Unloading Cycles Research Articles

Molecular dynamics analysis on bending mechanical behavior of alumina nanowires at different loading rates

The molecular dynamics (MD) model of α-Al2O3 nanowires in bending is established by using LAMMPS to calculate the atomic stress and strain at different loading rates in order to study the effect of loading rate on the bending mechanical behaviors of the α-Al2O3 nanowires. Research results show that the maximum surface stress—rotation angle curves of α-Al2O3 nanowires at different loading rates are all divided into three stages of elastic deformation, plastic deformation and failure, where the elastic limit point can be determined by the curve symmetry during loading and unloading cycle. The loading rate has great influence on the plastic deformation but little on the elastic modulus of α-Al2O3 nanowires. When the loading rate is increased, the plastic deformation stage is shortened and the material is easier to fail in brittle fracture. Therefore, the elastic limit and the strength limit (determined by the direct and indirect MD simulation methods) are closer to each other. The MD simulation result of α-Al2O3 nanowires is verified to be valid by the good agreement with the improved loop test results. The direct MD method becomes an effective way to determine the elastic limit and the strength limit of nanoscale whiskers failed in brittle or ductile fracture at arbitrary loading rate.

Effect of Ambient Humidity on the Elasticity and Deformation of Unweathered Granite

AbstractMoisture variation has been noted as a driver of strains in both natural and anthropogenic bedrock environments. Similarly, increasing water content has been shown to reduce elastic stiffness in a variety of rock materials, though evidence for granites is limited. This study presents strains of an axially stressed (0.1 MPa) Herrnholz granite cylinder with ambient relative humidity alternating between 20% and 90% in a stepwise manner. At each ambient humidity level, the Young's modulus and Poisson's ratio were determined by performing a series of load and unload cycles at 1–12 hr intervals. We observed that when the ambient humidity increased from 20% to 90%, Young's modulus declined by 13% on average, and nearly returned to the initial dry value when the ambient humidity was reduced to 20%. Poisson's ratio increased by 130% in response to the same humidity change, and approximately 60% of it was reversed when dried. These changes occurred linearly with a volumetric strain of up to 8 × 10−4 of the tested sample, equivalent to 24.5 MPa internal stress. This stress change is attributed to the generation of nanoscale adsorption stress, which includes surface adhesion pressure along pore walls and capillary pressures within characteristic pore spaces. Using a modified adsorption model, the two contributions were estimated to be 8.5 and 16 MPa, respectively. The agreement between laboratory‐derived and modeled stress change validates the proposed mechanism of adsorption‐induced strains and elastic property variations in unweathered granite.

Energy Evolution Characteristics of Sandstones During Confining Pressure Cyclic Unloading Conditions

Energy Evolution Characteristics of Sandstones During Confining Pressure Cyclic Unloading Conditions

Mechanical Characteristics of Sandstone under High Temperature and Cyclic Loading in Underground Coal Gasification

In the process of underground gasification of coal, the top rock of coal seam will experience a certain high temperature action and be in a complex stress environment. Therefore, it is of great theoretical and engineering significance to study the effect of cyclic unloading and loading on mechanical properties of rocks under high temperature action. In this thesis, the stress–strain curves of sandstone under different high-temperature treatments are obtained by conducting graded loading and unloading tests on sandstone treated at room temperature and at 200 °C, 400 °C, 600 °C, and 800 °C, respectively. The research content of this paper is as follows: the peak stress, peak strain, elastic modulus, Poisson’s ratio, internal friction angle, and cohesion of sandstone in the destruction stage of sandstone. The results show that the peak strain and cohesion of sandstone show an increasing trend with the increase of temperature from room temperature to 800 °C; the peak stress shows a decreasing trend with the increase of temperature from room temperature to 800 °C; the modulus of elasticity tends to increase from 200 °C to 400 °C and to decrease with temperature in the rest of the period; the Poisson’s ratio tends to increase from 600 °C to 800 °C and to decrease with temperature in the rest of the period; the internal friction angle increases sharply within room temperature to 200 °C, decreases slowly within 200–600 °C, and decreases sharply when the temperature exceeds 600 °C. The results of the study will provide important reference significance for the design and engineering application of the gasifier of a coal-bed underground gasification project.

