Creep Conditions Research Articles

Numerical implementation and application of an internal state variable model to analyze the time-dependent behavior of mining excavations in rock salt

The geomechanical behavior of rock salt has been investigated extensively over the years. Experimental studies have identified distinctive features associated with nonlinear inelastic response that shows strong time and loading history dependencies. In this paper, a relatively simple constitutive model is presented and implemented into the numerical code FLAC, and then applied to analyze the time-dependent behavior of excavations in salt mines. The unified creep-plasticity model includes an internal state variable (ISV) with an evolution law that induces progressive strain hardening (SH) until a steady state is reached. Numerical analyses are performed with the proposed ISV–SH model to evaluate material parameters, based on creep test results on natural rock salt, and to simulate the response of a circular opening and of rectangular mining excavations created in sequence. The results are analysed and compared with those obtained with the well-known Norton power law equation, commonly used in salt mine engineering. The time-dependent calculation results illustrate key aspects of rock salt behavior and highlight the major influence of transient inelastic behavior and stress redistribution on the response of underground openings. The proposed modelling approach constitutes an advantageous alternative to analyses based on stationary creep laws, often used in rock salt mining operations.

Characterization of creep constraint effects on creep crack growth behavior by Q-type parameters

The effect of creep constraint on creep crack growth behavior and the stress field around the creep crack-tip was studied under different specimen geometry conditions. The correlation between the constraint parameters Q and Qm with the stress triaxiality under creep conditions was investigated. The results showed that the Q and Qm depend linearly on the stress triaxiality. Moreover, based on the existing constraint parameters Q∗ and Qm, a new load-independent constraint parameter Qm∗ was proposed to quantify the creep constraint effect. The Qm∗ was superior to Q∗ in quantifying higher out-of-plane creep constraints induced by specimen width and thickness. Furthermore, the Q∗ and Qm∗ were employed to correlate with the normalized creep crack growth rate and initiation time. Finally, the constraint-dependent creep crack growth prediction model based on constraint parameters Q∗ and Qm∗ had been established.

Static Bending Creep Properties of Glass Fiber Surface Composite Wood

To study the static bending creep properties of glass fiber reinforced wood, glass fiber reinforced poplar (GFRP) specimens were obtained by pasting glass fiber on the upper and lower surfaces of Poplar (<i>Populus euramevicana</i>, P), the performance of Normal Creep (NC) and Mechanical Sorptive Creep (MSC) of GFRP and their influencing factors were tested and analyzed. The test results and analysis show that: (1) The MOE and MOR of Poplar were increased by 17.06% and 10.00% respectively by the glass fiber surface reinforced composite. (2) The surface reinforced P with glass fiber cloth only exhibits the NC pattern of wood and loses the MSC characteristics of wood, regardless of the constant or alternating changes in relative humidity. (3) The instantaneous elastic deformation, viscoelastic deformation, viscous deformation and total creep deflection of GFRP are positively correlated with the stress level of the external load applied to the specimen. Still, the specimen’s creep recovery rate is negatively correlated with the stress level of the external load applied to the specimen. The static creep deflection and viscous deformation of GFRP increase with the increase of the relative humidity of the environment. (4) The MSC maximum creep deflection of GFRP increased by only 7.41% over the NC maximum creep deflection, but the MSC maximum creep deflection of P increased by 199.25% over the NC maximum creep deflection. (5) The Burgers 4-factor model and the Weibull distribution equation can fit the NC and NC recovery processes of GFRP well.

РАСЧЕТНО-ЭКПЕРИМЕНТАЛЬНАЯ ОЦЕНКА РЕСУРСА КОМПОЗИТНОГО БАКА ВЫСОКОГО ДАВЛЕНИЯ В УСЛОВИЯХ ПОЛЗУЧЕСТИ

Numerical analysis and experimental results are employed to predict the long-term strength of a metal-composite high pressure vessel (MCHPV) used to store fuel in spacecraft electric propulsion systems. The research results indicate that the considered MCHPV has high resource indicators. This makes it possible to ensure the required characteristics of the vessel resource during a given service life under creep conditions in the material of load-bearing shell. A conclusion is made about the possibility for further optimization of the MCHPV structure and increasing its weight efficiency.

