Surface Skins Research Articles

Foam-filled aramid honeycomb sandwich composites with high acoustic insulation properties prepared using a unique thermal expansion molding process

Contemporary advancements in technology and the rapid development of transportation systems have led to the rise of noise pollution as a major environmental issue globally. Foam sandwich composites, distinguished by their lightweight yet robust attributes such as high-strength functionality, effectively combine the superior noise insulation properties of foam structures with the strength inherent in their surface skins. However, traditional foam core materials exhibit limitations in terms of mechanical durability and suboptimal interfacial bonding with the surface skin. This study addressed these shortcomings by employing a novel approach involving a self-developed thermally expandable epoxy resin and an aramid honeycomb composite. These components were co-cured with a fiber prepreg, culminating in the formation of enhanced sandwich composites. The composites fabricated through this method demonstrate an impressive average sound insulation capacity of 70 dB, alongside a significant enhancement in the mechanical properties of the core material, surpassing the performance of standalone foam structures.

Experimental and numerical analysis of the mechanical behaviors of large scale composite C-Beams fastened with multi-bolt joints under four-point bending load

Quasi-static 4-point bending behaviors and failure characteristics of a large scale composite C-beams fastened with multi-bolt joints were investigated experimentally and numerically. Strain gauge measurement and acoustic emission (AE) monitor system were used to monitor the mechanical response and damage development during loading process. The global and local characteristics of damage are presented and analyzed. A nonlinear numerical model with simplified bolt joints on ABAQUS platform was also established which could be used to efficient analyze the load distribution of bolts and accurately predict the nonlinear response of the composite C-section beams. The numerical results were verified by the experimental results within 10% error. From the finite element (FE) model results, the bolt load distribution and load transfer path were discussed. The results show that the surface skins of the beam take a higher portion of the load than beam web under pure bending load. The beam catastrophic failure is due to the bending load matches the compressive strength of the top composite skin. The FE model is capable to well catch mechanical response details of multi-bolts joint structures for engineers.

Flexural responses and pseudo-ductile performance of lattice-web reinforced GFRP-wood sandwich beams

Abstract This paper investigates the pseudo-ductile flexural behavior of innovative sandwich beams using wood as core material and glass fibre reinforced polymer (GFRP) as surface skins and inner lattice-webs. The beams were prepared by a vacuum assisted resin infusion process. An experimental study was carried out to evaluate the stiffness and strength properties of the composite sandwich beams and reference wood beams under four-point bending. Ten sandwich beams were tested with different core configurations in comparison to two reference wood beams. The results showed that implementation of GFRP webs in the sandwich beams considerably improved their flexural performance, in association with a pseudo-ductile failure process and certain residual load-carrying capacity. Analytical formulas were developed to predict the bending stiffness and capacity of GFRP-wood sandwich beams. A comparison of the predicted and experimental results showed good agreement for mechanical characteristics of the GFRP-wood sandwich beams tested under four-point bending.

A bio-inspired, active morphing skin for camber morphing structures

In this study, one kind of developed morphing skin embedded with pneumatic muscle fibers (PMFs) was manufactured and was employed for camber morphing structures. The output force and contraction of PMF as well as the morphing skin were experimentally characterized at a series of discrete actuator pressures varying from 0.15 to 0.35 MPa. The active morphing skin test results show that the output force is 73.59 N and the contraction is 0.097 (9.7%) at 0.35 MPa. Due to these properties, this active morphing skin could be easily used for the morphing structures. Then the proper airfoil profile was chosen to manufacture the adaptive airfoil in this study. The chord-wise bending airfoil structure was achieved by employing this kind of active morphing skin. Finally the deformed shapes of this chord-wise bending airfoil structure were obtained by 3-dimensions scanning measurement. Meanwhile the camber morphing structures were analyzed through the finite element method (FEM) and the deformed shapes of the upper surface skins were obtained. The experimental result and FEM analysis result of deformed shapes of the upper surface skins were compared in this paper.