Characteristics of Elastoplastic Consolidation by Compaction and Its Effects on Coal Permeability

This paper presents a combined experimental–analytical investigation of coal strain development under fluctuating applied hydrostatic stress. The laboratory setup mimics the isotropic volumetric compaction of coal under burial–uplift cycles in the absence of tectonic stress. Special emphasis is placed on the corresponding permeability evolution of the coal strata. Our results show that the stress–strain path is exponential, approaching a linear relation in the logarithmic stress–strain space with the monotonic increase in stress. A similar behavior is found for the strain–permeability path in the logarithmic strain–permeability space. The permeability recovery undergoes hysteresis with respect to the stress in a stress loading–unloading cycle, but the hysteresis is not manifest with respect to the strain. A theoretical geomechanical consolidation analysis was performed using an elastoplastic modelling framework. The analysis suggests that plastic strain is the cause of the hysteresis of the strain recovery in a stress loading–unloading cycle. The closed hysteresis loops manifested in stress loading–unloading–reloading cycles are promoted by the plastic strain during stress unloading and the difference in evolution rates of the elastic core between loading and unloading. The results of this study are helpful for understanding the mechanism of permeability evolution and optimizing water and coal seam gas production.

Achieved good mechanical properties and large elastocaloric effect in Ni-Mn-Ti-Cu-B alloy: Experiments and first-principles calculations

Solid-state refrigeration relying on the caloric effects of materials during phase transformation has attracted extensive attention for its great potential to replace the conventional vapor-compression technique. Here, we have optimized the mechanical properties of the Ni-Mn-Ti alloys via Cu and B co-doping. The reasons for the enhanced mechanical properties were systematically studied by experiments and first-principles calculations. The (Ni50Mn30.75Ti18.25Cu1)99.8B0.2 directionally solidified alloy can reach maximum compressive strength and strain of 2860 MPa and 20.4% at room temperature, respectively. A large adiabatic temperature change of up to 29.2 K can be achieved during stress-induced martensitic transformation. Besides, the adiabatic temperature change between loading and unloading cycles at 4% strain can be maintained at ∼11.3 K and basically do not decline after 650 cycles, which indicates that the present alloy has good cyclability of elastocaloric effect. Simultaneously achieved large elastocaloric effect and good mechanical properties at room temperature, Ni-Mn-Ti-Cu-B alloy has a great prospect for solid-state refrigeration applications.

IMPLEMENTING BREATH-BY-BREATH VOLUMETRIC CAPNOGRAPHY DEAD-SPACE COMPONENT CALCULATION DURING INCREMENTAL CYCLE EXERCISE

IMPLEMENTING BREATH-BY-BREATH VOLUMETRIC CAPNOGRAPHY DEAD-SPACE COMPONENT CALCULATION DURING INCREMENTAL CYCLE EXERCISE

ОПЫТ ПРОФИЛАКТИКИ ЭМОЦИОНАЛЬНОГО СТРЕССА С ПОМОЩЬЮ МЕТОДИК МУЗЫКОТЕРАПИИ И МЕДЛЕННЫХ МАНИПУЛЯЦИОННЫХ ДВИЖЕНИЙ У РЫБАКОВ ВО ВРЕМЯ ДЛИТЕЛЬНОГО МОРСКОГО РЕЙСА

Under the effect of extreme factors, compensatory and adaptive mechanisms are triggered in the body that change the metabolism and functional state of organs and tissues. With short-term exposure to these factors, homeostasis practically does not change, and with prolonged and repeated effects, stress and corresponding pathology may occur. In marine medicine, the problem of stress is particularly relevant. The adaptation of the seafarers' body should be carried out to a significant number of adverse factors in a relatively short time with sharp changes in the navigation regions. There are many ways to improve or correct a person's condition. Among them, relatively simple and inexpensive ones are of interest. The aim of the study is to evaluate the results of the use of various methods of psychological relaxation for the prevention and reduction of emotional stress among fishermen during a long sea voyage. Materials and methods: seafarers were examined during a long fishing voyage for psychoemotional stress detecting. Seances of psychological and emotional relaxation – music therapy and exercises of the technique of slow manipulative movements were conducted among individuals with identified signs of reactive anxiety. Results of the study: upon completion of all full cycles of emotional and psychological unloading, a positive effect was revealed in most of the subjects. 6 days after the start of the music therapy cycle, the level of reactive anxiety normalized in half of the participants, and after another 4 days, almost all individuals achieved a decrease in anxiety to at least moderate. In an experiment with the technique of slow manipulative movements (counting rosaries), after a week, the normalization of the level of anxiety occurred in 20% of the subjects, and in more than 2/3 of the sailors it decreased to moderate. A week later, almost all the subjects' anxiety levels returned to normal.

