Increasing Loading Rate Research Articles

Stress-strain relationship and damage constitutive model of wet concrete under uniaxial compression

The deformation and damage of wet shotcrete under geostress in deep mines is a common issue. To better understand the damage characteristics of rock deformation rates on wet shotcrete, uniaxial compression tests were conducted using an RMT-150 C press with five loading rates. Based on the damage mechanics theory, a segmental damage constitutive model was established, which defined the elastic modulus as the damage variable and considered the loading rate. The research findings indicate that the relationship between the compressive strength and elasticity modulus of wet concrete, as well as the loading rate, can be described by quadratic polynomial functions. The elasticity modulus and compressive strength increase slowly when the loading rate exceeds 0.1 mm/s. Additionally, the compressive strength is directly proportional to the elasticity modulus. When wet shotcrete reaches its maximum stress, a higher loading rate results in a faster damage value. However, exceeding the maximum stress drastically reduces the damage value. The relationship between the damage value D and axial strain is represented by a quadratic polynomial function. Although the deformation capacity of concrete increases slightly with an increase in loading rate, the effect is minimal. A segmental damage constitutive model, which considers the loading rate and uses the elastic modulus to define damage variables, has been developed based on the stress-strain curve of wet concrete. The accuracy of the model was verified by comparing theoretical and measured values. The theoretical model can serve as a useful reference for the design of wet shotcrete support.

Experimental study on dynamic mechanical and failure behavior of a jointed rock mass

In order to study the dynamic mechanics and failure behavior of jointed rock masses, impact tests were carried out on sandstone specimens with intersected jointed and unjointed intact sandstone specimens using a modified split Hopkinson pressure bar (SHPB) with confinement. The effects of intersected artificial joints on the dynamic properties and failure mechanism of sandstone samples were analyzed. The dynamic stress-strain curve of sandstone specimen was obtained from the test, as well as the fracture failure behavior of specimen under different loading rates and confining pressures. The results show that the dynamic compressive strengths of the unjointed intact rock specimens and the jointed rock specimens both increase with the increase of the loading rate and the confining pressure. With the increase of loading rate, the jointed rock specimens showed more obviously plastic deformation ability than the intact rock specimens, and the size of the broken pieces of the jointed rock samples became smaller. The dynamic failure modes of the jointed rock samples are irregular and mainly appear tensile and compressive shear failures. Next, the dynamic initiation and development process of cracks in the jointed rock specimens were analyzed by using the UDEC numerical simulation software. The process of initiation and development of meso-cracks is affected by intersected joints. Combined with the effective stress contour map, five typical crack types are defined to reveal the macroscopic fracture behavior of sandstone samples.

Mixed-mode fracture behavior in deep shale reservoirs under different loading rates and temperatures

In the last years, shale gas has gradually substituted oil and coal as the main sources of energy in the world. Compared with shallow shale gas reservoirs, deep shale is characterized by low permeability, low porosity, strong heterogeneity, and strong anisotropy. In the process of multi-cluster fracturing of horizontal wells, the whole deformation process and destruction modes are significantly influenced by loading rates. In this investigation, the servo press was used to carry out semi-circular bend (SCB) mixed-mode fracture experiments in deep shales (130, 160, 190 °C) with prefabricated fractures under different loading rates (0.02, 0.05, 0.1, 0.2 mm/min). The fracture propagation process was monitored using acoustic emission. The deformation characteristics, displacement–load curve, and acoustic emission parameters of shale under different loading rates were studied during the mixed-mode fracture propagation. Our results showed that during the deformation and fracture of the specimen, the acoustic emission energy and charge significantly increased near the stress peak, showing at this point the most intense acoustic emission activity. With the increase in loading rate, the fracture peak load of the deep shale specimen also increased. However, the maximum displacement decreased to different extents. With the increase in temperature, the effective fracture toughness of the deep shale gradually decreased. Also, the maximum displacement decreased. Under different loading rates, the deformation of the prefabricated cracks showed a nonlinear slow growth–linear growth trend. The slope of the linear growth stage increased with the increase in loading rate. In addition, as the loading rate increased, an increase in tension failure and a decrease in shear failure were observed. Moreover, the control chart showing the relationship between tension and the shear failure under different temperatures and loading rates was determined.

