Creep Values Research Articles

Experimental Study on Creep Characteristics of Microdefect Articular Cartilages in the Damaged Early Stage.

Traumatic joint injury is known to cause cartilage deterioration and osteoarthritis. In order to study the mechanical mechanism of damage evolution on articular cartilage, taking the fresh porcine articular cartilage as the experimental samples, the creep experiments of the intact cartilages and the cartilages with different depth defect were carried out by using the noncontact digital image correlation technology. And then, the creep constitutive equations of cartilages were established. The results showed that the creep curves of different layers changed exponentially and were not coincident for the cartilage sample. The defect affected the strain values of the creep curves. The creep behavior of cartilage was dependent on defect depth. The deeper the defect was, the larger the strain value was. The built three-parameter viscoelastic constitutive equation had a good correlation with the experimental results and could predict the creep performance of the articular cartilage. The creep values of the microdefective cartilage in the damaged early stage were different from the diseased articular cartilage. These findings pointed out that defect could accelerate the damage of cartilage. It was helpful to study the mechanical mechanism of damage evolution.

Investigation of adhesion recovery phenomenon using a scaled roller-rig

When high creep values occur on a braked/trailing wheelset running on a contaminated track, changes in the friction conditions take place. In fact, the work of the friction forces gradually removes the contaminant from the wheel and rail surfaces. Therefore, lower levels of adhesion are present on the leading wheelsets, while better conditions are restored on the following ones. A deep understanding of this phenomenon is needed to develop new algorithms for WSP and antiskid systems, in order to maximise the tractive/braking performances of railway vehicles. The research group of the Politecnico di Torino has recently designed an innovative multi-axle roller-rig to investigate the adhesion recovery phenomenon. First, the paper briefly shows the configuration of the new roller-rig, then focus is given to the experimental results obtained from adhesion recovery tests performed on the test bench.

Prediction Adequacy of Creep of Recycled Concrete Influenced by the Water Absorption in Recycled Aggregates

Abstract A recycled concrete aggregate (RCA) has much higher water absorption content compared to a natural aggregate and could significantly influence the creep properties and durability of concrete. This article investigates the influence of RCA and its water absorption content on the creep properties of concrete and develops a model to predict the creep of recycled aggregate concrete (RAC). Two types of RCA obtained from the lab and field with different water absorption contents were used to prepare RAC. RCAs were mixed with natural aggregates at four replacement ratios (0, 50 %, 70 %, and 100 %). Creep and shrinkage tests were conducted to measure the specific total creep, specific basic creep, coefficients of total creep and basic creep, and shrinkage of sealed and unsealed concrete specimens subjected to sustained compression loads. The existing total creep model was modified by considering the influence of water absorption of residual mortar in RCAs, and a new model was calibrated and verified with experimental data. The results show that both the total and basic creep of RAC increased with the RCA replacement ratio and the water absorption of RCA. The total creep values of RAC were much higher than the basic creep values of RAC. Type II RCA, with RCA that had a higher water absorption, produced higher creep values than Type I RCA. The modified total creep model presented high accuracy in predicting the total creep of RAC, and thus may be potentially used to predict total creep of RAC when mixed with dry RCAs in practice.

The influence of hygrothermal ageing on creep behavior of nanocomposite adhesive joints containing multi-walled carbon nanotubes and graphene oxide nanoplatelets

The effect of hygrothermal ageing on the creep behavior of multi-walled carbon nanotube (MWCNT) and graphene oxide nanoplatelet (GONP)-reinforced adhesive joints was investigated. The neat, MWCNT and GONP-reinforced adhesive single lap joints were manufactured and immersed in hot deionized water with three different temperatures for 24 h and then tested under creep loading. The results showed that the elastic and creep shear strain values of the neat adhesive joints increased by 14% and 25%, respectively, when the water temperature was increased from 30 to 50 °C. It was found out that 0.1 wt% MWCNTs had the maximum reinforcing effect against the creep behavior of adhesive joints pre-aged in hot water by 56% and 33% reductions in the elastic and creep strain values of the nanocomposite adhesive joints compared to the neat adhesive joints. Whereas, GONPs caused the maximum reductions of 45% and 20% in the elastic and creep strains of the nanocomposite adhesive joints compared to the neat joints. Furthermore, the Burgers rheological model was employed for simulating the creep response of adhesive joints. Semi-empirical models were proposed for the elastic and creep strains and the Burgers model parameters as functions of the water temperature and MWCNT/GONP weight percentage using the response surface methodology.

