Long-term Mechanical Behavior Research Articles

Effect of alignment and packing density on the stress relaxation process of carbon nanotube fibers spun from floating catalyst chemical vapor deposition method

In this study, we investigate the effects of the alignment and compaction of carbon nanotube (CNT) bundles on the time-dependent behaviors and relaxation process of the CNT fibers spun from floating catalyst chemical vapor deposition method. The long-term mechanical behavior of the CNT fibers shows a strong time-dependency in relaxation process that evidences a noticeable viscose force due to weak interfaces between their CNT bundles. The CNT fibers with higher alignment and compaction exhibit stronger CNT-CNT interfaces, resulting in better inter-tube load transfer efficiency, more difficult CNT bundles slippage, and higher elastic behavior. More importantly, among different viscoelastic models used to simulate the relaxation behavior of the CNT fibers, fractional order viscoelastic model is found to provide the highest accuracy at the entire tested time scale, indicating that the relaxation process of the CNT fibers follows the Mittag-Leffler function. This work suggests that the relaxation process of the CNT fibers spun from the floating catalyst method is a long-term process which occurs in multiscale structure at multi-time scales.

The Influence of Water Saturation on the Short- and Long-Term Mechanical Behavior of Red Sandstone

The presence of water greatly influences time-dependent rock deformation. An understanding of how water can affect the time-dependent mechanical behavior of rock is important when assessing the long-term stability of geotechnical projects. While the previous studies have performed brittle creep experiments on oven-dry or fully-saturated rocks, we report here on a study designed to better understand brittle creep at different levels of saturation. We performed brittle creep experiments on oven-dry samples of red sandstone (Hunan province, China) and samples of the sandstone pre-immersed in water for different durations (from 2 to 8 days). These samples were deformed at a constant stress in one of either two conditions: dry or submerged in water. Before performing creep experiments, we first performed a series of water absorption and constant stress rate experiments to guide the stresses required for our creep tests and to assist with their interpretation. Our creep experiments show that immersion in water greatly increased the minimum creep strain rate and greatly shortened the time-to-failure when compared to the dry state. In detail, the minimum creep strain rate and time-to-failure increased and decreased, respectively, as pre-immersion duration increased from 4 to 6 days, but did not change as the duration was further increased to 8 days. We attribute this to the saturation of microcracks between 4 and 6 days (i.e., water imbibition was complete, or close to completion, following 6 days). We also show that oven-dry samples deformed at a constant stress underwater fail at stresses much lower than oven-dry samples deformed under dry conditions, due to the imbibition of water during deformation. Samples pre-immersed in water, but deformed in the dry condition were characterized by lower strain rates and longer time-to-failure than those pre-immersed in water and deformed underwater. Our explanation for this is that, due to the availability of water, crack tips can remain hydrated when the sample is deformed underwater, thus increasing the efficacy of stress corrosion cracking. The relationships and data provided herein inform on the long-term stability of engineering structures.

Flexural creep tests and long-term mechanical behavior of fiber-reinforced polymeric composite tubes

An experimental investigation of the long-term creep behavior of fiber reinforced polymeric composite tubes subjected to flexural loading was performed. The tubes were first tested under a three-point bending scheme at room temperature to determine the ultimate flexural strength (UFS) and to identify the characteristic failure mode and possible failure locations. Creep tests were then carried out at stress levels of 45%, 55%, 65%, and 75% of the UFS at constant temperatures ranging from −60 °C to 100 °C for 500 h, and strain measurements over time were recorded. The tubes were further tested at varying temperatures by applying 22.5 thermal cycles between −60 °C and 100 °C and 9.5 thermal cycles between −160 °C and 80 °C. Similar flexural loads were applied simultaneously on the tubes. Long-term creep deformation of the tube was evaluated using the time-temperature-stress superposition principle, the derived creep main curves, and the Findley model. Finally, the creep deformation and mechanical strength of the tube at expected service conditions over its entire lifetime were predicted.

