Creep Reduction Research Articles

Study on creep characteristics and reduction coefficient of geomembrane under top pressure

A new top-pressure creep test device was developed to simulate the stress conditions of geomembrane in a real working state. CBR puncture and top pressure creep tests under different load levels were conducted using this device to study the creep characteristics of the geomembrane at top pressure. The top pressure creep reduction coefficients under different load levels and design lives were calculated and compared with the regular tensile creep reduction coefficients. Quantitative relationships between the creep-reduction coefficient, design life, and failure strain were established to predict the top-pressure creep-reduction coefficient. The results indicated that the top pressure creep reduction coefficient was greater than that of regular tensile creep. The creep reduction coefficient changed minimally with an increase in the design life. The top pressure creep reduction coefficient remained stable with an increase in the top pressure area strain for different design lives. Based on the results, to improve the safety and durability of engineering structures, the top pressure creep reduction coefficient should be used in the design of geosynthetics affected by the top pressure.

Stability bearing capacity of concrete filled steel tubular columns subjected to long-term load

Concrete-filled steel tube (CFST) are commonly used in modern building and bridge applications. Despite their popularity, studies on the investigation of the influence of long-term load on the stability bearing capacity of such elements are scarce. This study investigates how the key parameters including slenderness ratio (λ), axial load ratio (m), and eccentricity ratio (e/r) affect the stability bearing capacity of a CFST column under sustained load. Twenty three CFST columns were fabricated to investigate the effect of long-term load on the stability bearing capacity. Fourteen specimens were subjected to constant compressive loading for 462 days and then tested for failure. The remaining 9 were companion load-free specimens. A three-stage finite element method was used to predict the stability bearing capacity after creep. The results indicate that the stability bearing capacity of CFST columns decrease after being subjected to long-term load. Both the experimental and numerical results indicated that the load of steel tube for long-term load specimens reaching up to the elastic–plastic and plastic process was lower than that of the load-free specimens. Moreover, the corresponding strain of the creep specimens was greater than that of the load-free specimens when the member reached the maximum load. Benchmarking analyses have shown that the creep reduction coefficient (kcr) proposed for CFST columns can be used to predict the reduction of stability bearing capacity after creep. Furthermore, a collected database comprising 49 CFST specimens subjected to long-term load was used to investigate the proposed formulae for kcr. The results show that the formulae were consistent with the experiment results.

FRP-Reinforced/Strengthened Concrete: State-of-the-Art Review on Durability and Mechanical Effects

Fiber-reinforced polymer (FRP) composites have gained increasing recognition and application in the field of civil engineering in recent decades due to their notable mechanical properties and chemical resistance. However, FRP composites may also be affected by harsh environmental conditions (e.g., water, alkaline solutions, saline solutions, elevated temperature) and exhibit mechanical phenomena (e.g., creep rupture, fatigue, shrinkage) that could affect the performance of the FRP reinforced/strengthened concrete (FRP-RSC) elements. This paper presents the current state-of-the-art on the key environmental and mechanical conditions affecting the durability and mechanical properties of the main FRP composites used in reinforced concrete (RC) structures (i.e., Glass/vinyl-ester FRP bars and Carbon/epoxy FRP fabrics for internal and external application, respectively). The most likely sources and their effects on the physical/mechanical properties of FRP composites are highlighted herein. In general, no more than 20% tensile strength was reported in the literature for the different exposures without combined effects. Additionally, some provisions for the serviceability design of FRP-RSC elements (e.g., environmental factors, creep reduction factor) are examined and commented upon to understand the implications of the durability and mechanical properties. Furthermore, the differences in serviceability criteria for FRP and steel RC elements are highlighted. Through familiarity with their behavior and effects on enhancing the long-term performance of RSC elements, it is expected that the results of this study will help in the proper use of FRP materials for concrete structures.