Influence of Different Connection Types on Mechanical Behavior of Girder Trusses

The static test of two single light wood trusses and two girder trusses were carried out to explore the performance enhancement effect of girder trusses and the influence of different connection types on the mechanical behavior, which was studied through the comparison of the test results of four trusses. It is found that the girder trusses can show better load-carrying capacity and anti-deforming ability compared with which simply increases the cross-section size of the members. Because of the advantage of the connection materials and the non-fastening connection mechanism, the wood pin-connected truss has more ultimate bearing capacity which is 2.6 times of single truss, and better deformation resistance which is only 66.2% of the nailed one’s creep value.

Fatigue resistance of a simulated single LOCATOR overdenture system

Statement of problemThe incidence of fracture in a single-implant overdenture base increases in the region adjacent to the fulcrum implant. PurposeThe purpose of this in vitro study was to evaluate the effect of bidirectional woven electrical glass (E-glass) fiber reinforcements on the fatigue resistance of a simulated single LOCATOR–retained overdenture. Material and methodsTest specimens with a centrally positioned metal housing for a LOCATOR stud attachment were fabricated from autopolymerizing acrylic resin. Specimens for the control group were fabricated without glass fiber reinforcements. The 4L group specimens had 4 layers of E-glass fiber weaves and were divided according to the fiber location into the following 3 subgroups: 4L-A with 4 fiber layers above the metal housing; 4L-N with 4 fiber layers adjacent to the metal housing; and 4L-A+4L-N with 4 fiber layers above and 4 fiber layers adjacent to the housing. Specimens were stored in distilled water for 1 week at 23 °C before cyclic fatigue testing at 10 000 cycles by using a staircase approach (n=12). The results were analyzed with 1-way ANOVA and the Tukey multiple comparisons post hoc analysis (α=.05). A 2-way ANOVA (α=.05) was conducted to detect the effect of fatigue cyclic loading and the position of the fiber layers and their interaction on the fatigue resistance. ResultsThe results of the investigated compressive fatigue limits for the test groups were 190 ±15.9 N for the control group, 265 ±15.9 N for the 4L-A subgroup, 220 ±15.9 N for the 4L-N subgroup, and 275 ±15.9 N for the 4L-A+4L-N subgroup. A nonsignificant difference was found for creep values between the control group and reinforced subgroups (P>.05). The postfatigue flexural strength values in the 4L-A and 4L-A+4L-N subgroups were significantly higher than those in the control group (P<.001) and the 4L-N subgroup (P=.004 and P=.005). However, no significant difference was found in postfatigue flexural strength between the control group and the 4L-N subgroup (P=.828). ConclusionsPlacing 4 layers of bidirectional E-glass fiber weaves above the metal housing can increase the fatigue resistance and the postfatigue flexural strength of single LOCATOR–retained overdentures.

Investigation of Creep Behaviors of Wudang Mica Schist by Triaxial Compression Tests

Triaxial compression creep tests were conducted on Wudang mica schist samples to clarify the creep anisotropy characteristics. The mica schist samples were divided into parallel group and vertical group according to the angle of axial loading and schistosity plane. The creep tests were set up with step function loading under varied confining pressure conditions. The test results show that mica schist creep have 4 stages: instantaneous deformation stage, decay creep stage, steady creep stage and accelerating creep stage. The deformation is dominated by instantaneous deformation at low levels of stress. Then the creep value continued to increase as the stress grew. In comparison with the vertical group under same confining pressure and similar axial loads, the parallel group presented large instantaneous strain increments and axial creep value, high creep speed and short decay creep time. Experimental results showed that the vertical group was finally destructed in the form of shear fracture and the parallel group was finally destructed in the form of cleavage fracture, which indicated distinct creep anisotropy. A modified burgers creep model was introduced to describe mica schist’s creep characteristics and identify the creep parameters, and the result was proved to be efficient in description of mica schist’s creep features by the fitting results.