Influenced of different austenite finish temperature (Af) to the cyclic properties of pseudo elastic NiTi and NiTi hybrid with steel for self-recovery of seismic reinforcing bar

This research highlighting the novel properties of pseudo-elastic Ni-Ti bar owing to their ability to reverse macroscopically inelastic deformation during earthquake known as recentering capability and large elastic strain capacity which originated from the reversible austenite to martensite phase transformation. The homogeneity of Ni and Ti are extremely sensitive to the compositional in austenite finish temperature (Af) which led to phase change transformation behavior and the effect of different size of NiTi bar in design have not comprehensively considered in the previous work. Hence, this paper is presented and evaluates the cyclic properties of three different sizes and austenite finish temperature (Af) of pseudoelastic NiTi and reused NiTi to assess their prospective use as reinforcing bar to be exploited as seismic-resistant design and mitigation. The correlation of hysteretic behavior of Ni-Ti alloy in terms of cyclic loading number and history, mechanical properties at ambient temperature, equivalent damping, energy dissipation, and stress recovery were evaluated. The tensile cyclic test obtained demonstrated a rounded loading curve based on a 0.2% offset and increased with the increase of loading rate corresponding with the values of the stress plateau. The as-received bar and reused NiTi bar exhibited superior pseudo-elastic behavior and recentering through repeated cycling without significant degradation or permanent deformation but low energy damping due to narrow hysteresis while the steel rebar shows vice versa. Experimental results show potential for the use of SMAs in seismic applications and provide areas for continued research. It was concluded that the as-received and the reused pseudo elastic Ni-Ti reinforcing bar are suitable to use for seismic mitigation despite their ability to undergo cyclical strains is 6% to a maximum 8% with minimal residual strain, 0.004% to maximum less than 1%. Due to narrow hysteresis and phase transformation behavior, the 12.7 mm NiTi exhibit the perfect superelasticity with the residual strain (εR) was 0.1076% corresponding with strain recovery capacity of 98.21% for the range of cyclic strains up to 6%. The reused NiTi 12.7 mm exhibits the highest energy dissipation residual strains up to 99.95% for their wide hysteresis and successive to cyclic loading at an 8% strain cycle. In comparison, the strain recovery capacity (%) increased with a decrease in bar size. The energy dissipation and damping increasing with the increasing of the bar size, but decreasing with the increasing of the bar size for reused NiTi Af −6.3, hence correlate in a good agreement corresponding with 8 mm and 12 mm diameter of the bar. However, different relationship demonstrated for NiTi 12.7 mm due to different composition ratio that exhibits different phase transformation behavior.

Pollutant removal modeling in a hybrid wetland system for industrial wastewater treatment

Modeling of treatment wetlands allows the prediction of system behavior and facilitates large-scale design by evaluating the effects of modifying the operating parameters. The aim of this study was to calibrate and verify applicable models in vertical flow (VF) and horizontal subsurface flow (HSSF) wetlands -first and second stage of the hybrid wetland (HW) system- operating in site-specific environmental conditions. Stepwise multiple linear regression analysis, oxygen demand model, and k-C* model, were applied to each stage of the HW system as appropriate. All models were calibrated and verified based on the data observed by monitoring the HW system. Afterwards, simulations were performed at each stage of the HW system. The simulations show the dependence of nitrification on the availability of NH4+-N and also that both, the increase in Org-N and COD loading, affect the efficiency of nitrification in the VF stage. In the HSSF stage, simulations show that the increase in hydraulic or pollutants loading rate has a positive impact on the performance of the system. The fit achieved in the models was satisfactory and the results obtained by simulation provide a useful tool to evaluate the performance of the HW system in different possible operating scenarios.