Topological alternation from structurally adaptable to mechanically stable crosslinked polymer

ABSTRACT Stimuli-responsive polymers with complicated but controllable shape-morphing behaviors are critically desirable in several engineering fields. Among the various shape-morphing materials, cross-linked polymers with exchangeable bonds in dynamic network topology can undergo on-demand geometric change via solid-state plasticity while maintaining the advantageous properties of cross-linked polymers. However, these dynamic polymers are susceptible to creep deformation that results in their dimensional instability, a highly undesirable drawback that limits their service longevity and applications. Inspired by the natural ice strategy, which realizes creep reduction using crystal structure transformation, we evaluate a dynamic cross-linked polymer with tunable creep behavior through topological alternation. This alternation mechanism uses the thermally triggered disulfide–ene reaction to convert the network topology – from dynamic to static – in a polymerized bulk material. Thus, such a dynamic polymer can exhibit topological rearrangement for thermal plasticity at 130°C to resemble typical dynamic cross-linked polymers. Following the topological alternation at 180°C, the formation of a static topology reduces creep deformation by more than 85% in the same polymer. Owing to temperature-dependent selectivity, our cross-linked polymer exhibits a shape-morphing ability while enhancing its creep resistance for dimensional stability and service longevity after sequentially topological alternation. Our design enriches the design of dynamic covalent polymers, which potentially expands their utility in fabricating geometrically sophisticated multifunctional devices.

A new apparatus for the study of pullout behaviour of soil-geosynthetic interfaces under sustained load over time

At present, the design of geosynthetic-reinforced soil structures is executed with reference to the tensile strength of the reinforcement obtained from in-air short-term tensile tests, decreasing this value by means of several factors. Among these, the creep effect resulting from in-air tensile creep tests reduces tensile strength the most. Consequently, this procedure does not take into account the effects of soil confinement and interaction on the tensile response of the reinforcements. This paper illustrates a new large-scale pullout prototype apparatus, with the capacity to investigate the behaviour of a geosynthetic reinforcement embedded in a compacted soil and subject to a tensile load kept constant over time. The apparatus allows the verification of how the soil can modify the prediction of the long-term behaviour of geosynthetics. Results in terms of confined tensile strains were analysed, and the comparison of those values with the strains obtained by in-air tensile creep tests has led to the conclusion that the creep reduction factor might be conservative. Moreover, the confined tensile strains were related to the apparent coefficients of friction to propose a new procedure capable of determining the design interaction parameter under long-term pullout load as a function of the allowable reinforcement strains.

Elastin levels are higher in healing tendons than in intact tendons and influence tissue compliance.

Elastic fibers containing elastin play an important role in tendon functionality, but the knowledge on presence and function of elastin during tendon healing is limited. The aim of this study was to investigate elastin content and distribution in intact and healing Achilles tendons and to understand how elastin influence the viscoelastic properties of tendons. The right Achilles tendon was completely transected in 81 Sprague-Dawley rats. Elastin content was quantified in intact and healing tendons (7, 14, and 28days post-surgery) and elastin distribution was visualized by immunohistochemistry at 14days post-surgery. Degradation of elastin by elastase incubation was used to study the role of elastin on viscoelastic properties. Mechanical testing was either performed as a cyclic test (20× 10N) or as a creep test. We found significantly higher levels of elastin in healing tendons at all time-points compared to intact tendons (4% in healing tendons 28days post-surgery vs 2% in intact tendons). The elastin was more widely distributed throughout the extracellular matrix in the healing tendons in contrast to the intact tendon where the distribution was not so pronounced. Elastase incubation reduced the elastin levels by approximately 30% and led to a 40%-50% reduction in creep. This reduction was seen in both intact and healing tendons. Our results show that healing tendons contain more elastin and is more compliable than intact tendons. The role of elastin in tendon healing and tissue compliance indicates a protective role of elastic fibers to prevent re-injuries during early tendon healing. PLAIN LANGUAGE SUMMARY: Tendons transfer high loads from muscles to bones during locomotion. They are primarily made by the protein collagen, a protein that provide strength to the tissues. Besides collagen, tendons also contain other building blocks such as, for example, elastic fibers. Elastic fibers contain elastin and elastin is important for the extensibility of the tendon. When a tendon is injured and ruptured the tissue heals through scar formation. This scar tissue is different from a normal intact tendon and it is important to understand how the tendons heal. Little is known about the presence and function of elastin during healing of tendon injuries. We have shown, in animal experiments, that healing tendons have higher amounts of elastin compared to intact tendons. The elastin is also spread throughout the tissue. When we reduced the levels of this protein, we discovered altered mechanical properties of the tendon. The healing tendon can normally extend quite a lot, but after elastin removal this extensibility was less obvious. The ability of the healing tissue to extend is probably important to protect the tendon from re-injuries during the first months after rupture. We therefore propose that the tendons heal with a large amount of elastin to prevent re-ruptures during early locomotion.

