Creep Stability Research Articles

A double tunnels model test study on mechanical properties of surrounding rock during tunnels excavation and creep stages

The tunnel engineering has become increasingly common with the needs of urban development. However, excavation sequence has a significant effect on the stability of the tunnel perimeter rock, which can lead to more unpredictable perimeter rock hazards and more difficulty in maintaining perimeter rock stability. In this study, to investigate the effect of different excavation sequences on tunnel deformation and creep, biaxial compression experiments were conducted on muddy siltstone specimens by using the acoustic emission (AE) and digital image correlation (DIC) techniques. The results indicated that, as the distance from the tunnel portal increases, the amount of stress and strain change decreases, with the largest values of stress changes occurring in the tunnel vault and middle area of twin tunnels. The specimen undergoes two stages in the creep process: deceleration creep and steady-state creep. The steady-state creep rate of the measurement points depends on the degree of excavation disturbance impact it experiences. The AE events during excavation can be divided into two phases: the quiet period when the specimen is not excavated and the active period when the specimen is excavated, and the cumulative counting curve shows a “stepwise” increase with the number of excavations. There is a sudden drop in the b-value during excavation and a decrease in the b-value during the deceleration creep phase. The results of this study can reflect the creep patterns of the surrounding rock under different excavation sequences, which can help to choose a more suitable excavation plan in terms of rock creep stability.

A comprehensive feasibility evaluation of salt cavern oil energy storage system in China

Large-scale underground oil storage has a great effect on national energy safety. China's oil dependency has exceeded 70% for four consecutive years, so it is necessary for China to build strategic oil storage. The salt cavern is a good medium to store oil, and it is widely used in foreign countries. China is rich in salt mines and has basic conditions for building salt cavern oil storage (SCOS). In this paper, the necessity and advantages of SCOS are analyzed, the salt rock mechanics experiments of SCOS are conducted, the geological feasibility evaluation model is built, the 3D numerical creep stability model in three working conditions (long-term, oil pressure dynamic, injection and production) is built, and the economic feasibility evaluation model is built. The results show that the implementation of SCOS in China is feasible, the earthquake (51%) and fault (28%) have a great effect on geological selection, the SCOS has good stability under three working conditions, the storage cost of SCOS is lower by 64.3% and 84.3% contrasted to rock cavern and storage tank in 2025. The study has a reference value for the development of SCOS in China.

Effects of shear stress path and roughness on shear creep behavior of marine clay-concrete interface

Floating piles have been widely employed as foundations in coastal regions abounding with marine clay. A growing concern for these floating piles is their long-term performance of bearing capacity. To better understand the time-dependent mechanisms behind the bearing capacity, in this paper a series of shear creep tests was conducted to study the effects of load paths/steps and roughness on shear strain of the marine clay-concrete interface. Four main empirical features were observed from the experimental results. First, the creep process of the marine clay-concrete interface can be largely decomposed into the instantaneous creep stage, the attenuation creep stage and the uniform creep stage. Second, the creep stability time and the shear creep displacement generally increase as the shear stress level increases. Third, the shear displacement rises as the number of loading steps drops under the same shear stress. The fourth feature is that under the shear stress condition, the rougher the interface is, the smaller the shear displacement is. Besides, the load-unloading shear creep tests suggest that: (a) shear creep displacement typically contains both viscoelastic and viscoplastic deformation; and (b) the proportion of unrecoverable plastic deformation increases with increasing shear stress. These tests confirm that the Nishihara model can provide a well-defined description of the shear creep behavior of marine clay-concrete interfaces.

Enhanced creep stability in Inconel 718 deposited by induction-heating-assisted laser-directed energy deposition

This study investigated the creep stability enhancement of Inconel 718 by induction heating during the laser-directed energy deposition. The induction heating during the deposition fully dissolved the Laves phase, mitigated Nb segregation, and formed directionally solidified columnar grains. Based on the results of nanoindentation creep testing, the deposit utilizing induction heating exhibited high creep stability.

