Unloading Cycles Research Articles

A Stretchable Conductive Material with High Fatigue Resistance for Strain Sensors.

Intrinsically stretchable conductive materials based on elastic substrates and conductive components play important roles in biomedical applications, such as exercise rehabilitation monitoring and disease prediction. A persistent challenge is to combine high fatigue resistance with excellent mechanical properties in stretchable conductive materials. Herein, we present a stretchable conductive material with both good fatigue resistance and high tensile properties (∼3170%) based on poly(acrylic acid)-phytic acid-trehalose-polypyrrole (denoted as PPTP). The as-prepared PPTP hydrogel electrode showed no obvious cracking or delamination after 400 loading and unloading cycles and maintained good electrical signal transmission function after 1000 cycles. We further collected stable signals for human motion and handwriting using the stretchable hydrogel electrode as a strain sensor, demonstrating the potential application of the PPTP stretchable hydrogel electrode in biomedicine.

Rapid crack-based assessment of deep beams based on a single crack measurement

Deep beams in concrete infrastructure often exhibit wide diagonal shear cracks that extend from the supports to concentrated loads. A key problem in these cases is to assess the residual capacity against shear failure across such cracks. This problem can be solved by establishing a relationship between the opening of the crack and the residual capacity, expressed in percentage of the shear strength. The current study establishes such a relationship that requires minimal input information. The input consists of only three on-site measurements: the depth of the critical loading zone (CLZ) determined by the diagonal crack; the angle of the crack in the CLZ; and a single crack measurement: the vertical crack displacement in the vicinity of the CLZ. The physical basis of the proposed crack-based assessment (CBA) is established with the help of a targeted test and advanced modelling. It is shown that only three input parameters and two simple closed-form equations are sufficient to accurately evaluate the residual capacity of diagonally cracked deep beams with various properties. Furthermore, in the presence of loading and unloading cycles, the proposed CBA reveals how close the member has come to shear failure throughout its entire service life. These properties render the CBA not only suitable for rapid assessment, but also for long-term monitoring of deep beams with diagonal cracks.

Mode I crack propagation in polydimethylsiloxane-short carbon fiber composites

This paper presents a comprehensive analysis of reinforcement toughening and failure mechanisms in polydimethylsiloxane (PDMS)-carbon fiber (CF) composites employing an approach combining experiments and numerical simulations. Through a series of meticulously designed mechanical experiments, the behavior of the composite material under varying conditions is thoroughly examined. The introduction of CFs enhances the stiffness of the material while also leading to debonding and an increased Mullins effect. A constitutive model replicating the observed reinforcement and damage behavior is implemented. Our investigation extends to the analysis of crack growth through both numerical simulations and microscopic morphological examinations. A cohesive zone model is subsequently utilized to simulate crack propagation, providing enhanced understanding of the relationship between structural characteristics and mechanical behaviors. The process of crack propagation subjects the materials to cycles of loading and unloading, highlighted by the reinforcing action of CF pinning and stress transfer, alongside toughening mechanisms attributed to a variety of dissipative processes: interfacial debonding damage, energy loss due to CF pull-out, the Mullins effect, and viscous energy dissipation. This study elucidates the complex mechanical interplay within PDMS-CF composites and suggests pathways for their design optimization, significantly broadening their applicability in numerous domains.

Piezoresistive response and self-sensing properties of UHPC reinforced with recycled carbon fibers

This research explores Ultra-High-Performance Concrete (UHPC) reinforced with recycled carbon fibers to verify its potential application in self-sensing concrete for structural health monitoring. The improved electrical conductivity due to the inclusion of carbon fibers enables the emergence of piezoresistivity, transforming UHPC into a sensitive sensor for detecting mechanical stresses. By incorporating various characterization tests alongside investigations into fiber type (loose versus fibrillated sheets) and content variations, the study aims to establish a correlation between the electrical properties and the resulting piezoresistive response. Resistivity will be measured using impedance scanning, while piezoresistivity will be determined through loading and unloading cycles with a datalogger recording the variations in electrical resistance. The inclusion of carbon fibers suggests an improvement in mechanical performance, up to 42 % in flexural strength and 26 % in compression compared to UHPC without fibers. Resistivity ranges from 1 to 2 Ω⋅m and gauge factor reach 38. These results are comparable to those reported in other studies on UHPC with carbonaceous additions, highlighting the potential of recycled carbon fibers as a sustainable and effective reinforcement strategy.