Bioinspired Spinosum Capacitive Pressure Sensor Based on CNT/PDMS Nanocomposites for Broad Range and High Sensitivity.

Flexible pressure sensors have garnered much attention recently owing to their prospective applications in fields such as structural health monitoring. Capacitive pressure sensors have been extensively researched due to their exceptional features, such as a simple structure, strong repeatability, minimal loss and temperature independence. Inspired by the skin epidermis, we report a high-sensitivity flexible capacitive pressure sensor with a broad detection range comprising a bioinspired spinosum dielectric layer. Using an abrasive paper template, the bioinspired spinosum was fabricated using carbon nanotube/polydimethylsiloxane (CNT/PDMS) composites. It was observed that nanocomposites comprising 1 wt% CNTs had excellent sensing properties. These capacitive pressure sensors allowed them to function at a wider pressure range (~500 kPa) while maintaining sensitivity (0.25 kPa−1) in the range of 0–50 kPa, a quick response time of approximately 20 ms and a high stability even after 10,000 loading–unloading cycles. Finally, a capacitive pressure sensor array was created to detect the deformation of tires, which provides a fresh approach to achieving intelligent tires.

Study on Permeability Characteristics of Gas Bearing Coal under Cyclic Load

With the depletion of shallow coal mine resources, the mining depth of coal mines is deepening year by year, therefore, gas explosion and other disasters occur from time to time. Gas drainage is the main measure to prevent gas disasters, and the permeability of coal is one of the main factors affecting gas drainage. In order to explore the seepage characteristics of coal under different confining pressure loading and unloading conditions and different gas pressure, the seepage test of raw coal samples was carried out by using the coal rock triaxial loading seepage test device. The results show that the permeability of coal samples decreases with the increase of confining pressure in the loading stage, and increases with the unloading of confining pressure in the unloading stage; Through calculation, it is found that the permeability loss of coal body in the loading stage decreases with the increase of loading times, and during unloading, this value also decreases with the increase of unloading times, and reaches the maximum value at the first loading and unloading. With the increase of loading and unloading cycles, the permeability loss of coal samples decreases, and the first loading and unloading is the main stage of permeability loss of coal samples. At the same time, it is found that when the confining pressure increases, the permeability loss of coal samples decreases, and the initial permeability of coal samples maintains a good linear growth relationship with the increase of gas pressure.

Extraordinary mechanical properties and successive caloric effects with ultrahigh cyclic stability in directionally solidified Ni36.6Co12.8Mn34.7Ti15.9 alloy

Caloric effects based on multiferroic phase transformation have attracted extensive attention for their great potential to replace the conventional vapor-compression technique. However, most multiferroic phase transforming materials are inherently brittle and their caloric effects exhibit poor cyclic stability. Here, the Ni36.6Co12.8Mn34.7Ti15.9 alloy was manufactured by the directional solidification technique. The alloy can reach maximum compressive strength and strain of 2823 MPa and 17.7% at room temperature, respectively. Under magnetic field changes of 7 T, the maximum magnetic entropy change induced by a magnetic field is 28.1 J·kg−1·K−1. A large adiabatic temperature change of 20.1 K can be obtained by applying a stress field. Besides, the adiabatic temperature change between loading and unloading cycles can be maintained at ∼11.3 K for more than 10,000 cycles. A broad refrigeration temperature range of over 80 K can be realized by combining magnetocaloric and elastocaloric effects. With the simultaneously achieved extraordinary mechanical properties and large successive caloric effects with excellent cyclic stability, the present alloy has a great prospect for solid-state refrigeration applications.