Fabrication method and compressive properties of CFRP isogrid cylindrical shells

Since isogrid structures are composed of triangular grid stiffeners and a surface skin, they have a high specific strength and stiffness. These structures are now being mainly used for aerospace applications. In order to produce a metal isogrid cylindrical shell, most of the thick metal plates must be cut off while the remaining thin surface skins with the stiffeners and both ends of the skins are welded to each other. This process needs a long lead time and a high cost, and unstable behaviors are observed in the joint parts. On the other hand, carbon fiber reinforced plastics (CFRP) isogrid cylindrical shells can be fabricated with an integral method by using a filament winding apparatus. In this study, a fabrication method of the CFRP isogrid cylindrical shells is proposed and their compressive tests are carried out to investigate the reinforcement effect of the grids on the CFRP isogrid cylindrical shells. Furthermore, the results of the static compressive tests are compared with numerical ones obtained by finite element method (FEM).

Layer and orientation resolved bond relaxation and quantum entrapment of charge and energy at Be surfaces

The chemistry and physics of under-coordination at a surface, which determines the process of catalytic reactions and growth nucleation, is indeed fascinating. However, extracting quantitative information regarding the coordination-resolved surface relaxation, binding energy, and the energetic behavior of electrons localized in the surface skin from photoelectron emission has long been a great challenge, although the surface-induced core level shifts of materials have been intensively investigated. Here we show that a combination of the theories of tight binding and bond order-length-strength (BOLS) correlation [C. Q. Sun, Prog. Solid State Chem., 2007, 35, 1-159], and X-ray photoelectron spectroscopy (XPS) has enabled us to derive quantitative information, by analyzing the Be 1s energy shift of Be(0001), (1010), and (1120) surfaces, for demonstration, regarding: (i) the 1s energy level of an isolated Be atom (106.416 ± 0.004 eV) and its bulk shift (4.694 eV); (ii) the layer- and orientation-resolved effective atomic coordination (3.5, 3.1, 2.98 for the first layer of the three respective orientations), local bond strain (up to 19%), charge density (133%), quantum trap depth (110%), binding energy density (230%), and atomic cohesive energy (70%) of Be surface skins up to four atomic layers in depth. It is affirmed that the shorter and stronger bonds between under-coordinated atoms perturb the Hamiltonian and hence the fascinating localization and densification of surface electrons. The developed approach can be applied to other low-dimensional systems containing a high fraction of under-coordinated atoms such as adatoms, atomic defects, terrace edges, and nanostructures to gain quantitative information and deeper insight into their properties and processes due to the effect of coordination imperfection.

Electric-Current-Induced Liquid Al Deposition, Reaction, and Flow on Cu Rails at Rail–Armature Contacts in Railguns

When a railgun is fired, the aluminum armature typically undergoes melting at the rail-armature contacts and deposits liquid metal on the rails. Rail life is affected by these deposits. Localized aluminum armature melting has been attributed to current crowding and localized Joule heating. This paper reports on 1) the characterization of debris on fired rails and their role in rail damage and 2) experiments to elucidate the role of electric current in the transport of liquefied armature material along the rail surfaces. A miniature static railgun model was constructed by wrapping an Al foil around a quartz core and placing the Al foil in contact with two thin-film Cu stripes deposited on Si substrates. In this arrangement, the Cu thin films simulated the surface skins on the two rails, whereas the Al foil represented the skin of the armature. The circuit was subjected to a constant current under high vacuum and various ambient temperatures. The part of the Al foil in contact with Cu melts quickly due to Joule heating, and the liquefied Al starts flowing down the negative rail. In all instances, the direction of liquid flow was from the positive to the negative terminal. The kinetics of Al migration was measured, and it was found that the activation energy for Al migration is close to that for self-diffusion in liquid Al. The observed liquid transport is attributable to liquid electromigration under the influence of the applied current.

FACTORS CONTROLLING THE STRONGEST SIZES IN THE INVERSE HALL–PETCH RELATIONSHIP

Incorporating the bond–order–length–strength correlation mechanism [C. Q. Sun, Prog. Solid State Chem.35, 1 (2007)] and Born's criterion for melting [J. Chem. Phys.7, 591 (1939)] into the conventional Hall–Petch relationship has turned out an analytical expression for the size and temperature dependence of the mechanical strength of nanograins, known as the inverse Hall–Petch relationship (IHPR). Reproduction of the measured IHPR of Ni , NiP , and TiO 2 nanocrystals revealed that: (i) the competition between the size-induced energy–density gain and atomic cohesive energy loss in the surface skins of nanograins originate from the IHPR; (ii) the competition between the activation and inhibition of atomic dislocations motion activate the entire IHPR behavior; (iii) the bond nature involved and the T/Tm ratio between the temperature of operating and the temperature of melting dictate the measured strongest sizes of a given specimen; (iv) a quasimolten phase present before melting determines the size-induced softening and the superplasticity of nanostructures.

