Unloading Cycles Research Articles

Degradation of RC short beams under monotonic and repeated loads after cryogenic freeze-thaw cycles

ABSTRACT To investigate the effects of cryogenic freeze-thaw cycles and repeated loading on the bearing capacity and deformation properties of reinforced concrete (RC) short beams, RC short beams with different reinforcement ratios were subjected to Minus 160°C cryogenic freeze-thaw cycles. Subsequently, a four-point bending tests were carried out under monotonic and repeated loading with the aim of exploring the performance characteristics of the test beams, such as failure mode, load‐deflection response, and ductility. Experimental results showed that the failure mode of RC short beams changed from ductile bending to brittle shear failure after cryogenic freeze-thaw cycles. Additionally, the load-carrying capacity, deformation properties and energy dissipation performance exhibited a significant degradation trend. There is decreasing trend for the ductility and ultimate bearing capacity of beams with the increase in freeze-thaw cycles, and the decrease degree was most obvious in the first three cycles. Moreover, compared with monotonic loading, the ductility index of normal temperature beam and freeze-thaw cycle short beam decreased more significantly under repeated loading, and the bearing capacity decreases more obviously under the last stage of multi-level loading and unloading cycles.

Failure evaluation mechanism of cement sheath sealing integrity under casing eccentricity during multistage fracturing

A microannulus (MA) is the primary reason for sustained casing pressure in multi-stage fractured-shale gas wells. However, the effect of the casing eccentricity on the long horizontal section has not been considered. In this study, a full-scale integrity tester for cement sheaths is adopted to measure the cumulative plastic deformation. Numerical models are applied to evaluate the development of the cumulative plastic deformation and quantify the MA width considering casing centralization and eccentricity in the context of multiple loading and unloading cycles. Subsequently, the influences of the eccentricity distance and angle, cement-sheath mechanical variables, and different well depths on the cumulative sheath plastic deformation and sheath MA development are explored. The research results demonstrate that casing eccentricity significantly increases the cumulative sheath plastic deformation compared with that of the casing-centered condition. Consequently, the risk of sealing integrity failure increases. The accumulated plastic deformation increases when the eccentricity distance increases. In contrast, the initial plastic deformation increases as the eccentricity angle increases. However, the cumulative plastic deformation decreases after a specific loading and unloading cycle count. Affected by the coupled influence of the internal casing pressure and fracturing stages, the width of the MA in the horizontal section increased from the toe to the heel, and the casing eccentricity significantly increased the MA width at each stage, thus increasing the risk of gas channeling. Finally, an engineering case is considered to study the influence of casing eccentricity. The results show that cement slurries that form low and high elastic moduli can be applied to form a cement sheath when the fracturing stage is lower or higher than a specific value, respectively. The results of this study offer theoretical references and engineering support for the integrity control of cement sheath sealing.

An adhesion model for contact electrification

Contact electrification (CE) is a universal phenomenon that occurs at the contact interface where tribo-charges transfer from one surface to another. These CE-driven charges accumulate at the interface and can significantly affect the soft adhesive contact. So far, only a few analytical and numerical models have been proposed to partially solve the CE-induced electroadhesive contact problem. In this study, we have developed a full self-consistent contact model to study the CE-induced electroadhesion between a dielectric elastic axisymmetric parabolic surface and a dielectric rigid flat. Both surfaces are initially neutralized and undergo only one loading–unloading cycle. The surface interaction is determined by both the Lennard-Jones and the electrostatic interaction laws. The electrostatic problem is numerically solved to accurately calculate the electrostatic traction due to both the tribo-charges and bound charges. The normal traction, interfacial gap, and hysteresis loop of the force curve are thoroughly investigated. The effect of non-uniform tribo-charge density on the force curve and electrostatic traction is explored. The numerical model developed in the present work can serve as the foundation for more complex electroadhesive contact models. It also sheds light on the interfacial design of CE-related devices, e.g., contact-separation triboelectric nanogenerator.

