Characterization of irregular open-cell cellular structure with silicone pore filler

The paper presents the results of studying the influence of silicone polymer pore filler on the macroscopic quasi-static and dynamic compressive behaviour of aluminium foam with irregular open-cell structure. The study is based on a mechanical experimental testing programme, where the deformation mechanism and mechanical energy absorption capacity of aluminium foam with silicone pore filler have been observed for the first time. As plastic yielding is accompanied by significant heat energy dissipation, this study was additionally supported by thermal imaging, which enables visualization of plastification to better understand the deformation process of observed specimens. The influence of specimen size on the behaviour of aluminium foam specimens has also been investigated. The results show that introduction of silicone pore filler considerably increases the energy absorption capacity at almost unchanged densification strain under both quasi-static and dynamic loading conditions. The silicone pore filler also significantly influences the deformation behaviour of aluminium foam specimens, which is manifested in a different stress distribution and a significant transverse deformation with conical plastification front. However, only a minor difference in response of different size specimens has been observed.     

Shape and eccentricity effects in adhesive contacts of rodlike particles

The effects of shape and eccentricity on adhesion and detachment behavior of long, rodlike particles in contact with a half-space are analyzed using contact mechanics. The particles are considered to have cross sections that are squarish, oblate, or prolate rather than circular. Such cross sections are represented very generally by using superellipses. The contact mechanics model allows deduction of closed-form expressions for the contact pressure, load-contact size relation, detachment load, and detachment contact size. It is found that even relatively small deviations in shape from a cylinder have a significant influence on the detachment load. Eccentricity also affects the adhesive behavior, but to a lesser extent, with oblate shapes requiring larger separation loads than prolate shapes. The load-contact size solution reduces to that for a right-circular, cylindrical rod when the appropriate limit is taken. The detachment behavior of right-circular cylinders is also found to be mimicked by an entire family of rod shapes with different cross sections.     

Localized damage response of carbon fiber reinforced polymer composite sandwich panel after thermal exposure

An experimental study was conducted to investigate the effect of thermal exposure on indentation behavior of carbon fiber reinforced polymer composite sandwich panel (CFRPCSP) with pyramidal truss cores. Composite sandwich panels were fabricated by the hot press molding method. Subsequently, composite sandwich panels were exposed to different temperatures for 6 h. After thermal exposure, quasi-static indentation tests were carried out at room temperature. Then, the effect of thermal exposure on the failure mechanism, indentation load and energy absorption were analyzed and discussed. The results showed that the indentation load and energy absorption decreased as exposure temperature increased, which was caused by the degradation of the matrix properties and fiber-matrix interface properties at high temperature. In addition to the decrease of the indentation load and energy absorption, the failure modes also changed with exposure temperature. It is expected that this study can provide useful information for the design and application of composite sandwich panel with pyramidal truss cores at high temperature.     

Spectroscopic analysis of single-walled carbon nanotubes and semiconjugated polymer composites

Interactions between arc discharge single-walled carbon nanotubes within polymer composites have been well documented. Here hybrid systems of the conjugated organic polymer poly(p-phenylene vinylene-co-2,5-dioctyloxy-m-phenylene vinylene) (PmPV) and HiPco SWNTs are explored using UV/vis/NIR and Raman spectroscopy at 514.5 and 632.8 nm to determine specific interactions. An examination of the radial breathing modes at 514.5 nm shows similar tube diameters of 1.28 and 1.35 nm selected for both the arc discharge and HiPco composites. The corresponding G lines of both composites show no specific type of tubes being selected. At 514.5 nm, the G line of the HiPco composite (1% mass fraction) shows contributions from semiconducing and metallic tubes, and the arc discharge composite (1% mass fraction) is dominated by semiconducting nanotubes. At 632.8 nm, the G line of the HiPco composite (1% mass fraction) is dominated by semiconducting tubes, and the arc discharge composite (1% mass fraction) shows strong contributions from metallic tubes. This finding is a strong indication that the selection process is dependent on tube diameter rather than backbone structure. The solubility limits of both composites are determined by investigating the G lines of both composites and have been found to be greater than 1% mass fraction by weight for the arc discharge composite and greater than 0.1% mass fraction by weight for the HiPco composite.     

