccentric loading leads to different tensile and compressive creep through the cross section, which contradicts the current design approach that assumes that tensile and compressive creep are identical. The proposed model also predicts accurately the long-term behaviour of tests on reinforced concrete elements available in the literature. This study contributes towards further understanding of the long-term behaviour of concrete structures, and towards the development of advanced creep models for the design/assessment of concrete structures. [ABSTRACT FROM AUTHOR]

The detrimental influence of high levels of sustained load on the compressive strength of concrete has been acknowledged and investigated since the 1950s. Despite the potential significance of this phenomenon in many situations, current design codes still provide limited guidance on how to account for this effect at ultimate limit state. In addition, the practical case of a significant permanent stress followed by stress increases due to variable loads is not addressed in most codes of practice. The aim of this paper is to present a general but simple design approach to account for the influence of sustained loading on the uniaxial behavior of concrete in compression, both in terms of its strength and deformation capacity. This approach is based on a previously developed mechanical model accounting for nonlinear creep and material damage development on the long‐term response of concrete. On that basis, a design approach is formulated in a simple code‐like manner and used to investigate several practical design situations involving both sustained and variable actions. This approach is shown to be consistent and applicable to any general loading pattern, accurately reproducing the results of the general mechanical model and of available experimental results. [ABSTRACT FROM AUTHOR]

This work proposes an effective algorithm for description of nonlinear deformation of hereditary materials based on Rabotnov’s method of isochronous creep curves. The notions have been introduced for experimental and model rheological parameters and similarity coefficients of isochronous curves. It has been shown how using them, one can find instantaneous strains at various stress levels for description of nonlinear deformation of hereditary materials at creep. Relevant equations have been determined from the nonlinear integral equation of Yu. N. Rabotnov for the application cases of Rabotnov’s fractional exponential kernel and Abel’s kernel for nonlinear deformation of hereditary materials at creep. The improved methods have been given for determination of creep parameters α, ε0, δ, β, and λ. By processing and using test results for material Nylon 6 and glass-reinforced plastic TC 8/3-250, the process has been shown for sequential implementation of the developed methods for description of linear and nonlinear deformation of these materials at creep. From the results of the experimental investigation performed by the authors of this paper, it has been determined that fine-grained, dense asphalt concrete at the temperature of 20 ± 2 °C and stresses up to 0.183 MPa at direct tension is deformed considerably in a nonlinear way. It has been shown in an illustrative way by construction of isochronous creep curves at various load durations and curves of experimental rheological parameter at various stresses. Nonlinear deformation of asphalt concrete at creep is adequately described by the proposed methods. [ABSTRACT FROM AUTHOR]

The time-temperature-stress equivalent (TTSE) principle refers to the phenomenon that the time-dependent mechanical properties of materials rely on the variations of temperature and stress level. Thus, it is reasonable to predict the long time material mechanical properties based on their relationships with rising temperatures and stress levels. According to the single-integral nonlinear constitutive equation proposed by Schapery, a general expression of the TTSE principle for nonlinear viscoelastic creep property is deduced. The specific expression for time-temperature-stress shift factor is presented by assuming quadratic polynomial form of the Doolittle formula for the viscosity as a function of free volume. Creep curves of granite at different temperatures and stress levels are introduced, and the validation of applying time-temperature-stress principle to predict the long time creep property of granite is provided. The master curves are generated through vertical shift modification and horizontal shift equivalence of all creep curves, and corresponding coefficients in the shift equations are determined by curve fittings. The results indicate the modified time-temperature-stress equivalent relation is appropriate to predict the long time creep property of granite. DOI: http://dx.doi.org/10.5755/j01.ms.20.4.6294