Experimental Study to Determine the Development of Axial Stiffness of Wood Screws with Increasing Load Cycles

123 withdrawal tests were conducted to investigate the change in axial stiffness of fully threaded screws under axial loading and up to four loading cycles. The screws were initially loaded in two cycles within the elastic range, followed by two cycles up to 90% of the characteristic load-carrying capacity. Several parameters relevant to construction practice were varied. The angle between the screw axis and the grain ranged from 30° to 90°, the timber material was varied between glued laminated timber (glulam) and laminated veneer lumber (LVL) made of beech, and the screw diameter ranged from 8 mm to 12 mm. The test results indicate that axial stiffness increases upon reloading compared to the initial loading. On average, axial stiffness increases by 11% between the first and second loading and remains at this level during unloading and further load cycles. However, if the load exceeds the linear–elastic range, the axial stiffness is reduced due to plastic deformation. A comparison with tests on the composite axial stiffness of fully threaded screws in glulam shows that even with a different test setup and testing objective, there is a slight increase in axial stiffness from the first to the second load cycle in the range of 4 to 8%.

Flexible piezoelectric sensor based on PAN/MXene/PDA@ZnO composite film for human health and motion detection with fast response and highly sensitive

Polyacrylonitrile (PAN) nanofiber-based flexible piezoelectric sensors are recognized for their applicability in wearable electronic devices, personal health monitoring, and motion detection. This research explores the impact of dual fillers, MXene and polydopamine-modified zinc oxide (PDA@ZnO), on the characteristics of PAN-based piezoelectric composite nanofiber films, including planar zigzag conformation content, dielectric, ferroelectric, piezoelectric, and mechanical properties. By integrating PDA@ZnO as a filler, PAN/MXene/PDA@ZnO-5 (PMPO) piezoelectric sensors were developed, showcasing enhanced piezoelectric sensing capabilities. These sensors achieved a sensitivity of 28.56 V/N across a broad linear range, with superior mechanical stability and durability across 3000 loading–unloading cycles, and exhibited swift response and recovery times of 49 ms and 40 ms, respectively. Furthermore, their unique fiber structure and flexibility, when interfaced with the human body, enable the detection of minute physiological movements and the conversion of varied motions into distinct voltage outputs. Thus, these sensors offer considerable promise for applications in human health monitoring and motion posture correction.

Experimental Research on Anisotropy Characteristics of Shale under Triaxial Incremental Cyclic Loading and Unloading

Shale is a common rock type that is associated with underground engineering projects, and several important factors, such as bedding structure, confining pressure, and the loading and unloading path, significantly influence the anisotropy of shale. Triaxial monotonic loading tests and triaxial incremental cyclic loading and unloading tests of shale under three kinds of confining pressures and five types of bedding inclination angles (θ) were thus performed to investigate the anisotropy of shale in terms of mechanical behavior, acoustic emission (AE), and energy evolution, and reveal the mechanism by which shale anisotropy is weakened. The results show that (1) the compressive strength and elastic modulus of shale decrease and then increase as the θ increases, and that both σ3 and incremental cyclic loading and unloading reduce the anisotropy in terms of the compressive strength and elastic modulus of shale, with the ratio of plastic strain to total strain reaching its maximum at a θ of 60° during each loading and unloading cycle. (2) The failure modes of shale with θ of 0°, 30°, and 90° under triaxial monotonic loading are similar to the counterparts under triaxial incremental cyclic loading and unloading, while the failure modes of shale with θ of 45° and 60° differ significantly under the two loading conditions, and interestingly, the degree to which the bedding plane participates in shale crack evolution under incremental cyclic loading and unloading is considerably lower than that under triaxial monotonic loading. (3) The cumulative AE count and AE b-value of shale first decrease and then increase as the θ increases, while the Felicity ratio decreases as the number of cycles increases. (4) As the θ increases, the total energy density U0 and the parameter m, which reflects the accumulation rate of elastic energy, first decrease and then increase, with both reaching a minimum at a θ of 60°. (5) The mode by which cyclic loading and unloading leads to failure in shale with a θ of 60° is similar to that at a θ of 0° and is the main mechanism by which shale anisotropy weakening occurs as a result of cyclic loading and unloading. The results provide experimental support and a theoretical basis for safer and more efficient underground engineering projects that involve shale.