Harsh Environment-Tolerant, High Performance Soft Pressure Sensors Enabled by Fiber-Segment Structure and Plasma Treatment.

As the demand for specialized and diversified pressure sensors continues to increase, excellent performance and multi-applicability have become necessary for pressure sensors. Currently, flexible pressure sensors are primarily utilized in fields such as health monitoring and human-computer interaction. However, numerous complex extreme environments in reality, including deep sea, corrosive conditions, extreme cold, and high temperatures, urgently require the services of flexible devices. Here, a piezoresistive flexible pressure sensor based on expanded polytetrafluoroethylene/functionalized carbon nanotubes (EPTFE/FCNT) is proposed. Benefiting from the unique fiber-segment architecture, chemical stability, and strong chemical binding force between EPTFE and FCNT, the fabricated sensor exhibits remarkable sensing capabilities and can be employed in multifarious extreme environments. It demonstrates a sensitivity of 862.28kPa-1, a response time of 6-7ms, and a detection limit below 1Pa. Furthermore, it possesses a pressure resolution of 0.0018% under 111kPa and can withstand over 10,000 loading and unloading cycles under 1MPa. Additionally, the EPTFE/FCNT sensor retains its outstanding pressure response and work efficiency in extreme conditions such as an ultra-low temperature of -80°C, high temperature (200°C), acidic and alkaline corrosion, and underwater. These notable attributes enormously broaden the sensors' real-world application range.

Mechanical Properties of Latex-Modified Cement Stone under Uniaxial and Triaxial Cyclic Loading.

During the cyclic injection and extraction process in underground storage wellbores, the cement sheath undergoes loading and unloading stress cycles. In this study, we investigated the mechanical properties of latex-modified cement stone (LMCS), widely used in oil and gas wells, through uniaxial and triaxial cyclic loading and unloading tests. The aim of the study was to determine the effect of various loading conditions on the compressive strength and stress-strain behavior of LMCS. The results show that the stress-strain curve of LMCS exhibits a hysteresis loop phenomenon, with the loop intervals decreasing throughout the entire cyclic loading and unloading process. As the number of cycles increases, the cumulative plastic strain of the LMCS increases approximately linearly. Under uniaxial cyclic loading and unloading conditions, the elastic modulus tends to stabilize. However, under triaxial conditions, the elastic modulus increases continuously as the number of cycles increases. This result provides data for engineering predictions. Furthermore, a comparison of the uniaxial and triaxial cyclic loading and unloading of LMCS shows that its cumulative plastic strain develops rapidly under uniaxial conditions, while the elastic modulus is larger under triaxial conditions. These findings provide a valuable reference for constructing underground storage wellbores.

Cardiac dysfunction in dialysing adults with end-stage kidney disease is associated with exercise intolerance: A pilot observational study.

