Compression after impact test (CAI) on NOMEX™ honeycomb sandwich panels with thin aluminum skins

Sandwich panels are used in industrial fields where lightness and energy absorption capabilities are required. In order to increase their exploitation, a wide knowledge of their mechanical behavior also in severe loading conditions is crucial. Light structures such as the one studied in the present work, sandwich panels with aluminum skins and Nomex^™ honeycomb core, are exposed to a possible decrease of their structural integrity, resulting from a low velocity impact. In order to quantitatively describe the decrease of the sandwich mechanical performance after an impact, an experimental program of compression after impact tests (CAI) has been performed. Sandwich panel specimens have been damaged during a low velocity impact test phase, using an experimental apparatus based on a free fall mass tower. Different experimental impact energies have been tested. Damaged and undamaged specimens have been consequently tested adopting a compression after impact procedure. The relation between the residual strength of the panel and the possible relevant parameters has been statistically investigated. The results show a clear reduction of the residual strength of the damaged panels compared with undamaged ones. Nevertheless, a reduced dependency between the impact energy and the residual strength is found above a certain impact energy threshold.     

Effective properties for sandwich plates with aluminium foil honeycomb core and polymer foam filling – Static and dynamic response

The estimation of static and eigenvibration properties of honeycomb sandwich reinforced by polymeric foam were investigated in the paper. A new “real microstructure” numerical 3D FEM model was proposed for the analysis in which the face materials and the honeycomb were modelled by shell elements, whereas filling foam was modelled by solid elements. Two variants of the honeycomb sandwich panel were considered: with and without polymer foam filling. Static and modal analyses have been performed in both, filled and hollow cases, to observe the effect of core stabilization with foam, particularly for higher natural frequencies. The effective properties of the honeycomb sandwich panels were estimated for both considered cases. Similar calculations have been made for the core materials without top and bottom faces and for the sandwich plate without honeycomb core structure (only polymer foam). One can observe: (1) the substantial increase of the effective elastic properties of the plate; (2) that the eigenvibration properties depend strongly on: the face material, honeycomb core and filling materials properties. The above conclusions are important for design process of structural parts.     

Vacuum-bag processing of sandwich structures: Role of honeycomb pressure level on skin–core adhesion and skin quality

Non-autoclave processing of honeycomb sandwich structures generally leads to poor compaction and high porosity of the skins, along with a decreased skin–core adhesion. The pressure level inside the honeycomb cells plays an important role and is controlled by the permeability to air of the skins. In this work, an initial range of skin permeability to air was explored by perforating the prepregs and adhesive layer selectively. The role of the resulting pressure inside the honeycomb on skin–core adhesion and skin quality was evaluated. Prepreg air permeability was found to control skin–core adhesion through the pressure in the honeycomb and potential outgassing of the adhesive layer. An optimal range of initial pressure inside the honeycomb was found to be 40–70 kPa. A universal process window was proposed to determine the time frame of vacuum application leading to an optimal initial honeycomb pressure level.     

Testing and modeling of a novel FRP-encased steel–concrete composite column

A composite column consisting of steel, concrete and fiber reinforced polymer (FRP) is presented and assessed through experimental testing and analytical modeling. The composite column utilizes a glass FRP (GFRP) composite tube that surrounds a steel I-section, which is subsequently filled with concrete. The GFRP tube acts as a stay-in-place form in addition to providing confinement to the concrete. This study investigates the behavior of the proposed composite columns under axial loading. A total of seven specimens were tested. The influence of concrete shrinkage on the compressive behavior of the composite columns was also investigated. Significant confinement and composite action resulted in enhanced compressive behavior. The addition of a shrinkage reducing agent was found to further improve the compressive behavior of the composite columns. An analytical model was developed to predict the behavior of the composite columns under axial loading.     

Structural and composition analysis of Apex™ and Foturan™ photodefinable glasses

Little is known about photostructurable glasses when compared to quartz and the other glass families. This article investigates optical and thermal behavior of the two commercially produced Apex? and Foturan? photosensitive glasses in relation to their composition. A composition analysis is performed on the two glasses using Rutherford backscattering spectrometry, and UV spectroscopy. Cerium and silver were found to exist at higher concentrations in Foturan than in Apex glass. Difference in transmission in the 240–340 nm window is mainly attributed to the different concentrations of cerium and silver in the glasses. Infrared transmission in the range of 2.7–5.0 μm is improved following an annealing process. Structural stability over a different range of temperatures in the two photosensitive glasses is investigated, and is attributed to the silica content at the expense of lithium oxide. Raman spectroscopy shows that the UV-exposed-then-baked photosensitive glass, results in the formation of a uniform crystalline-phase lithium metasilicate with a preponderantly Q₂ species.     