Effect of multi‐walled carbon nanotubes on rheological behavior and electrical conductivity of poly(ethylene‐co‐vinyl acetate)/acrylonitrile‐butadiene rubber/multi‐walled carbon nanotubes nanocomposites

AbstractIn this work, the effect of multi‐walled carbon nanotubes (MWCNTs) on electrical conductivity and rheological behavior of poly(ethylene‐co‐vinyl acetate) (EVA)/acrylonitrile‐butadiene rubber (NBR) blends containing 0 to 7 wt% MWCNTs are investigated. The theoretical calculations according to the interfacial free energy of the components revealed that the nanofillers had a tendency to locate in NBR domains. However, the rheological, electrical, and morphological studies indicated that the majority of the MWCNTs remained in the EVA phase. Shear creep measurements of the molten materials showed that the creep stability improved steadily with the increase in MWCNTs content. The creep behaviors of the materials, except EVA, were analogous to the Burgers model prediction and all four parameters of the model increased in value with the increase in the nanofillers content in comparison with those of the unfilled blend. The rheological studies indicated that the damping factor (tanδ) for the materials containing small amounts of the nanofillers reached a maximum and subsequently decreased with rising temperature. However, for the materials having 1 wt% MWCNTs and higher, the tanδ values were smaller at the highest temperature (200°C) than those of the lowest temperature (100°C) investigated in this work. The electrical percolation threshold was also found to take place at about 2.80 wt% MWCNTs loading.

3D printing of ABS Nanocomposites. Comparison of processing and effects of multi-wall and single-wall carbon nanotubes on thermal, mechanical and electrical properties

• Multi-Wall and Single-Wall carbon nanotubes affect thermoelectrical behavior. • Fuse Filament Fabrication of conductive nanocomposites at selected low resistivity. • 3D manufactured nanocomposites with applications for thermoelectric devices/sensors. • Anisotropy in electro-mechanical behaviour of multilayer nanocomposites. • Conductivity of CNT composites depends on filler content, process and geometry. The following paper reports on a comparative study of the effects of two types of carbon nanotubes, namely multiwall (MWCNT) and single-wall (SWCNT) carbon nanotube, on the properties of 3D-printed parts produced with acrylonitrile-butadiene-styrene (ABS) nanocomposites with various CNT loadings of 5-10 wt.%. Quasi-static tensile properties and Vicat softening temperature of 3D-printed parts were enhanced with the increasing CNT content. The highest enhancement in tensile properties was observed for the ABS/CNT nanocomposites at 10 wt.% filler loading. 3D-printed ABS/SWCNT composites showed higher tensile modulus, better creep stability and higher Vicat temperature. However, the strength of ABS/SWCNT 3D samples is relatively lower than that of ABS/MWCNT. In addition, 3D-printed parts exhibited anisotropic electrical conductive behaviour, which has a conductivity of through-layer of about 2-3 orders of magnitude higher than cross-layer. The highest conductivity of 3D-printed samples reached 25.2 S/m, and 9.3 S/m for ABS/MWCNT and ABS/SWCNT composites at 10 wt.%, respectively. The results obtained, i.e. the successful fuse filament fabrication and the consequent electromechanical properties, confirm that these 3D printable nanocomposite could be properly utilized for the production, and application up to about 90 °C, of thermoelectric devices and/or resistors for flexible circuits.

Creep stability of the DART/Hera mission target 65803 Didymos: II. The role of cohesion