Compressive response of composite ceramic particle-reinforced polyurethane foam

Quasi-static and dynamic compressive tests are undertaken on the polyurethane (PU) foam and fumed silica reinforced polyurethane (PU/SiO₂) foam experimentally. The ceramic microspheres with varying mass fractions are adopted to mix with the PU/SiO₂ foam to fabricate the composite particle-reinforced foams. The effects of strain rate and particle mass fraction are discussed to identify and quantify the compressive response, energy-absorbing characteristic, and the associated mechanisms of the composite foams. The results show the initial collapse strength and plateau stress of the foams are improved significantly by reinforcing with the ceramic microsphere within 60 wt% at quasi-static compression. The rate sensitivity is observed on all the foams, but in different patterns due to the influence of ceramic microsphere. The compressive response affected by ceramic microsphere can be attributed to the particle cluster effect and stress wave propagation. Together with the deformation, the compressive characteristic experiences non-monotonic change from the low to high strain rates. The specific energy absorption (SEA) of the foam with 41 wt% ceramic microsphere show the largest magnitude at quasi-static compression. With the increasing strain rate, the ceramic reinforced foam exhibits superior energy absorption efficiency at high strain rates to that of the pure foams.     

Influence of diameter on the degradation profile of multiwall carbon nanotubes

Nanofluid and coiled tubes have been employed as two passive methods for enhancing the heat transfer. In the present study, the turbulent flow of CuO–water nanofluid in helical and conical coiled tubes was numerically investigated with constant wall temperature through mixture model. The thermophysical properties of base fluid (water) were considered as temperature-dependent functions, while Brownian effects were adopted in thermal conductivity and dynamic viscosity of nanofluid. Simulation results were validated using experimental data for heat transfer coefficient and pressure drop in helical coiled tube for different Reynolds numbers. Four different geometries were simulated and compared. The first one was a conical coiled tube; the others were helical coiled tubes whose coil diameters were minimum, maximum, and median of the conical coiled tube pitch coil diameter. The velocity profiles indicated stronger secondary flow in conical coiled tube at a specified Dean number. The obtained results also showed higher heat transfer enhancement in the conical coiled tube in comparison with helical coiled tube with the same average pitch coil diameter. Moreover, the nanoparticle-induced heat transfer enhancement was more effective in conical coiled tube.

     

Effect of magnesium nitrate vaporization on gas temperature in the graphite furnace

The vaporization of magnesium nitrate was observed in longitudinally-heated graphite atomizers, using pyrocoated and Ta-lined tubes and filter furnace, Ar or He as purge gas and 10–200-μg samples. A charge coupled device (CCD) spectrometer and atomic absorption spectrometer were employed to follow the evolution of absorption spectra (200–400 nm), light scattering and emission. Molecular bands of NO and NO₂ were observed below 1000°C. Magnesium atomic absorption at 285.2 nm appeared at approximately 1500°C in all types of furnaces. The intensity and shape of Mg atomization peak indicated a faster vapor release in pyrocoated than in Ta-lined tubes. Light scattering occurred only in the pyrocoated tube with Ar purge gas. At 1500–1800°C it was observed together with Mg absorption using either gas-flow or gas-stop mode. At 2200–2400°C the scattering was persistent with gas-stop mode. Light scattering at low temperature showed maximum intensity near the center of the tube axis. Magnesium emission at 382.9, 383.2 and 383.8 nm was observed simultaneously with Mg absorption only in the pyrocoated tube, using Ar or He purge gas. The emission lines were identified as Mg ³P°–³D triplet having 3.24 eV excitation energy. The emitting species were distributed close to the furnace wall. The emitting layer was thinner in He than in Ar. The experimental data show that a radial thermal gradient occurs in the cross section of the pyrocoated tube contemporaneously to the vaporization of MgO. This behavior is attributed to the reaction of the sample vapor with the graphite on the tube wall. The estimated variation of temperature within the cross section of the tube reaches more than 300–400°C for 10 μg of magnesium nitrate sampled. The increase of gas temperature above the sample originates a corresponding increase of the vaporization rate. Fast vaporization and thermal gradient together cause the spatial condensation of sample vapor that induces the light scattering.     