Thermal Creep on Mars: Visualizing a Soil Layer under Tension

At low ambient pressure, temperature gradients in porous soil lead to a gas flow called thermal creep. In this regard, Mars is unique as the conditions for thermal creep to occur in natural soil only exist on this planet in the solar system. Known as a Knudsen compressor, thermal creep induces pressure variations. In the case of Mars, there might be a pressure maximum below the very top dust particle layers of the soil, which would support particle lift and might decrease threshold wind velocities necessary to trigger saltation or reduce angles of repose on certain slopes. In laboratory experiments, we applied diffusing wave spectroscopy (DWS) to trace minute motions of grains on the nanometer scale in an illuminated simulated soil. This way, DWS visualizes pressure variations. We observe a minimum of motion, which we attribute to the pressure maximum ∼2 mm below the surface. The motion above but especially below that depth characteristically depends on the ambient pressure with a peak at an ambient pressure of about 3 mbar for our sample. This is consistent with earlier work on the ejection of particle layers and is in agreement with a thermal creep origin. It underlines the supporting nature of thermal creep for particle lift, which might be especially important on Mars.

An Orthotropic Nonlinear Thermoviscoelastic Model for Polymeric Battery Separators

Structural responses of separators are critical to battery safety. Commonly used separators are dry-processed polymers with anisotropic microstructures. Due to the material anisotropy, rate dependence, and temperature dependence, developing a model for predicting the thermomechanical response of polymeric battery separators has been challenging. This paper presents an orthotropic nonlinear thermoviscoelastic model for predicting the response of polymeric battery separators in thermal ramp scenarios. The model was based on the Schapery nonlinear viscoelastic model and the temperature dependence was introduced through the time-temperature superposition principle (TTSP). The model was implemented in LS-DYNA® finite element (FE) package as a user-defined material model. The model parameters were determined for a polypropylene (PP) separator. The predicted material responses under large deformations in isothermal and non-isothermal temperature conditions for stress relaxation, creep and tensile loadings at different rates agree well with the experimental data.

Study on Mechanical Properties of Simply-Supported Composite Beams Considering Creep and Slip

In the current industry, steel–concrete composite beams are used in large-span bridges and super-high-rise building structures due to their excellent overall performance. Concrete’s creep and slip effects in the combined structure can adversely affect the structure, thus affecting the safe use of bridges and buildings. It is necessary to study the mechanical properties of the combined structure considering creep and slip. In order to further study the mechanical properties of steel–concrete composite beams under creep–sliding coupling, in this study, based on the energy variational method principle, the energy equation of a composite beam considering creep and slip coupling is established. The second-order differential equation of the axial force of steel–concrete composite beams is derived by introducing basic assumptions. The calculation formulas for the axial force, deflection, and slip of simply-supported composite beams under different loads are obtained using different boundary conditions. Then, the creep effect of composite beams is simulated using the creep criterion in the ANSYS finite element software when the concrete material parameters change with time. The results show that a simply-supported composite beam considering both slip and creep will have a significant effect on the structure; the more strongly the studs constrain the concrete slab, the greater the adverse effect of concrete creep on the combined beam. The formula derived in this paper is consistent with the numerical simulation solution and is suitable for different creep and slip conditions. The research results can provide a theoretical basis for the calculation of the axial force, deflection, and slip of combined beams under uniform and concentrated loads in practical engineering considering slip and creep.

