A fiber fracture model for metal matrix composite monotape consolidation

The consolidation of plasma sprayed monotapes is emerging as a promising route for producing metal and intermetallic matrix composites reinforced with continuous ceramic fibers. Significant fiber fracture has been reported to accompany the consolidation of some fiber/matrix systems, particularly those with creep resistant matrices. Groves et al. [Acta metall. mater.42, 2089 (1994)] determined the predominant mechanism to be bending at monotape surface asperities and showed a strong dependence of damage upon process conditions. Here, a previous model for the densification of monotapes [Elzey and Wadley, Acta metall. mater.41, 2297 (1993)] has been used with a stochastic model of the fiber failure process to predict the evolution of fiber fracture during either hot isostatic or vacuum hot pressing. Using surface profilometer measured roughness data for the monotapes and handbook values for the mechanical properties of different matrices and fibers, this new model is used to elucidate the damage dependence on process conditions, monotape surface roughness, and the mechanical properties both of the fiber and matrix. The model is used to investigate the “processibility” of several currently important matrix and fiber systems and to identify the factors governing this. An example is also given of its use for the simulation of a representative consolidation process cycle. This approach to the analysis of a complex, nonlinear, time-varying process has resulted in a clear understanding of the causal relationships between damage and the many process, material and geometric variables of the problem and identified new strategies for its elimination.     

The experimental behaviour of bolted joints in pultruded glass/ polyester material. Part 2: Two-bolt joints

Results reported in the literature for the behaviour of multi-bolt joints in composite materials can be contradictory, owing basically to the variation of the load distribution between the bolts. Therefore, the present investigation of the behaviour of two-bolt joints made from pultruded glass/polyester composite materials concentrates on the load distribution between the bolts. The results of this work show a tendency for the loads to equalize with increasing load. The achievement of full load redistribution has been found to depend on the amount of local deformation that can develop before the joint fails.     

Progressive fracture processes around tunnel triggered by blast disturbances under biaxial compression with different lateral pressure coefficients

To investigate the progressive fracture processes around a tunnel triggered by static stress and dynamic disturbance, experiments and numerical simulations were performed. The results show that the spatial distributions of acoustic emission (AE) events become very different as lateral pressure coefficients change. The combined effect of static stress and dynamic disturbance causes the damage around the tunnel, and initial stress conditions control the damage morphology. The blast disturbance cannot fundamentally change the damaged area but will deepen the extent of damage and accelerate the failure speed. The more significant the difference between the vertical and horizontal stresses is, the higher the impact on the tunnel by the dynamic disturbance is. The AE activity recovers to a relatively stable state within a short time after the blast and conforms to power-law characteristics.     

GFRP筋土钉支护钉头的锚固性能

土钉钉头为土钉墙整体结构中的薄弱部位,为研究土钉钉头锚固性能,以一种装配式柔性面层GFRP筋土钉墙为例,通过室内钉头锚固性能试验及三维数值模拟对外径32 mm的中空GFRP筋钉头的极限抗拉强度、钉头变形及破坏规律进行研究。研究结果表明,该GFRP筋钉头极限承载力在240~290 kN之间,当螺母拧紧时螺纹副的应力主要分布在螺纹牙前3环,其中以钉头第1环螺纹牙应力集中现象尤为明显;此外,采用螺纹展开法建立钉头螺纹牙的力学模型,得出了第1环螺纹极限剪切强度与钉头极限荷载之间的关系,并通过计算得出该GFRP筋钉头极限承载力为244.54 kN,与试验结果相符。通过室内钉头锚固性能试验的脆性破坏特点,确定GFRP筋钉头安全系数在1.8~2.0之间,并判定此种GFRP筋安全荷载在125~135 kN之间。     

Using fuzzy MCDM technique to find the best location in Qatar for exploiting wind and solar energy to generate hydrogen and electricity

Hydrogen has been a promising energy carrier to meet the world's energy needs as well as reduce pollutant emissions. Although many countries have policies and programs to expand hydrogen production, the potential for hydrogen production in different regions of Qatar has not yet been evaluated. Therefore, this paper, for the first time, evaluates the possibility of an average annual cogeneration of 14 kWh of electricity and 85 kg/day of hydrogen by a home-scale solar-wind system connected to the grid in Qatar. NASA's 20-year average of meteorological data, the electricity tariff and gasoline price in 2018, along with annual real interest rate, were used as inputs to HOMER software. The techno-econo-enviro analysis was done over a one-year period hour by hour. From the results, it was found that the lowest prices of hydrogen and electricity generated, with $ 2.092/kg and $ 11.495/kWh, were related to Grid and PV-Wind-Grid scenarios, respectively. Also, results indicated that Ar-Ruways station and PV-Wind-Grid scenario were the most environmentally suitable options that resulted in a CO₂ emission rate of 1434 kg annually. To select just one station among five areas, a fuzzy method was deployed as a prioritization technique. Its results suggested that Doha Intl Airport site is the most suitable one for constructing solar-wind hybrid energy generation system.     

Fate of Escherichia coli experimentally injected in a drinking water distribution pilot system

Detection of coliforms and E. coli in drinking water distribution systems can be explained by accidental contaminations and/or growth of these bacteria in the network. This last point still remaining debatable, an experiment was carried out with two E. coli strains (E. coli O126:B16 and one isolated from a drinking water distribution system) separately injected in a drinking water distribution pilot plant continuously fed with drinking water. The work aimed to study (i) the partition of the injected bacteria between the water phase and the indigenous biofilm phase and (ii) the kinetic of disappearance or growth of these bacteria.Depending on the strain, 1–50% of injected bacteria adsorbed within a few hours to the indigenous bacterial biofilm. During the first 5–7 days, both E. coli populations behaved similarly: the total number of injected bacteria decreased more quickly than the theoretical washout (death, lysis, predation, ...). Then the number of these bacteria slowly increased and at day 9–12, the number of recoverable injected bacteria became higher than the predictable number calculated from the theoretical washout. This result clearly proves that both E. coli strains are able to grow at 20°C in the absence of residual chlorine in a distribution network system largely colonized with an autochthonous population. However, colonization of the network by E. coli strains was only partial and transient.     

Volatile solids reduction in two-phase and conventional anaerobic sludge digestion

Success of two-phase anaerobic systems for primary and secondary sludge treatment has been reported based on both directly and indirectly measured volatile solids (VS) reduction, total gas and methane generation, COD reduction, etc. The objective of this research was to determine whether phase separation increases directly measured VS reduction compared to conventional anaerobic sludge digestion. Two-phase and conventional digesters were operated with sludge feeds from three sources; both 1:1 sludges (primary:waste activated, solids basis) and 100% waste activated sludges (WAS) were studied. The maximum difference between VS reductions in conventional and two-phase systems was about 8.7% with waste activated sludge. The increase in volatile solids reduction in two-phase systems with the 1:1 sludge ranged from 1.9 to 6.0% as compared to conventional systems. This relatively small increase in VS reduction may not be worth the additional cost of operating two-phase digesters at full scale.     

时滞动力学与控制研究进展

与一般动力系统不同,无穷维时滞系统的研究方法并不成熟,使得时滞系统的分析和控制都很困难.与此同时,时滞广泛存在于包括神经网络、人工智能、机械加工、多智能体、机器人控制等众多领域,使得时滞动力学与控制的研究至关重要.因此,本专刊聚焦时滞引起的稳定性、非线性动力学和控制问题,着重讨论了神经、网络、机械和减振等领域的时滞动力学问题,希望能为相关领域的学者提供一些借鉴和参考.