Experimental investigation on effect of loading rate on fracture behavior of coal-seam roof rock

The fracture behavior of coal-seam roof rock in coal mining is a key and controlling factor for the mode optimization of the artificial roof caving. However, the fracture mechanism of roof rock under loading is not clear. In the work, the split Hopkinson pressure bar (SHPB) experiment was carried out using semicircular bending samples from the sandstone of coal-seam roof rock in the Junger mining area of Inner Mongolia at the loading rate of 0.35–3.78 GPa·m0.5·s−1, and the dynamic fracture behavior and energy dissipation mechanism of samples under different loading rates were investigated. The result shows that the dynamic stress–strain process of the hard roof rock includes four stages: linear instantaneous compaction, linear elastic compression, failure, and fracture extension, in which the failure forms changes from brittle fracture to ductile fracture with the increase of loading rate. The mode I fracture toughness increases linearly under confining pressure. In addition, the propagation orientation of induced fractures is parallel to the loading direction, and the gravel in the samples can inhibit fracture extension, resulting in changing the fracture extension path. Further, the energy absorption efficiency of the samples during the fracture process decreases with the increase in the loading rate.

Biogas Production from Arthrospira platensis Biomass

Biogas production by fermentation is a relatively low-cost and simple method for the transformation of a substrate into an energy carrier with a wide range of possible applications. The aim of this study was to determine the potential of Arthrospira platensis biomass as a source of bioenergy produced during anaerobic digestion (AD). The studies were carried out on a fractional-technical scale. Biogas yield and composition were analyzed as a function of the amount of biomass subjected to anaerobic digestion, the substrate dosing frequency in the digester and the use of biomass pre-hydrolysis in the mixing compartment. The energy efficiency of the process was also compared for each sample. In addition, a biomass conversion power index was developed and determined. It was found that A. platensis biomass had significant energy potential, and the amount of biogas obtained and its calorific value changed depending on the applied treatments. The maximum cumulative biogas production was 505 L kg−1 volatile solids (VS), while the maximum average methane (CH4) content was 67.32%. A two-fold increase in the organic loading rate from 1 g VS·L−1 volatile solids (VS) to 2 g VS·L−1 had a positive effect on methane concentration. The highest energy efficiency of the AD process was obtained for 2 g VS·L−1, with a single feedstock input into the digester, in a single-stage process (2/s/-), while the highest conversion power ratio was for a feedstock of 1 g VS·L−1, under the same process conditions (1/s/-). Moreover, the energy efficiency of the microalgae fermentation process obtained in the study is higher compared to conventional substrates used in biogas plants. This energy analysis can support the selection of cogeneration power engines in a biogas plant and help to determine the potential output of the biogas plant, especially with varying energy and heat demand.

Effect of Grain Size on the Dynamic Flexural Tensile Strength of Granite: Insight from GBM3D-PFC Simulations

For reproducing the heterogeneous structure of granites, a novel three-dimensional grain-based model based on particle flow code (GBM3D-PFC) was proposed in this paper. Then, dynamic semicircular bending tests under different impact velocities were conducted on the numerical samples with different grain sizes by the coupled split Hopkinson pressure bar system. On the one hand, with increasing grain size, the dynamic flexural tensile strength and proportion of transgranular cracks increase, while the total crack number decreases. On the other hand, as the loading rate increases, the dynamic flexural tensile strength and transgranular crack proportion increase. Based on the GBM3D-PFC, the evolution of the load applied to the sample can be reasonably described. The relationship between the contact force network and the sample’s strength can be established. The microcracking behavior obtained by numerical simulations can be used to explain the variation of the macroscopic mechanical parameters.

Measurement and Classification Criteria of Strength Decrease Rate and Brittleness Indicator Index for Rockburst Proneness Evaluation of Hard Rocks.

Rockburst is one of the common geological hazards. It is of great significance to study the evaluation indexes and classification criteria of the bursting liability of hard rocks, which is important for the prediction and prevention of rockbursts in hard rocks. In this study, the evaluation of the rockburst tendency was conducted using two indoor non-energy indexes, namely the brittleness indicator (B2) and the strength decrease rate (SDR). The measuring methods of B and SDR as well as the classification criteria were analyzed. Firstly, the most rational calculation formulas for B and SDR were selected based on previous studies. The B2 equaled to the ratio between the difference and sum of uniaxial compressive strength and Brazilian tensile strength of rocks. The SDR was the average stress decrease rate of the post-peak stage in uniaxial compression tests and equaled the uniaxial compressive strength dividing the duration time of post-peak rock failure in uniaxial compression tests. Secondly, the uniaxial compression tests of different rock types were designed and carried out, and the change trend of B and SDR with the increase of loading rate in uniaxial compression tests were studied in detail. The results showed that after the loading rate was greater than 5 mm/min or 100 kN/min, the B value was affected, limited by the loading rate, while the SDR value was more affected by the strain rate. The displacement control, with a loading rate of 0.1-0.7 mm/min, was recommended for the measurement of B and SDR. The classification criteria of B2 and SDR were proposed, and four grades of rockburst tendency were defined for SDR and B2 according to the test results.