In 1967-1985, a research campaign comprising a unique set of long-term experimental data on concrete beams was conducted in joint collaboration with four Belgian research institutes to determine the influence of creep and shrinkage on the long-term behavior of reinforced concrete members. The main aim of the research campaign was the determination of the long-term behavior of cracking and deformations subjected to permanent loads considering the influence of the magnitude of the loads and various reinforcement ratios. The objective of this article is twofold: to provide an overview of the measured data of the reinforced beams of the research campaign, which has never been published before, and to propose a simplified calculation method based on available models in literature that can predict the available measurement data. A simplified calculation model is proposed, which accounts for nonlinear creep strains due to high stresses, shrinkage, aging, and cracking in reinforced concrete beams. This numerical method is based on a cross-sectional analysis formulated using the layered Euler-Bernoulli beam theory, allowing fast and accurate predictions of strains, stresses, and deflections as a function of time based on fib Model Code 2010 and EN1992-1-1. The measurements of the beams subjected to high permanent loads during a time period of 4 years are compared to the results evaluated with the proposed simplified calculation model. The results show that the proposed simplified calculation method based on the current models of EN1992-1-1 and fib Model Code 2010 can predict the long-term behavior of reinforced concrete beams subjected to high loads in good agreement with the measu [ABSTRACT FROM AUTHOR]

The time-dependent stress-strain behavior of clayey soil is a great concern for predicting long-term settlement especially for large-scale land reclamation projects. In this paper, long-term time-dependent behavior of Hong Kong Marine Deposits (HKMD) under the loading stage and unloading stage is investigated by multi-staged loading oedometer test. Special emphasis has been given to study creep and swelling behavior. It is found that the clayey soil demonstrates the creep behavior during the loading stage, whereas it shows the swelling behavior during the unloading stage. The analysis of the creep behavior shows that there exists a linear trend between creep strain and time (in log-scale) within a certain period, whereas a nonlinear relationship in a long term view. Similar result is found for the swelling behavior. A nonlinear function is found to be suitable to predict the long-term creep and swelling behavior. The creep strain limit and swelling strain limit can be obtained if the nonlinear function is adopted to fit the experimental data. In addition, a new modified Elastic Visco-Plastic model considering Swelling (EVPS model) is proposed and presented, which considers the nonlinear creep and swelling behavior. In this modified EVPS model, the final creep and swelling equilibrium stress-strain state can also be obtained when the time is infinite. [ABSTRACT FROM AUTHOR]

This work proposes an effective algorithm for description of nonlinear deformation of hereditary materials based on Rabotnov’s method of isochronous creep curves. The notions have been introduced for experimental and model rheological parameters and similarity coefficients of isochronous curves. It has been shown how using them, one can find instantaneous strains at various stress levels for description of nonlinear deformation of hereditary materials at creep. Relevant equations have been determined from the nonlinear integral equation of Yu. N. Rabotnov for the application cases of Rabotnov’s fractional exponential kernel and Abel’s kernel for nonlinear deformation of hereditary materials at creep. The improved methods have been given for determination of creep parameters α, ε0, δ, β, and λ. By processing and using test results for material Nylon 6 and glass-reinforced plastic ТС 8/3-250, the process has been shown for sequential implementation of the developed methods for description of linear and nonlinear deformation of these materials at creep. From the results of the experimental investigation performed by the authors of this paper, it has been determined that fine-grained, dense asphalt concrete at the temperature of 20 ± 2 °С and stresses up to 0.183 MPa at direct tension is deformed considerably in a nonlinear way. It has been shown in an illustrative way by construction of isochronous creep curves at various load durations and curves of experimental rheological parameter at various stresses. Nonlinear deformation of asphalt concrete at creep is adequately described by the proposed methods.

The material behavior of concrete is highly nonlinear and time-dependent. In particular, the evolution of deformation due to creep is crucially dependent on the acting stress and the environmental conditions. Numerous experimental campaigns for determining the complex material behavior of concrete in various loading conditions and environmental conditions can be found in the literature. However, the experimental investigation of the time-dependent deformation due to high sustained loading, i.e., nonlinear creep, has received less attention. Especially, the evaluation of the impact of different environmental conditions on nonlinear creep is still pending. In this contribution an experimental study on basic and drying creep under moderate and high uniaxial compression is presented. The experimental results indicate that the nonlinearity of the deformation due to creep in high uniaxial compression is dependent on the environmental conditions. Additionally, the capability of selected constitutive models for representing nonlinear creep is assessed in a numerical study based on the novel experimental results. • Experimental study on nonlinear creep considering both sealed and drying conditions. • Distinct influence of the humidity conditions on the nonlinear creep behavior. • Extension of the well-known B4 model to nonlinear creep. • Comparison of the test results with the prediction of modern creep models. • Calibration of the extended B4 model using the experimental results. [ABSTRACT FROM AUTHOR]