Creep prediction with temperature effect and experimental verification of rubber suspension components used in rail vehicles

Rubber anti-vibration components are widely used in rail vehicles as either a primary or a secondary suspension system with long-term service. An essential requirement is to limit the deflections of suspension components not to exceed their structural limitation due to creep in order to avoid early failure over its service life. Temperature is the main environmental factor to account for creep. Traditional hyperelastic approach can only be used for mechanical loading analysis without reference to time. In this article, the primary creep of rubber structures is obtained by introducing time as an additional variable into hyperelastic models. A deviation function from the primary creep due to temperature change is established and also integrated into the proposed model so that the real creep values can be obtained at specified time. The proposed approach has been validated experimentally using a Vee mount, a Metacone component and a Circular mount. It has been shown that real creep value due to temperature change can be obtained from this approach. It is also revealed that the creep due to temperature change is mainly attributed to alternation of the rubber elastic parameters. It is suggested that this approach can be used in a design stage for similar polymer applications.

Monotonic behaviour of typical Al-Cu-Mg alloy pre-strained at elevated temperature

This paper presents results of monotonic tensile and creep tests conducted on typical Al-Cu-Mg alloy (commercial 2024) specimens. Tensile tests carried out at room (20◦C) and elevated (100◦C, 200◦C, 300◦C) temperatures made it possible to determine strength properties of the material (Young’s modulus, yield stress, ultimate tensile strength). Creep tests were performed at elevated temperature (100◦C, 200◦C and 300◦C) with a constant force. In order to obtain material creep characteristics, creep-rupture tests were carried out. Then creep tests were conducted with two different strain values: one corresponding to the beginning of the secondary creep and the other corresponding to a certain value of the tertiary creep. After preliminary creep deformation at two various strain levels, specimens were cooled at ambient temperature and then subjected to monotonic tensile tests. The characteristics of the material were obtained for pre-strained specimens at different tempe- ratures. Specimens fracture surfaces obtained as a result of tensile (at elevated and room temperature), creep and combined tests were analyzed.

Lightweight Aggregate Concrete as an Alternative for Dense Concrete in Post-Tensioned Concrete Slab

The paper presents the results of tests conducted on two lightweight aggregate concretes made of new national Certyd artificial aggregate. This research is intended to first application of lightweight concrete to construct large-span post-tensioned slab. In addition to mechanical properties development, shrinkage and creep during 3 months of loading were tested. The obtained results are compared with theoretical results predicted by standard. Conducted tests indicated, that measured values of shrinkage and creep are significantly lower than predicted ones. This is promise for application of tested concrete in construction of post-tensioned slabs.

Prediction of the Creep of Elastomers Taking into Account the Forces of Entropic Elasticity of Macromolecules (Prediction of Creep of Elastomers)

ABSTRACTAccounting for the increase in the energy of activation of viscous flow of polymers by an amount proportional to the force of entropic elasticity of the stretched macromolecules, allows for a quantitative description of the curves of deformation of elastomers in the models of Kelvin-Voigt and the standard linear solid. For these models, application of a rubber-like elasticity modulus, increasing with the increase of deformation, enables calculation of Flory's correction factor for the network defects for the deformation curves of a rubber. The curves of creep of silicone rubbers under constant load for various temperatures were measured. For the Kelvin-Voigt model differential equations the dependence of the coefficients of linear approximations of these equations on loadings and temperatures were measured. The possibility to predict the time of achieving given creep values based on the proposed Kelvin-Voigt model equations were demonstrated.

Theoretical and numerical analysis of creep crack initiation combined with primary and secondary stresses

In this work, the creep crack initiation time (CCI) considering the residual stress effects were investigated by numerical simulation and theoretical approaches based on the ductility exhaustion model. The redistribution of stress under the combined loading condition (i.e. the tensile residual stress generated by the pre-compressed loads and primary loads) was taken into account. The elastic follow-up factor, Z, as well as the reference stress method, was considered as part of the theoretical method. The suitability of the theoretical method to the conditions that included combined primary and secondary stresses was then investigated. Reasonable and conservative predictions of CCI time could be obtained from the theoretical solutions when the elastic follow-up factor, Z, was taken into account within the K-Riedel-Rice (K-RR) controlled model (initiation initially under the elastic K field and then under transient creep stress conditions, the Riedel-Rice (RR)) and the Hutchinson-Rice-Rosengren-Riedel-Rice (HRR-RR) controlled model (initiation initially under the elastic-plastic Hutchinson-Rice-Rosengren (HRR) field and then under RR controlled stress field) compared with finite element (FE) solutions. K-RR was more suitable when the primary stress intensity factor KIP was below 15 MPa m1/2, and HRR-RR was more accurate when KIP was beyond 15 MPa m1/2. However, RRss (initiation under the steady state creep) values were much higher than the FE solutions, but K (initiation under the elastic stress intensity factor K-controlled stress field) controlled values seriously underestimated the initiation times. HRR (initiation under the elastic-plastic HRR-controlled stress field) values underestimated the initiation time when load level KIP was less than 27 MPa m1/2, but slightly overestimated when KIP was larger than 27 MPa m1/2. The HRR-RR and HRR solutions were much close and may be a little unconservative at the high load levels (KIP > 27 MPa m1/2). The CCI time including the elastic follow-up factor Z was much more accurate than when ignoring the residual stress.

