关键词： Wood composites   heat treatment   injection molding   water absorption   diffusion coefficient   Fickian diffusion   mechanical properties  

摘要： The hygroscopic nature of wood limits the use of wood thermoplastic composites (WTC) in outdoor industrial and domestic applications. To reduce this tendency, red balau saw dust was heat treated at 180 and 200 degrees C for 1 h and compounded with Low Density Poly(ethylene) (LDPE) into 20 and 37 wt% and then molded into test specimens by injection molding. Samples were immersed in distilled water at room temperature for 4 months. Heat-treated wood composites showed remarkable water resistance relative to untreated ones. Wood composites made from wood treated at 180 and 200 degrees C exhibited almost similar water absorption pattern. Reduced water absorption of heat-treated wood composites relative to untreated ones indicates that heat treatment has resulted in a degree of modification of the wood. Most of the composites displayed the Fickian mode of water absorption with n values close to 0.5. Also, the diffusion coefficient reduced with wood content in untreated wood composites due to interaction of water with the polar groups in wood through hydrogen bonding. Untreated wood composites exhibited poorer mechanical properties with water immersion as a result of degradation due to moisture. The mechanical properties of the heat-treated wood composites were not adversely affected with water absorption. Therefore, heat treatment can reduce the proneness to water absorption in WTCs and alleviate the detrimental effects on mechanical properties.

关键词： Boron nanosheet   Grain size   Doped polycrystalline graphene   Fracture behavior   Molecular dynamic simulation   Initial crack  

摘要： This article investigates the effect of different initial crack sizes on the mechanical response of single-layer boron doped polycrystalline graphene nanosheets by Molecular Dynamics (MD) simulations. We study 1%, 3%, 6% and 10% of boron doped polycrystalline graphene nanosheets with grain sizes of 10 and 15 nm for eight different initial crack lengths of 0.02L, 0.04L, 0.08L, 0.12L, 0.16L, 0.2L, 0.24L, and 0.32L, where L is the initial length of the nanosheet. We found that 1. brittle fracture for boron doped polycrystalline graphene as the failure occurs without any sign of plastic deformation and low energy absorption and 2. the ultimate tensile strength is independent of the initial crack size. For identical grain sizes and doping of boron atoms, the ultimate tensile stress and strain decrease as the crack lengths increases. In contrast, a clear trend was not observed in the ultimate tensile strength for the same crack length and doping of boron atoms as the grain size increases. The highest difference of 13.4% in the ultimate tensile strength was observed between grain sizes of 10 and 15 nm for 3% of boron doped nanosheets with an initial crack size of 20 angstrom.

关键词： drill-string dynamics   delay differential equations   specific energy  

摘要： This paper considers the effect of an antistall tool on the dynamics of deep drilling systems. Field results show that the antistall tool increased the rate of penetration (ROP) of drilling systems when compared with ROP in offset wells drilled without this tool. We developed a model-based approach to investigate the effect of this downhole tool on the ROP and on the mechanical specific energy. Toward this, a drillstring model including the antistall tool was constructed;it describes the coupled axial/torsional dynamics in the form of delay differential equations. Simulation results and a dynamic analysis based on averaging the obtained steady-state response show that an increased drilling efficiency was obtained using the antistall tool, resulting in a higher ROP.