A mechanical characterisation on multiple timescales of electroconductive magnetorheological elastomers

Magnetorheological elastomers are a type of smart hybrid material which combines elastic properties of a soft elastomer matrix with magnetic properties of magnetic micro particles. This leads to a material with magnetically controllable mechanical properties of which the magnetorheological effect is the best known. The addition of electroconductive particles to the polymer mix adds electrical properties to the material behaviour. The resulting electrical resistance of the sample can be manipulated by external magnetic fields and mechanical loads. This results in a distinct interplay of mechanical, electrical and magnetic effects with a highly complex time behaviour. In this paper a mechanical characterisation on multiple time scales was conducted to get an insight on the short and long-term electrical and mechanical behaviour of this novel material. The results show a complex resistivity behaviour on several timescales, sensitive to magnetic fields and strain velocity. The observed material exhibits fatigue and relaxation behaviour, whereas the magnetorheological effect appears not to interfere with the piezoresistive properties.

Long-Term Mechanical Behavior of Nano Silica Sol Grouting.

The longevity of grouting has a significant effect on the safe and sustainable operation of many engineering projects. A 500-day experiment was carried out to study the long-term mechanical behavior of nano silica sol grouting. The nano silica sol was activated with different proportions of a NaCl catalyst and cured under fluctuating temperature and humidity conditions. The mechanical parameters of the grout samples were tested using an electrohydraulic uniaxial compression tester and an improved Vicat instrument. Scanning electron microscope, X-ray diffraction, and ultrasonic velocity tests were carried out to analyze the strength change micro-mechanism. Tests showed that as the catalyst dosage in the grout mix is decreased, the curves on the graphs showing changes in the weight and geometric parameters of the samples over time could be divided into three stages, a shrinkage stage, a stable stage, and a second shrinkage stage. The catalyst improved the stability of the samples and reduced moisture loss. Temperature rise was also a driving force for moisture loss. Uniaxial compressive stress-strain curves for all of the samples were elastoplastic. The curves for uniaxial compression strength and secant modulus plotted against time could be divided into three stages. Sample brittleness increased with time and the brittleness index increased with higher catalyst dosages in the latter part of the curing time. Plastic strength-time curves exhibit allometric scaling. Curing conditions mainly affect the compactness, and then affect the strength.

Long-term undrained mechanical behavior of overconsolidated clay under cyclic loading with a large number of cycles

Long-term undrained mechanical behavior of overconsolidated clay under cyclic loading with a large number of cycles

Experimental and Numerical Studies of the Excess Pore Pressure Field Surrounding an Advancing Spudcan Footing

The rapid jack-in process of spudcan footing tends to generate substantial build-up of excess pore pressure below the spudcan underside. An intimate knowledge of the excess pore pressure concentration is thus of significance for understanding both the short-term and long-term mechanical behavior of spudcan. As yet there is a dearth of information in this area. This paper presents a comprehensive research to explore the pattern and magnitude of the excess pore pressure field surrounding an advancing spudcan. Both centrifuge experiments and effective-stress large deformation finite element analyses (LDFEAs) were involved, while the former served mainly to benchmark the latter and the latter provided a complete information database of the excess pore pressure. The bulb-shaped excess pore pressure concentration zone was identified below the spudcan underside. Sizes of pressure bulbs as well as the magnitudes of excess pore pressures inside them bore close relationship to the penetration depths and soil's properties. The results of parametric study led to the development of a simple yet useful approach to estimate the spudcan-penetration-induced excess pore pressure in soil without carrying out the complicated computation. Necessary charts and tables were provided to facilitate its usage. It is expected that the development of such an approach can aid the practicing engineer to obtain a quick and rough estimate of excess pore pressure generated in the midst of the jack-up spudcan penetration and prepare for its short-term and long-term implications.