Axial Impact Load of a Concrete-Filled Steel Tubular Member with Axial Compression Considering the Creep Effect.

The dynamic loads acting on concrete-filled steel tubular members under axial impacts by rigid bodies were studied herein by FEM. The whole impact process was simulated and the time history of the impact load was obtained. The effects of eight factors on the axial impact load were studied; these factors were the impact speed, mass ratio, axial pressure ratio, steel ratio, slenderness ratio, concrete strength, impact position, and boundary conditions. Besides this, the effects of concrete creep on the impact load were also considered by changing the material parameters of the concrete. The results show that axial impact load changes with time as a triangle. The peak value of impact load increases and the impact resistance improves with the growth of the axial pressure ratio, steel ratio, slenderness ratio, and concrete strength after creep occurs. As the eccentricity of the axial impact acting on a concrete-filled steel tubular member increases, the peak value of the impact load decreases. The enhancement of constraints at both ends of the member can improve the impact resistance. The creep reduction coefficients for the peak axial impact load of a concrete-filled steel tubular member under axial compression and considering the creep effect over 6 months and 30 years are 0.60 and 0.55, respectively. A calculation formula for the peak value of impact load was suggested based on the existing formula, and its accuracy was proved by finite element calculation in this study.

Research on the Long-term Tensile Strength of Geogrid in Reinforced Soil Structure

The long-term tensile strength of reinforcement in geogrid reinforced soil structure is an important design parameter. The reduction factors of geogrid, such as installation damage, creep, chemical and micro-biological degradation during construction and operating in the soil must be considered to decide the long-term tensile strength. According to the lab test results, the strength of geogrid in soil and air is compared, and the tensile properties of HDPE geogrid with different strain rates have been studied. In addition, the installation damage reduction factors in five poorly-graded granular fill have been put forward. The creep test method and creep reduction factor have been raised. Finally, the durability reduction factor is discussed in this paper. These results can be used as a reference for future design of geogrid reinforced soil structure.

Extended Microprestress-Solidification Theory for Long-Term Creep with Diffusion Size Effect in Concrete at Variable Environment

The solidification theory has been accepted as a thermodynamically sound way to describe the creep reduction due to deposition of hydrated material in the pores of concrete. The concept of self-equilibrated nanoscale microprestress has been accepted as a viable model for marked multi-decade decline of creep viscosity after the hydration effect became too feeble, and for increase of creep viscosity after any sudden change of pore humidity or temperature. Recently, though, it appeared that the original microprestress-solidification theory (MPS) predicts incorrectly the diffusion size effect on drying creep and the delay of drying creep behind drying shrinkage. Presented here is an extension (XMPS) that overcomes both problems and also improves a few other features of the model response. To this end, different nano- and macro-scale viscosities are distinguished. The aforementioned incorrect predictions are overcome by a dependence of the macro-scale viscosity on the rate of pore humidity change, which is a new feature inspired by previous molecular dynamics (MD) simulations of a molecular layer of water moving between two parallel sliding C-S-H sheets. The aging is based on calculating the hydration degree, and the temperature change effect on pore relative humidity is taken into account. Empirical formula for estimating the parameters of permeability dependence on pore humidity from concrete mix composition are also developed. Extensive validations by pertinent test data from the literature are demonstrated.

