Characterization and numerical evaluation of vibration on elastic–viscoelastic sandwich structures

The objective of this paper is to study the vibration characteristic for a sandwich beam with silica/polymer blend as principal material, and pure polymer matrix as surface laminate. It is anticipated that high stiffness and structure damping of viscoelastic layer can be obtained by taking advantage of fascinating network of densely packed between silica and polymer matrix. Spherical particles of size 12–235 nm at various filler fraction (10–50 wt.%) and three different polymer matrices, polyacrylate, polyimide and polypropylene, were selected as the matrix materials. The mechanical damping and stiffness of the sandwich cantilever beam are recorded by using a Dynamic Mechanical Thermal Analyzer (DMTA). The silica’s small particle size feature and strain difference between principal and surface layers could highly enhance the energy dissipation ability of the beam structure. A numerical model is then developed and validated for the vibration of a symmetric elastic–viscoelastic sandwich beam. Experimental results show that the structure deformation for these sandwich beams with contiguous and constraining layers are in reasonable agreement with the prediction of the model. Both higher resonant vibrations are well damped in accordance with the symmetric motion of the elastic layers and relative little motion of the constraining layer.     

Nanoindentation measurement of core–skin interphase viscoelastic properties in a sandwich glass composite

Mechanics of Time-Dependent Materials - Debonding at the core–skin interphase region is one of the primary failure modes in core sandwich composites under shear loads. As a result, the...     

The influence of core properties on the perforation resistance of sandwich structures – An experimental study

The aim of this study is to investigate the perforation resistance of a range of foam-based sandwich structures. Nine foams, based on a crosslinked PVC, a linear PVC and PET, have been combined with thin glass fibre reinforced plastic skins to produce a range of lightweight sandwich structures, Initially, the mechanical properties of the different foams are characterised. Here, a new test geometry is used to evaluate the toughness characteristics and strengths of the foams under shear loading, a condition similar to that encountered during the impact perforation event.The influence of the plastic collapse stress of the foam in determining the failure thresholds of the front and rear composite skins is established. Here, an existing model has been used to successfully predict failure of the top surface composite skin in the sandwich structures. In addition, the force associated with perforating the lightweight core has been shown to be strongly dependent on the shear strength of the polymer foam. Finally, the perforation resistance of the sandwich structure has been shown to be closely linked to the Mode II work of fracture of the foam material. Here a unique relationship has been established between these two parameters, with all of the experimental points lying on one curve.     

Simulation of delamination–migration and core crushing in a CFRP sandwich structure

Following the onset of damage caused by an impact load on a composite laminate structure, delaminations often form propagating outwards from the point of impact and in some cases can migrate via matrix cracks between plies as they grow. The goal of the present study is to develop an accurate finite element modeling technique for simulation of the delamination–migration phenomena in laminate impact damage processes. An experiment was devised where, under a quasi-static indentation load, an embedded delamination in the facesheet of a laminate sandwich specimen migrates via a transverse matrix crack and then continues to grow on a new ply interface. Using data from this test for validation purposes, several finite element damage simulation methods were investigated. Comparing the experimental results with those of the different models reveals certain modeling features that are important to include in a numerical simulation of delamination–migration and some that may be neglected.     

Investigations on sandwich core properties through an experimental–numerical approach

Sandwich structures are increasingly used in the aerospace field, also for primary parts. However, due to the extensive manufacturing process, the evaluation of their precise mechanical behaviour is not straightforward. This evaluation is, at present, a key task to enable future exploitation of these structures. Numerical simulations are powerful and flexible tools to study and identify the mechanical behaviour of such components in detail. Thus, in the present work, an extensive and highly accurate identification/validation strategy based on experimental tests has been developed. Finite element simulations (virtual tests) have been carried out in order to investigate, in detail, the effect that several manufacturer’s parameters have on the crushing mechanics of sandwich panels containing a Nomex? honeycomb core. Numerical scale modelling has also been evaluated with the aim to increase the understanding of the crushing phenomena, particularly the reciprocal effect of the cells.     