关键词： Epoxy resin   Activation energy   Mechanical properties   Fast curing  

摘要： Fast curing resins are the material basis for large-scale production of fiber reinforced polymer (FRP) composites. In this study, diglycidyl ether of bisphenol A (DGEBA) was cured with 2-ethyl-4-methylimidazole (2E4MI) as the main curing agent, and allyl glycidyl ether (AGE) was added to improve the processability. Based on this curing system, the effect of isophorone diamine (IPD) content on curing kinetics and mechanical properties was studied to obtain the optimal fast curing resin formulation. Both the curing kinetics and the glass transition temperature (T-g) were characterized by non-isothermal DSC, and the impact, flexural, and tensile behaviours of the cured products were tested. The gel and curing time of the curing system were characterized by rheometry and isothermal DSC test, respectively. The results demonstrated that the increasingly evident first shoulders from the non-isothermal DSC curves indicated the incorporation of addition reaction of IPD with epoxy groups in the first curing stage. The correlation between activation energy (E-a) and degree of curing (alpha) determined by Ozawa-Flynn-Wall methods strongly depended on the content of IPD. Moreover, the curing time of the optimal resin system was shortened to 15 min at 120 degrees C with guaranteed mechanical properties and the T-g was 102.50 degrees C.

关键词： Shear stress  

摘要： This paper reports on results for the thermal and mechanical states at the interface of two-layer thermal convection in two-dimensional (2-D) spherical geometry solved by numerical calculations. The two-layer system was composed of a highly viscous layer (HVL) and a low-viscosity layer (LVL) underneath. The two end-member convection regimes were studied by varying two free model parameters, which control the degree of layering in HVL convection and separate the HVL into the upper and lower parts. One of the regimes was a nearly whole-layer convection regime in which the upwelling and downwelling plumes easily penetrated into another layer in the HVL, while the other was a so-called hybrid convection regime, which represented a transitional regime between the whole-layer convection and the double-layer convection. The spatiotemporal analyses of convection behavior showed that the lateral scale of HVL convection and the resultant lateral scale of thermal heterogeneity beneath the HVL-LVL interface tended to be larger in the hybrid convection regime than those in the whole-layer convection regime. On the other hand, the fluctuation of shear-stress at the HVL-LVL interface was more time-dependent in the hybrid convection regime, whereas the mechanical heterogeneity near the HVL-LVL interface was larger in the whole-layer convection regime. The present results on the differences in the scale of dynamically determined thermal and mechanical states beneath the HVL-LVL interface between the two end-member convection regimes may apply to issues on the relationship between thermal and mechanical conditions at the Earth's core-mantle boundary and the strength of the geomagnetic field. Published under license by AIP Publishing.

关键词： Molecular dynamics simulations   Graphdiyne nanotubes   Mechanical properties   Temperature   Vacancy defect  

摘要： In this paper, the mechanical properties of the graphdiyne nanotubes are investigated. For this purpose, molecular dynamics simulations are used to simulate the tensile test on the graphdiyne nanotubes. The effects of chirality, diameter, length, temperature, and vacancy defect on the stress-strain curves and Young's modulus of the graphdiyne nanotubes are investigated. Two different behaviors are observed in the stress-strain behavior of the nanotubes. In the first type, the fracture happens suddenly by breaking all of the bonds and in the second type, the fracture happens gradually in several steps by breaking some bonds and forming some newer bonds in each step. Investigating the influence of the chirality on the Young's modulus of the graphdiyne nanotubes, it is seen that the armchair nanotubes have smaller Young's modulus than the zigzag ones with the same diameter and length.

关键词： ionic block copolymer   molecular dynamics simulation   mechanical property  

摘要： Polymerized ionic copolymers have recently evolved as a new class of materials to overcome the limited range of mechanical properties of ionic homopolymers. In this paper, we investigate the structural and mechanical properties of charged ionic homopolymers and di-block copolymers, while using coarse-grained molecular dynamics simulation. Tensile and compressive deformation are applied to the homopolymers and copolymers in the glassy state. The effect of charge ratio and loading direction on the stress-strain behavior are studied. It is found that the electrostatic interactions among charged pairs play major roles, as evidenced by increased Young's modulus and yield strength with charge ratio. Increased charge ratio lead to enhanced stress contribution from both bonding and pairwise (Van der Waals + coulombic) interaction. The increase in the gyration of the radius is observed with increasing charge ratio in homopolymers, yet a reversed tendency is observed in copolymers. Introduced charge pairs leads to an increased randomness in the segmental orientation in copolymers.

