Constitutive model for gas hydrate-bearing soils considering different types of hydrate morphology and prediction of strength-band

The present study proposes a new elasto-plastic constitutive model that considers different types of hydrates in pore spaces. Many triaxial compression tests on both methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have been carried out over the last few decades. It has been revealed that methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have different strength and dilatancy properties even though they have the same hydrate contents. The reason for this might be due to the different types of hydrate morphology. In this study, therefore, the effect of the hydrate morphology on the mechanical response of gas-hydrate-bearing sediments is investigated through a model analysis by taking into account the different hardening rules corresponding to each type of hydrate morphology. In order to evaluate the capability of the proposed model, it is applied to the results of past triaxial compression tests on both methane hydrate-containing and carbon dioxide hydrate-containing sand specimens. The model is found to successfully reproduce the different stress–strain relations and dilatancy behaviors, by only giving consideration to the different morphology distributions and not changing the fitting parameters. The model is then used to predict a possible range in which the maximum deviator stress can move for various hydrate morphology ratios; the range is defined as the strength-band. The predicted curve of the maximum deviator stress obtained by the constitutive model matches the empirical equations obtained from past experiments. It supports the fact that the hydrate morphology ratio changes with the total hydrate saturation. These findings will contribute to a better understanding of the relation between the microscopic structures and macro-mechanical behaviors of gas-hydrate-bearing sediments.     

Fabrication and properties of diatomite ceramics with hierarchical pores based on direct stereolithography

Porous diatomite ceramics with hierarchical pores and high apparent porosity (50.29–56%) were successfully fabricated via direct stereolithography. The pre-ball-milling time, dispersant type and dispersant concentration were systematically investigated to prepare diatomite pastes with high solid loading, low viscosity and a self-supporting effect. The results showed that a pre-ball-milling time of 24 h was more suitable to prepare diatomite pastes with high solid loading, and Span80 at 2 wt% was the optimal dispersant to obtain 40 vol% diatomite paste with a low viscosity and a self-supporting effect. To restrain the formation of defects, a heating rate as low as 0.2 °C/min was allowed to control the pyrolysis rate in the multistage debinding process. At sintering temperatures ranging from 900 °C to 1000 °C, porous diatomite ceramics exhibited a typical bimodal porosity, high apparent porosity and great flexural strength.     

Preparation of self-healing Cf/SiBCN(O) composite using a novel polyborosilazane

In this study, novel polyborosilazane-derived SiBCN(O) ceramic was used as self-healing component in self-healing C_f/SiBCN(O) composite, which was prepared by polymer infiltration and pyrolysis (PIP) process. Molecular-level structure design of boron-containing ceramic precursors was utilized to achieve uniform dispersion of boron-containing self-healing components in prepared composites. No elemental diffusion was observed at the interface of ceramic matrix and carbon fibers, which resulted in stable SiBCN(O) structure. In addition, boron was uniformly distributed in C_f/SiBCN(O) composite ceramic matrix, which was beneficial for self-healing of cracks. Cracks and indentations were able to heal at high temperatures in air. The best crack-healing behavior occurred in air atmosphere at 1000 °C, with nearly complete crack healing. This excellent self-healing behavior was achieved because silicon and boron atoms in SiBCN(O) ceramic reacted with available oxygen at high temperatures to form SiO₂(l), B₂O₃(l), and B₂O₃·xSiO₂ liquid phases, which effectively filled cracks. In general, as-prepared C_f/SiBCN(O) composite exhibited excellent self-healing properties and shows great application potential in high-temperature environment applications such as aviation, aerospace, and nuclear power.     

Direct ink writing of vancomycin-loaded polycaprolactone/ polyethylene oxide/ hydroxyapatite 3D scaffolds

Novel inks were formulated by dissolving polycaprolactone (PCL), a hydrophobic polymer, in organic solvent systems; polyethylene oxide (PEO) was incorporated to extend the range of hydrophilicity of the system. Hydroxyapatite (HAp) with a weight ratio of 55–85% was added to the polymer-based solution to mimic the material composition of natural bone tissue. The direct ink writing (DIW) technique was applied to extrude the formulated inks to fabricate the predesigned tissue scaffold structures; the influence of HAp concentration was investigated. The results indicate that in comparison to other inks containing HAp (55%, 75%, and 85%w/w), the ink containing 65% w/w HAp had faster ink recovery behavior; the fabricated scaffold had a rougher surface as well as better mechanical properties and wettability. It is noted that the 65% w/w HAp concentration is similar to the inorganic composition of natural bone tissue. The elastic modulus values of PCL/PEO/HAp scaffolds were in the range of 4–12 MPa; the values were dependent on the HAp concentration. Furthermore, vancomycin as a model drug was successfully encapsulated in the PCL/PEO/HAp composite scaffold for drug release applications. This paper presents novel drug-loaded PCL/PEO/HAp inks for 3D scaffold fabrication using the DIW printing technique for potential bone scaffold applications.     

Image sequence-based risk behavior detection of power operation inspection personnel

A novel image sequence-based risk behavior detection method to achieve high-precision risk behavior detection for power maintenance personnel is proposed in this paper. In this method, the original image sequence data is first separated from the foreground and background. Then, the free anchor frame detection method is used in the foreground image to detect the personnel and correct their direction. Finally, human posture nodes are extracted from each frame of the image sequence, which are then used to identify the abnormal behavior of the human. Simulation experiment results demonstrate that the proposed algorithm has significant advantages in terms of the accuracy of human posture node detection and risk behavior identification.     