People with end-stage kidney disease (ESKD) often exhibit impaired cardiac structure and function, which may contribute to poor exercise capacity. This study used multimodal exercise testing to investigate the central and peripheral mechanisms of exercise limitation in adults with ESKD, also comparing in-centre hemodialysis (ICHD) to home hemodialysis (HHD). Seventeen adults (55.5 ± 14.5 years; n = 14 male; n = 12 HHD) participated. Resting cardiac examinations, followed by submaximal cycling cardiopulmonary exercise testing (CPET) and functional exercise testing, revealed cardiac structural abnormalities (increased left ventricular mass) and cardiac injury. Aerobic fitness in adults with ESKD was low, with pulmonary oxygen uptake (V̇O2) at the gas exchange threshold (GET) occuring at 39 ± 8% predicted V̇O2peak. O2 pulse, an estimate of stroke volume (SV), was higher in HHD at rest (p = 0.05, ES = 0.58) and during unloaded cycling (p = 0.05, ES = 0.58) compared to ICHD. However, thoracic bioreactance derived SV at the GET was significantly higher in adults receiving ICHD versus HHD (p = 0.01, ES = 0.74). In adults with ESKD, cardiac output was positively associated with V̇O2 at the GET (r = 0.61, p = 0.04). This study highlights prevalent exercise dysfunction in adults with ESKD undergoing dialysis, with potential distinct differences between in-centre and home hemodialysis, mechanistically linked to underlying cardiac abnormalities.

Performance and behaviour of prebored and precast pile with floating pile tip based on A full-scale field static axial load test

This study investigates the load transfer mechanism of a Prebored and Precast pile (PP pile), constructed installed in accordance with the rules applicable to the Hyper-Straight pile method (HS pile), in clay soils. While the HS pile method, developed in Japan, typically results in high bearing capacity piles in various soil types, its performance in clay soils remains understudied. Our research focuses on a unique configuration where the pile tip “floats” within a soil–cement mixing (SCM) column near the bottom of the borehole, a condition that significantly influences the system’s performance.We conducted a full-scale axial static load test on a 500 mm diameter and 140 mm thickness straight shaft precast prestressed concrete spun pile. The pile was instrumented with vibrating wire strain gauges (VWSG) and displacement measuring devices (tell-tales), embedded 15 m deep in a 750 mm diameter SCM column (15.75 m long). The pile tip was positioned 75 cm above the bottom of the borehole, creating a floating condition within the SCM material. Both the pile and the surrounding SCM were instrumented to provide comprehensive data on the system’s behavior.The test involved two loading–unloading cycles. The 1st Cycle reached a maximum load of 3627 kN, resulting in a 75.52 mm pile head settlement. The 2nd Cycle achieved a maximum load of 4181 kN, leading to a 118.04 mm pile head settlement. In the 1st Cycle, we observed upward movement of the SCM material around the shaft after the pile skin friction reached its maximum capacity. Stress at the pile tip exceeded the unconfined compressive strength of the SCM material, indicating potential local shear failure.Contrary to expectations based on HS pile performance in other soil types, the ultimate bearing capacity of our pile was determined to be 2000 kN, comprising 545 kN from skin friction and 1455 kN from end bearing. This result aligns more closely with the behavior of conventional bored pile rather than the “hyper” capacity typically associated with HS pile. Consequently, we classify our pile as a “prebored and precast pile,” like systems used in China and Korea.Our study concludes that the strength of the SCM material and the pile tip location significantly influence the pile’s bearing capacity in clay soils. These findings highlight the critical impact of soil type on the performance of piles constructed using the HS method. The observed behavior suggests that current design methods for HS pile may overestimate capacity in clay conditions, emphasizing the importance of soil-specific analysis and testing.This research contributes to the understanding of PP pile behavior in clay soils, providing valuable insights for geotechnical engineers. It underscores the need for refined prediction models and design methods specific to these soil conditions, paving the way for more accurate and reliable foundation designs in regions with predominant clay soils.