Linear and quadratic solid–shell finite elements SHB8PSE and SHB20E for the modeling of piezoelectric sandwich structures

In this article, hexahedral piezoelectric solid–shell finite element formulations with linear and quadratic interpolation, denoted by SHB8PSE and SHB20E, respectively, are proposed for the modeling of piezoelectric sandwich structures. Compared to conventional solid and shell elements, the solid–shell concept reveals to be very attractive, due to a number of well-established advantages and computational capabilities. More specifically, the present study is devoted to the modeling and analysis of multilayer structures that incorporate piezoelectric materials in the form of layers or patches. The interest in this solid–shell approach is shown through a set of selective and representative benchmark problems. These include numerical tests applied to various configurations of beam, plate and shell structures, both in static and vibration analysis. The results yielded by the proposed formulations are compared with those given by state-of-the-art piezoelectric elements available in ABAQUS, in particular, the C3D20E quadratic hexahedral finite element with piezoelectric degrees of freedom.     

Structural properties and Mössbauer spectroscopy of Finemet™ doped with Ge

Structural and magnetic properties of amorphous and crystalline alloys Fe_73.5Cu₁Nb₃B₉Si_13?xGe_{x=1;5;10;13.5} were studied by means of energy dispersive X-rays (SEM + EDX), X-ray diffraction (XRD), differential scanning calorimetry (DSC), Mössbauer spectroscopy (MS), as-quenched (a-q) and after annealing (a). EDX show agreement between the nominal and the measured atomic contents for all alloys. DSC provided three phase transition temperatures of the materials, MS spectra reveal amorphous structure of as-quenched alloys and a mixture of amorphous and crystalline structures of annealed alloys. The XRD spectra of annealed alloys allow us to resolve three crystalline phases of Fe.     

Fluid–structure interaction modeling of ringsail parachutes with disreefing and modified geometric porosity

Fluid?Cstructure interaction (FSI) modeling of parachutes poses a number of computational challenges. These include the lightness of the parachute canopy compared to the air masses involved in the parachute dynamics, in the case of ringsail parachutes the geometric porosity created by the construction of the canopy from ??rings?? and ??sails?? with hundreds of ??ring gaps?? and ??sail slits,?? in the case of parachute clusters the contact between the parachutes, and ??disreefing?? from one stage to another when the parachute is used in multiple stages. The Team for Advanced Flow Simulation and Modeling (T?AFSM) has been successfully addressing these computational challenges with the Stabilized Space?CTime FSI (SSTFSI) technique, which was developed and improved over the years by the T?AFSM and serves as the core numerical technology, and a number of special techniques developed in conjunction with the SSTFSI technique. The quasi-direct and direct coupling techniques developed by the T?AFSM, which are applicable to cases with nonmatching fluid and structure meshes at the interface, yield more robust algorithms for FSI computations where the structure is light. The special technique used in dealing with the geometric complexities of the rings and sails is the homogenized modeling of geometric porosity (HMGP), which was developed and improved in recent years by the T?AFSM. The surface-edge-node contact tracking (SENCT) technique was introduced by the T?AFSM as a contact algorithm where the objective is to prevent the structural surfaces from coming closer than a minimum distance in an FSI computation. The recently-introduced conservative version of the SENCT technique is more robust and is now an essential technology in the parachute cluster computations carried out by the T?AFSM. As an additional computational challenge, the parachute canopy might, by design, have some of its panels and sails removed. In FSI computation of parachutes with such ??modified geometric porosity,?? the flow through the ??windows?? created by the removal of the panels and the wider gaps created by the removal of the sails cannot be accurately modeled with the HMGP and needs to be actually resolved during the FSI computation. In this paper we focus on parachute disreefing, including the disreefing of parachute clusters, and parachutes with modified geometric porosity, including the reefed stages of such parachutes. We describe the additional special techniques we have developed to address the challenges involved and report FSI computations for parachutes and parachute clusters with disreefing and modified geometric porosity.     

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.     