The binary asteroid 65803 Didymos-Dimorphos is the target of the first asteroid deflection test (NASA's Double Asteroid Redirection Test, DART) and the first binary asteroid system that will be characterized by a rendezvous mission (ESA's Hera). The cohesive strength of the fast-spin-primary Didymos is a key factor that could affect the impact outcome and stability of this system. To support the preparation and data interpretation of these missions and gain a better understanding of the formation and evolution of this system, we investigate the structural stability and cohesive strength of Didymos based on current observational information. We use the Didymos radar shape model to construct rubble-pile models consisting of ~40,000 to ~100,000 particles with different arrangements and size distributions. To investigate the effect of cohesion on the structural stability and dynamical behaviors of Didymos, we explicitly simulate the YORP spin-up process of these rubble-pile models from a slow spin state to Didymos' current spin state using a high-efficiency soft-sphere-discrete-element-model code, pkdgrav. We test the creep stability of Didymos' rubble-pile representation with different values of cohesion and derive the critical amount of cohesion to maintain stability. The results show that Didymos should at least have a minimum bulk cohesion on the order of 10 Pa to maintain its structural stability if the interparticle tensile strength is uniformly distributed. Since the surface particles are less bonded by cohesive contacts than the interior particles, the internal macroscopic cohesion is about three times the surface macroscopic cohesion. We find that the bulk density and particle arrangement and size distribution of Didymos have significant influences on its critical cohesion and failure behaviors, indicating different binary formation pathways. With the critical cohesion, Didymos is at the edge of maintaining a stable shape, and a rapid small decrease in its spin period would excite its rubble-pile structure and lead to reshaping or mass shedding. Whether the DART impact could partially or globally destabilize this system requires further investigation of the full two-body gravitational dynamics and the ejecta evolution. With the expected measurements returned by DART's onboard cubesat LICIACube in 2022 and Hera in 2027, the correlations between Didymos' physical properties and failure behaviors found in this study may be possible to constrain the mechanical properties and evolutionary history of this binary system.

Creep properties and resistivity-ultrasonic-AE responses of cemented gangue backfill column under high-stress area

To investigate the creep and instability properties of a cemented gangue backfill column under a high-stress area, the uniaxial compression creep tests were conducted by single-step and multi-step loading of prismatic samples made of cemented gangue backfill material (CGBM) under the high stress-strength ratio. The creep damage was monitored using an electrical resistivity device, ultrasonic testing device, and acoustic emission (AE) instrument. The results showed that the CGBM sample has a creep hardening property. The creep failure strength (CFS) is slightly larger than the uniaxial compressive strength (UCS), ranging in ratio from 108.9% to 116.5%. The instantaneous strain, creep strain, and creep rate increase with increasing stress-strength ratio in the single-step loading creep tests. The instantaneous strain and creep strain decrease first and then increase during the multi-step loading creep process. The axial creep strain of the CGBM column can be expressed by the viscoelastic-plastic creep model. Creep instability is caused by the accumulation of strain energy under multi-step loading and the continuous lateral expansion at the unconstrained middle position during the creep process. The creep stability of a CGBM column in a high-stress area can be monitored based on the variation of electrical resistivity, ultrasonic pulse velocity (UPV), and AE signals.

Time-dependent reliability analysis for out-plane stability of CFST arches

Concrete filled steel tubular (CFST) arches have been widely used in bridge structures for the advantages of material properties and construction technologies. However, studies for out-plane stability with the consideration of uncertainties due to the concrete creep have been limited. In this paper, the limit state function (LSF) for out-plane creep stability of CFST arches is established. The stastical information of creep coefficient of concrete is obtained based on the Monte Carlo Simulation (MCS) method. Based on the models of out-plane creep stability, the time-dependent reliability method and the finite element reliability method (FERM) are combined to investigate the reliability deteriotion for the out-plane stability of CFST arch ribs. The results show that the reliability indices for out-plane creep stability decreases sharply with time; the deteriotion of time-dependent reliability is significant in the first 200 days; the effects of steel ratios on the time-dependent reliability index is remarkble.

The bulk compressive creep and recovery behavior of human dentine and resin-based dental materials

ObjectiveTo evaluate and compare the viscoelastic properties of dentine and resin-based dental materials by bulk compressive test and the Burgers model. Materials and methodsSound dentine, three resin composites as well as a resin-based cement were prepared into cylindrical specimens (n = 8). A bulk compressive creep test was applied with a constant load of 300 N (23.9 MPa) for 2 h, followed by another 2 h recovery. The maximum strain, creep stain, percentage of recovery and permanent set was measured using a linear variable displacement transducer. The viscoelastic properties were characterized via the Burgers model, and the instantaneous elastic, viscous as well as elastic delayed deformation were separated from the total strain. Data were analysed via ANOVA (or Welch's Test) and Tukey (or Games–Howell Test) with a significance level of 0.05. ResultsSound dentine presented the lowest maximum strain, creep strain, permanent set and the highest percentage of recovery, followed by 3 resin composites with comparable parameters, while the cement showed a significantly higher maximum strain, permanent set and lower percentage of recovery (p < 0.001). The Burgers model presented acceptable fits for characterization viscoelastic processes of both dentine and resin-based dental materials. Viscous and elastic delayed strain of dentine was significantly lower than those for tested materials (p < 0.001) with the highest instantaneous elastic strain percentage. Similar viscous and delayed strain was found among the 4 resin-based materials (p > 0.05). SignificanceSound dentine exhibited superior creep stability compared to resin-based dental materials. The viscous deformation in sound dentine could be ignored when loading parallel to dentine tubules.