Investigating the influence of graphene coating thickness on Al0.3CoCrFeNi high-entropy alloy using molecular dynamics simulation

Graphene was proven to be an excellent surface-strengthening material. However, there needs to be more literature explaining its strengthening mechanism at the atomic level. This study systematically investigated the surface strengthening mechanism of graphene coating on the Al0.3CoCrFeNi high-entropy alloy (HEA) and the influence of loading-unloading cycles on the formation of internal defects in the material through molecular dynamics (MD) simulations at the atomic level. Research results show that the internal stress concentration in HEA without graphene coating leads to a more considerable residual depth after unloading. With the addition of a graphene coating, the material's stress area increases, effectively alleviating stress concentration and reducing residual depth. Furthermore, the lengths of Lomer-Cottrell dislocation locks and Hirth dislocation locks inside the material increase, enhancing the material's deformation resistance. The more layers of graphene, the larger the stress distribution area and the greater the hardness. Under the same indentation force, the number of HCP structures and amorphous structures inside the material is inversely proportional to the thickness of the graphene layers. In addition, after multiple loading and unloading cycles, the hardness of HEA/GP materials basically remains unchanged, while internal defects increase and residual depth increases.

Biobased, degradable and directional porous carboxymethyl chitosan/lignosulfonate sodium aerogel-based piezoresistive pressure sensor with dual-conductive network for human motion detection

The rapid growth of wearable electronics, healthcare monitoring, and human–computer interaction has sparked growing interest in flexible piezoresistive pressure sensors, which in turn raised considerable concerns about the increasingly serious environmental pollution caused by electronic waste. Herein, a radial inward directional freezing-drying method was proposed to prepare biobased, degradable and directional porous carboxymethyl chitosan/lignosulfonate sodium aerogel with dual-conductive network constructed by carboxylated multiwalled carbon nanotubes (C-CNTs) and MXene for piezoresistive pressure sensor. Owing to the synergistic conductive network of C-CNTs and MXene as well as directional porous structure, the sensor exhibited high sensitivity (2.33 kPa−1 in 0–10 kPa, 1.04 kPa−1 in 10–31 kPa, 0.53 kPa−1 in 31–53 kPa), fast response (response/recovery time of 140/60 ms), and excellent repeatability (2,000 loading–unloading cycles). Moreover, the sensor was successfully applied for human motion detection, healthy monitoring, micro-expression identification, and speech recognition. In addition, the aerogel for sensor was able to be completely degraded within 70 h in 1 M hydrochloric acid solution or partially degraded in boiling water within 2.5 h for the recycling of conductive materials, which was beneficial for not only environment protection but also saving resource. Our findings conceivably stand out as a new strategy to prepare environmentally friendly and high-performance piezoresistive pressure sensor and will promote the further development and application of flexible electronics.

Elastoplastic mechanical behavior analysis of surface-coated Zircaloy-4 cladding under multi-field coupling

The elastoplastic mechanical behavior of Zircaloy-4 (Zr-4) cladding, coated with chromium (Cr) or FeCrAl on its surface, is explored under the coupled effects of multi-field coupling. Utilizing the Finite Element Software ABAQUS, simulations are conducted to calculate the evolution of stress and strain over two complete fuel cycles. Comparisons are drawn between the coated and uncoated Zircaloy-4 cladding materials. The results indicate that the application of surface coatings significantly mitigates stress levels in the cladding during the first fuel cycle. During the second fuel cycle, all three types of cladding exhibit relatively minor plastic strain, which is attributed to the unloading and reloading process between cycles. Notably, the plastic zone propagates from the interior to the exterior of the cladding. When compared to traditional Zircaloy-4 cladding, the coated cladding exhibits improved elastoplastic mechanical behavior. The operational mechanism of the coating for different stresses in cylindrical coordinates and its response to unloading and reloading cycles are also investigated. Specifically, the coated claddings exhibit an evident delay in reaching full plasticity compared to uncoated claddings. Furthermore, FeCrAl coating material initially shows good performance, and it needs to be verified in more aspects in the future. Results and Conclusions in this paper can provide reference and guidance for future experiments.