Nanocavity strengthening: Impact of the broken bonds at the negatively curved surfaces

The atomistic origin of the atomic vacancy or nanocavity induced hardening in hollow nanotubes and nanoporous structures has long been a puzzle. An analysis from the perspective of bond-order–length-strength correlation mechanism [C. Q. Sun, Prog. Solid State Chem. 35, 1 (2007)] has led to solutions that show that the shortened and strengthened bonds between the undercoordinated atoms in the negatively curved surface skins dominate the observed nanocavity strengthening and thermal instability of the porous structures. It is suggested that the broken bond derived local strain and quantum trapping and the associated energy densification provide pinning centers for inhibiting atomic dislocations and that the broken bond induced cohesive energy dropping dominate the thermal instability. On the other hand, nanocavities also provide sites that initiate the structure failure under plastic deformation. The agreement between predictions and the experimentally observed size dependence of mechanical strength of some nanoporous materials and the well-known phenomenon of hollow tube strengthening evidences for the proposed mechanism.

Indentation depth dependence of the mechanical strength of Ni films

The indentation depth effect has been systematically examined on the mechanical properties of electrodeposited nickel films under 0% and 10% tensile strains. It is found that the indentation depth is proportional to the square root of the loads applied and the depth profiles of hardness and elastic modulus follow the similar trend of change with maximal values at the surface skins. The hardness and modulus then attenuate to a value of about half of the maximum, which follows the model proposed by Graca et al., Surf. Coat. Technol. (in press) with the mechanism of geometrically necessary dislocations and surface free energy. We suggest that the effect of surface oxidation and surface bond contraction [C. Q. Sun, Prog. Solid State Chem. 35, 1 (2007)] contributes intrinsically to the anomalous skin strengthening because of the local strain and energy trapping caused by surface bonds breaking.

Fabrication, lattice strain, corrosion resistance and mechanical strength of nanocrystalline nickel films

Nanocrystalline nickel films of 17–40 nm grain sizes were prepared using pulsejet electrodeposition. Structure, corrosion and lattice strain were analysed by transmission electron microscope, electrochemical workstation and X-ray diffraction, revealing that with decreasing of grain size, the lattice strain, corrosion rate of the films are enhanced. The observations can be consistently understood in terms of the bond-order-length-strength correlation mechanism indicating that the shortened and strengthened bonds between the under-coordinated atoms modify the energy density and the atomic cohesive energy in the surface skins of the grains. The surface energy density gain is responsible for the residual atomic cohesive energy for the activation energy of corrosion. Additionally, a novel algorithm was proposed to extract the elastic-plastic properties of nickel films and results that the nickel film has much higher yield strength than bulk nickel.

Size-induced acoustic hardening and optic softening of phonons in InP,CeO2,SnO2, CdS, Ag, and Si nanostructures

It has long been a puzzle that the Raman optical modes shift to lower frequency (or termed optical mode softening or redshift) associated with the creation of Raman acoustic modes that shift to higher energy (or called acoustic hardening or blueshift) upon a nanosolid being formed and its size being reduced. Understandings of the mechanism behind the size-induced Raman shifts have been quite controversial. On the basis of the bond-order--length--strength (BOLS) correlation mechanism [Phys. Rev. B 69, 045105 (2004)], we show that the optical softening arises from atomic cohesive energy weakening of atoms in the surface skins, whereas the acoustic mode hardening is predominated by intergrain interactions. Agreement between predictions and observations has been realized for Ag, Si, CdS, InP, ${\mathrm{TiO}}_{2}$, ${\mathrm{CeO}}_{2}$, and ${\mathrm{SnO}}_{2}$ nanostructures with elucidation of vibration frequencies of the corresponding isolated dimers from fitting the optical softening.