Effect of water and temperature on short-term and creep mechanical behaviors of coal measures mudstone

In the present study, a series of uniaxial compression tests and multistage creep tests were performed on specimens of coal measures mudstone with different water contents and at different temperatures using an electrohydraulic servo-controlled testing system (MTS816), a specially developed cantilever-type multiarm lever creep testing device and a thermostatic water tank. The effects of water and temperature on short-term strength, failure mode, and creep properties of the mudstone were systematically analyzed. The uniaxial compression test results showed that the presence of water has a significant effect on the mechanical strength and weakens the short-term mechanical properties of the mudstone specimens, whereas high temperatures can exacerbate this water-weakening effect. The uniaxial compressive strength of the specimens decreased exponentially, as the water saturation coefficient increased at room temperature, whereas the strength of saturated specimens decreased linearly with the increase in temperature. The multistage creep test results indicated that increases in the water saturation coefficient and temperature accelerate the creep deformation and creep strain rate of the mudstone specimens. Under different water and temperature conditions, the creep strain rate increased exponentially, as the creep stress level increased. The sensitivity of the creep strain rate to the creep stress level increased with the increase in water saturation coefficient and temperature. Based on the creep test results, linear rheological models were employed to fit the creep behavior of the mudstone. The results showed that the Burgers model can replicate the primary and secondary creep behaviors of the mudstone under different water content and temperature conditions. The parameters of the Burgers model (E m , E k , and η m ) generally decreased with the increase in water saturation coefficient and temperature. The reasons why water and temperature affect the short-term and long-term mechanical behaviors of the mudstone are the physicochemical effects and mechanical effects.

Long-term deflection test and theoretical analysis on cracked prestressed concrete box beams

A long-term deflection test is performed for 470 days on two simply supported prestressed concrete box beams under sustained uniform load. The deflection growth rules of the box beams under both cracked and uncracked states are studied. The measured longterm deflection of the cracked beam is 18.4% larger than that of the uncracked beam, whereas the deflection growth coefficient of the cracked beam is 65.3% that of the uncracked beam. The larger long-term deflection of the cracked beam is mainly attributed to the huge value of the instantaneous deflection caused by the crack. Then, the cracks lead to the release of concrete stress, thereby reducing the creep effect. For the uncracked beam, the influences of concrete shrinkage, creep, and prestress loss on the long-term mechanical behavior of components are considered, and then deflection is predicted using the blended method based on the age-adjusted modulus method, the prestressed equivalent load method, and the calculation method for prestress loss. Results show that the theoretical and measured values are in good agreement. For the cracked beam, the crack influence coefficient is introduced to revise its effective elastic modulus by considering the influence of cracks on the long-term deflection of the cracked beam. Then, a formula for calculating the long-term deflection of cracked beams that considers stiffness reduction for instantaneous deflection correction is proposed. The formula exhibits high precision in calculating and analyzing the long-term deflection of the cracked beam. The deviation between the calculated and measured values is within 10%.

Assessment of the resistance to environmentally assisted cracking (EAC) of C-steel casing and overpack in the COx claystone

ABSTRACTThe French national radioactive waste management agency (Andra) is in charge of studying the disposal of high level wastes (HLW) in deep geological repositories. The reference concept for HLW disposal cells consisted of a multi-barrier system: horizontal boreholes drilled in the Callovo Oxfordian (COx) claystone, cased with carbon steel (C-steel) and containing C-steel overpacks with the nuclear waste packages. Mechanical strength is required for the metallic structures to ensure safety. This study presents the work performed on C-steel to assess in situ mechanical loading, long-term mechanical behaviour based on modelling and environmentally assisted cracking (EAC) susceptibility. The results from in situ experiments have demonstrated anisotropy of the mechanical loading of the casing. Long-term calculations revealed local plastic strain after a few years to a few decades, which highlighted the need to assess the potential risk of EAC. Eventually, the results on EAC assessment in the COx claystone confirmed that the microstructure of the casing and overpack plays a key role on the mechanical resistance.This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.