Progressive failure numerical simulation and experimental verification of carbon-fiber composite corrugated beams under dynamic impact

To explore the axial impact energy absorption capacity of bidirectional carbon pre-impregnated (prepreg) composite corrugated beams, a solid 3D finite element model with different trigger mechanism settings and different ply designs was established. Numerical simulation of dynamic impact was performed on the model. An in-plane damage model considering shear failure was created based on continuum damage mechanics and Hashin's criteria, and a stiffness degradation model of damage failure for G803/5224 is proposed. The cohesive zone model is used and the bilinear traction-separation constitutive model is considered to simulate inter-laminar delamination failure, thereby accurately reflecting the anisotropic progressive damage characteristics of bidirectional carbon-fiber prepreg composite corrugated beams. The results show that progressive failure and damage occur under impact loading of corrugated beams. The energy-absorbing load-displacement curve and specific energy absorption were obtained through simulation. Simulation results were validated by comparison with test results. With the maximum relative error of its average crushing load less than 11%, the damage morphology and test results of the beam has improved in uniformity. Furthermore, the validity of 3D finite element models considering inter-laminar delamination damage has been validated.     

Absorption spectra of small niobium and gold clusters measured by photodepletion of their rare gas van der Waals complexes: some preliminary experiments

The optical absorption spectra of small niobium clusters have been determined over the wavelength range 260 – 740 nm by photodetaching Krypton atoms from the corresponding neutral van der Waals, vdW, complexes, Nb_nKr_m, n=5–15, m=1–3. Cross sections for small gold clusters were determined by photodetachment experiments oncharged vdW complexes [Au_nXe_m]⁺, m=1, 2. The absorption cross sections are observed to increase monotonically with decreasing wavelength. At the long wavelength end of the range, the cross section is practically independent of the cluster nuclearity, n; whereas, at the short wavelength end of the range, the cross section increases monotonically with n.     

Deformation mechanisms of constrained polymer chains. I. Geometrically induced correlations in the deformation response mechanisms of tie molecules

The geometrical constraints acting on sections of tie molecules in noncrystalline regions severely limit the number and type of available polymer chain conformations. It is shown that these constraints induce explicit correlations in the rotations about the backbone bonds. These correlated rotations, in turn, specify distinct structural conversion paths which define the molecular mechanisms underlying the deformation response of tie molecules. Application of these constraining relationships to highly oriented polyethylene shows that the kink and jog structures of tie molecules can be decomposed into combinations of three primary conformational building blocks. Each of the basic conformational subunits follow an explicit set of dihedral angle correlations and, consequently, imparts specific characteristics to the composite structure of tie molecules. It is proposed that the composite response characteristics of tie molecules can be described as linear combinations of the response characteristics of these three primary conformational subunits.     

Ab initio potential energy surfaces, total absorption cross sections, and product quantum state distributions for the low-lying electronic states of N(2)O

Adiabatic potential energy surfaces for the six lowest singlet electronic states of N(2)O (X (1)A('), 2 (1)A('), 3 (1)A('), 1 (1)A("), 2 (1)A(") and 3 (1)A(")) have been computed using an ab initio multireference configuration interaction (MRCI) method and a large orbital basis set (aug-cc-pVQZ). The potential energy surfaces display several symmetry related and some nonsymmetry related conical intersections. Total photodissociation cross sections and product rotational state distributions have been calculated for the first ultraviolet absorption band of the system using the adiabatic ab initio potential energy and transition dipole moment surfaces corresponding to the lowest three excited electronic states. In the Franck-Condon region the potential energy curves corresponding to these three states lie very close in energy and they all contribute to the absorption cross section in the first ultraviolet band. The total angular momentum is treated correctly in both the initial and final states. The total photodissociation spectra and product rotational distributions are determined for N(2)O initially in its ground vibrational state (0,0,0) and in the vibrationally excited (0,1,0) (bending) state. The resulting total absorption spectra are in good quantitative agreement with the experimental results over the region of the first ultraviolet absorption band, from 150 to 220 nm. All of the lowest three electronically excited states [(1)Sigma(-)(1 (1)A(")), (1)Delta(2 (1)A(')), and (1)Delta(2 (1)A("))] have zero transition dipole moments from the ground state [(1)Sigma(+)(1 (1)A('))] in its equilibrium linear configuration. The absorption becomes possible only through the bending motion of the molecule. The (1)Delta(2 (1)A('))<--X (1)Sigma(+)((1)A(')) absorption dominates the absorption cross section with absorption to the other two electronic states contributing to the shape and diffuse structure of the band. It is suggested that absorption to the bound (1)Delta(2 (1)A(")) state makes an important contribution to the experimentally observed diffuse structure in the first ultraviolet absorption band. The predicted product rotational quantum state distribution at 203 nm agrees well with experimental observations.     