Impact of bond-order loss on surface and nanosolid magnetism

Incorporating the recent bond-order-length-strength correlation mechanism [Sun CQ, Bai HL, Li S, Tay BK, Jiang EY, Acta Mater 2004;52:501] into the Ising convention and the Brillouin function has enabled the unusual magnetic behavior of a ferromagnetic nanosolid and a surface to be reproduced using Monte Carlo simulations. Examination of the size and temperature dependence of the saturation magnetization ( M S) of a solid of various structures reveals that: (i) at low temperatures, the M S increases inversely with solid size due to the contribution from the localized charges that are trapped by the deepened potential well of the lower-coordinated atoms in the surface skins; (ii) at the ambient temperatures, the M S drops with solid size because of the bond-order loss that suppresses the Curie temperature of the specimen; (iii) the quantized features of the surface to volume ratio of the solid is responsible for the observed M S oscillations of smaller clusters at low temperatures.

Quartz microtextures and microstructures owing to deformation of glaciolacustrine sediments in the northern Venezuelan Andes

AbstractGlaciolacustrine sediments, ranging in age from Early to Late Mérida (Wisconsinan) in sections in the northern Venezuelan Andes near Pedregal, were studied to assess the effect of small‐scale folding and faulting on the surface microtextures of individual grains. At site PED5, faulting of sediments beneath 35 m of Middle to Late Mérida glaciolacustrine, glaciofluvial and probable till deposits, has resulted in a maximum displacement of 1 m and many crushed sand grains are found along the shear zone. These dominantly quartz particles are typically smoothed and/or faceted with surface skins split from parent grains to produce adhering blade or wafer‐shaped coarse silts. Faulting with displacement of< 40 cm occurs in Late Mérida glaciolacustrine beds at sites PED13 and PED11, beneath ca. 1–5 m of sediment and is thought to result from the removal of lateral support during valley erosion and possibly seismic disturbance. Associated sediments exhibit a greater incidence of small‐scale fold structures and convoluted bedding but with similar numbers and types of crushed grains. All of the fault traces examined exhibit microstructural modifications, including microfaulting, folding and multiple domains associated with fluidisation and liquefaction. The microtextures described are distinct from those produced by glacial or glaciofluvial transport processes and potentially may be used to infer glaciotectonic or other post‐depositional disturbance of sediments. Rare euhedral quartz grains with distorted crystallographic axes may derive from the shear zone of the regional transform Boconó Fault, although many of these grains appear to be overprinted by glacial and/or recent deformation processes. Copyright © 2004 John Wiley & Sons, Ltd.

Effective Diffusion Coefficients of Soil Aggregates with Surface Skins

Dual‐permeability model simulations and sensitivity studies indicate that preferential solute leaching in structured soil is intensified by a limited solute mass transfer between preferential flow paths and matrix. It is currently unknown how much aggregate skins may affect solute mass transfer. The objective of this study was to evaluate the effect of a skin layer on the effective diffusion coefficient, De, of water saturated soil aggregates. In a diffusion experiment, the concentration decrease of Br− and Cl− in a solution being in contact with soil aggregates was measured. Aggregates with intact and removed surface skins were studied. Two different theoretical approaches were compared: (i) fitting a modified analytical solution of Crank for diffusion out of a solution of limited volume into a plane sheet to obtain De for intact and for skin‐removed aggregates; and (ii) calibrating a numerical solution for diffusion through a two‐layer system to estimate De of the skin layer and the interior part, separately. The De‐values of the skins differed for Br− and Cl− Using the first approach, De was six (Br−) and 15 (Cl−) times smaller for the intact than for the skin‐removed aggregates. The second approach resulted in effective diffusion coefficients of the skin layer being 30 (Br−) and 45 (Cl−) times smaller than those of the interior parts. Aggregate skins may reduce diffusive anion transfer between interaggregate region and soil matrix, an effect which may significantly influence preferential solute transport in structured soil.