Experimental study and modeling of shear rheology in sandstone with non-persistent joints

Understanding the long-term mechanical behavior of cracks in rock masses is important for engineering projects and for controlling rock mass stability, particularly on rock slopes. In this study, laboratory-prepared cubic sandstone samples with non-persistent joints were subjected to shear creep testing using a rock shear rheometer. The results indicate that long-term shear strength is influenced by the long-term internal friction angle and cohesion, and decreasing cohesion is a key factor for changes in long-term shear strength under constant load. Additionally, damage to the shear modulus is related to the initial damage conditions and joint persistence of the samples. A new creep model is developed that considers the effect of crack or joint length on time-dependent rock behavior. The elasto-viscoplastic behavior of non-linear creep is also described. The modified shear creep model shows promising results for estimation of failure time, shear creep deformation and long-term stability of a rock slope for cases where planar-type failure occurs along a crack or joint surfaces in which no cohesion exists in the crack or joint surface contacts.

Development of a viscoelastoplastic model for a bedded argillaceous rock from laboratory triaxial tests

Argillaceous rocks are candidate host and (or) cap formations for the geological disposal of nuclear wastes in many countries, including Canada, France, and Switzerland. The understanding of the long-term mechanical behaviour of such rocks is an essential requirement for the assessment of their performance as a barrier against radionuclide migration. The French Institute for Radiological Protection and Nuclear Safety (IRSN) operates an Underground Research Laboratory (URL) in Tournemire, France, in a rock formation known as the Tournemire shale. Many types of experiments are conducted at the Tournemire URL to better understand the physical and chemical behaviour of this shale and its interaction with seal materials intended to be used in the geological disposal of radioactive wastes. The Canadian Nuclear Safety Commission (CNSC) collaborates with the IRSN and CanmetMINING laboratories to perform experimental and theoretical research on the mechanical behaviour of the Tournemire shale. Using data from creep tests, and monotonic and cyclic triaxial tests performed at CanmetMINING Laboratories, we developed constitutive relationships for the mechanical behaviour of the Tournemire shale. The model is based on the theory of plasticity, and takes into consideration the inherent anisotropy due to the existence of bedding planes, hardening behaviour before the peak strength, and viscosity.

Long-term mechanical behaviour of skeletal muscle tissue in semi-confined compression experiments

In this study, porcine skeletal muscle tissue was tested until 112 hours post mortem using a semi-confined compression device that induces fascicles to enter one of the states of compression (mode I), tension (mode II), or constant length (mode III). Based on the authors׳ previous studies (Böl et al., 2014, 2015a), the anisotropic mechanical behaviour of the tissue was analysed, with a special focus on the testing time post mortem. The results suggest that the tissue exhibits significant anisotropic behaviour during the first hours of post mortem but that this anisotropy becomes insignificant at later time points. Interestingly, the compressibility of the tissue is more or less unaffected by the testing time. These results are discussed especially with respect to tissue microstructure.

Chemical–mechanical coupling observed for depleted oil reservoirs subjected to long-term CO2-exposure – A case study of the Werkendam natural CO2 analogue field

Geological storage of CO2 is one of the most promising technologies to rapidly reduce anthropogenic emissions of carbon dioxide. In order to ensure storage integrity, it is important to understand the effect of long-term CO2/brine/rock interactions on the mechanical behaviour of a storage complex. As most of these reactions are too slow to reproduce on laboratory timescales, we studied a natural CO2 analogue reservoir (the Röt Fringe Sandstone, Werkendam field, the Netherlands; 125–135 Ma of CO2-exposure) and its unreacted counterpart. We focused on CO2-induced mineralogical and porosity-permeability changes, and their effect on mechanical behaviour of both intact rock and simulated fault gouge. Overall, CO2-exposure did not lead to drastic mineralogical changes. The CO2-exposed material shows a stronger dependence of permeability on porosity, which is attributed to differences in diagenesis (closed-system diagenesis and hydrocarbon emplacement) taking place before CO2 charging. The limited extent of reaction was in part the result of bitumen coatings protecting specific mineral phases from reaction. In local, mm-sized zones displaying significant anhydrite cement dissolution, enhanced porosity was observed. For most of the reservoir the long-term mechanical behaviour after CO2-exposure could be described by the behaviour of the unreacted sandstone, while these more ‘porous’ zones had a lower rock strength. In addition, CO2-exposure did not affect the fault friction behaviour, and slip is expected to result in stable sliding. Simple stress path calculations predict that reservoir failure due to depletion and injection is unlikely, even for the ‘porous’ zones, nor will fault reactivation occur for realistic injection scenarios.