Increased hip adductor activation during sit-to-stand improves muscle activation timing and rising-up mechanics in individuals with hemiparesis

Long sit-to-stand (STS) time has been identified as a feature of impaired functional mobility. The changes in biomechanics of STS performance with simultaneous hip adductor contraction have not been studied, which may limit indications for use of hip adductor activation during STS training. Ten individuals with hemiplegia (mean age 61.8years, injury time 29.8±15.2months) performed the STS with and without squeezing a ball between two legs. The joint moments, ground reaction force (GRF), chair reaction force and movement durations and temporal index of electromyography were calculated from the control condition for comparison with those from the ball squeezing condition. Under the squeeze condition, reduced peak vertical GRF during the ascension phase with increased loading rate was observed in the nonparetic limb, and the peak knee extensor moment occurred earlier in the paretic. Earlier activation of tibialis anterior and gluteus maximus, and gluteus medius were found in squeeze STS. Squeezing a ball between limbs during STS increased the contraction timing of tibialis anterior, gluteus maximus, gluteus medius, and soleus as well as a more symmetric rising mechanics encourage the use of squeezing a ball between limbs during STS for individuals with hemiparesis.

Experimental study on the influence of loading rate on the directional propagation law of rock mode-I cracks

To investigate the influence of the loading rate on the propagation behavior of rock mode-I cracks, a simple device for simulating rock-crack directional propagation was developed. Subsequently, loading tests were performed for rock mode-I crack propagation at different loading rates. The entire crack propagation process was monitored by applying acoustic emission (AE) and digital image correlation (DIC), and the influence of the loading rate on the mode-I crack propagation mechanism was investigated. The results showed that the peak load increased as the loading rate increased; however, the crack initiation angle and crack initiation time both tended to decrease. According to the evolution law of the deformation field, the crack propagation process can be divided into four stages: local strain band initiation, local strain band development, main control crack initiation, and main control crack coalescence. The deformation separation degree of the main control crack increased with an increase in the loading rate. The evolution process of AE can be divided into four stages: quiet, slowly increasing, booming, and decreasing periods. When the loading rate increased, the intensity of the released AE energy increased. The energy evolution law of the crack propagation process was closely related to the loading rate. The energy dissipated before the peak load decreased with an increasing loading rate, whereas the releasable elastic energy increased after the peak load increased. Scanning electron microscopy analysis revealed that when the loading rate increased, the crack propagation changed from a low-energy-efficiency fracture mode with high energy consumption to a high-energy-efficiency fracture mode with low energy consumption.

Effects of cyclic loading and unloading rates on the energy evolution of rocks with different lithology

The deformation-to-failure process of rocks is accompanied by energy dissipation and release, indicating that it is an energy conversion process. To investigate the influence of lithology as well as the loading and unloading rates on the evolution and distribution of rock energy, the MTS 816 rock mechanics testing system was used for performing uniaxial compression and uniaxial cyclic loading–unloading tests on the 165 samples of four rock types. The total energy density, elastic energy density, and dissipated energy density absorbed by rocks of different lithology were obtained, and the evolution and distribution laws of lithology and loading and unloading rates on the accumulation and dissipation of rock energy were investigated. The results revealed that the energy density of all rock samples increased nonlinearly with the increase in axial stress, and the elastic energy density increased gradually first and subsequently rapidly with the increase in the axial stress. The evolution curves were not affected by the loading and unloading rates. The dissipated energy density increased gradually with the increase in axial stress, and the discrepancy of the evolution curve was large. The proportion of the elastic energy density varied nonlinearly with the increase in axial stress or the cycle index, exhibiting the evolutionary process of increasing, then stabilizing, and finally decreasing. However, the proportion of the dissipated energy density exhibited an opposite trend. The elastic energy accumulated in the rock sample at the pre-peak stage was considerably higher than the dissipated energy, and the proportion of the elastic energy density was large. With the increase in loading and unloading rates, the proportion of elastic energy exhibited an approximate growth trend, and the proportion of dissipated energy exhibited an approximate decreasing trend.