Evaluation of Creep Reduction Factor for Geosynthetic Strip Reinforcement with Folding Grooves

Evaluation of Creep Reduction Factor for Geosynthetic Strip Reinforcement with Folding Grooves

Prediction of creep behaviour from load relaxation behaviour of polymer geogrids

Evaluation of creep behaviour of polymer geosynthetic reinforcement is important for predicting the long-term performance of geosynthetic-reinforced soil (GRS) structures and determining the creep reduction factor of geosynthetic reinforcement for the design lifetime of a GRS structure. The inherent link between the creep behaviour and the stress relaxation behaviour was confirmed by performing a comprehensive series of sustained loading (SL) tests and stress relaxation (SR) tests and numerical simulations of SL and SR tests on several geogrid types. In the framework of the non-linear three-component (NTC) model, the creep strain in SL is entirely irreversible, while the tensile load decrement in SR is associated with the developed positive irreversible strain with zero total strain rate. Measured systematic relationships between the elapsed times necessary to reach the same irreversible strain rate in SL and SR tests and those between the irreversible strain increments necessary to reach the same irreversible strain rate in SL and SR tests are presented and simulated by the NTC model. It is shown that, based on these relationships, the time history of creep strain can be predicted from given results of SR tests performed for a much shorter period. This method is validated by successful simulations of experimental results.

Reducing Tram Car's Curve-Pasing Resistance by Double Treaded Wheel Profile

Special attention is taking place in the environment of public transport, for which higher amount of small radius curves being applied is specific. The outcome of such operation of vehicles is an increase in vehicle's effects on the track in the rail-wheel contact resulting in increased ride resistance, creep in the rail-wheel contact patch, speeding up the process of wear in the contact pair as well as noise generation. At present, a variety of technical solutions for the vehicle bogie design as well as track designs focused on decreasing of these negative effects exists. Their use in smaller radius curves however, cannot give acceptable results and often causes complications in bogie design. The authors give a concept of creep reduction in rail-wheel contact by using double wheel tread, which doesn't require complicated bogie design. The proposed solution is supported by dynamical simulation of the tram car vehicle ride with considering of active wheel tread changes, and is also registered under Patent Application Nr. a201701589.

Briefing: Efficiency of stone-column groups installed in very soft clays

Field measurements from stone-column groups installed in very soft clays show that there is a direct relationship between the stress split between columns and soil and the column spacing/diameter ratio, with the proportion of stress carried by the columns decreasing with decreasing spacing/diameter ratio. This results in the settlement reduction achieved in practice falling increasingly short of theoretical predictions with increasing stone replacement. Comparison of the theoretical variation in column efficiency with stone replacement with that achieved in practice enables the effect of column installation to be quantified, demonstrating that the efficiency of stone columns in very soft clays, and thus the amount of settlement reduction achieved, is primarily dependent on the amount of installation-induced soil disturbance. Theoretical methods are therefore inappropriate for predicting settlement reduction and a practical upper-bound settlement reduction/stone replacement relationship is proposed for design purposes. Corresponding creep reduction factors, to be used in conjunction with the proposed primary settlement reductions, are presented, suggesting that stone columns are unlikely to be effective in soils for which creep is the major component of settlement.

Effect of limestone powder on creep of high-strength concrete

Effect of limestone powder on creep characteristics and internal relative humidity of high-strength concrete has been investigated. The results indicate that limestone powder can reduce creep of concrete, with a greater reduction in creep at higher replacement levels. Limestone powder delays the variation of internal relative humidity of concrete, and there exists a good linear relationship between creep and internal relative humidity of concrete, so creep can be predicted according to internal relative humidity. The creep prediction data of CEB-FIP 90 model obviously overestimate the test data of high-strength concrete with limestone powder always.