A Review on the Mullins Effect in Tough Elastomers and Gels

AbstractTough elastomers and gels have garnered broad research interest due to their wide-ranging potential applications. However, during the loading and unloading cycles, a clear stress softening behavior can be observed in many material systems, which is also named as the Mullins effect. In this work, we aim to provide a complete review of the Mullins effect in soft yet tough materials, specifically focusing on nanocomposite gels, double-network hydrogels, and multi-network elastomers. We first revisit the experimental observations for these soft materials. We then discuss the recent developments of constitutive models, emphasizing novel developments in the damage mechanisms or network representations. Some phenomenological models will also be briefly introduced. Particular attention is then placed on the anisotropic and multiaxial modeling aspects. It is demonstrated that most of the existing models fail to accurately predict the multiaxial data, posing a significant challenge for developing future anisotropic models tailored for tough gels and elastomers.

Characterization of a NEG cartridge under high pressure conditions

The development of advanced sintered pumping elements based on the ZAO® alloy, characterized by a high affinity towards hydrogen and its isotopes, make Non Evaporable Getter (NEG) pumps particularly appealing for applications in fusion research, which typically deal with large fluxes of these gaseous species. NEG getters show high pumping speeds and capacity, excellent reliability with ability to withstand a high number of loading and unloading cycles without showing any type of failure, ease of integration within a complex system (such as a fusion reactor) because of their high specific properties and the simplicity of the mechanism that regulates its operation. These properties make NEG technology particularly suitable for the development of compact, efficient and easily scalable solutions, which are also very safe, since the release of absorbed hydrogen is prevented in the case of power outage or subsystem failures, as it can occur only by providing an adequate amount of heat to the getter material. This paper reports the results of an experimental campaign conducted on a new type of NEG pump, aimed to obtaining an accurate characterization in response to different operating conditions. Hydrogen pumping speed was investigated in a range of pressures of particular interest for the activities of the Divertor Tokamak Test facility (between 0.1 and 10 Pa). The influence of nitrogen absorption and getter material operating temperature on the pumping performances of the NEG pump were also examined. Finally, numerical simulations have been performed to estimate the nominal pumping speed of the NEG pump, that is, the one acting at the absorbing surface, stripped of the effects of the conductance of the paths from the pump NEG to the pressure gauges.

A Low‐Cost and Do‐It‐Yourself Pressure Sensor Enable Human Motion Detection and Human–Machine Interface Applications

AbstractIn this work, cost‐effective and do‐it‐yourself capacitive pressure sensors are fabricated using readily available commercial components. The sensors are created in a single‐step process ‐by simply applying electrically conductive paint onto both sides of a porous melamine sponge. These sensors exhibit a wide‐range pressure sensing capability, spanning from 10 Pa to 100 kPa. The sensors showcase an impressively low limit of detection, detecting pressures as low as 10 Pa, and exhibit a moderate response time of 123 ms. Moreover, the sensors display remarkable repeatability and stability over 10 000 loading and unloading cycles without experiencing fatigue. Notably, these exceptional qualities come at an exceptionally low material cost, with the sensor measuring 20 × 20 × 2 mm. To showcase their potential applications, the fabricated sensors are successfully employed in real‐time human motion detection, proximity detection, and wearable keyboard applications.

Inverse regulation of SOS1 and HKT1 protein localization and stability by SOS3/CBL4 in Arabidopsis thaliana

To control net sodium (Na+) uptake, Arabidopsis plants utilize the plasma membrane (PM) Na+/H+ antiporter SOS1 to achieve Na+ efflux at the root and Na+ loading into the xylem, and the channel-like HKT1;1 protein that mediates the reverse flux of Na+ unloading off the xylem. Together, these opposing transport systems govern the partition of Na+ within the plant yet they must be finely co-regulated to prevent a futile cycle of xylem loading and unloading. Here, we show that the Arabidopsis SOS3 protein acts as the molecular switch governing these Na+ fluxes by favoring the recruitment of SOS1 to the PM and its subsequent activation by the SOS2/SOS3 kinase complex under salt stress, while commanding HKT1;1 protein degradation upon acute sodic stress. SOS3 achieves this role by direct and SOS2-independent binding to previously unrecognized functional domains of SOS1 and HKT1;1. These results indicate that roots first retain moderate amounts of salts to facilitate osmoregulation, yet when sodicity exceeds a set point, SOS3-dependent HKT1;1 degradation switches the balance toward Na+ export out of the root. Thus, SOS3 functionally links and co-regulates the two major Na+ transport systems operating in vascular plants controlling plant tolerance to salinity.