This study deals with nonlinear creep and the time taken for completion of the stress redistribution process within a borehole scenario. Specifically, the pressuremeter test, with emphasis on a step loading of long duration, in ice saturated frozen soils is analyzed. A closed form stationary creep solution based on the model of a thick cylinder under plane strain condition, that considers large strains, is first developed. Subsequently non-stationary creep finite element analyses were carried out via this model and an assumption-relaxed model that considers the actual geometry of a typical pressuremeter test (where the net cavity pressure is applied only on a portion of the borehole cavity). Finally estimates of the extent to which stress redistribution occurs is numerically considered by revisiting classical theories and proposing a time scale by introducing a redistribution index, RI. [ABSTRACT FROM AUTHOR]

The article proposes equations for the problem of bending circular symmetrically loaded flexible plate in creep. The problem was reduced to a system of two nonlinear differential equations. This system was solved using the successive approximations method in combination with the method of finite differences. The calculations were performed numerically in Matlab software package. Maxwell-Gurevich physical law was used as an equation, establishing the connection between stresses and creep strains. The results obtained with the geometric nonlinearity and while neglecting it were compared. The calculation of the plates on the geometrically linear theory leads to overestimate deflections. Another effect caused by the geometric nonlinearity is nonlinear stress distribution through the thickness of the plate. [ABSTRACT FROM AUTHOR]

This paper presents a mixed finite element (FE) model for the nonlinear time-dependent analysis of composite beams with partial shear connection. The key idea is to consider, as a first approach, a viscoelastic/plastic model for the concrete slab in order to simulate the interaction between the time effects of concrete, such as creep and shrinkage, and the concrete cracking. Creep is taken into account via linear aging viscoelasticity, while cracking is modeled using an elastoplastic model with softening. A nonlinear isotropic/kinematic hardening model is adopted for steel behavior and an appropriate nonlinear constitutive relationship is utilized for the shear stud. A consistent time integration is performed by adopting the Euler backward scheme. Finally, comparisons between the numerical results and experimental data available in the literature are undertaken to validate the accuracy of the model. It is shown that the interaction between cracking and time effects (creep and shrinkage) significantly increases the deflection. [ABSTRACT FROM AUTHOR]

• Minutes-long flexural test is used to measure nonlinear creep of early-age mortar. • Nonlinear creep coefficient of early-age mortar varies with holding time. • Curing age affects the creep rupture time more significantly than stress level. Nonlinear creep normally defined as the time-dependent deformation of materials under high stress condition, is highly coupled with the damage condition. The microstructure evolution of early-age cementitious materials due to the cement hydration influences the accurate measurement of nonlinear creep. Besides, the commonly-used assumption in the uniaxial creep test that the shrinkage of the loaded concrete is the same as that of the unloaded concrete is questionable for the high stress case. To reduce the influence of cement hydration and shrinkage on the measured results, a minutes-long flexural test was designed in this study to investigate the nonlinear creep of cement mortar with w / c ratio of 0.45 at the ages of 7 and 28 days under drying condition with relative humidity of 70 %. The maximum stress applied to the mortar beam was set as 30 %, 40 %, 70 %, 85 %, and 90 % of the flexural strength. The results show that cement mortar subjected to less than 40 % of the flexural strength exhibits a linear creep property, while a significant nonlinear creep is observed when the stress level is greater than 70 %. The influence of curing time on the creep rupture time seems more significant than that of the stress level. The nonlinear creep coefficient of early-age cement mortar decreases gradually and tends to converge to a constant if it is not ruptured during the minutes-long creep test, which cannot be characterized by the existing models where the nonlinear creep coefficient depends only on the stress level. A new model is proposed in this study with the function form of η (t) = η 0 + k e - x t , which is capable of capturing the evolution of the nonlinear creep coefficient of early-age cement mortar with good accuracy. [ABSTRACT FROM AUTHOR]