Confined-Accelerated Creep Tests to Determine the Creep Reduction Factor

The creep characteristics of geogrids is one of the most important factors affecting its permanent in reinforced soil structures. currently, most of the creep tests are under unconfined conditions, the results deviates significantly from the real status in the application . And conventional creep tests are time-consuming and costing.In this paper, time-temperature superposition principle of geosynthetics is used to accelerate creep and shorten test time. This paper presents a new device to perform simultaneously confined and accelerated creep tests on HDPE50 geogrid . According to the results and time-temperature superposition principle of geosynthetics,the master curves are obtained . Finally the creep reduction factor of confined and unconfined conditions is analysed in the actual project .

Creep behaviour of HDPE/wood particle composites

The effect of particle type, size, content and manufacturing process on the creep behaviour of wood particles/High Density Polyethylene (HDPE) composites has been investigated. Short-term creep tests at different temperatures were carried out and modelled using the Burger's model and the Findley power law. The creep of the composites was found to increase with temperature due to the mobility of the amorphous bulk and tie HDPE molecules. Increased wood particle content generally decreased the creep level. Jack pine composites exhibited the highest creep reduction due to the chemical composition of the fibres surface and the efficiency of adhesion mechanism between fibres and the HDPE. Injection and compression processes led to better creep behaviour than the extrusion process due to differences in the composites microstructures. Particle size did not show important impacts on the creep properties. Findley power law led to better prediction of long time creep behaviour of the composites.

Experimental and modeling results of creep–fatigue life of Inconel 617 and Haynes 230 at 850 °C

Creep–fatigue testing of Ni-based superalloy Inconel 617 and Haynes 230 were conducted in the air at 850°C. Tests were performed with fully reversed axial strain control at a total strain range of 0.5%, 1.0% or 1.5% and hold time at maximum tensile strain for 3, 10 or 30min. In addition, two creep–fatigue life prediction methods, i.e. linear damage summation and frequency-modified tensile hysteresis energy modeling, were evaluated and compared with experimental results. Under all creep–fatigue tests, Haynes 230 performed better than Inconel 617. Compared to the low cycle fatigue life, the cycles to failure for both materials decreased under creep–fatigue test conditions. Longer hold time at maximum tensile strain would cause a further reduction in both material creep–fatigue life. The linear damage summation could predict the creep–fatigue life of Inconel 617 for limited test conditions, but considerably underestimated the creep–fatigue life of Haynes 230. In contrast, frequency-modified tensile hysteresis energy modeling showed promising creep–fatigue life prediction results for both materials.

감소계수 상호영향을 고려한 지오그리드의 장기성능 해석

지오그리드의 장기허용강도를 산출할 때 사용되는 총 감소계수는 내시공성 감소계수(<TEX>$RF_{ID}$</TEX>), 내화학성 감소계수(<TEX>$RF_D$</TEX>), 크리프 감소계수(<TEX>$RF_{CR}$</TEX>) 등이 적용된다. 지오그리드의 단기인장강도에 대한 감소계수를 고려한 허용인장강도 산출 모델의 경우 감소계수들 사이의 상호 작용력을 고려하지 않는 한계를 가지고 있다. 접점강도는 인장강도와 마찬가지로 시공 시 손상이나 화학적 분해에 의하여 감소하게 된다. 기존의 단일접점강도 시험 방법은 치수효과를 고려할 수 없기에 결과의 편차가 큰 시공 시 손상된 시험편의 접점강도를 측정하는데 적합하지 않다. 또한 시공 시 손상에 의한 전단강도 변화에 대한 연구도 전혀 이루어지지 않은 실정이다. 따라서 본 연구에서는 다양한 조건을 고려하여 지오그리드의 장기성능에 영향을 미치는 감소계수들을 재평가하고 감소계수 사이의 상호 작용을 고려하여 정확한 장기허용강도를 구하려고 한다. 내시공성 시험과 내화학성 시험 후 크리프 시험결과 총 감소계수는 GRI GG-4 시험값보다 작게 나타났다. 내시공성 시험과 내화학성 시험 후 접점강도의 감소계수는 인장강도 감소계수보다 더 작게 나타났다. 내시공성 시험후 전단강도 차이가 나타나지 않거나 증가함을 나타내었다. Total reduction factor that is used when calculating allowable tensile strength of geogrids is made by multiplying the installation damage reduction factor (<TEX>$RF_{ID}$</TEX>), chemical degradation reduction factor (<TEX>$RF_D$</TEX>), and creep reduction factor (<TEX>$RF_{CR}$</TEX>) etc. In case of a model estimating allowable tensile strength considering reduction factor over the short-term tensile strength of geogrids, it has a limit of not considering interaction force between reduction factors. Junction strength comes to be reduced by installation damages or chemical degradation in the same way as tensile strength. Single junction test method cannot properly test damaged samples and shows large deviations as it does not consider scale effect. Besides, regarding calculating shear strength, no reasonable study on reduction factors was conducted yet. Therefore, in this study, reduction factors that may affect the long-term performance of geogrids were revaluated considering various conditions and accurate long-term allowable tensile strength was calculated considering interrelation between reduction factors. Creep results after installation damage and chemical resistance test showed lower value than calculated value according to GRI GG-4. After the installation damage test and the chemical resistance test, the reduction factor of junction strength was less than that of tensile strength. Shear strength before and after installation damage showed no change or increase.