Investigating the Performance of Lithium-Doped Bismuth Ferrite [BiFe1−xLixO3]-Graphene Nanocomposites as Cathode Catalyst for the Improved Power Output in Microbial Fuel Cells

In this study, multifunctional lithium-doped bismuth ferrite [BiFe1−xLixO3]-graphene nanocomposites (x = 0.00, 0.02, 0.04, 0.06) were synthesized by a sol-gel and ultrasonication assisted chemical reduction method. X-ray diffraction and FESEM electron microscopy techniques disclosed the nanocomposite phase and nanocrystalline nature of [BiFe1−xLixO3]-graphene nanocomposites. The FESEM images and the EDX elemental mapping revealed the characteristic integration of BiFe1−xLixO3 nanoparticles (with an average size of 95 nm) onto the 2D graphene layers. The Raman spectra of the [BiFe1−xLixO3]-graphene nanocomposites evidenced the BiFe1−xLixO3 and graphene nanostructures in the synthesized nanocomposites. The photocatalytic performances of the synthesized nanocomposites were assessed for ciprofloxacin (CIP) photooxidation under UV-visible light illumination. The photocatalytic efficiencies of [BiFe1−xLixO3]-graphene nanocomposites were measured to be 42%, 47%, 43%, and 10%, for x = 0.00, 0.02, 0.04, 0.06, respectively, within 120 min illumination, whereas the pure BiFeO3 nanoparticles were 21.0%. BiFe1−xLixO3 nanoparticles blended with graphene were explored as cathode material and tested in a microbial fuel cell (MFC). The linear sweep voltammetry (LSV) analysis showed that the high surface area of BiFeO3 was attributed to efficient oxygen reduction reaction (ORR) activity. The increasing loading rates of (0.5–2.5 mg/cm2) [BiFe1−xLixO3]-graphene composite on the cathode surface showed increasing power output, with 2.5 and 2 mg/cm2 achieving the maximum volumetric power density of 8.2 W/m3 and 8.1 W/m3, respectively. The electrochemical impedance spectroscopy (EIS) analysis showed that among the different loading rates used in this study, BiFeO3, with a loading rate of 2.5 mg/cm2, showed the lowest charge transfer resistance (Rct). The study results showed the potential of [BiFe1−xLixO3]-graphene composite as a cost-effective alternative for field-scale MFC applications.

PDMS/CNB-impregnation treatment for improving the electrical and piezoresistive properties of recycled fine aggregate mortar

Self-sensing mortar prepared by recycled fine aggregate (RFA) has great significance for recycling of waste concrete. However, recycled fine aggregates have not been widely used in self-sensing mortar due to their inferior properties to natural fine aggregates. Therefore, the purpose of this study is to improve the properties and conductivity of RFA by the impregnation pretreatment in polydimethylsiloxane emulsion (PDMS) solution containing nano carbon black (NCB), and use modified RFA to prepare self-sensing RFA mortar. The effects of several factors, including modified RFA replacement ratio, temperature and humidity on the resistivity of as-prepared RFA mortar were studied by four-electrode method. The results show the micro-pores of RFA have been filled by PDMS, reducing the water absorption by 1.64%. At the same time, the NCB particles in PDMS have successfully been coated on the surface of RFA by this impregnation pretreatment. The resistivity of mortar decreases with the increases of modified RFA replacement ratio. When the replacement ratio of modified RFA is between 50% and 60%, the threshold percolation concluded by the sharp decline of mortar resistivity begins to appear. Besides, the conductivity of modified RFA mortar has good temperature sensibility and humidity adaptability. Under cyclic loading of stress, the modified RFA mortar has 1.82%/MPa stress sensitivity, 9.47% linearity, 176.1 sensitivity, 10.95% repeatability and 6.39% hysteresis in percolation zone, which of the piezoresistive properties are better than those in insulation and conductive zones. In addition, the stress sensitivity of modified RFA mortar in different zones decreases with the increase of loading rate.