Gelling behavior of MgAl2O4 slurries with a common dispersant

MgAl₂O₄ transparent ceramics were shaped by a commonly used polyacrylic acid (PAA), which acted as both dispersant and gelling agent. The spinel slurries were prepared by ball-milling MgAl₂O₄ powder, PAA, and water in an attrition mill. The gelling of slurries happened at room temperature in air atmosphere without any other organic additive. The gelling mechanism was the formation of chelates between Mg²⁺ and carboxyl groups (-COO⁻) of PAA. The frequency-based testing method was applied to investigate the gelling process of the as-prepared slurry. In addition, a novel in situ characterization method based on a modified indentation testing was invented to better understand the strengthening of the wet green body with time and to guide when demolding could be carried out. After sintering, transparent MgAl₂O₄ ceramics with high in-line transmittance were resulted.     

建筑结构控制爆破拆除的力学分析与应用

应用运动学、动力学理论建立建(构)筑物控爆坍落过程的力学模型,进而研究在非定常约束条件下坍落结构的力学特征。首次提出了采用延时起爆技术时,非自由质点系及其广义坐标应是建立在每一段位上的瞬间虚位移。在此基础上,分析求解运动过程中时控单元的质点自由度,为精确选定爆破部位及时序提供了设计理论和计算依据。同时,结合振动理论,将控爆时差、结构载荷分布调制为振动系统,形成坍落结构梁板单元的强迫振动,充分实现在运动中进一步解体、破碎其自身结构,并形成具有明显缓冲效果的下落过程。应用实践结果证明力学分析是正确的,为工程结构爆破拆除计算机辅助设计及动态模拟技术的有效应用奠定了基础。     

基于FLAC3D的破碎围岩巷道支护效果分析

为了解破碎围岩分别采用锚杆支护、锚喷支护以及锚喷+锚索耦合三种支护方式下的支护效果,进而为破碎围岩巷道选择合理的支护方式提供参考。通过借助FLAC^3D软件建立数值模型,分析不同支护条件下的破碎围岩巷道位移量、应力分布以及塑性区的时空演化特征。结果表明,采用锚喷+锚索耦合支护时,可以较好的控制巷道围岩的位移量、减小应力集中效应、缩小塑性区的影响范围。     

Meal patterns of weaned dairy calves are affected by previous dietary experience and associated with competition surrounding individual feed bins

The objective of this study was to evaluate how meal patterns of recently weaned and group-housed dairy calves are subject to effects of previous dietary experience and associated with competition for feed. During the preweaning period, Holstein heifer and bull calves were provided diets differing in provision and presentation of forage as follows: (1) pelleted starter only (n = 12), (2) starter and chopped coastal bermudagrass provided in separate buckets (n = 13), (3) starter and chopped coastal bermudagrass mixed together in the same bucket (n = 15), or (4) starter, chopped coastal bermudagrass, and liquid molasses mixed together in 1 bucket (n = 13). At 58 ± 2 (mean ± standard deviation) days of age, following weaning, calves were mingled between treatments and moved into group housing in weekly age-based cohorts (7 ± 2 calves/group; 8 groups total), and all were provided the mixed diet (without molasses). Within group pens, calves were fed individually using the Calan Broadbent feeding system (American Calan Inc.). Calves were monitored for 1 wk following introduction to the group pen. Feed intake was measured daily. Behavioral data, describing feeding times and competition for feed, were recorded continuously for 48 h beginning after a 5-d adaptation to the group pen. Calves previously provided starter only had longer, less frequent meals than calves previously provided forage in any form, and they tended to consume less feed compared with those previously provided starter and hay separately, with calves previously provided mixed diets having intermediate intakes. We observed occurrences of displacements at bins, which were followed by replacements and feed stealing on some occasions, where 34% of calves consumed feed from a bin assigned to another calf on at least 1 occurrence, and 64% of calves were stolen from at least once. Competition at feed bins was not affected by previous dietary treatments, but was associated with meal patterns. Actor displacement rate was negatively associated with meal frequency and duration, but calves that were displaced more often were those that spent more time feeding. These results suggested a possible carryover effect of previous exposure to forage on postweaning meal patterns following a dietary change. Further, we found that competitive behavior varied considerably between individuals, occurring frequently despite feeding calves using individually-assigned feeding bins, and was associated with meal characteristics. These results highlighted the importance of considering both previous dietary experience and social factors when evaluating feeding behavior.     

Influence of density on the post-suffusion behavior of gap-graded sand with fines

Suffusion erosion, characterized as the selective detachment and transportation of finer particles by seepage flow, is hazardous to the stability and serviceability of geotechnical structures. The removal of finer particles deteriorates the structure and fabric of the soil, leading to the degradation of its mechanical properties. Studies into the effects of suffusion on mechanical behavior have so far produced disparate results depending on the pre-erosion relative density of the specimens tested. To investigate this issue, small cyclic and monotonic loading tests were performed on intact and eroded gap-graded silty sand specimens in three dispersed density states, using a triaxial cell modified for the purpose of erosion. The variation of Young's modulus showed an inverse relationship with the pre-erosion density of the specimen, as the small strain stiffness decreased in the dense cases and increased in the loose cases. Conversely, Poisson's ratio increased in value as suffusion progressed regardless of the initial density of the specimens. In the contractive phase of monotonic loading, the densification of the coarse soil skeleton by the downward seepage flow resulted in a decrease in contractiveness and an increase in secant stiffness. In the dilative phase, the increase in porosity by the erosion of finer particles reduced the dilatancy and peak strength of the specimens. The results suggest that the pre-suffusion density determines the primary locus of affected mechanical behavior in triaxial compression, which shifts from the contractive phase to the dilative phase with the increase in pre-suffusion density. The critical state strength, inferred using stress–dilatancy theory, was largely unaffected by the erosion of finer particles. As the results indicate, pre-erosion density may be of practical significance in assessing the vulnerability to deterioration and collapse of geotechnical formations and structures subjected to suffusion erosion.