Flexible and sensitive three-dimension structured indium tin oxide/zinc aluminum oxide/Cu composites thin-films pressure sensors for healthcare monitoring

Flexible pressure sensors play a crucial role in large-scale commercial applications, such as health monitoring, robotics, and smart devices. However, mass production of piezoelectric sensors for industrial applications is limited by their complex and cost-intensive fabrication processes. In this study, sensitive, simple, and wearable piezoelectric sensors were prepared by magnetron sputtering of indium tin oxide/zinc aluminum oxide (AZO)/Cu composite thin films onto three-dimension structured polydimethylsiloxane) substrate. The prepared flexible sensors delivered output voltages of −200 mV and +200 mV during 1000 loading and unloading cycles, respectively, showing an outstanding sensitivity and cycling performance. The piezoelectric performance of AZO films is primarily determined by the doping concentration of aluminum in AZO; the output voltage of the sensor with the optimal doping concentration was approximately 340 % higher than that of the undoped sample. Finite-element simulations demonstrated that the substrate with a square platform exhibited the most uniform stress distributions as well as the highest stress on its microstructure surface, which resulted in a high stress and sensitivity of the thin-film sensors. Experiments revealed that the incorporation of a microstructure enhanced the output voltages of the sensors. In particular, the output voltage delivered by the flexible sensors with square microstructures was approximately 60 % higher than that exhibited by those without microstructures.

Superelasticity with narrow stress hysteresis and high cyclic stability in Co–V–Al polycrystalline alloy

Superelasticity of shape memory alloy associated with the martensitic transformation is extremely attractive for applications in sensors and biomedical. This application requires narrow stress hysteresis to reduce energy dissipation and a stable superelasticity cycle is an important factor in their practical applications. Co–V–Al alloys have inimitable advantages due to their narrow stress hysteresis, low-cost, and corrosion resistance, but the low number of superelasticity cycles restricts its development. Here, we report an intrinsic Co58V29Al13 polycrystalline alloy with a superelasticity stress hysteresis of 30 MPa at room temperature, the lowest among the alloys in this system, and a superelasticity strain of 3.6%. In addition, the sample shows no significant decay trend after 500 loading and unloading cycles. The high number of superelasticity cycles is mainly due to the elastic stress field generated around the dislocation during the cycle, which provides internal stress for martensite nucleation and reduces energy dissipation during martensitic transformation. This work provides essential guiding significance for the design of high-performance superelasticity alloys.

Experimental study on full-scale glass roof panels with embedded laminated connections under the short-term and long-term loading

This experimental study covers a problematic of full-scale roof panels with four embedded laminated point connections under two types of loads - short-term loads on intact panels, and long-term uniform loads after the fracture of a thermally treated glass pane. Two types of panels consisting either of fully tempered, or heat strengthened glass plies in combination with float glass, were tested, and a specific type of Ethylene Vinyl Acetate foil was used as the interlayer. The first part of the study investigates short-term load bearing capacity of horizontally suspended panels. The load was applied in several loading and unloading cycles until the collapse of any connection. The second part of the study is focused on the residual load-bearing capacity of the roof panels after a partial failure, whereas a first step, the thermally treated glass plies were intentionally fractured. The long-term part of the experiment was performed within 65 days. During this period, the load was increasing until a collapse of any connection. In both types of experiment, a difference between specimens with fully tempered and heat strengthened glass was observed. In the short-term part of the experiment, the specimens with fully tempered glass failed in two steps and therefore, showed considerable post-failure load bearing capacity. On the other hand, the specimens with heat strengthened glass collapsed suddenly in one moment giving no post-failure capacity. In the long-term part of the experiment, the resistance of specimens with heat strengthened glass was higher than the resistance of specimens with fully tempered glass. Therefore, both types of panels might find its purpose in real applications if the needs of the structure are properly considered.