Biofilm Reactor Performance and Shifts of Microbial Community during the Start-Up and Operation Phase of MBBRs at 8 °C: Effect of Exogenous Quorum Sensing Bacteria

Quorum sensing (QS) plays a critical role in the enhancement of biofilm performance at low temperatures. In this study, a uni-QS (Sphingomonas rubra) and three bisynergistic-QS [composed of S. rubra and one of the isolated QS aerobic denitrifying bacteria (QS-ADB) Pseudomonas mendocina QQQC, P. mendocina ABDF, and Enterobacter aerogenes] were added to biofilm reactors in the start-up phase at 8 °C. Results indicate that compared to the control reactor, bisynergistic-QS exhibited maximum TN removal efficiency (93.50 ± 5.44, 93.97 ± 3.93, and 92.35 ± 5.04%), largest functional genes amoA, narG, and norB abundance (15-fold, 15-fold, and 75-fold), and uprising C10-HSL and C14-HSL concentrations (22.32- to 39.28-fold and 3.87- to 8.66-fold). Network analysis revealed that the biofilm thickness which possessed strong relationships with its dominant bacteria might contribute to great performance in bisynergistic-QS reactors under low temperature and increased loading rate. Proteobacteria play an important role in the AHL-based QS system. The abundance of keystone taxa Raoultella, Micropruina, and Zoogloea deserved more attention utilizing exogenous aerobic denitrifier addition. This study provided a helpful guideline for the design of exogenously enhancing biofilm denitrification systems at low temperatures.

Influence of Drying-Wetting Cycles on Dynamic Tensile Properties of Red-Sandstone Using the Brazilian Test

The phenomenon of the drying-wetting cycle is a component of the weathering process and has a significant impact on the characteristics of rock materials. In order to examine the physical and dynamic tensile properties of red sandstone subjected to drying-wetting cycles, physical tests and dynamic splitting tensile tests were performed on red sandstone specimens under varying drying-wetting conditions. First, the split Hopkins pressure bar was used to perform dynamic tensile tests on red sandstone samples after varying numbers of drying-wetting cycles (0, 20, 40, 60, and 80). Second, the effects of the loading rate and drying-wetting cycles on dynamic tensile strength and energy dissipation of red sandstone were studied. Finally, taking into account the effect of the loading rate, a model was developed to predict the trend of decay in the dynamic tensile strength of red sandstone following drying-wetting cycles. The findings indicate that as the number of drying-wetting cycles increases, there is a decrease in longitudinal wave velocity and a marked degradation of the red sandstone due to the effects of the drying-wetting cycles. The dynamic tensile strength and dissipation energy density of red sandstone samples increase with the increasing loading rate. Under consistent loading rate conditions, the dynamic tensile strength and dissipative energy density of red sandstone samples exhibit a decrease as the number of drying-wetting cycles increases.

Dynamic Mechanical Properties and Failure Characteristics of Sandstone with Pre-Flaws Parallel to the Loading Direction

Investigations on rock dynamics have been comprehensively focused on when the dynamic impact is perpendicular to the trend of the flaws, while understanding the dynamic mechanical properties and failure characteristics of rock with pre-flaws parallel to the loading direction remains unrevealed. In this study, impact tests are performed to experimentally study the dynamic mechanical properties, failure characteristics, and energy evolution of pre-flawed sandstone using the split Hopkinson pressure bar (SHPB) apparatus, in which the pre-flaws are parallel to the loading direction. The results show that for specimens, the dynamic loading rate and the number of flaws greatly influence the dynamic strength, peak strain, energy absorption rate, and failure behaviors. The dynamic strength increases exponentially with an increase in loading rate and decreases with an increase in flaw numbers. The fractal dimension of triple-flawed specimens is the largest at the same loading rate range. The failure modes of different pre-flawed specimens show a transition from tensile failure to tensile-shear failure with the increase of dynamic loading rate.