Transition from source- to stress-controlled plasticity in nanotwinned materials below a softening temperature

Nanotwinned materials have been widely studied as a promising class of nanostructured materials that exhibit an exceptional combination of high strength, good ductility, large fracture toughness, remarkable fatigue resistance, and creep stability. Recently, an apparent controversy has emerged with respect to how the strength of nanotwinned materials varies as the twin thickness is reduced. While a transition from hardening to softening was observed in nanotwinned Cu when the twin thickness is reduced below a critical value, continuous hardening was reported in nanotwinned ceramics and nanotwinned diamond. Here, by conducting atomistic simulations and developing a theoretical modeling of nanotwinned Pd and Cu systems, we discovered that there exists a softening temperature, below which the material hardens continuously as the twin thickness is reduced (as in nanotwinned ceramics and diamond), while above which the strength first increases and then decreases, exhibiting a maximum strength and a hardening to softening transition at a critical twin thickness (as in nanotwinned Cu). This important phenomenon has been attributed to a transition from source- to stress-controlled plasticity below the softening temperature, and suggests that different hardening behaviors may exist even in the same nanotwinned material depending on the temperature and that at a given temperature, different materials could exhibit different hardening behaviors depending on their softening temperature.

Long‐Term Deformation Analysis of Recycled Construction Waste Subgrade Filler

The application of construction waste to subgrade has important economic and environmental significance. In order to study the long‐term deformation characteristics of recycled construction waste filler, a series of laboratory tests and field measurement have been carried out. Through compaction, sieving, and California bearing ratio (CBR) tests, the gradation and CBR value of recycled construction waste filler meet the requirements of expressway subgrade. Long‐term deformation tests include laboratory creep tests and field settlement measurement. The creep tests were carried out for about 500 days by a self‐made consolidation creep instrument, the creep type of recycled construction waste filler under the test load is stable creep, as the load increases, not only the creep deformation increases but also the time to reach creep stability is longer, and the breakage and compaction of particles are the main reasons for long‐term deformation. Through measurement and analysis, it is found that the settlement stability of the construction waste subgrade takes a long time; after the operation of road, the settlement of the subgrade has grown rapidly. Furthermore, the Burgers model can be used to predict the deformation of creep tests, and the maximum settlement of construction waste subgrade can be predicted. The research can provide reference for the application of recycled construction waste filler, and it is conducive to the promotion of construction waste subgrade.

Low-temperature Performance and its Microstructure of SBS Modified Asphalt under Condition of Aging

The effects of USP warm-mixed agent, Evo warm-mixed agent and aging degree on the low temperature performance and micro-morphology of SBS modified asphalt were analyzed by ductility test, bending creep stiffness test (BBR) and atomic force microscope (AFM) scan test. The test results show that: The ductility test analysis shows that USP warming agent can improve the low temperature performance of SBS modified asphalt in the positive temperature zone, and aging reduces the ductility of warm mixed asphalt, in which Evo warm mix asphalt has the strongest ability to resist thermal aging. BBR test analysis shows that USP warm mixed agent improves the low temperature performance of the negative temperature zone of SBS modified asphalt, but this improvement is limited by the aging effect, the comparison analysis found that creep loading time had a significant impact on the creep compliance derivative, it is more than the effect of warm-mixed agent, and aging can accelerate warm mix asphalt into the creep stability period; AFM micro-scanning found that the "bee-like structure" appeared on the front topography of SBS modified asphalt and Evo warm asphalt. The bee-like structure of Evo warm mix asphalt is larger and more numerous. The larger the size of the bee structure, the smaller the number of bee structures; however, the surface of the USP warm mix asphalt is more undulating; the aging of the two types of asphalt is more serious, and the size of the honeycomb structure is larger, but the number of honeycomb structure is reduced; while the surface of USP warm asphalt is undulating.