Lithospheric structure of Venus from gravity and topography

There are many fundamental and unanswered questions on the structure and evolution of the venusian lithosphere, which are key issues for understanding Venus in the context of the origin and evolution of the terrestrial planets. Here we investigate the lithospheric structure of Venus by calculating its crustal and effective elastic thicknesses (Tc and Te, respectively) from an analysis of gravity and topography, in order to improve our knowledge of the large scale and long-term mechanical behaviour of its lithosphere. We find that the venusian crust is usually 20–25km thick with thicker crust under the highlands. Our effective elastic thickness values range between 14km (corresponding to the minimum resolvable Te value) and 94km, but are dominated by low to moderate values. Te variations deduced from our model could represent regional variations in the cooling history of the lithosphere and/or mantle processes with limited surface manifestation. The crustal plateaus are near-isostatically compensated, consistent with a thin elastic lithosphere, showing a thickened crust beneath them, whereas the lowlands exhibit higher Te values, maybe indicating a cooler lithosphere than that when the venusian highlands were emplaced. The large volcanic rises show a complex signature, with a broad range of Te and internal load fraction (F) values. Finally, our results also reveal a significant contribution of the upper mantle to the strength of the lithosphere in many regions.

Strength properties of an epoxy resin and cement-stabilized silty clay soil

The use of new materials for soil strengthening is crucial for geotechnical engineering, especially in the methods commonly used for ground improvement, such as deep mixing. The main objective of this study was to investigate the potential use of two-component water-soluble epoxy resins to improve the mechanical properties (compressive strength, splitting tensile strength, elastic modulus, shear strength parameters, and long-term mechanical behavior) of silty clay soils because the efficacy of these resins for soil strengthening has not yet been properly investigated. The experiments were conducted using soil mixes with different epoxy resin-to-water ratios, solid-to-water ratios and cement contents, cured at different ages. The results of this study indicate that most of the epoxy resin-treated soil samples, even with the adverse influence of water content on the development of strength, appeared to have noticeable strength improvement after a curing period of 90days. The addition of cement significantly improved the strengths of all mixes at all curing ages. The values of the strength parameters increased with cement content. This increment in strength for epoxy resin/cement-treated specimens could be mainly attributed to the consumption of large quantities of water from cement compounds, which results in the promotion of polymerization of the epoxy resin with the hardener.

An experimental fatigue study of a porous scaffold for the regeneration of articular cartilage

The aim of this experimental study is to predict the long-term mechanical behavior of a porous scaffold implanted in a cartilage defect for tissue engineering purpose. Fatigue studies were performed by up to 100,000 unconfined compression cycles in a polycaprolactone (PCL) scaffold with highly interconnected pores architecture. The scaffold compliance, stress–strain response and hysteresis energy have been measured after different number of fatigue cycles, while the morphology has been observed by scanning electron microscopy at the same fatigue times. To simulate the growing tissue in the scaffold/tissue construct, the scaffold was filled with an aqueous solution of polyvinyl alcohol (PVA) and subjected to repeating cycles of freezing and thawing that increase the hydrogel stiffness. Fatigue studies show that the mechanical loading provokes failure of the dry scaffold at a smaller number of deformation cycles than when it is immersed in water, and also that 100,000 compressive dynamic cycles do not affect the scaffold/gel construct. This shows the stability of the scaffold implanted in a chondral defect and gives a realistic simulation of the mechanical performance from implantation of the empty scaffold to regeneration of the new tissue inside the scaffold׳s pores.