Tensile creep behavior of a heat treated β-solidified γ-TiAl alloy: The development of dynamic recrystallization and the role of B2 phase

The tensile creep behavior of a heat treated β-solidified γ-TiAl alloy has been investigated at 800 °C under 250–300 MPa. Results indicate that the heat treatment procedure proposed in this study can improve creep resistance of the alloy. Combined with kinetic analysis (stress exponent n = 3.36) and TEM observation, it can be inferred that creep deformation is dominated by viscous dislocation glide, and assisted by mechanical twinning. It is found that both twin boundary and TiB can hinder dislocation movement and strengthen creep resistance. During creep, there are obvious dynamic recrystallization (DRX) and B2 phase precipitation. On the one hand, DRX volume fraction increases with the increase of creep stress or the extension of creep time. Meanwhile, TiB can act as nucleation sites of recrystallized γ grains. On the other hand, there are two kinds of B2 phase precipitation behaviors: (ⅰ) thin B2 laths are formed in α2 lamellae; (ⅱ) blocky B2 phases are formed at colony boundary. Creep deformation is mainly completed by γ phase, B2 phase can involve in the deformation only under specific creep conditions. Due to poor deformation ability of B2 phase, creep voids prefer to nucleate at the γ-B2 interface and B2-colony interface. When voids are connected, cracks are formed and begin to propagate, eventually leading to creep failure in brittle-ductile mixed fracture.

The Influence of Mortar's Poisson Ratio and Viscous Properties on Effective Stiffness and Anisotropy of Asphalt Mixture.

This paper presents the results of a research study and analysis conducted to determine the degree of anisotropy of asphalt concrete in terms of its initial elastic properties. The analysis of asphalt concrete was focused on determining the effective constrained stiffness modulus in three mutually perpendicular directions based on the finite element method. The internal structure of the asphalt concrete was divided into the mortar phase and the mineral aggregate phase. Static creep tests using the Bending Beam Rheometer were conducted for the mortar phase to fit the rheological model. The aggregate arrangement and orientation were analysed using an image analytical technique for the mineral phase. The Finite Element Method (FEM) meshes were prepared based on grey images with an assumption of plane strain in 2D formulation. Using the FEM model, the tension/compression tests using selected characteristic directions were conducted, and the effective constrained stiffness moduli were estimated. This study showed a dominant horizontal direction for all coarse aggregates resulting from the normal force of the road roller and paving machines during laying and compaction on a road site. Depending on the values of the mortar's mechanical parameters and the load direction, the effective stiffness modulus might differ by ±20%. Based on the FEM analysis, this result was proven and commented on through an effective directional modulus evaluation and a presentation of internal stress distribution. Depending on the shape and orientation of the aggregates, it was possible to observe local "stress bridging" (transferring stresses from aggregate to aggregate when contacting). Moreover, the rheological properties of the mortar were considered by assuming two limiting situations (instantaneous and relaxed moduli), determining the bands of all possible solutions. In the performed FEM analysis, the influence of the Poisson ratio was also considered. The analysed asphalt concrete tends to be isotropic when the Poisson's mortar ratio is close to the value of 0.5, which agrees with the physical expectations. The obtained results are limited to particular asphalt concrete and should not be extrapolated to other asphalt mixture types without prior analysis.

Rubber Creep Model and Its Influence on Mounting Stiffness

The mount of the engine will creep under the action of long-term load. Creep will change its original structure, resulting in changes in static and dynamic characteristics and fatigue life. In order to solve the problem of mounting rubber creep, the creep characteristics were studied in this paper. In order to study the influence of creep characteristics on engine mounting stiffness, a theoretical model of mount creep was established based on the creep mechanism of rubber. The effects of different loads on the creep characteristics of engine mount were studied. The static viscoelastic parameters and creep analysis of rubber mounting were analyzed numerically. The creep variation law of suspension under different loads is obtained. By analyzing the static and dynamic characteristics of no creep, 2.98 mm creep and 3.83 mm creep of engine mount, the creep characteristics and the variation law of mount stiffness of rubber mount were revealed. The results show that the static stiffness of suspension increases with the increase of creep. When the frequency is constant and the creep increases, the dynamic stiffness of the suspension increases obviously. In this paper, the creep characteristics of rubber mount are analyzed, and the results of the analysis provide a design method for rubber mount design.