Mechanical properties and deformation behaviors of the hydroxyl-terminated polybutadiene and ammonium perchlorate interface by molecular dynamics simulation

The interfacial structure and deformation behaviors have considerable influences on the structural reliability and durability of the hydroxyl-terminated polybutadiene (HTPB) solid rocket propellant. In this work, an all-atom molecular dynamics (MD) simulation was adopted to investigate mechanical properties and damage behaviors of the HTPB and ammonium perchlorate (AP) interface. The modified polymer consistent force field was implanted to describe intra- and intermolecular interactions. The effects of loading rate, temperature, and moisture content on the interfacial strength and tensile curves were discussed. The simulation results show that the interfacial strength and modulus exhibit an increasing tendency when the loading rate increases. However, increasing the temperature and moisture content will lead to a decrease in the interfacial strength. Two interfacial tensile modes distinguished by the mass density curve were adopted and compared to reveal the internal deformation process. The AP-HTPB interfacial deformation initially occurs inside the HTPB bulk due to its lower mechanical strength than the interface and then gradually expands to the interface region. Furthermore, a linear extrapolation based on phenomenological results was used to reveal the discrepancy in scale between experiments and the MD simulation.

Research on the Biomechanical Characteristics of Salted Wakame (Undaria pinnatifida)

At present, there has been no mechanized equipment designed for the separation of the leaves from the stems of wakame in China, and the stem–leaf separation is mainly completed by manual work, which suffers from problems such as low efficiency, high cost, and poor quality. To develop mechanized stem–leaf separation equipment of wakame, it is necessary to have a preliminary understanding of the biomechanical characteristics of wakame. In this study, we adopted an electronic universal testing machine, a texture tester, and a friction tester to investigate the mechanical characteristics of the stems and leaves of salted wakame. Analysis was performed to clarify the effects of salted wakame thickness and loading speed on the tension, compression, shear, and separation mechanical properties as well as the effects of the loading speed, normal force, and contact material on the friction characteristics. As shown in the results, the average tensile strength, shear strength, and resilience of salted wakame stems were 2.27 MPa, 6.34 MPa and 0.27, respectively; the average tensile strength and shear strength of salted wakame leaves were 1.67 MPa and 2.93 MPa; the separation strength of the stems and leaves was 1.78 MPa, and the friction coefficient between salted wakame and stainless steel, silicone rubber, and vulcanized rubber was 0.38, 0.44 and 0.40 on average. In general, the increase in the loading rate, the shear strength, and the stem–leaf separation strength of salted wakame showed a downward trend, while the changes in the friction coefficient showed an upward trend, with no significant influence on the recovery and tensile strength. The tensile strength, resilience, shear strength, stem–leaf separation force, and strength increased as the thickness of the salted wakame improved. The friction coefficient of the salted wakame stems decreased with increasing the normal force, while the friction coefficient of the salted wakame leaves increased. In general, the increase in wakame thickness improved its mechanical properties, and the increase in the test loading rate led to the decrease in the mechanical properties of salted wakame. The research results in this paper can provide suggestions for the research and development of stem–leave separation equipment for saline salted wakame.

Effect of Loading Rate on the Mechanical Properties of Weakly Cemented Sandstone

Weakly cemented rocks are characterized by low strength, loose structure, and easy disintegration. High-intensity mining activities can damage and rupture such rock bodies and induce damage, such as flaking and roofing on roadways. To reveal the mining intensity influence on the weakly cemented rocks’ deformation and damage, a numerical particle flow model of weakly cemented sandstone was established based on particle flow theory. Uniaxial compression simulation tests were conducted at four loading rates of 0.01, 0.1, 0.5, and 1 mm/min to study the weakly cemented sandstone’s stress–strain relationship, damage rupture, acoustic emission, and energy evolution. The results show that, with an increased loading rate, the uniaxial compressive strength of weakly cemented sandstone increases exponentially, and the rupture mode transforms from brittle damage to ductile damage; the greater the loading rate, the greater the degree of damage and crushing range of the rock. Further, with an increased loading rate, the peak hysteresis of rock acoustic emission events decreases, and the number of events increases; the energy accumulated in the rock increases, thus intensifying the degree of rock damage. Therefore, the possibility of engineering disasters should be considered when conducting high-speed underground mining activities.