Nonlinear creep is one of the generic features of deformation response of glassy polymers, where a rich set of strain vs time responses is observed that depend upon the applied stress and the thermal history used to form the glass. Although existing constitutive models may be able to describe some aspects of nonlinear creep, these models are unable to provide a unified description of the diverse nonlinear creep behaviors exhibited by glassy polymers, where predicting the effects of thermal history has proved especially challenging. In this paper the ability of a recently developed Stochastic Constitutive Model (SCM) to describe nonlinear creep is critically examined, where it is demonstrated that the SCM qualitatively captures all of the major features of nonlinear creep of glassy polymers, including the tertiary creep. In particular the SCM predicts how the duration of the secondary creep and the range of the tertiary creep depend on the aging time prior to deformation. The SCM predicts the existence of Stage IV creep, which comes after the tertiary creep – where examination of nonlinear creep data clearly indicate the presence of Stage IV creep, although heretofore not recognized, in the experimental data. The SCM unifies the description of nonlinear creep, and provides a detailed mesoscopic picture of the processes that underlie both strain controlled and stress controlled experiments. [ABSTRACT FROM AUTHOR]

Tensile wood creep has not been measured previously in the temperature region 135°C-150°C for a range of moisture content (MC) up to fiber saturation point due to equipment and measurement challenges. Yet this is the region where the most dramatic softening effects can be observed. The aim of this study was to develop specialized equipment to measure tensile solid wood creep over 70°C-150°C for a range of MC and loads. Creep displacement was successfully able to be isolated from elastic and mechanosorptive strains and statistically modeled by regression analysis. This proved more accurate than complex series expansions consisting of spring- and dash-pot-type components. The best creep displacement relationship was a power law with a strain root mean square error of 0.28%. The amplitude of the power law was nonlinear with respect to stress and temperature and changed by a stress factor of up to 0.9 for a stress of up to 1.2 MPa and by a temperature factor 1.2-6.3 over the investigated range. The creep relationship provides a valuable tool for predicting time-dependent distortion and internal stress of wood during drying or thermohygromechanical modification below 150°C. [ABSTRACT FROM AUTHOR]

For high stress levels, the dependence of the creep strain rate of concrete on the acting stress is nonlinear. Furthermore, very high stress levels may lead to a critical growth of microcracks resulting in material failure. For the lifetime assessment of creep sensitive concrete structures by means of numerical simulations, the realistic description of nonlinear creep is crucial. However, many concrete models are formulated for either nonlinear time-independent material behavior, or time-dependent material behavior, restricted to linear creep. This is the motivation for extending a damage-plasticity model, aiming at a unified and computationally efficient approach for representing the highly nonlinear time-dependent material behavior of concrete. Well established approaches for modeling the evolution of material properties, inelastic deformation, damage, creep and shrinkage serve as basis for considering nonlinear creep and material failure due to high sustained acting stress. The extended material model for concrete is validated by comprehensive experimental results from the literature. • Extension of a damage-plasticity model for concrete to nonlinear creep. • A novel approach for modeling nonlinear creep and failure due to creep. • Application of a gradient-enhanced formulation for mesh-insensitive results. • Extensive validation of the proposed model by uniaxial and multiaxial test data. • Application to 3D FE simulations of a beam characterized by failure due to creep. [ABSTRACT FROM AUTHOR]

Abstract: Creep deflections that greatly exceed the predicted values by the linear creep law are being found in measurements on actual PC bridge viaducts. In this study, structural creep deformations were reproduced by using the multi-scale coupled thermo-hygro and mechanical modeling which enables to deal with an interaction of chemo-physical events of differing dimensions ranging from the kinematics of moisture in micro-pores to the macroscopic structural mechanics, and the effect of various factors was analytically investigated. The numerical analysis approximately reproduced the excessive deflection measured on an actual bridge viaduct. It was confirmed that the creep bending of the viaduct having the hollow cross-section varies significantly due to the ambient temperature, humidity and the structural specific surface area. The macroscopic structural responses in association with the thermodynamic state of moisture in the micro-pores are also discussed. [Copyright &y& Elsevier]