Short-term environment-dependent creep and shrinkage of mature concrete

The objective of this research was to experimentally assess the relative significance of short-term environment-dependent creep and shrinkage strains on the relief of temperature-induced restraint stresses in bridges. Although substantial theoretical and experimental works have been performed on long-term creep and shrinkage effects under varying environmental conditions, controlled experimental works on short-term creep and shrinkage effects under realistic outdoor ambient conditions are lacking. In this study, multiple short-duration tests were performed on concrete cylinders (in Chicago, IL, USA) to evaluate the short-term environment-dependent creep and shrinkage of mature concrete caused by outdoor environmental variations. The results indicate that short-term environment-dependent creep and shrinkage of concrete are each limited to a maximum of approximately 15% of the corresponding changes in temperature strains. However, the values of short-term environment-dependent creep and shrinkage can be high compared with indoor (laboratory) creep and shrinkage strains occurring within the same brief time frame. Environment-dependent creep tends to compensate for an increase in temperature strains. Environment-dependent shrinkage also tends to compensate for an increase in temperature strains, except when concrete temperatures are below the freezing point. Overall, the beneficial influence of short-term environment-dependent creep and shrinkage on daily thermal strains and stresses is relatively small.

EXTENDED KALMAN FILTER DESIGN FOR RAILWAY TRACTION MOTOR

Monitoring the adhesion force between a railway wheel and a rail surface is very essential in maintaining the high acceleration and braking performance of railway vehicles. Due to the difficulties encountered in direct measurement of friction coefficient, creepage and adhesion force, state observers are used as indirect estimation methods. This paper proposes an effective estimation method, which exploits railway traction motor behaviour to give an assistance for realizing wheel slip and adhesion control in order to be used in railway applications. This method plays an active role in optimizing the use of the existing adhesion and reducing wheel wear by decreasing high creep values. With this method, adhesion force can be indirectly estimated by measuring stator currents, and angular speed of the AC traction motor and using dynamic relationships based on the extended Kalman filter (EKF) simulation model. The re-adhesion controller can be designed to regulate the motor torque command according to the maximum available adhesion depending on the estimated results. To test the proposed method, simulations were performed under different friction coefficients.

The influence of the unloading rate on creep recovery of force transducers

This paper presents a study of the effect of unloading rate on the creep recovery of force transducers. The creep values increase with increase loading rate and this increasing phenomenon follows a Natural Logarithm function. The creep is highly dependent on the unloading rate from 0.5 mm/min to 5 mm/min while above 5 mm/min up to 20 mm/min the creep values are less dependent. An equation represents the time-loading/unloading rate effect on the creep/creep recovery of the force transducers, is introduced.

Bayesian approach for quantifying silica fume and water reducer impact on creep of concrete

The behaviour of concrete structures is affected by the time-dependent volume changes of concrete caused by sustained loads and moisture loss. Creep deformations have an important effect on long-term integrity of reinforced concrete structures. Incorrect or inaccurate prediction of these volume changes leads to undesirable and harmful consequences for the structures. Knowing that the Eurocode 2 does not take admixtures and additives into consideration in predicting creep deformations, the aim of this study is to apply correction coefficients to the Eurocode 2 creep formulas to take into account the effect of additives and admixtures, especially silica fume and water reducer. A large database from international laboratories and research centres is used in this study. The first step consists of comparing experimental measurements of creep deformations with creep values predicted by the Eurocode 2 formulas. The second step lies in introducing correction coefficients to the creep formulas of Eurocode 2, taking into consideration the percentage of admixtures and additives. These new correction coefficients are introduced in the Eurocode 2 formulas using an approximate Bayesian computation method based on a rejection algorithm.