Ionic Strength Impacts the Physical Properties of Agarose Hydrogels.

Agarose is a natural polysaccharide known for its ability to form thermoreversible hydrogels. While the effects of curing temperature and polysaccharide concentration on mechanical properties have been discussed in the literature, the role of ionic strength has been less studied. In the present manuscript, we investigate the effects of supporting salt concentration and the role of cation (i.e. Na+ or Li+, neighbors in the Hofmeister series), on the setting and performance of agarose hydrogels. Compressive and rheological measurements show that the supporting salts reduce the immediate elastic response of agarose hydrogels, with Li+ showing a stronger effect than Na+ at high ionic strength, while they significantly increase the extent of linear stress-strain response (i.e., linear elasticity). The presence of increasing amounts of added supporting salt also leads to a reduction in hysteresis during mechanical deformation due to loading and unloading cycles, which is more pronounced with Li+ than with Na+. The combination of rheological measurements and NMR relaxometry shows a mesh size in agarose hydrogels in the order of 6-17 nm, with a thickness of the water layer bound to the biopolymer of about 3 nm. Of note, the different structuring of the water within the hydrogel network due to the different alkali seems to play a role for the final performance of the hydrogels.

Research on Damage Evolution Law of Glazed Hollow Beads-Cement/Sodium Silicate Grouting Materials under Different Cycles of Loading and Unloading.

With the depletion of shallow resources, deep resource mining has become a trend. However, the high temperature and complex stress environment in deep mines make resource extraction extremely challenging. This paper developed a thermal insulation grouting material made of glazed hollow beads, sodium silicate, and cement and tested the compressive strength, gelation time, and stone rate under various curing days in light of the issue of high temperature heat damage in high ground temperature mines and the impact of mining on roadway grouting bolt support. Fatigue strength, fatigue deformation, load-residual strain, energy evolution and microscopic features were studied and analyzed in relation to the damage law of graded cyclic loading and unloading under the number of varying cycles. The findings demonstrate that cyclic loading and unloading strength is lower than uniaxial compressive strength. The fatigue strength is significantly decreased when the number of cycles reaches its limit. Residual strain is less sensitive to changes in stress than load strain. The fitting correlation coefficients of total output energy and elastic energy are higher than 0.71.

Electric field–tunable self-sensing nanocomposites with aligned CNTs for in-situ pavement health monitoring: Electrodynamic alignment, sensor development, and performance validation

As urban societies increasingly rely on road infrastructures, the health of pavements becomes a pressing concern. Accurate in-situ monitoring is essential for early detection and maintenance, with tensile strain and temperature being pivotal health indicators. While traditional sensors, such as resistive strain gauges and fiber Bragg gratings, offer solutions, they face challenges in durability and sensitivity. This study introduces electric field-tunable self-sensing nanocomposites with aligned carbon nanotube (CNT) networks in an epoxy matrix as a promising solution for precise pavement health assessment. Through meticulous examination of electric field, environmental, and material parameters, an optimal alignment of CNTs was achieved. Optimization led to sensors that excel in capturing minute strains, showcasing impressive attributes like a gauge factor over 8, rapid response and recovery times of 8 ms and 7 ms respectively, and exceptional durability withstanding 20,000 loading–unloading cycles. Another distinctive feature is the dual-sensing capability, enabling concurrent monitoring of strain and temperature without mutual interference. In real-world applications, the developed sensors actively monitor dynamic pavement responses under moving rollers and vehicles at varied loading models. This innovative approach promises significant advancements in the realm of pavement health monitoring, addressing existing challenges and opening avenues for next-generation pavement sensor development.