The viscoelastic-plastic creep experiments on soft ore-rock in Jinchuan Mine III were performed under circular increment step load and unload. The experimental data were analyzed according to instantaneous elastic strain, visco-elastic strain, instantaneous plastic strain and visco-plastic strain. The result shows that instantaneous deformation modulus tends to increase with the increase of creep stress; soft rocks enhance the ability to resist instantaneous elastic deformation and instantaneous plastic deformation during the multi-level of load and unload in the cyclic process. In respect of specimen JC1099, the ratio of visco-elastic strain to visco-plastic strain varies from 3.15 to 6.58, and the ratio has decreasing tendency with stress increase as a whole; creep deformation tends to be a steady state at low stress level; soft rocks creep usually embodies accelerated creep properties at high stress level. With the damaging variable and the hardening function introduced, a nonlinear creep model of soft rocks is established, in which the decay creep is described by the nonlinear hardening function H of viscidity coefficient. The model can describe the accelerated creep of soft rocks since the nonlinear damaging evolvement variable D of deformation parameter of rocks is introduced. Three stages of soft rocks creep can be described with the uniform creep equation in the nonlinear creep model. With this nonlinear creep model applied to the creep experiments of the ore-rock of Jinchuan Mine III, the nonlinear creep model’s curves are in good agreement with experimental data. [ABSTRACT FROM AUTHOR]

The paper is concerned with the problem of predicting nonlinear creep strains and time to ductile rupture of prismatic rods under constant tension. The material of the rod is assumed isotropic, homogeneous, and perfectly plastic. The problem is solved using models that take into account the change in the geometry of the rod during creep, the finiteness of the creep strains, and the effect of the initial and actual elastic strains. The conditions whereby the characteristic dimension of the rod tends to infinity and the accumulated and real strains in the viscous flow are limited are used as a failure criterion. The calculated results are compared with experimental data for a number of steels and alloys to formulate the conditions for the ductile rupture and embrittlement of metallic materials under uniaxial creep [ABSTRACT FROM AUTHOR]

Creep strains in unidirectional organic-fiber-and organic-glass-fiber-reinforced plastics subject to tension in the reinforcement direction are predicted. Based on homogeneity condition and statistical criteria, it is shown that the creep of the composites is essentially nonlinear. To predict creep strains, use is made of a nonlinear creep model based on a modified Rabotnov’s similarity hypothesis for isochrones and of material constants determined from tests on specimens made of a composite as a whole and specimens made of its separate components, the mixing rule applied in the latter case. The calculated results and the experimental data are in satisfactory agreement [ABSTRACT FROM AUTHOR]

Stress induced change in intrinsic time scale was investigated by nonlinear creep tests of polypropylene (PP) at room temperature of 27 °C. The time-dependent axial elongations of the specimen were measured at 5 different stress levels, from 10.2 to 20 MPa, and modeled according to the time-stress superposition principle. The test duration was only 1 h. The test results show that the creep compliance vs logarithm time curves at different stresses depart from each other, indicating nonlinear viscoelastic behavior, and can be horizontally shifted to overlap onto a smooth master curve up to 51.5 h at the reference stress of 10.2 MPa. It is demonstrated that the time-stress superposition principle provides an accelerated test technique to evaluate the materials’ long-term mechanical properties. [ABSTRACT FROM AUTHOR]

The nonlinearity of the creep of nylon fibers is justified based on the similarity of a set of isochronous creep curves, which also includes the instantaneous deformation curve. Nonlinear hereditary constitutive equations of creep are derived. The real values of the influence function are determined as the basic rheological characteristic of the material. The applicability of Boltzmann's, Abel's, Rzhanitsyn's, and Rabotnov's kernels is estimated quantitatively. The choice of an Abel-type kernel is justified. The calculated and experimental data are in satisfactory agreement for a loading duration of up to 1,000 hours and an order of magnitude change in the stresses [ABSTRACT FROM AUTHOR]