Relaxation of Structural Concrete due to its Shrinkage in Terms of Age-Adjusted Effective Modulus Method

This paper focuses on the calculation of residual stresses due to shrinkage with a tensile creep effect. Whereas the shrinkage of concrete causes stresses in the material, the tensile creep counteracts the shrinkage as a stress relaxation mechanism. The main objective of this paper is to evaluate the ageing coefficient c (referred to as Trost-Bazant Coefficient) reflecting the load history. The coefficient is used for the residual stress analysis by means of a simplified method called Age-adjusted Effective Modulus Method. The tensile creep effect was evaluated according to the rheological model provided by Eurocode 2. Although the Eurocode predicts the creep for the structural members subjected to compressive stresses, this study proves that it can be used for the tensile creep prediction as well. We tested three types of concrete: reference concrete, high-performance concrete with reduced shrinkage magnitude by means of special admixtures, and fibre concrete with the content of polypropylene fibres. From the obtained results, it can be stated, that the ageing coefficient can be considered to be the value of 0.45 for any shrinkage development. It was also proved, that the tensile creep value essentially affects the magnitude of residual stresses, even in the “early age” concrete. The correctness of the calculated residual stresses was verified by means of a Ring-test.

Research on the Relative Stable Time and Stable Value of Concrete Creep

Abstract The safety coefficients of structures change due to the creep behavior of concrete, which causes increased deformation and cracks in structures, redistribution of internal stresses, and the loss of prestress. It is an important and significant problem to determine when concrete creep is stable and how much of the creep will occur once it is stable over the projected life of the structure. The relationship between compressive concrete creep and time was obtained during creep experiments, which were conducted using the compressive concrete cube with stress levels of 0.160, 0.261, 0.370, 0.470, 0.568, and 0.668, a loading age of 126 days, and a time under loading of 589 days. The creep compliance function model of linear and non-linear compressive concrete creep under stress levels lower than 0.8 was unified by introducing a creep compliance function amplification coefficient Aj. Relative stable time and relative stable value of concrete creep, with different stress levels, were obtained according to the creep model and regular relative stable time.

Estabilidad física de bases emulsionadas e hidrosolubles con quitosana y acetato de quitosana

Context: Chitosan has received great attention because it is a functional, biodegradable, renewable and non-toxic biopolymer with multiple pharmaceutical applications, including stabilizing agent. Aims: To evaluate the physical stability of emulsified bases and hydrophilic gels containing chitosan or chitosan acetate as stabilizing agents. Methods: Stability of shelf-life formulations at room temperature and in refrigeration was evaluated over a period of 60 days and by thermal stress testing and centrifugal destabilization. The organoleptic characteristics, pH, conductivity and flow behavior were evaluated, the latter through the analysis of the rheograms, the determination of rheological parameters (consistency index, apparent viscosity, creep value and flow index), as well as their comparison statistics. The possible correlations between these parameters and the concentration of the biopolymers were also evaluated. Results: The bases elaborated with chitosan or its soluble derivative showed adequate physical stability during the study time. The effect of the storage temperature, as well as the type and concentration of the stabilizing agent used was evidenced. Emulsifier combinations provided less stability. A linear correlation between the rheological parameters and the biopolymer concentration was evidenced. Conclusions: Chitosan and chitosan acetate can be used as emulsifying agents in semi-solid and gelling bases in hydrophilic gels, due to the electrostatic stabilization and the viscosity they contribute to the system in relation to its concentration.

Computer Simulation of Porous Material Plastic Deformation

This paper reports on a computer simulation of plastic deformation of refractory material porous billets when rolling in four-roll-pass mills. This model allows calculating the geometrical, kinematic, and energy parameters of the rolling process accounting for the preset law of porous billet compacting in the deformation zone. The theoretical study of porous billet rolling process using the mathematical model has proved the decrease of billet elongation compared with that of non-porous ones. The computer analysis has shown that in the process of rolling falling of creep value, the rolling force and rolling moment take place. The experimental test of the computer simulation results has shown a good correspondence of main rolling process parameters.