Testing and modeling of Nomex™ honeycomb sandwich Panels with bolt insert

Nomex™ honeycomb core sandwich panels with a bolt insert were load tested and modeled. The objective was to predict the honeycomb local buckling load and to identify a Nomex™ honeycomb constituent material model. Sandwich specimens were subjected to bolt pull-out load tests. The same sandwich structure was also tested in flat-wise tension with strain gages installed on the honeycomb walls. Finite element models of the flat-wise tension and bolt pull-out tests were built. The honeycomb geometry and strain gages were modeled with shell elements. An orthotropic honeycomb material model was identified by comparing the two test models to the experimental data. The material parameters identified are in the mid-range of previously published values. The pull-out test model was used to predict honeycomb wall buckling with a nodal rotation vector sum criterion. The buckling loads predictions closely corresponded to the start of the experimental load/displacement slope transition zone.     

Preparation and high-temperature properties of skin–core structure SiC ceramic fibers

Journal of Materials Science - A new type of skin–core structure SiC ceramic fibers was prepared from polycarbosilane by employing the curing process of alternating air and vacuum atmosphere...     

Computational study of the effect of core–skin structure on the mechanical properties of carbon nanofibers

Journal of Materials Science - The effect of the core–skin structure on the mechanical properties of carbon nanofibers is investigated in large-scale molecular dynamics simulations of tensile...     

Behavior of Steel–Concrete–Steel sandwich structures with lightweight cement composite and novel shear connectors

In Steel–Concrete–Steel (SCS) sandwich structure, mechanical shear connectors are commonly used to transfer longitudinal shear forces across the steel–concrete interface. In this paper, novel shear connectors such as J-hook and cable shear connectors are proposed and their performance to achieve composite strength of SCS sandwich structures is investigated. The use of these connectors together with ultra-lightweight cement composite core reduces the overall weight of SCS sandwich system making it suitable for the construction of marine and offshore structures. Static tests were carried out on SCS sandwich beams with J-hook, cable shear connectors and headed studs. Their ultimate strengths were reported and their respective failure modes were discussed. An analytical method to predict the ultimate strength of the Steel–Concrete–Steel sandwich beams with various types of shear connectors was developed and its accuracy was ascertained by comparing with the test results. Deign recommendations are made on minimum connector spacing to prevent shear cracking of concrete core and local buckling of face plates.     

Modal numerical–experimental identification method for characterising the elastic and damping properties in sandwich structures with a relatively stiff core

In this paper, a mixed numerical–experimental identification procedure for characterising the storage and loss properties in sandwich structures with a relatively stiff core is developed. At the computational level, the proposed method is based upon an original structurally damped shell finite element model derived from the higher-order shear deformation theory and, at the experimental level, upon an accurate contact-free measurement setup with a loudspeaker-based excitation and a scanning laser interferometer for capturing the time responses. From the modal information extracted from two specimens with different geometries, the procedure allows the simultaneous estimation of the skin and core constitutive parameters through adequate objective functions measuring the discrepancy between the experimental data and the numerical predictions. For validation purposes, the method is then applied to two test cases for which all the influent properties could be estimated with a fairly good accuracy.     

Effect of nanoclays on physico-mechanical properties and adhesion of polyester-based polyurethane nanocomposites: structure–property correlations

Polyester–polyurethane nanocomposites based on unmodified and modified montmorillonite clays were compared in terms of their morphology, mechanical, thermal, and adhesive properties. Excellent dispersion of the modified nanoclay in polymer with 3 wt% loading was confirmed from X-ray diffraction, and low-, and high-magnification transmission electron micrographs. The properties of the clay-reinforced polyurethane nanocomposites were a function of nature and the content of clay in the matrix. The nanocomposite containing 3 wt% modified clay exhibits excellent improvement in tensile strength (by ~100%), thermal stability (20 °C higher), storage modulus at 25 °C (by ~135%), and adhesive properties (by ~300%) over the pristine polyurethane.     

Laser and GTAW torch processing of Fe–Cr–B coatings on steel

A comparison has been made of the relationship between microstructure and microhardness developed by surface melting Nanosteel SHS 7170 Fe–Cr–B alloy powder onto a plain carbon steel surface. This powder was initially developed as a high velocity oxyfuel sprayed coating, giving a strength 10 times that of mild steel, and is particularly suitable for surface protection against wear and corrosion. In the present study, the alloy powder was injected into the laser melted surface, while a preplaced powder was melted using the gas tungsten arc welding (GTAW) technique. The laser track consisted of fine dendrites and needle-like microstructures, which produced a maximum hardness value of over 800 HV, while the GTAW track produced a mixture of equiaxed and columnar grain microstructures with a maximum hardness value of 670 HV. The lower hardness values are considered to be associated with dilution and grain size.     