New Open Loop Control Improves Linearity of Piezoelectric Actuators

The piezoelectric material is subject to hysteresis and creep resulting in a nonlinear relationship between the applied voltage and the output mechanical displacement. An approach for compensation of both the hysteresis and creep characteristics of a piezoelectric stack actuator is proposed by utilizing a switched capacitor charge pump. The new charge pump transfers the same amount of charges to the piezoelectric actuator quantitatively, and the actuator will be excited to change its length with constant step. Compared with voltage driving mode, experiments show that both creep and hysteresis of the piezoelectric stack driven by the charge pump are effectively reduced. A hysteresis reduction of 86.10% at 0.01Hz and 94.36% at 5Hz is achieved. At the maximum driving voltage, a creep reduction of 77% is obtained.

An Interlocking Ligamentous Spinal Disk Arthroplasty with Neural Network Infrastructure

An elastomeric spinal disk prosthesis design (BioFI™) with vertebral interlocking anchors has been modified using an embedded TiNi wire array. Bioinert styrenic block copolymer (Kraton®) and polycarbonate urethane (Bionate®) thermoplastic elastomer (TPE) matrices were utilized. Fatigue resistant NiTi wire was pretreated to induce superelastic martensitic microstructure. Stent-like helical structures were produced for incorporation within homogenous TPE matrix. Composite prototypes were fabricated in a vacuum hot press using transfer moulding techniques. Implant prototypes were subject to axial compression using a BOSE ® ELF3400. The NiTi reinforced implants exhibited reduction in axial strain, compliance, and creep compared to TPE controls. The axial properties of the NiTi reinforced Bionate® BioFI™ implant best approximated those of a spinal disk followed by Kraton®-NiTi, Bionate® and Kraton® prototypes. An ovine lumbar segment biomechanical model was used to characterize the disk prosthesis prototypes. Specimens were subject to 7.5Nm pure moments in axial rotation, flexion-extension and lateral bending with a custom jig mounted on an Instron® 8874. The motion preserving ligamentous nature of this arthroplasty prototype was not inhibited by NiTi reinforcement. Joint stiffness for all prototypes was significantly less than the intact and discectomy controls. This was due to lack of vertebral anchor rigidity rather than BioFI™ motion segment matrix type or reinforcement. Implant stress profiles for axial compression and axial torsion conditions were obtained using finite element methods. The biomechanical testing and finite element modelling both support existing BioFI™ design specifications for higher modulus vertebral anchors, endplates and motion segment periphery with gradation to a low modulus core within the motion segment. This closer approximation of the native spinal disk form translates to improvements in prosthesis biomechanical fidelity and longevity. Axial compressive strain induced within a TiNi reinforced Kraton® BioFI™ was found to be linearly proportional to the NiTi helical coil electrical resistance. This neural network capability delivers opportunities to monitor and telemeterize in situ multiaxis joint structural performance and in vivo spine biomechanics.