Tuning Hydrogen Storage Properties in La1−xYxNi4.5Cu0.5 (x = 0.1; 0.2; 0.3; 0.4, 0.5) Alloys

Metal hydrides of AB5 compositions have been investigated over the years as they offer extraordinary volumetric hydrogen densities with high cycling stability and purity of released hydrogen. Moreover, by doping with different elements, the sorption properties of alloys can be significantly changed according to their foreseen applications. In this contribution, we report the synthesis routes and hydrogenation characteristics of La1−xYxNi4.5Cu0.5. The synthesized alloys exhibit excellent structural purity with all reflections indexed by the hexagonal P6/mmm structure. It was found that the Y content can easily tune (raise) the equilibrium pressures of the pressure–composition isotherms, while overall gravimetric density remains at a level exceeding 1.5 wt.% up to x = 0.3 then strongly decreases. The pressure range for desorption can be tuned from 1.5 to 5 bars at room temperature. Some alloys (x = 0; 0.2) exhibit very good stability during 1000 cycles of hydrogen loading and unloading. Furthermore, activation of the alloys is prompt, making them good candidates for stationary hydrogen storage, non-mechanical hydrogen compressors, or soft actuators.

Peripheral hypercapnic chemosensitivity at rest and progressive exercise intensities in males and females.

Peripheral hypercapnic chemosensitivity (PHC) is the ventilatory response to hypercapnia and is enhanced with acute whole-body exercise. However, little is known about the mechanism(s) responsible for the exercise related increase in PHC and if progressive exercise leads to further augmentation. We hypothesized that unloaded cycle exercise (0W) would increase PHC but progressively increasing the intensity would not further augment the response. Twenty healthy subjects completed 2 testing days. Day 1 was a maximal exercise test on a cycle ergometer to determine peak power output (Wmax). Day 2 consisted of six 12-minute stages: (i) rest on chair, (ii) rest on bike, (iii) 0W unloaded cycling, (iv) 25% Wmax, (v) 50% Wmax, and (vi) ~70% Wmax with ~10 min of rest between each exercise stage. In each stage, PHC was assessed via 2 breaths of 10% CO2 (~21% O2) repeated 5 times with ~45s between each to ensure end-tidal CO2 (PETCO2) and ventilation returned to baseline. Pre-stimulus PETCO2 was not different between rest and unloaded cycling (p=0.478). There was a significant increase in PHC between seated rest and 25% Wmax (0.71±0.37 vs. 1.03±0.52 LmmHg-1min-1, respectively p=0.0006), and between seated rest and unloaded cycling (0.71±0.37 vs. 1.04±0.4 LmmHg-1min-1, respectively p=0.0017). There was no effect of exercise intensity on PHC (1.03±0.52 vs. 0.95±0.58 vs. 1.01±0.65 LmmHg-1min-1for 25, 50 and 70% Wmax, p=0.44). The increased PHC response from seated rest to unloaded and 25% Wmax, but no effect of exercise intensity suggests a possible feedforward/feedback mechanism causing increased PHC sensitivity through the act of cycling.

Experimental evaluation of cyclic performance of steel slag and grids of truck scrap tire as a novel reinforced ballast material

Introducing a novel ballast material based on reinforcing steel slag aggregates with strips of scrap tire can be beneficial due to decreasing the crushing and increasing the damping of the ballast materials. Moreover, recycling the two waste materials included in this novel ballast has a positive impact on environmental preservation. In this regard, the Waste Tire Reinforcement (WTR) consists of strips of truck scrap tire (385/80/R22.5) with a width of 70 mm utilized in grid shape (with various dimension grids of 21 ×7, 11.5 ×7, and 7 ×7 cm) as a reinforcement element. To evaluate the performance of suggested reinforcement with Steel Slag Ballast (SSB), a series of ballast box tests were performed up to 100,000 loading and unloading cycles with a magnitude and frequency of 30 kN and 3 Hz at three different levels of 5, 10, and 15 cm for WTR placement in ballast. The experimental results revealed that the WTR reinforcement with grids of 11.5 × 7 cm and a placement height of 5 cm (WTR11.5–5) has the best performance compared to other configurations. Compared to SSB without reinforcement, WTR11.5–5 may reduce breakage by more than 90% and increase the damaging ratio by 160%.