Mechanical properties and damage constitutive model of concrete under low-temperature action

• Mechanical properties of concrete subjected to cyclic loading and unloading after low-temperature action are studied. • A damage constitutive model of concrete after low-temperature action considering temperature and load effects is proposed. • Variations of the damage amount of concrete with strain are investigated. To study the effect of temperature on the mechanical properties and damage characteristics of concrete, a static uniaxial compression test, splitting tensile test, and cyclic loading and unloading tests were conducted on concrete specimens cured for three different periods (3 d, 7 d, and 28 d) under three different temperatures (20 ℃, −10 ℃, and −15 ℃). Subsequently, a damage constitutive model and a damage evolution equation for the concrete at different temperatures were established based on the equivalent effect variation hypothesis, Weibull and Lognormal distribution statistical theory. The stress–strain curves of the concrete specimens at different temperatures were found to be similar at the same age, all of them exhibiting five stages. With decreasing temperature, the mass loss rate of concrete increased, whereas the longitudinal wave velocity, compressive strength, and splitting tensile strength decreased. Under cyclic loading and unloading, the loading modulus of concrete at different temperatures and curing ages was slightly lower than the unloading modulus in the last loading and unloading cycle. The input energy density required for concrete cured for 3 d and 7 d at different temperatures was less than that required for concrete cured for 28 d. The findings of this study provide a reference for the analysis and design of concrete structures following low-temperature action.

Online measurement method of FeO content in sinter based on infrared machine vision and convolutional neural network

Sintering is an important process in iron and steel manufacturing. FeO content of sinter is a key index for both sintering and blast furnace ironmaking, but it needs manual sampling and testing in practical production. In this research, an online measurement method based on machine-tailed infrared images and convolutional neural network (CNN) for the FeO content is proposed. Firstly, a target detection model of sinter bed cross-section (SBC) is established, to obtain the key frame image (KFI) of SBC in each unloading cycle. Then, the KFI is labeled with the offline chemical analysis of FeO content, and a modified CNN is proposed to build the online measurement model for FeO content. The experimental results show that the proportion of samples with detection error of FeO content within ± 0.5 % reaches 91.2 % and the RMSE is less than 0.3 %. It provides a novel measurement approach for online detection of FeO content.

High-performance and superhydrophobic piezoresistive pressure sensor based on mountain ridge-like microstructure by silver nanoparticles and reduced graphene oxide

Ingenious microstructure design is crucial for piezoresistive pressure sensor to achieve excellent sensing performances and water repellency. Herein, we propose a new strategy to prepare high-performance polydimethylsiloxane (PDMS) sponge-based piezoresistive pressure sensor with superhydrophobicity. The silver nanoparticles and reduced graphene oxide on the skeleton surface of PDMS sponge jointly constructed mountain ridge-like microstructure and endowed the sensor with conductive network. The sensor exhibited high sensitivity, wide detection range, fast response and exceptional repeatability (4,000 loading–unloading cycles) and was applied for detecting human motions and daily scenarios. Owing to the hierarchical microstructure and the surface modification by 1H,1H,2H,2H-perfluorooctyltriethoxysilane, the sensor showed a high water contact angle of 156°, and the response signal kept stable even being sprayed by water. The prepared high-performance and superhydrophobic piezoresistive pressure sensor in this work has great potential in wearable electronics, healthcare monitoring and smart robot.

Numerical and Experimental Analysis of the Rotational Stiffness of a Timber Semi-Rigid Dowel-Type Connection

The paper deals with the analysis of the rotational stiffness of a semirigid connection created from a system of two stands and a rung. The connection was made from glued laminated timber with metal mechanical dowel-type fasteners. Not only a common combination of bolts and dowels but also fully threaded screws were used for the connection. The aim of the research and its motivation was to replace commonly used fasteners with more modern ones, to shorten and simplify the assembly time, and to improve the load-carrying capacity of this type of connection. Each of these two types of connection was loaded to the level of 60%, 80%, and 100% of the ultimate limit state value. Subsequently, the rotational stiffness was determined for each load level after five loading and unloading cycles. This paper presents the results and comparison of the experimental testing and the numerical modeling. The obtained results were also compared with the assumption according to the currently valid standard.