Absorption cross sections of formaldehyde at wavelengths from 300 to 340 nm at 294 and 245 K

Absorption cross sections for the A1A2-X1A1 electronic transition of formaldehyde have been measured by ultraviolet (UV) laser absorption spectroscopy in the tropospherically significant wavelength range 300-340 nm, over which HCHO is photochemically active. Absorption cross sections are reported at two temperatures, 294 and 245 K and at a spectral resolution of 0.0035 nm (0.35 cm-1). At this resolution, greater peak absorption cross sections are obtained for many of the sharp spectral features than were previously reported. To simulate atmospheric conditions in the troposphere, the effects of adding a pressure of nitrogen of up to 500 Torr and of reduced sample temperature were investigated. The overall magnitudes of peak absorption cross sections are largely unaffected by the added pressure of nitrogen, but a modest degree of pressure broadening (0.2-0.3 cm-1 atm-1) is evident in the line shapes. Computer simulations of spectra have been optimized by comparison with wavelength-dependent formaldehyde absorption cross sections for each major vibronic band in the chosen wavelength range. Experimental and computer simulated spectra at 294 and 245 K are compared to test the reliability of the computer simulations for quantification of the effects of temperature on absorption cross sections. All experimental absorption cross section data and tables of input parameters for spectral simulations are available as Supporting Information.     

Production and crosslinking of multi-layer tubes (PE & metal) by E-beam

Irradiation crosslinking of PE-tubes has been used for heating floors for about 25 years. Such tubes are also used today for drinking water supply. A further development has been the coating of such tubes with Ethylene-Vinyl-Alcohol-Copolymers (EVAL), in order to prevent oxygen diffusion into the water through the PE tube. For about 15 years composite tubes made of PE and aluminum have been available. These tubes are crosslinked with electron beams. The energy of the accelerated electrons must be adjusted for the particular tube configuration, so that the inner PE-layer will be crosslinked. This paper will concern itself with the manufacture and the crosslinking of composite tubes.     

Computation of some characteristics of gas or liquid turbulent flow in a circular pipe

The dependence of the type of velocity profile in turbulent flow of liquids and gases in a circular tube outside the viscous sublayer and buffer area on the Reynolds number was examined. On this basis, the dependence of the ratio of the velocity averaged over a pipe cross section to maximum velocity, as well as parameters in the balance equation of mechanical energy and momentum on the Reynolds number were studied.     

Flexural properties of sandwich composite panels with glass laminate aluminum reinforced epoxy facesheets strengthened by SMA wires

Sandwich composite panels are widely used and significant in structural applications such as aerospace, shipbuilding and transportation, etc. This is due to their specific properties such as specific stiffness, strength and energy absorption. Still, many innovations are required to develop and upgrade their mechanical properties in various loadings and conditions, specifically in bending loads. One of the methods to enhance the properties of sandwich structures is to employ various advanced materials in these structures to change their acquired properties. In the present research work, sandwich composite panels made by fiber metal laminate like glass laminate aluminum reinforced epoxy (GLARE) as the facesheets and PVC polymer foam as the core material are investigated in flexural (bending) loading condition. To change or enhance the behaviour of sandwich panel in bending loads, shape memory alloy wires are also embedded in between glass fiber reinforced epoxy composite layers in fiber metal laminate facesheets. The shape memory wires are also pre-strained in fiber reinforced epoxy composite in sandwich panels. To study the flexural properties of sandwich panels with fiber metal laminate facesheets, the effect of shape memory alloy wires and also the effect of pre-straining of the wires, three types of sandwich panels are considered and made including panels without shape memory alloy wire, two wires with 0% tensile pre-strain, and two wires with 5% tensile pre-strain for the same cross section. Due to the importance of bending properties in structural applications, the sandwich composite specimens are subjected to flexural test according to ASTM standards. The maximum of 13% increase in maximum bending load and 84% increase in breaking load for the specimens with 0% pre-strained wires are achieved. Also, the maximum displacement and the energy absorption for the specimen with 5% pre-strain was enhanced by 26.5% and 37%, respectively. The energy absorption during the flexural test is greater in case of the specimen with pre-strained wires. Moreover, the specimens with pre-strained wires show better integrity of the structure after the failure in bending. The results represent the advantage effect of shape memory alloy wires on sandwich composite panel's behaviour in bending.     