Excellent elastocaloric and mechanical performance in Mn-doped Co–V-Ga as-cast polycrystalline alloy with simple preparation

With the rapid development of solid-state refrigeration technology, elastocaloric materials face higher requirements, such as a higher elastocaloric effect (eCE), better mechanical properties, and simpler and faster large-scale preparation. In this work, we propose a composition design strategy for Mn-doped Co–V-Ga shape memory alloys, which can enhance both the eCE and the mechanical properties. We combine first-principles calculations and experiments to demonstrate this strategy. Our results reveal that the lattice distortion caused by the difference in atomic radii produces a solid-solution-strengthening effect within the alloy, impeding dislocation motion and significantly improving the mechanical properties. Through composition optimization, the low-cost Co51.7V31.3Ga14.75Mn2.25 as-cast polycrystalline alloy exhibits a large adiabatic temperature change (ΔTad) of −11.2 K at room temperature, while its maximum compressive strength and strain can reach up to 1859 MPa and 26% at room temperature, respectively. Importantly, the alloy exhibits excellent cycle stability, and its ΔTad does not decay after 100 loading and unloading cycles, indicating that it has the potential for practical application. Further comparison with the annealed sample shows that the secondary defects deteriorate the ΔTad of the alloy. Our research provides an effective strategy for the simple, rapid and large-scale production of as-cast eCE alloys.

Impact induced compression and decompression waves in porous meta-materials modeled using a continuum theory of phase transitions

Porous meta-materials with both regular and random microstructure are of intense research interest today due to their interesting dynamical properties, including but not limited to, their acoustic band structure, shock absorption properties, and fracture toughness. Some of these materials can exist in a rarefied or densified state depending on the state of stress, and recover their original configuration after a cycle of loading and unloading. Often, they exhibit a hysteretic stress–strain response under quasistatic uniaxial compression. As such, many aspects of their mechanical behavior can be captured using a continuum theory of phase transitions. In this work, the dynamical behavior of such materials is explored. It is shown that the impact problem for these materials can result in propagating shocks, phase boundaries, and fans. The impact problems admit multiple solutions for the same set of initial and boundary conditions leading to non-uniqueness. This non-uniqueness can be remedied using a nucleation criterion and kinetic laws, as is known from the continuum theory of phase transitions. The fan solutions which arise in decompressive impact problems have not received much attention in the literature and may be regarded as a novel contribution of this work. The analysis presented here may have applications in the dynamic behavior of a broad class of porous materials including architected truss-like metastructures and random fiber networks.

Designing a Compact High-precision Positioner with Large Stroke Capability for Nanoindentation Devices

A new design of a fine positioner or high precision driven unit with a large positioning range is proposed for a custom-made in-situ indenter device equipped inside an SEM chamber. The design configuration of the proposed system is size-effective for the confined working area of the SEM chamber. The indentation depths can be precisely varied by controlling the fine positioner driven by a piezoelectric actuator. The main goal is to achieve very deep penetrations toward the bottom layers of tall or large-size scale specimens by single indentation, without the need for sequential indentations. Thus, the proposed design can eliminate the need for sequential adjustments of the specimen position with respect to the indenter tip as currently being practiced by the researchers. The specimen position adjustment after each indentation heavily depends on the coarse positioner and its accuracy level in a sub-millimeter regime which could result in position errors and unwanted lateral forces in the nanoindentation process. Therefore, the sequential indentations technique could lead to considerable variations in the outcomes of nanoindentation tests done on similar specimens. The proposed design will be realized to deploy in the Continuous Stiffness Measurement (CSM) techniques generally used to evaluate elastic properties as a function of continuous penetration depth with high-frequency loading and unloading cycles.

Balloon inflation revisited

In the present work, an inexpensive setup of an experiment of balloon inflation is described in order to measure its pressure and diameter. As already children know from blowing into such a (toy) balloon, the initially necessary high pressure level decreases with increasing balloon diameter. The original intention of this project has thus been to quantify this effect, which from a theoretical point of view is known as material instability. With a corresponding instrumentation of the test setup, it is additionally possible to realise and analyse further results for different load scenarios such as loading and unloading cycles, which are able to (re)produce by the presented system quite as easy.