Damping Behavior of Hybrid Composite Structures by Aeronautical Technologies

Hybrid composite laminates are manufactured by using technologies and raw materials of the aeronautic sector with the aim to improve the damping behavior of composite structures. Matrix hybridization was achieved by laminating carbon fiber reinforced (CFR) plies with elastomer interlayers. Up to 10 different composite sandwich architectures were investigated by changing the stacking sequence, the thickness of the elastomer layers, and the elastomer typology, whereas the total number of the CFR plies was fixed to six for all the hybrid composites. Square panels with the size of 300 × 300 mm2 were autoclave molded with vacuum bagging, and rectangular samples were extracted for static and dynamic tests. Dynamic mechanical analyses were performed to measure the storage modulus and loss factor of hybrid materials, which were compared with static and dynamic performances of the composite structures under bending. Repeated loading–unloading cycles and free oscillation tests allowed us to the energy loss per unit of volume, and the acceleration damping, respectively. Results show that softest elastomer interlayers lead to big loss of stiffness without any positive effect in the damping behavior, which worsens as well. By using soft elastomers, complex architectures do not provide any additional benefit in comparison with the traditional sandwich structure with soft core and hard skins.

Fabrication, characterization and modelling of the fabric electrode-based highly stretchable capacitive strain sensor

Highly stretchable and capacitance-based strain sensors were fabricated utilizing highly stretchable silicone elastomer as dielectric film sandwiched between the conducting and stretchable cloth-based top and bottom electrodes. Electromechanical characterizations for the proposed sensor after multiple and reversible loading and unloading cycles up to 150% applied strain were conducted followed by the modelling of the sensing performance. It has been shown that the sensor exhibits reproducible electromechanical performance under multiple reversible stretching and relaxation cycles. There was a very small deviation (< 10%) in the observed values of the capacitance upon the applied strain of 150% up to the 1000 loading and unloading cycles. Tensile measurement for the sensor under various applied strains, not only exhibits almost constant force indicating negligible Mullins effect and negligible time-dependent changes at a fixed strain up to 100% suggesting that viscoelastic losses can also be neglected. From uniaxial tensile experiments, constitutive model of the dielectric elastomer sensor was validated. For the fitting of the experimental data obtained from the uniaxial tensile measurement, a variety of strain energy formulations have been compared for predicting the constitutive behaviour of sensor using the Mooney– Rivlin, Neo-Hookean, Yeoh and Ogden models, where 2nd term of the Ogden model was found to explain the experimental tensile data most accurately. To explain some observed losses at 150% of applied strain, the Prony series-based time dependent model was used for predicting stress at any value of time and it was found that experimental data was well fit with 4th order of Prony series.

Comparison between thermodilution and Fick methods for resting and exercise-induced cardiac output measurement in patients with chronic dyspnea.

Studies comparing thermodilution (TD) and the direct Fick method (dFM) for cardiac output (CO) measurement are rare. We compared CO measurements between TD (2–5 cold water injections), the dFM, and indirect Fick method (iFM) at rest and during exercise, and assessed the effect of averaging different numbers of TD measurements during exercise. This retrospective study included 300 patients (52.3% women, mean age 66 ± 11 years) having pulmonary hypertension (76.0%) or unexplained dyspnea. Invasive hemodynamic and gas exchange parameters were measured at rest (supine; n = 300) and during unloaded cycling (semi‐supine; n = 275) and 25‐W exercise (semi‐supine; n = 240). All three methods showed significant differences in CO measurement (ΔCO) at rest (p ≤ 0.001; ΔCO > 1 L/min: 45.0% [iFM vs. dFM], 42.0% [iFM vs. TD], and 45.7% [TD vs. dFM]). ΔCO (TD vs. dFM) was significant during unloaded cycling (p < 0.001; ΔCO > 1 L/min: 56.6%) but not during 25‐W exercise (p = 0.137; ΔCO > 1 L/min: 52.8%). ΔCO (TD vs. dFM) during 25‐W exercise was significant when using one or two (p ≤ 0.01) but not three (p = 0.06) TD measurements. Mean ΔCO (TD [≥3 measurements] vs. dFM) was −0.43 ± 1.98 and −0.06 ± 2.29 L/min during unloaded and 25‐W exercise, respectively. Thus, TD and dFM CO measurements are comparable during 25‐W exercise (averaging ≥3 TD measurements), but not during unloaded cycling or at rest. Individual ΔCOs vary substantially and require critical interpretation to avoid CO misclassification.