The one-dimensional constitutive equations of strain-hardening materials subject to nonlinear creep are derived. The solution is found using the hypothesis of unified deformation curve based on the similarity of the tensile and isochronic creep curves. A generalized rheological model is constructed which accounts for the instantaneous strain rate, loading rate, and the mode of strain hardening. This model is used to derive one-dimensional constitutive equations for linear viscoelastic, nonlinear viscoelastic, and linear- and nonlinear-hardening viscoelastoplastic materials, it is shown that the creep of linear viscoelastic and linear-hardening viscoelastoplastic materials is transient. For nonlinear viscoelastic and nonlinear-hardening viscoelastoplastic materials, all the characteristic stages of creep are present. [ABSTRACT FROM AUTHOR]

The creep responses of (bisphenol A) polycarbonate at 80°C undercombined two-dimensional shear with superposed tensile and compressivestress states were measured on Arcan specimens in the nonlinearlyviscoelastic regime. Of particular interest is the influence of thedilatational deformation component on the nonlinearly viscoelastic creepbehavior. Because the nonlinear material response determines the stressdistribution under fixed deformation or load, but is not known a priori,a re-estimation of the latter is essential to verify or adjust thestress state(s). This is accomplished by approximating isochronalstress-strain relations derived from shear creep behavior, encompassingthe nonlinear domain, by a classical incremental elasto-plastic materialdescription at appropriate times. To the extent that the two-dimensionalcharacter of the test configuration permits accessing three-dimensionalinformation, a coherent representation of the results is examined interms of maximum shear and/or octahedral representation. It is found that the creep behavior under shear and normal stressor deformation imposition differ significantly: When viewed as aresponse to the imposition of a maximum shear stress, the creepresponses differ depending on whether one or the other dominates. On theother hand, if the response is formulated in terms of an octahedraldescription the representation becomes less sensitive to normal vs.shear behavior. It is clear in either case, however, that normal strainhas a disproportionately large effect on creep response in shear. Withinthe precision underlying the measurements it is found that the shear andnormal strain components accumulate under creep in nearly constantratios. Under this scenario it is demonstrated clearly that theinfluence of negative dilatational stress (or deformation) on pure sheardeformation leads to distinctly lower creep rates. The converse is true,if positive dilatational stresses are added, though not monotonically so. [ABSTRACT FROM AUTHOR]

The mechanical behavior of asphalt mixture under high stresses presents nonlinear viscoelasticity and permanent deformation. In this paper, a nonlinear fractional viscoelastic plastic (NFVEP) creep model for asphalt mixture is proposed based on the Nishihara model, with a Koeller spring-pot replacing the Newton dashpot. The NFVEP model considers the instantaneous elasticity, viscoelasticity with damage and time-hardening viscoplasticity with damage concurrently, and the viscoelastic response is modeled by fractional derivative viscoelasticity. To verify the model, uniaxial compressive creep tests under various stresses ranging from 0.4 MPa to 0.8 MPa were carried out at room temperature. The NFVEP model predictions are in good agreement with the experiments. The comparison with the modified Nishihara model and the Burgers model reveals the advantages of the NFVEP model. The results show that the NFVEP model, with the same set of parameters, can not only describe the primary and steady-state creep stages of asphalt mixture under low stress levels but also the whole creep process, including the tertiary creep stage, of asphalt mixture under high stress levels. [ABSTRACT FROM AUTHOR]