Effect of thermal processing on the microstructure and composition of Cu–Sb–Se compounds

We report on the effects of thermal processing on the microstructure and composition of a system with overall stoichiometry of 3Cu:1Sb:3Se with the aim of producing single-phase Cu₃SbSe₃. It was found that slow cooling from the melt produced a multiphase material consisting of Cu₂Se and CuSbSe₂, but devoid of Cu₃SbSe₃. Cooling rapidly from the melt resulted in three-phase microstructures consisting of Cu₂Se, CuSbSe₂, and Cu₃SbSe₃. Subsequent annealing of the three-phase material between 325 and 400 °C shifted the composition toward nearly pure Cu₃SbSe₃—the target compound of this work. The kinetics of the transformation into Cu₃SbSe₃ was successfully described using a modified Avrami model which suggests that diffusion is the rate-controlling step. Values of Young’s modulus and hardness, obtained by nanoindentation, are reported for Cu₂Se, CuSbSe₂, and Cu₃SbSe₃.     

Effect of processing routes on mechanical and thermal properties of copper–graphene composites

In this paper, copper–graphene composites were fabricated by using two different processing routes (ball milling (BM) and ultrasonication) followed by spark plasma sintering. Vickers hardness and anisotropic thermal conductivity of the composites were measured and observed that ultrasonicated fabricated composites gave better result compared with BM composite and even from pure copper. The hardness values obtained for ultrasonicated copper–graphene composite were 69?HV (57% higher) and thermal conductivity 387?W/m?K (13% higher) by using only 0.5?wt-% of graphene, while for pure copper the values were 44?HV and 341?W/m?K. The value of anisotropic thermal conductivity ultrasonicated composites was also 1.97 which is much higher than pure copper 0.94.     

Controlled synthesis and catalytic properties of mesoporous nickel–silica core–shell microspheres with tunable chamber structures

In 550 °C and H₂ current, the mesoporous nickel–silica composite core–shell microspheres with tunable chamber structures have been successfully prepared by reduction of Ni₃Si₂O₅(OH)₄ microspheres, which are synthesized by the reaction between Ni(Ac)₂·4H₂O and SiO₂ microspheres via a self-template approach. The chamber (SiO₂ core sizes) and shell thickness (40–150 nm) of the nickel–silica microspheres can be controlled by adjusting the synthetic parameters of Ni₃Si₂O₅(OH)₄, such as the reaction time. After reduction, these microspheres still have the same sizes, morphologies, and core–shell structures with porous shell as before. These mesoporous nickel–silica microspheres with large BET surface area, exhibit good catalytic activity in m-dinitrobenzene (m-DNB) and high selectivity of m-phenylenediamine (m-PDA) after 3.5 h, but different selectivity of m-PDA in the progress, showing good potential in the catalyst industry.     

Numerical modeling on concrete structures and steel–concrete composite frame structures

A steel–concrete composite fiber beam-column model is developed in this study. The composite fiber beam-column model consists of a preprocessor program that is used to divide a composite section into fibers and a group of uniaxial hysteretic material constitutive models coded in the user defined subprogram UMAT in ABAQUS. The steel–concrete composite fiber beam-column model is suitable for global elasto-plastic analysis on composite frames with rigid connections subjected to the combined action of gravity and cyclic lateral loads. The model is verified by a large number of experiments and the results show that the developed composite fiber model possesses better accuracy and broader applicability compared with a traditional finite element model. Although the fiber beam-column model neglects the slip between the steel beam and concrete slab, there are essentially no effects on the global calculation results of steel–concrete composite frames. The proposed model has a simple modeling procedure, high calculation efficiency and great advantage when it is used to analyze composite frames subjected to cyclic loading due to earthquake.     