Hydromechanical response of a bedded argillaceous rock formation to excavation and water injection

Opalinus clay is a candidate host formation for the geological disposal of nuclear wastes in Switzerland. The understanding of its long-term mechanical (M) and hydraulic (H) behaviour is an essential requirement for the assessment of its performance as a barrier against radionuclide transport. To study the HM response of Opalinus clay, a microtunnel, 13 m in length and 1 m in diameter, was excavated in that formation at the Mont Terri Underground Research Facility. The rock mass was equipped with sensors to measure the deformation and pore pressure in the rock mass during and after the excavation. A mathematical model that couples the equations of flow and mechanical equilibrium was developed to simulate the HM response of the rock mass. An anisotropic plastic constitutive relationship, based on a microstructure tensor approach, was incorporated in the model. Creep was also considered, as well as the anisotropy of permeability. It is shown that the model satisfactorily predicts the shape and extent of the excavation damage zone (EDZ), deformation, and pore pressure in the rock mass. It is also shown that anisotropy and creep play an important role in the HM response of the rock mass to excavation. The model was further used to simulate water injection tests performed at the test section in the microtunnel. The results show that EDZ, due to its high permeability, is a preferential groundwater flow path along the microtunnel.

Microstructural evolution of an incipient fault zone in Opalinus Clay: Insights from an optical and electron microscopic study of ion-beam polished samples from the Main Fault in the Mt-Terri Underground Research Laboratory

Slickensided shear surfaces are ubiquitous in many fault zones. However the internal structure, the micromechanics and the evolution of these structures are not fully understood and the contributions of crystal plasticity, grain-boundary sliding, microfracturing, solution-precipitation and mineral transformation under different conditions are subject of debate.We studied well- preserved core samples from the Main Fault, an up to 3 m wide zone of approximately 10 m offset in the Mont Terri Underground Research Laboratory (CH), a site to evaluate long-term safety of radioactive waste disposal. The drill core breaks easily along many slickensided shear surfaces indicating reverse slip, which form an anastomosing network connected by branch lines.Broad ion beam polishing and scanning electron microscopy shows that the slickensides are invariably revealed by fracture of the drill core along a few μm thick shear zone, which acts as a crack guide for fracturing the samples. In this zone, a complex set of processes is inferred, leading to extreme localization of strain, development of strong particle preferred orientation, the formation of nanoparticles, and local precipitation of calcite veins in releasing sections. In lenses between shear zones, homogeneous gouge is formed with a well-developed oblique foliation and removal of calcite grains by pressure solution. We infer that with progressive deformation, the number and density of slickensided shear surfaces increases, generating tectonically derived scaly clay and more homogeneous gouge. In all deformed elements of the Main Fault, porosity is much smaller than in the undeformed Opalinus Clay. An interesting observation is the almost complete absence of cataclastic microstructures. Transmission electron microscopy (TEM) of focused ion beam lamellae of this micron-wide shear zone shows a strong preferred orientation of clay minerals, including nano-sized illite particles. In TEM, the shear zones envelop hard particles and confirm an almost complete loss of porosity compared to the protolith.We propose that inter- and transgranular microcracking, pressure solution, clay neoformation, crystal plasticity and grain boundary sliding are important micro-scale processes during the early stages of faulting in Opalinus Clay and thus need to be considered in extrapolating laboratory results to long-term mechanical behavior.

Characterisation and modelling of anisotropic thermo-mechanical behaviour of oriented polyethylene terephthalate

The long-term and short-term anisotropic mechanical behaviour of a biaxially stretched polyethylene terephthalate foil is measured. The orientation of the crystalline phase is characterized and the representative foil microstructure is discussed. Using the obtained information, a mean-field model is used to simulate the elasto-viscoplastic behaviour of the oriented polymer foil, taking into account the different constitutive behaviour of the phases. The material is modelled as an aggregate of connected two-phase domains. The parameters of the constitutive behaviour of the crystalline and non-crystalline phases have been determined, and the ability to simulate the large-strain anisotropic behaviour of polyethylene terephthalate in the strain-rate-controlled regime and the long-term creep has been demonstrated. The model is extended to include pre-orientation of the non-crystalline phase. In addition, deformation at the microscopic level is analysed using the model results.