• Uniaxial nonlinear creep test of concrete under different ages and stress-strength ratios. • Quantitative relationship between creep characteristics and control variables. • Establish a concrete three-phase numerical simulation model. • Simulate the development process of concrete nonlinear creep, damage and crack. • Verify the mesoscopic mechanism of concrete nonlinear creep. In order to study the nonlinear creep mechanism of concrete materials, predict and evaluate the safety of the structure, uniaxial nonlinear creep tests of concrete specimens under different ages and stress-strength ratios (σ / f c) were carried out. Based on the discrete element theory (DEM), a concrete three-phase numerical simulation model is established, which considers the real aggregate shape and the strength difference of each phase material. The study found that the nonlinear creep duration of the early-age concrete specimens increases exponentially with age, and decreases exponentially with the increase of stress-strength ratio. Age has the greatest influence on total deformation of concrete nonlinear creep. The quantitative relationship between creep characteristics (duration, total deformation) and control variables (age, stress-strength ratio) is established. The numerical simulation model simulates the uniaxial nonlinear creep process of the concrete specimen. Compared with the measured data, it is found that the creep curve is in good agreement. At the same time, the overall trend of the measured damage value defined by the wave velocity and the damage simulation value defined by the number of cracks is in good agreement, which objectively reflects the general law of damage and crack development in the nonlinear creep process. Through the analysis of the failure mode and fabric diagram of the concrete specimen, the microcracks first occurred on the interface. At the same time, the cement gel in the cement mortar undergoes viscous flow, the stress is redistributed, and the microcracks continue to produce, expand, and connect, which ultimately leads to the complete failure of the specimen. The quantitative relationship and numerical simulation model proposed in this paper can be applied to the creep prediction and safety calculation of components or buildings under the condition of high stress-strength ratio, and has a good engineering application prospect. [ABSTRACT FROM AUTHOR]

Salt rock is widely used as an excellent material for energy storage owing to its low permeability, high safety, and stable mechanical properties. In this study, salt rock was subjected to a gradual loading creep test through the conventional uniaxial compression test. The loading time of each stage was approximately 14 days, and the total creep time exceeded five months. The steady-state creep rate of salt rock under different stresses and the corresponding creep strain law were positively correlated with the increase in stress and time. In addition, the long-term strength of the salt rock, determined via the isochronous stress–strain curve inflection point method, was 12 MPa. Furthermore, the viscoelastic–plastic damage-creep model of salt rock was established based on the theory of fractional derivatives. This proposed model was compared with the Nishihara model, and a sensitivity analysis of the parameters was performed based on the results of the nonlinear fitting of the fractional derivative. The rationality of the model was verified based on the results, especially for the accelerated creep phase. Moreover, the constitutive relationship of the model was straightforward and easy to apply. The proposed model provides a theoretical basis for future creep laws based on experimental data. The results reflected the creep law of salt rock to a certain extent and are expected to serve as a reference to studies on the long-term stability of deep salt rock. • Different creep stages of salt rocks under gradual stress loadings were determined. • Fractional derivative used to fit stress–strain data of different creep stages. • The fitted model describes effects of parameters on the stress–strain curve. • The model offers a superior correlation coefficient and is simple and convenient. [ABSTRACT FROM AUTHOR]

• Nonlinear creep test on circular RACFST stub columns. • Influence of the RCA on the nonlinear creep of circular CFST. • Variation of confinement effects with time for RACFST. Encasing recycled aggregate concrete (RAC) in circular steel tube can improve the mechanical behaviours of the RAC without considerably increasing the cost. The confinement effects, which significantly increase the strength and the ductility of the RAC, also influence the nonlinear creep behaviour of RAC, which has not been well investigated. This paper presented nonlinear creep tests on recycled concrete filled steel tubes (RACFST) for up to 500 days. The load level varied between 0.41 and 0.79. Two concrete strengths were included in the tests (i.e. 30 MPa and 50 MPa). All these members used 100% of the recycled coarse aggregates (RCA) and have the steel ratio (the ratio of the steel area over the concrete area) of 0.08. The tested time-dependent deformations were compared against those reported for the circular CFST members to investigate how the full use of RCA would influence the nonlinear creep behaviour of the composite sections. The confinement effects were also carefully monitored during the long-term tests. It was found that the nonlinear creep can increase the creep coefficient of the RACFST members by 52%–115%, which needs to be carefully considered in the design. Confined by steel tubes, the encased RAC could stand for up to 500 days without creep failure under the stress level of 0.78–0.98 f cm , where f cm is the uniaxial strength of the RAC. The use of recycled aggregates can considerably increase the nonlinear creep effects by up to 22%, and such influence increased with the sustained loading level to the maximum value at the stress level of approximately 0.68 and then started to decrease. [ABSTRACT FROM AUTHOR]