Stability and economic evaluation of multi-step horizontal salt caverns with different step distances in bedded salt formations

A multi-step horizontal salt cavern (MSHSC) for energy storage has been drawing more attention, using retractable water-injection tubes to mine larger storage spaces between two boreholes. The retract distance per step best determines the shape of the cavern roof, and thus has an important influence on the stability of MSHSCs. This paper discusses the impact of the step distance on the stability and economy of MSHSCs. The geometry of MSHSCs with different step distances is obtained using our previously developed solution mining model. Geo-mechanical models are developed based on the geometry and the geo-conditions of Jintan salt mine in China. Then a series of static creep analyses are performed using FLAC3D software. Results show that smaller step distances mean higher stability risks during long-term operation. As the step distance is reduced, both the volume shrinkage and plastic zone volume increase. In the displacement contours after 30 years of operation, the large displacement zone increases significantly with decreasing step distance. However, the cavern volume and expected revenue increase with decreasing step distance, contrary to the stability parameters. Economic analysis with integrated stability considerations is conducted to give a balanced step distance. Volume shrinkage is considered in the calculation of effective cavern volume. Plastic zone volume is reflected in the space cost of MSHSCs, since it influences the thickness of the cavern top plate and the pillar width between adjacent caverns. Further, a comprehensive indicator of economic cost/expected revenue per unit volume of rock salt versus time is proposed, calculated, and compared. According to the comparison results, a medium step distance of 100 m or 125 m is recommended for the design of MSHSCs in a given salt layer, with the stability and economic considerations best balanced.

Modeling the Behavior of an Aggregate Skeleton during Static Creep of an Asphalt Mixture Based on a Three-Dimensional Mesoscale Random Model

The aggregate skeleton and its evolution under external loads are crucial to evaluate the mechanical properties of the asphalt mixture. This study developed a method to identify the three-dimensionally (3D) aggregate skeleton based on geometry information. The proposed method can identify the evolution process of the aggregate skeleton during loading in the finite element method (FEM). Random models based on the Gilbert–Johnson–Keerthi (GJK) algorithm are used to study the influence of the concave–convex degree of the gradation curve and the magnitude of the creep load on the aggregate skeleton during the creep process. The results indicate that the aggregate skeleton undergoes three stages during creep: aggregate redistribution, skeleton formation, and skeleton failure after reaching the ultimate load, while the increase of load causes the aggregate skeleton to lose its bearing capacity faster. More coarse aggregate raises the strength of the aggregate skeleton, but it is more likely to break down quickly once it reaches the strength. More fine aggregate enhances the stability of the aggregate skeleton. The proposed identification method can serve as a general tool to investigate the evolution process of the internal aggregate structure of the asphalt mixture.

Study on Bending Creep Performance of GFRP-Reinforced PVC-Based Wood-Plastic Composite Panels.

Wood-plastic composites (WPCs) are environment-friendly materials, which have broad application prospects in structures. They cannot be used for bearing structures because of poor mechanical performance and creep deformation. In order to enhance the mechanical behavior and decrease the long-term creep deformation, glass fiber reinforced plastics (GFRP) sheets and rebar reinforcement design methods are proposed. The bending static tests and creep performance tests of WPCs were conducted. The results showed that GFRP sheets and rebars improved the ultimate flexural loading capacity and deformation capacity by 257% and 165%, respectively, decreased the creep deflection effectively, and avoided shear failure. When the load level was very low, the creep deformation of WPC panels unreinforced, or reinforcement developed stably with time, and the damage did not occur within 1100 h. When the load increased to 80% of the ultimate load level, all specimens were damaged in the compression zone, the creep deformation increased quickly and unstably, bending shear failure of the unreinforced specimen occurred after 7 h, shear failure of the GFRP-sheets-reinforced specimen occurred after 1100 h, and the rebar-reinforced specimen failed after 720 h with excessive deflection deformation in the span. The reinforced effect of GFRP sheets is better. The creep strain growth rate of all specimens increased quickly at the first stage and gradually decreased at the second stage and tended to be stable. The creep calculation model was built based on the four-element model, which is simple and efficient and can make scientific and reasonable predictions of the two phases of structural transient creep and deceleration creep.