Protein docking using spherical polar Fourier correlations

We present a new computational method of docking pairs of proteins by using spherical polar Fourier correlations to accelerate the search for candidate low-energy conformations. Interaction energies are estimated using a hydrophobic excluded volume model derived from the notion of "overlapping surface skins," augmented by a rigorous but "soft" model of electrostatic complementarity. This approach has several advantages over former three-dimensional grid-based fast Fourier transform (FFT) docking correlation methods even though there is no analogue to the FFT in a spherical polar representation. For example, a complete search over all six rigid-body degrees of freedom can be performed by rotating and translating only the initial expansion coefficients, many unfeasible orientations may be eliminated rapidly using only low-resolution terms, and the correlations are easily localized around known binding epitopes when this knowledge is available. Typical execution times on a single processor workstation range from 2 hours for a global search (5 x 10(8) trial orientations) to a few minutes for a local search (over 6 x 10(7) orientations). The method is illustrated with several domain dimer and enzyme-inhibitor complexes and 20 large antibody-antigen complexes, using both the bound and (when available) unbound subunits. The correct conformation of the complex is frequently identified when docking bound subunits, and a good docking orientation is ranked within the top 20 in 11 out of 18 cases when starting from unbound subunits. Proteins 2000;39:178-194.

Influence of surface coverage on the effective optical properties of porous silicon modeled as a Si-wire array

The effective dielectric function, effective absorption coefficient and effective refractive index for a model of porous silicon (PS) are calculated using the volume and surface averaging method. The model consists of periodic Si wires with different surface coverages. This approach allows to obtain analytical results within certain approximations. The method uses experimental parameters to characterize the bulk and the surface. We choose the bulk c-Si, and cover it with three different possible surface skins: siloxanes, a-Si:H and SiO2. The results are compared with PS experimental data and other theoretical approaches for silicon wires. We obtain good agreement for certain coatings. Our results emphasize the important role of surface coatings in the effective response of porous silicon.

Fiber orientation in simple injection moldings. Part II: Experimental results

AbstractExperimental measurements of fiber orientation are reported for two parts injection molded from nylon 6/6 reinforced with 43 wt% of glass fibers. The parts are a center‐gated disk and a film‐gated strip. Orientation is measured from polished cross sections and reported as a function of position, both across the thickness and in the flow direction. Both parts have a layered structure, with outer shell layers of flow‐aligned fibers surrounding a central core of either random‐in‐plane (strip) or transversely aligned fibers (disk). The disk also has surface skins with less alignment. The experiments are compared with predictions of the simulation presented in Part I. The simulation predicts the presence, nature, and location of the layers very well. However, it overpredicts the small out‐of‐plane fiber orientation and places the core‐shell transition too close to the midplane. A comparison with selected experimental results suggests that the major source of error is the closure approximation used by the fiber orientation equation. The simulation is exercised for a variety of cases to show the importance of material and process parameters. A polymer matrix with a small power‐law index or a large heat of fusion gives a thicker core and is less likely to have a skin. Injection time is an important parameter, but injection temperature and mold temperature have little effect on fiber orientation.

Experimental Verification of Turbulent Skin Friction Reduction with Compliant Walls

Preliminary compliant wall skin friction test results obtained in a low-turbulence pressure tunnel are reported. Compliant surface skins consisted of 0.0025 cm thick mylar, stretched under tension and area-bonded or longitudinally strip-bonded with silicone rubber adhesive to polyurethane foam. Mean velocity and fluctuating survey data were obtained with a single slanted hot wire. Mean velocity profiles for the area-bonded mylar surface skins indicated up to a 20% reduction in boundary-layer thickness (and lower momentum thickness) over rigid surfaces. This reduction in boundary-layer thickness indicates that a drag reduction occurred. In addition, a 16% reduction in wall shear was evident for the mylar/compressor foam compliant surface.

Mathematical analysis of factors influencing the skin thickness of asymmetric reverse osmosis membranes

The formation of the dense surface skin of asymmetric reverse osmosis membranes is analyzed in terms of a composite process involving solvent diffusion and polymer relaxation. The analysis indicates that films with minimum skin thickness result from (i) rapid polymer relaxation; (ii) strongly concentration-dependent solvent diffusion coefficients. The numerical results are discussed in terms of experimental variables. It indicates great difficulty in producing asymmetric films with extremely thin surface skins. The implications of this finding are discussed. Experimental NMR self-diffusion studies of acetone in cellulose acetate are reported.