Impact of synthesis conditions on meso- and macropore structures of resorcinol–formaldehyde xerogels

Xerogels were prepared by the sol–gel polymerization of resorcinol with formaldehyde at different conditions. The effects of different synthesis factors (namely, resorcinol-to-formaldehyde ratio, resorcinol-to-water ratio, resorcinol-to-catalyst ratio, and initial solution pH) on the surface areas, pore volumes, pore size distributions, and adsorption capacity of liquid nitrogen were studied. Factorial design was also used to investigate the relative significance of these factors on the resulting xerogels properties, and the possible interactions between them. The mean effects of the most significant factors and factor interactions on determining the pore structures and adsorption capacities were evaluated. The synthesized xerogels were characterized by Fourier transmission infrared spectroscopy, thermal gravimetric analysis, scanning electron microscopy, and surface area analyzer. The results showed that surface areas, pore size distributions, and nitrogen adsorption capacity are dependent completely on the recipes used to prepare the xerogels. Pore structure results put xerogel samples as candidates for adsorption technology, ultra-filtration, and nano-filtration fields based on pore sizes and pore volume scales.     

Influences of processing methods and chemical treatments on fracture toughness of halloysite–epoxy composites

This study presented experimental reports on effects of processing methods (mechanical mixing and ball milling homogenization) on the fracture toughness of as-received and phenylphosphonic acid (PPA) treated halloysite–epoxy composites. It was demonstrated that with halloysite added, the composites held higher fracture toughness value than that of neat epoxy. Large-sized particle clusters occurred during mechanical mixing process can significantly decrease by ball milling homogenization and much more uniform dispersion was obtained in the cured composites. With PPA treatment, the morphology of halloysite changed from nanotubes to nanoplatelets with a substantial increase in the total contact area between halloysite and epoxy, it further enhanced fracture toughness in PPA-treated composites. The fracture toughness mechanisms of each nanocomposite with different processing methods and chemical treatments have been illustrated.     

Effect of processing parameters on textural and bioactive properties of sol–gel-derived borate glasses

A compositional range of recently developed bioactive sol–gel-derived borate glasses (SGBGs) have demonstrated remarkably rapid rates of conversion to hydroxy-carbonated apatite (HCA) in simulated body fluid (SBF). Although the composition of SGBGs did not greatly impact HCA conversion rates, it is still unknown how the sol–gel processing parameters affect the textural properties and thus bioactivity of the glass. In this study, a borate-substituted Bioglass^® “45S5” formulation [(46.1)B₂O₃-(26.9)CaO-(24.4)Na₂O-(2.6)P₂O₅; mol%] was fabricated using different sol–gel processing parameters including precursor materials, ageing time and temperature, along with calcination rate and temperature. It was found that a higher calcination temperature led to a partially crystallized glass with almost a magnitude decrease in specific surface area relative to the other glasses. All processing routes resulted in highly bioactive glasses according to Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy, which confirmed HCA formation in SBF in as little as 2 h. The majority of ion-exchange occurred within 30 min, facilitating this rapid conversion to bone-like HCA. Interestingly, the partially crystallized glasses (i.e., glass–ceramics) also underwent full conversion to HCA in SBF. Furthermore, ageing time and temperature did not affect the bioactive properties of these glasses, which allow for significantly reduced processing times. In summary, this study demonstrates that SGBGs can be tailored for targeted tissue engineering applications by varying the sol–gel processing parameters.     

Investigation of effects of Argenine–Glycine–Aspartate (RGD) and nano-scale titanium coatings on cell spreading and adhesion

This paper examines the effects of physical and chemical surface modifications on the biocompatibility of silicon surfaces that are relevant to implantable silicon Bio-micro-electro-mechanical systems (BioMEMS). Two types of surface modifications were explored. The first involved the deposition of nano-scale biocompatible layers of pure titanium on silicon, while the second explored the covalent attachment of the binding peptide Argenine–Glycine–Aspartic acid (RGD) for improved cell adhesion. Improvements in biocompatibility were assessed through examination of cell areas after culture, as well as the measurements of adhesion strengths, as determined by shear assay techniques. The titanium nanolayers and the RGD coating resulted in improvements in biocompatibility. Increased cell spreading areas and improved adhesion strength were obtained from short and long-term studies of Human Osteosarcoma (HOS) cells cultured on the coated surfaces. RGD functionalization resulted in the greatest improvement in cell spreading area and adhesion strength for short culture times. The effects of the titanium, while less than those of RGD for short culture times, appeared to be greater after 48 h of culture.