Probabilistic characterisation of transition points in three-stage permanent deformation curves using Bayesian inference approach

ABSTRACT Multiple specimens are tested to evaluate their rutting propensity by applying static and dynamic creep loads until a three-stage permanent deformation curve is obtained. This three-stage curve consists of two transition points, i.e. one from the primary to the secondary stage and the other one from the secondary to the tertiary stage of deformation. Due to replication, these transition points behave like random variables rather than deterministic. This stochasticity in the transition points is characterised by the Probabilistic -Stress-Number of cycles (P-S-N) approach. A power law model is used to describe the trajectory of transition points with respect to the input stress level. The probabilistic nature of these power law parameters is characterised by the Bayesian approach rather than the classical approach because the latter considers distribution parameters to be constant and does not reflect the replicate stochasticity. Posterior distributions are determined based on data translated likelihood method. Based on the posterior distributions of the data sets, obtained power law trajectories at various reliability levels, ranging from highest to lowest, are constructed. One advantage of this approach is its capability to extrapolate secondary transition points (Flow number or Flow time) when they cannot be readily obtained under desired testing conditions.

Eco-friendly, disposable bamboo fiber dishware: Static and dynamic mechanical properties and creep behavior

The global ban on plastics and the COVID-2019 epidemic have accelerated the demand for eco-friendly disposable dishware. As an alternative to plastic, the characterization of low-cost, widely-sourced, and sustainable plant fibers for degradable, eco-friendly dishware is urgently needed. In this study, bamboo fiber dishware (BFD) and poly lactic acid (PLA) dishware were investigated by exploring how the properties of their microstructures affect their mechanical properties. Thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) were used to characterize the thermal decomposition behavior and dynamic viscoelasticity of both dishware types. Their modulus reductions were predicted using the accelerating creep mode, according to the time-temperature equivalence principle. The results showed that (1) the eco-friendly BFD was light weight and strong. Its specific strength, specific modulus were 4.50 and 3.09 times higher than those of PLA dishware, respectively. The efficient three-dimensional structure formed by the bamboo fibers and held together by hydrogen bonding played an important role in maintaining the excellent physical and mechanical properties. (2) The BFD and PLA dishware exhibited glass transition temperatures of 63.2 °C and 54.7 °C, respectively, according to the loss modulus curves, and their activation energies were 70.76 kJ/mol and 132.53 kJ/mol, respectively. Compared to the PLA dishware, the thermal decomposition behavior and storage modulus of the BFD were showed a lower sensitivity to temperature. (3) The time-temperature principle could be applied to the long-term creep prediction of BFD and PLA dishware. It showed that BFD exhibited a greater creep resistance in protecting food. • BFD’s specific strength and modulus were 4.50 and 3.09 times higher than PLA. • BFD could maintain high storage modulus at high temperature than PLA. • BFD had better creep resistance than PLA after a long time.

Microstructural Evolution of T91 in High Temperature Conditions and under Combined Effect of Stress and Temperature

ASTM A213 T91 steel is widely used in power plants and petrochemical industry for long-term service components. Due to its high resistance to creep, thermomechanical fatigue and corrosion, the use of grade 91 steel allows usual plant service parameters to be raised up to ultra-supercritical conditions (600 °C, 300 bar) so that performances are remarkably increased. The strongest factors that affect performances are the time of exposure, the temperature and the applied stress: such parameters can dramatically decrease the service life of a plant component. The improved mechanical properties of grade 91 are strictly related to its specific microstructure: a tempered martensite matrix with fine precipitates embedded in. Two typologies of secondary phases are present: M23C6 carbides (where M = Cr/Fe/Mo/Mn) and finely dispersed MX-type carbonitrides (where M = V/Nb and X = C/N). This study is focused on the microstructure evolution of grade 91 steel under creep conditions. First, three sets of laboratory-aged specimens heated in oven at 550 °C, 600 °C and 650 °C were examined; the exposure time was up to 50,000 h. Furthermore, the influence of stress on the microstructure in two sets of samples was evaluated: a first batch of specimens cut from an ex-service tube of a petrochemical plant (over 100,000 h of service at 580 °C and 19–25 bar) and a second set of samples coming from another ex-service tube under ultra-supercritical conditions (605 °C, 252 bar) in a power plant. The microstructures were characterized by optical, scanning electron and transmission electron microscopy and the results were compared to the literature. Some interesting trends were evidenced, in terms of secondary phases precipitation and coarsening, as well as martensite recovery. Furthermore, the applied stress seems to influence size and number of Laves phase particles.