关键词： Metal organic frameworks   Chemical structure   Solvents   Nuclear magnetic resonance spectroscopy   Materials  

摘要： Zinc(II) metal-organic framework (MOF) materials were prepared using a porphyrin-based tetracarboxylate linker and pyridine-terminated [2]rotaxane linkers as pillars. Two pillaring rotaxanes (R = H or CH3 at pyridine 2-position) were used to create two MOFs alpha-UWDM-6 and alpha-UWDM-7, and their structures were determined by single-crystal X-ray diffraction. Both MOFs undergo reversible phase transitions upon solvent removal to generate gamma-UWDM-6 and beta-UWDM-7. The dynamics of the 24-membered macrocycle ([24]crown-6) inside each MOF phase was studied using variable-temperature H-2 solid-state NMR spectroscopy. Interestingly, only solvated alpha-UWDM-7 demonstrates unrestricted rotation of the macrocycle inside the MOF pores;studies on alpha-UWDM-7 with various solvents showed that the dynamics of the wheel is essentially independent of the solvent.

关键词： Animal occupied zone   Baffle placement   Cross-ventilated barn   Heat and mass transfer   Numerical simulation  

摘要： Baffles are often installed in mechanically cross-ventilated barns to combat heat stress by enhancing the cooling effect produced by the barn's design. How effectively these baffles operate during warm weather often depends on where each baffle is located and at what angle it is set. Currently, their location and orientation are determined primarily according to the personal experience of the contractor in charge of installing the barn's ventilation system. This study seeks to quantify the extent to which the location and orientation may affect the efficiency of the installed baffles. Computational fluid dynamics was used to create a three-dimensional model of a typical low-profile, cross-ventilated dairy barn. Differences were evaluated using both heat transfer and heat stress index (the Equivalent Temperature Index for Cattle - EITC). The results obtained from the tested scenarios indicated that installing the baffles above the animal occupied zone (AOZ) could indeed increase the air velocity (from less than 0.5 m s(-1) to around 3 m s(-1) once baffles were in place), and could also greatly increase (by 21.1%-50.9%) the amount of heat removed, while reducing the EITC by similar to 6 degrees C. In comparisons between the most commonly used scenario (in which baffles were installed vertically in the middle of the AOZ), and the other with baffle-scenarios, the rate at which heat was removed from the cows varied by -3.0% to 2.6%, with the maximum difference being 172.4W. However, these scenario produced no significant improvement comparing with the commonly used one. The results obtained should provide a more reliable reference for engineers engaged in designing large-scale dairy barns.

关键词： Polyimide   Molecular dynamics simulation   Hierarchical multiscale  

摘要： The molecular dynamics simulations are adopted to investigate the thermo-mechanical properties of pyromellitic dianhydride(PMDA)/2-(4-aminophenyl)-1H-benzimidazole-5-amine(BIA), PMDA/p-phenylene diamine(PDA), PMDA/4,4'-oxidianiline(ODA), 3,3',4,4'-biphenyldianhydride(BPDA)/BIA, BPDA/PDA, and BPDA/ ODA polyimide films in hierarchical multiscale level. The results indicate that the properties follow the order of BIA > PDA > ODA. At a small scale, the hydrogen bonding in the BPDA/BIA polyimide strengthens the chain interactions and results in a coiling-shaped chain conformation, endowing the polyimide with superior properties. But the ODA increases the helical-shaped segments conformation, therefore possessing large elongation under small tension. At the scale of the entire polymeric chain, the chain mobility was discussed by the ratio of < Ree(2) > to < s(2) > and MSD. And higher chain mobility corresponds to lower tensile modulus. At a large scale, the torsion angle distribution changes apparently before and after stretching, which increases the d-spacing and elongates the entire molecular chain simultaneously, serving as the main cause of polyimide tensile deformation.