Theoretical Analysis on Optical Limiting Properties of Newly Synthesized Graphene Oxide-Porphyrin Composites (cited:2)

Optical limiting (OL) properties and two-photon absorption (TPA) of a series of covalently linked graphene oxide-porphyrin composite materials have been investigated by numerically solving the rate equations and field intensity equation with an iterative predictor-corrector finite-difference time-domain technique in nanosecond time domain. Our results show that graphene oxide-porphyrin composites exhibit enhanced OL behavior and possess larger TPA cross section compared with individual porphyrins. Interestingly, unlike the previous result that porphyrin with heavier central metal shows better nonlinear abilities than that with-out metal substitute, graphene oxide-metal free porphyrin composite has stronger nonlinear absorption properties compared with graphene oxide-metal porphyrin composite. The com-putational results are in reasonable agreement with the experimental ones. Special attention has been paid to the influence of thickness of the medium and pulse width on TPA cross sections, which presents that larger TPA cross sections are obtained as the medium is thicker or the pulse duration is wider.     

Unveiling electronic transitions in three novel chiral azo-compounds using linear and nonlinear circular dichroism: a theoretical-experimental study

Herein, we report on the experimental and theoretically study of the linear absorption, electronic circular dichroism (ECD) spectra, as well as the two-photon absorption circular-linear dichroism measurements of three different chiral azo derivatives in dimethylsulfoxide solution. Using potential energy surfaces and frontier orbital analysis, we established the most stable conformation for each molecule and elucidated their different electronic transitions. Our theoretical calculations allowed us to unambiguously identify the spectral position of such transitions and correlate them with the spectral profiles observed in the two-photon absorption spectra. To further elucidate the characteristics of the main electronic transitions in terms of spectral shape and position, we carried out measurements of the polarization dependent two-photon absorption cross sections and determined the two-photon circular-linear dichroism spectra of these azo dyes.     

From molecule to bulk material: optical properties of hydrogen-bonded dimers [C12H12N4O2AgPF6]2 and [C28H28N6O3AgPF6]2 depend on the arrangement of the oxime moieties

The dependence of the optical properties of [C(12)H(12)N(4)O(2)AgPF(6)](2) (dimer-1) and [C(28)H(28)N(6)O(3)AgPF(6)](2) (dimer-2) on the arrangement of the oxime moieties in the molecule and in bulk crystals was investigated by means of time-dependent density functional theory. Dimer-1 with simple pyridine oxime ligands and a wavy arrangement has a smaller dipole moment and larger transition energy between the two states, and thus smaller third-order polarizabilities and two-photon absorption cross sections. Dimer-2 with extended pyridine oxime ligands and a ladder arrangement has a larger dipole moment and smaller transition energy between the two states, and thus larger third-order polarizabilities and two-photon absorption cross sections. The lowest energy absorption band is red-shifted for dimer-2 as compared with dimer-1, due to more pronounced pi-pi delocalization interactions and weaker hydrogen bonding in dimer-2. The electronic absorption spectra, frequency-dependent third-order polarizabilities, and two-photon absorption cross sections involve significant contributions from charge transfers from pi/pi* orbitals of the pyridine oxime ligands but no contribution from PF(6) (-) ions or H(2)O molecules in the wavelength range studied for the monomers and dimers of the C(12)H(12)N(4)O(2)AgPF(6) and C(28)H(28)N(6)O(3)AgPF(6) molecules. The third-order susceptibilities and two-photon absorption coefficients of bulk solids were estimated on the basis of the optical properties of the corresponding dimers, and the bulk material constructed from dimer-2 has the larger optical parameters of the two.     

A CFD analysis on the effect of tube curvature,hot flow control valve profile,and inlet swirl on the thermal performance of curved vortex tubes

Journal of Thermal Analysis and Calorimetry - The influence of tube curvature, conical valve geometry, and initial swirl on the thermal performance of vortex tubes is numerically investigated....     

Solvent Effects on Two-Photon Absorption of Alkyne and Alkene π-bridging Chromophores

The present work concerns the study of solvent effects on the geometrical structures, as well as one- and two-photon absorption (TPA) processes, for two series of alkyne and alkene π-bridging molecules, within the framework of the polarization continuum model. Particular emphasis was put on the characterization of solvent effects on the molecular geometrical structures and geometric distortion, which were measured by the bond-length-alternation parameter. The π centres in the compounds are seen to play a decisive role in increasing the TPA cross section and nonlinear optical properties. All studied molecules have relatively strong TPA characteristics, while the alkyne π-bridging ones yield larger TPA cross sections.