Revisiting analytic shear-lag models for predicting creep in composite materials

Analytic shear-lag models enjoy great popularity for assessing and interpreting microstructure dependent stationary creep in fibrous metal composites, especially the formulation of Kelly-Street (Kelly and Street, 1972 [2]). Beyond the original model's scope, i.e. large aspect ratios of inclusions, predictions are highly inaccurate, which was recently pointed out by Wicht et al. (Wicht et al., 2022 [10]), by comparing model predictions to micromechanical Fast-Fourier-Transform-based simulations. In this study we therefore modify basic Kelly-Street model assumptions, concerning effective creep rate, stress transfer and inclusion spacing to arrive at a modified model with an extended scope. To validate the modified model, we benchmark the model against Fast-Fourier-Transform-based micromechanical simulations. We show, that the proposed modifications are successful in extending the model's scope to inclusions with small aspect ratios ≤20. Thus, the reformulated model is a powerful tool to describe and interpret microstructure dependent creep.

Salinity, force chains, and creep in muddy sediments

The salinity of sediments is not usually measured. Experience with sandy sediments shows that it has no significant effect on the acoustic properties. In muddy sediments, salinity is critical to the skeletal frame, because it causes the clay particles to flocculate, forming an aggregate of larger particles with significant water fraction. It behaves like a granular medium, in which stress is transmitted along random force chains. Mud is known to suffer from creep, and the force chain model fits neatly into creep theory. It may be modeled as a form of stationary creep, which is linear in many respects. No net strain-hardening is involved. The result is a creep model of the skeletal frame that naturally couples into the Biot theory of porous media. It predicts an attenuation that increases linearly with frequency at low frequencies, which is overtaken by viscous attenuation that increases as the second power of frequency and high frequencies. [Work supported by ONR, Ocean Acoustics Program.]

Rough surface contact under creep conditions

A pressing concern in superplastic forming (SPF) is the ability to quantitatively model to effectively design tooling and assess process viability. Current finite element method simulations for SPF generally do not include the evolutionary nature of friction under creep conditions. The presence of creep strain leads to junction growth and saturated contact areas at longer time scales than conventional metal forming; therefore, friction models that ignore creep-induced strains will underpredict asperity contact and friction levels. In this work, a new micro-contact model of a rough surface in contact with a rigid flat surface is created to predict asperity flattening under creep behavior. A semi-empirical equation of the real contact area and friction at different SPF conditions is established.

Experiment of static creep behavior of aggregate-asphalt interfaces

In order to investigate the fatigue failure of compacted asphalt mixtures, the static creep mechanical behavior of aggregate-asphalt interfaces was conducted which especially constructed the creep equation between the deformation-time of aggregate-asphalt interfaces. The hard sandstone cylinder specimens of 38 mm and 21 mm in diameter were used to measure the static creep and strength of aggregate-asphalt, respectively. Through the gravity of asphalt itself, the excess asphalt flowed away from the end face of samples. A thin layer film was adhered to the end surface of hard sandstone cylinder samples. Shear strength and direct tensile strength between the aggregate-asphalt interfaces were measured with the electronic universal testing machine (EUTM) at the loading velocity of 50 mm/min and at 15∘C and 25∘C, respectively. The shear creep between the aggregate-asphalt interfaces was firstly done with EUTM successfully. The direct tensile creep was done with the universal testing machine (UTM). The shear creep deformation-time of the aggregate-asphalt interfaces was the power function relation. The direct tensile creep deformation-time of the aggregate-asphalt interfaces was also the power function relation. These power function relations of the aggregate-asphalt interfaces contribute to the following research about the fatigue failure of compacted asphalt mixtures.