Effects of the Nanofillers on Physical Properties of Acrylonitrile-Butadiene-Styrene Nanocomposites: Comparison of Graphene Nanoplatelets and Multiwall Carbon Nanotubes.

The effects of carbonaceous nanoparticles, such as graphene (GNP) and multiwall carbon nanotube (CNT) on the mechanical and electrical properties of acrylonitrile–butadiene–styrene (ABS) nanocomposites have been investigated. Samples with various filler loadings were produced by solvent free process. Composites ABS/GNP showed higher stiffness, better creep stability and processability, but slightly lower tensile strength and electrical properties (low conductivity) when compared with ABS/CNT nanocomposites. Tensile modulus, tensile strength and creep stability of the nanocomposite, with 6 wt % of GNP, were increased by 47%, 1% and 42%, respectively, while analogous ABS/CNT nanocomposite showed respective values of 23%, 12% and 20%. The electrical percolation threshold was achieved at 7.3 wt % for GNP and 0.9 wt % for CNT. The peculiar behaviour of conductive CNT nanocomposites was also evidenced by the observation of the Joule’s effect after application of voltages of 12 and 24 V. Moreover, comparative parameters encompassing stiffness, melt flow and resistivity were proposed for a comprehensive evaluation of the effects of the fillers.

3D printable thermoplastic polyurethane blends with thermal energy storage/release capabilities

The aim of this work was to develope novel 3D printable thermoplastic polyurethane (TPU) blends with thermal energy storage (TES) capabilities. The target are potential applications for winter sport equipment. Different amounts of an encapsulated paraffin were added to a TPU matrix, and the resulting blends were then used to produce 3D printed samples. FESEM observation evidenced a homogeneous distribution of the capsules in the polymer matrix and a good adhesion between the layers in the 3D printed parts. DSC tests indicated that an effective energy storage/release capability was obtained in the 3D printed parts, with melting enthalpy values up to 70 J/g. The hard shells of the microcapsules, made of melamine formaldehyde resin, induced an increase of the stiffness, of the creep stability and of the Shore A hardness of the material, accompanied by a decrease of the elongation at break.

Stability Evaluation of Old Goaf Treated with Grouting Under Building Load

In this paper, stability of overlying strata and grouting entities are determined as the two key factors affecting the stability of old goaf treated with grouting under building load by theoretical analysis, the additional stress influence depth and the grouting entities strength are determined as the two key indexes for the stability of old goaf treated with grouting under building load. Through theoretical analysis and field investigation, the calculation method of the influence depth of the additional stress in the old goaf under the building load is revised, and the calculation formula of the grouting entities strength is confirmed by theoretical analysis and the experimental results. The uniaxial compression and uniaxial creep stability of the grouting entities under dry and saturated conditions are verified by mechanical experiments. And the revised parameter reference value is provided for the calculation formula of the grouting entities strength. Finally, the stability of the grouting reinforcement project in China is verified by numerical simulation.

Selective localization of organic montmorillonite in poly( l -lactide)/poly(ethylene vinyl acetate) blends and the resultant properties

A small quantity of organic montmorillonite (OMMT) was introduced into an immiscible poly(l-lactide)/poly(ethylene vinyl acetate) (PLLA/EVA) blend. The selective location state of OMMT, the blend morphologies and the microstructure of the blend composites were systematically investigated. The theoretical calculation based on interfacial tensions and experimental observations showed that OMMT selectively located in the rigid PLLA component. Apparent morphological changes were observed for the blend composites. Rheological measurements showed that OMMT at relatively high content formed the percolated network structure in the blend composites. Tensile measurements showed that OMMT exhibited excellent reinforcement effect for the immiscible PLLA/EVA blends. Furthermore, tensile creep behavior measurements showed that incorporating OMMT into the immiscible PLLA/EVA blends facilitated the apparent enhancement of creep stability. The microstructure-performance relationship of the blend composites was then analyzed.