EXPERIMENTAL DETERMINATION OF FLOOR PANELS DEFORMABILITY OF A LARGE-PANEL SYSTEM BUILDING AFTER RENOVATION

This paper deals with a field tests of prefabricated reinforced concrete floor panels of a multistory building by the method of hydrostatic loading. Object of study is presented by 3 damaged floor slabs of a 9-storey residential building which was hit during the russian invasion. After renovation of panels using the constructive solution which consists of a reinforced build-up layer in the compressed zone (upper part) and additional external reinforcement in the stretched zone (lower part) connected to each other with the help of anchor connections, a series of tests with water loading were set. For creation of loading an inventory pool was used, which was gradually filled with water at a step of 1kN/m2. Maximum created loading level was 5kN/m2 which corresponds to the operational load. After reaching the maximum load, the panels were left under load for 15 hours and then fully unloaded in steps of 1kN/m2. For each panel two type of tests were established: first when panel had both upper and lower part of reinforcement, and second – when lower reinforcement within steel beams was removed. The idea of dismantling of steel beams connected with previously performed numerical analysis which gave to little difference in deflections of panels for these two cases. Deflection measurements at critical locations along with thorough visual inspection were conducted during the loading and unloading cycles. No damage was observed during the in-situ loading and unloading cycles. Maximum vertical deflection for panel before dismantling of beams lies in the diapason of 1.95-2.1mm and 2.31-2.4mm after they were removed. These results meet the requirements of regulatory restrictions on the maximum deflection equal to 20 mm for this structure. Difference in deflection for two cases doesn’t exceed 20% which allows to abandon the use of lower reinforcement in the form of steel beams in the future. As the size of the pool doesn’t cover the whole panel area, the equivalent uniformly distributed load was also calculated using Lira software. Keywords. Large panel system house, reinforcement, floor panel, method of hydrostatic loading, deflection.

Breathing reserve as a predictor of postoperative pulmonary complications in patients with lung cancer

The objective was to assess the possibility of using breathing reserve (BR) to evaluate the individual risk of postoperative pulmonary complications (PPC) in patients who underwent open surgery for lung cancer.Materials and methods. The study involved 185 patients who underwent open surgery for lung cancer in the clinic of the Pavlov University in 2018–2020. All patients underwent cardiopulmonary exercise testing (CPET) in the preoperative period to determine the BR. All patients were retrospectively divided into 2 groups depending on the presence of PPC during 7 days after the surgery. To assess the information content of BR for predicting PPC and their outcome, the data were statistically processed: the Mann–Whitney U-test, Fisher’s exact test, Youden index and linear regression method were used.Results. PPC developed in 7 patients (3.8%), in 3 of them (42.9% of the group with PC and 1.6% of the total group) they were accompanied by acute respiratory failure (ARF), requiring reintubation and mechanical ventilation; these patients died. At the anaerobic threshold (AT), there were significant differences in BR (p = 0.003). A direct correlation was found between BR at the AT not only at the peak load but also during the unloaded cycling (UC) (closeness of connection on the Chaddock scale BR (AT) – BR (peak) ρ = 0.724, BR (AT) – BR (UC) ρ = 0.734, p < 0.001). The chances to develop PC changed as follows: in the group of patients with BR (UC) < 72.025% were 21.4 times higher (95% CI: 2.499 – 182.958); with BR (AT) < 44.136% were 27.2 times higher (95% CI: 4.850 – 152.167); with BR (peak) < 36.677% were 7.6 times higher (95% CI: 1.426 – 40.640).Conclusions. Dynamic measurement of the BR is informative at all stages of CPET. The risk of PPC and their unfavorable outcome increases when the BR is below 72.025% at the unloaded cycling, below 44.136% at the anaerobic threshold and below 36.377% at the peak load. BR can be used as a marker of the development of PPC in patients undergoing lung cancer surgery.