Muscle Fatigue Is Attenuated When Applying Intermittent Compared With Continuous Blood Flow Restriction During Endurance Cycling.

The aim of this study was to identify a blood-flow-restriction (BFR) endurance exercise protocol that maximizes metabolic strain and minimizes muscle fatigue. Twelve healthy participants accomplished 5 different interval cycling endurance exercises (2-min work, 1-min rest) in a randomized order: (1)control, low intensity with unrestricted blood flow (CON30); (2)low intensity with intermittent BFR (i-BFR30, ∼150mm Hg); (3)low intensity with continuous BFR (c-BFR, ∼100mm Hg); (4)unloaded cycling with i-BFR0 (∼150 mm Hg); and (5)high intensity (HI) with unrestricted blood flow. Force production, creatine kinase activity, antioxidant markers, blood pH, and potassium (K+) were measured in a range of 5 minutes before and after each cycling exercise protocol. HI showed the highest reduction (Δ = -0.26 [0.05], d = 5.6) on blood pH. Delta pH for c-BRF30 (Δ = -0.02 [0.03], d = 0.8) and ΔpH for i-BRF30 (Δ = -0.04 [0.03], d = 1.6) were different from each other, and both were higher compared with CON30 (Δ = 0.03 [0.03]). There was significant before-to-after force loss following HI (Δ = 55 [40]N·m-1, d = 1.5) and c-BFR30 (Δ = 27 [21]N·m-1, d = 0.7) protocols only, which were accompanied by significant increases in K+ (HI: Δ = 0.94 [0.65]mmol·L-1, d = 1.8; c-BFR30: Δ = 0.72 [0.85]mmol·L-1, d = 1.2). Moreover, all BFR conditions elicited slight increases in plasma creatine kinase, but not for HI and CON30. Glutathione changes from before to after were significant for all BFR conditions and HI, but not for CON30. The attenuation in fatigue-induced reductions in maximal force suggests that i-BFR exercise could be preferable to c-BFR in improving exercise capacity, with considerably less biologic stress elicited from HI exercises.

On Strain-rate Independent Fracture Toughness and Relaxation in Cyclic Tensile Behavior: A Case Study for Toughened Biobased Polyamide 410 Blends

The equivalence in strain-rate dependent fracture toughness of toughened biobased polyamide 410 blends under high-strain conditions as in Izod impact toughness and low strain conditions as in essential work of fracture (EWF) assessment is discussed in this work. An escalation of ∼57% in resistance to crack initiation (we) (EWF parameter) and an increase of ∼356% Izod impact strength confirms the semi-ductile to ductile transition in the range of 10-15 wt.% of elastomer loading. The isotropic responses, i.e., strain-rate independent fracture-toughness of the blends, have indicated three factors (a) toughness originating from shear yielding/banding or craze bridging of dispersed elastomer domain, (b) gradual reduction in matrix stiffness and strength with greater extent of plastic deformation and (c) crack tip-blunting based fracture work to be responsible behind toughening of the blends. The crack opening displacement (COD) has indicated out-of-plane, i.e., vertical extension of advancing crack tip to be exceeding the in-plane crack extension in the composition range of 10-15 wt.% of the blends. The uniaxial cyclic-loading till twenty cycles with a 10 N increment (in every next cycle) showed the plastic-strain contribution to net deformation of the material. The phase-lag in the loading and unloading cycle (work loss hysteresis) increased with the number of cycles and plastic strain owing to viscosity-controlled energy dissipation in the form of cyclic plastic deformation showing elasto-viscoplastic nature of the PA 410 blend system.