Creep stability of the proposed AIDA mission target 65803 Didymos: I. Discrete cohesionless granular physics model

As the target of the proposed Asteroid Impact & Deflection Assessment (AIDA) mission, the near-Earth binary asteroid 65803 Didymos represents a special class of binary asteroids, those whose primaries are at risk of rotational disruption. To gain a better understanding of these binary systems and to support the AIDA mission, this paper investigates the creep stability of the Didymos primary by representing it as a cohesionless self-gravitating granular aggregate subject to rotational acceleration. To achieve this goal, a soft-sphere discrete element model (SSDEM) capable of simulating granular systems in quasi-static states is implemented and a quasi-static spin-up procedure is carried out. We devise three critical spin limits for the simulated aggregates to indicate their critical states triggered by reshaping and surface shedding, internal structural deformation, and shear failure, respectively. The failure condition and mode, and shear strength of an aggregate can all be inferred from the three critical spin limits. The effects of arrangement and size distribution of constituent particles, bulk density, spin-up path, and interparticle friction are numerically explored. The results show that the shear strength of a spinning self-gravitating aggregate depends strongly on both its internal configuration and material parameters, while its failure mode and mechanism are mainly affected by its internal configuration. Additionally, this study provides some constraints on the possible physical properties of the Didymos primary based on observational data and proposes a plausible formation mechanism for this binary system. With a bulk density consistent with observational uncertainty and close to the maximum density allowed for the asteroid, the Didymos primary in certain configurations can remain geo-statically stable without requiring cohesion.

Thermo-mechanical characterization of banana particulate reinforced PVC composite as piping material

A banana particulate reinforced polyvinyl chloride (PVC) composite was developed with low cost materials having an overall light-weight and good mechanical properties. The specimen composite material was produced with the banana (stem) particulate as reinforcement using compression molding. The composition with optimum mechanical property of 42MPa was estimated to have a long term stress value of 25MPa corresponding to 40% loss in strength over a period of 32years. This composition has a formulation of 8%, 72% and 20% of banana stem particulates (reinforcement), PVC (matrix) and Kankara clay (filler) respectively with corresponding water absorption of 0.79%, Young’s Modulus of 1.3GPa and a density of 1.24g/cm3. Thermogravimetric analyses showed that constituents of reinforcement/filler in the composite increased the thermal stability of the composite by 38.6% over that of pure PVC. The composite has better creep stability at elevated temperatures than PVC. Long term TTS (time–temperature-superposition) performance prediction at 50°C showed the composite satisfied WLF assumption with reduced stiffness to 0.65GPa over an estimated period in excess of 100years of usage indicating better long term performance than PVC pipe material – the stiffness could be much higher when used below 50°C. Comparison with conventional piping materials showed the composite offered a price savings per meter length of 84% and 42% when compared with carbon steel and PVC material.

Enhanced tensile creep stability of immiscible poly(l-lactide)/poly(ethylene vinyl acetate) blends achieved by adding carbon nanotubes

Abstract Creep stability is usually adopted to evaluate the dimensional stability of polymers under the load condition. However, the creep behavior of polymer blend composites, which has been already used in many fields, is seldom investigated. In this work, immiscible poly( l -lactide) (PLLA)/poly(ethylene vinyl acetate) (EVA) blend composites with different contents of carbon nanotubes (CNTs) were prepared. The crystalline structure of matrix, the dispersion of CNTs as well as the morphologies of the blend composites were comparatively investigated. The results demonstrated that CNTs selectively located at the blend interface and induced the morphological changes from quasi-cocontinuous structure to cocontinuous structure. Mechanical property measurements demonstrated that CNTs exhibited toughening effect for the immiscible PLLA/EVA blends. The creep behavior was investigated under different external forces. The results demonstrated that the creep stability of the blend composites was apparently enhanced by adding CNTs. The mechanisms for the enhanced creep stability were then proposed.