Wearable skin integrated flexible PVA‐nickel nanocomposite patch for temperature sensor

AbstractThe study focuses on developing a noninvasive, flexible polyvinyl alcohol (PVA)‐nickel (Ni) nanocomposite thin film as a passive skin‐integrated patch sensor for wearable temperature monitoring, marking a significant advancement in the field of wearable sensors. This flexible PVA‐Ni nanocomposite thin film serves as a temperature‐sensitive material, offering several advantages over commercially available active‐type sensors. Field emission scanning electron microscopy (FE‐SEM) images show that Ni nanoparticles (NPs) are broadly dispersed throughout the PVA matrix, indicating the effectiveness of the conductive patch in detecting temperature variations. A PVA‐Ni nanocomposite patch with a thickness of 0.08 mm demonstrated superior flexibility, breathability, and lower tearing strength compared to a pure PVA patch. The film also exhibited excellent repeatability in bending tests, maintaining performance after 120 bending and unloading cycles, suggesting its durability for long‐term use as a wearable sensor. Furthermore, the fabricated sensors function as thermistors, with conductivity increasing linearly with temperature. The performance of these temperature sensors was compared, revealing a highest temperature coefficient of resistance (TCR) and thermal index of −1.08%/°C and 1271 K, respectively. The sensors showed high temperature sensitivity between room temperature and 50°C, outperforming typical commercial platinum temperature sensors. The stability and response time of the PVA‐Ni nanocomposite film were evaluated by adhering the patch to a human wrist and capturing thermal images using a FLIR thermal imaging camera. The observed maximum temperature difference of approximately 1.9–2.1°C highlights the patch's sensitivity in detecting temperature changes. Additionally, the antimicrobial properties of the conductive film were tested to assess its biocompatibility, confirming its potential for applications in energy storage, thermal management, and early breast cancer detection.

RUTTING IN ASPHALT MIXTURES – A REVIEW

Vehicles that pass over a pavement structure induce load and unload cycles that generate recoverable (resilient) and permanent (plastic) deformations within layers. Pavement has been developing studies since the decade of the 60´s with the purpose of attempting to understand the visco-elasto-plastic behavior of asphalt mixtures. This has the purpose of evaluating and understanding how the main damage mechanisms that are sought to be controlled in flexible pavement design methods are generated. One of the main damage mechanisms for these types of pavements is rutting. Based on the reviewed literature in relation to the phenomenon of rutting in asphalt mixtures, this article depicts and describes in summary, the main variables that influence the generation of said phenomenon in pavement. The end of the article presents the evolution of equations developed through theoretical and experimental studies in order to attempt to predict the phenomenon of rutting.

Strong and tough conductive hydrogels via an amphiphilic macromolecular crosslinker for flexible strain sensor

The strength and toughness of hydrogel are usually achieved through the synergy of chemical cross-linking and physical cross-linking from multi-component contributions. However, there are few reports on hydrogels that rely on a single component to obtain two cross-linked structures. Herein, we successfully synthesized an amphiphilic macromolecular crosslinker (polyoxyethylene 20 oleyl ether acrylate (O20AC)), and introduced it into a system composed of acrylamide (AAm), sodium chloride (NaCl) and water to prepare the P(AAm-O20AC)/NaCl hydrogel with high strength, outstanding toughness, and excellent good conductivity. Notably, the O20AC possess both crosslinker properties, which not only provide hydrogels with a stable chemical cross-linking network, but also its long hydrophobic chain act as physical crosslinking points to give the hydrogel a new pathway of energy dissipation.Depending on the O20AC crosslinker, the P(AAm-O20AC)/NaCl hydrogels exhibit good strength (tensile stress 714.46 kPa), toughness (elongation at break 3005 %), and fatigue resistance (overlap of hysteresis curves in repeated loading–unloading cycles). Besides, the existence of sodium ions and chloride ions also provides the hydrogel with highly sensitive deformation-dependent conductivity (GF is 5.8 in the 400 %-500 % strain range). Collectively, the strategy of preparing chemical and physical double-crosslinked hydrogels by amphiphilic O20AC will provide a bran-new insights for the development of hydrogel sensing materials.

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.