Design of auxetic sandwich panel faceplates comprising cellular networks with high stiffness and negative Poisson’s ratio

In the present work, a novel design is presented for producing auxetic laminated faceplates for structures that contain auxetic cores in order to produce fully auxetic sandwich structures. The design is based on the use of periodic cellular networks that are embedded in a fibre-reinforced polymer matrix. These networks have a high in-plane negative Poisson’s ratio and a high in-plane stiffness. Two auxetic networks were chosen for this purpose: a classical re-entrant hexagonal network and an anti-tetra chiral auxetic network. The finite elements method was used to model the auxetic network and the faceplate. For the auxetic network, the relative modulus (E_s/E_m) was investigated to determine its effect on the behaviour of the faceplate. The auxetic behaviour of the faceplate occurs when the auxetic network has a high relative modulus. For example, for a classical re-entrant hexagonal network faceplate, the auxetic behaviour starts to appear when E_s/E_m > 33, while for an anti-tetra chiral auxetic network faceplate, the auxetic behaviour starts to appear when E_s/E_m > 24. Analytical expressions for the elastic constants of the faceplates were developed using the representative volume element model (RVE) and a semi-empirical formula of the rule of mixtures (ROMs). The results of the analytical expressions were compared with the finite elements results for various values of the relative density parameter ρ^*. The relative density had a significant effect on the elastic constants of the faceplate. The model produced with the representative volume element method had higher values for the elastic constants than did that created with the finite elements model, but the semi-empirical rule of mixtures gave more accurate results for both types of faceplates. A modified design for the classical re-entrant hexagonal network that increases the in-plane stiffness of the auxetic network is presented since it has been found that it plays a major role in producing a high negative Poisson’s ratio. The modified networks are an oval re-entered and a stiffened re-entered network. Experiments were carried out on the oval re-entered faceplates to obtain the in-plane moduli and the Poisson’s ratios. The tested faceplate samples clearly showed auxetic behaviour. A good agreement between the finite elements results and the experimental data was obtained.     

Mechanical behavior of sandwich panels with hollow Al–Si tubes core construction

A new type of lightweight sandwich panels consisting of vertically aligned hollow Al–Si alloy tubes as core construction and carbon fiber composite face sheets was designed. The hollow Al–Si alloy tubes were fabricated using precision casting and were bonded to the face sheets using an epoxy adhesive. The out-of-plane compression (i.e. core crushing), in-plane compression, and three-point bending response of the panels were tested until failure. The hollow Ai–Si alloy tubes core configuration show superior specific strength under crushing compared to common metallic and stochastic foam cores. Under in-plane compression and three-point bending, the buckling of face sheets and debonding of hollow cores from the face sheets were observed. Simple analytical relationships based on the concepts of mechanics of materials were provided for the compression tests, which estimate the sandwich panels’ strength with high fidelity. For three-point bending, detailed finite element analysis was used to model the response and initial failure of the sandwich panels.     

Structural performance of composite sandwich bridge decks with hybrid GFRP–steel core

This paper presents the static and fatigue performance of composite sandwich bridge decks with hybrid GFRP–steel core. The composite sandwich bridge deck system is comprised of wrapped hybrid core of GFRP grid and multiple steel box cells with upper and lower GFRP facings. Its structural performance under static loading and fatigue loading with a nominal frequency of 5 Hz was evaluated. The responses from laboratory testing were compared with the ANSYS finite element predictions. The failure mode of the proposed composite sandwich bridge deck was more favourable because of the yielding of the steel tube when compared with that of all-GFRP decks. The ultimate failure of the composite sandwich deck panels occurs by shear of the bonded joints between GFRP facings and steel box cells. Results from fatigue load test indicated no loss in stiffness, no signs of de-bonding and no visible signs of deterioration up to 2 million load cycles. The thickness of the composite sandwich deck retaining the similar stiffness may be decreased to some extent when compared with the all-GFRP deck. This paper also presents design of a connection between composite sandwich deck and steel girder.     

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.     

Microwave Near-Field Imaging with Open-Ended Waveguide—Comparison with Other Techniques of Nondestructive Testing

Abstract

The results of an experimental study of microwave imaging with an open-ended waveguide at 30 GHz for NDE of dielectric materials are presented. Using a hybrid measurement method, the near-field point spread function is investigated. Improvement of sensitivity and resolution via deconvolution also is discussed. A comparison of results obtained with more prominent NDE methods as ultrasonics, X-rays, and thermal waves demonstrates the performance of the technique.     

Effects of tangential and radial velocity on fluid flow and heat transfer for flow through a pipe with twisted tape insert—laminar flow

The present study reports the numerical analysis of fluid flow and heat transfer in a pipe with full length twisted tape insert. The investigation is carried out for five different twist ratios of 4, 5, 6, 8 and 10 at 100 ≤ Re ≤ 1000. The velocity field in terms of streamwise, tangential and radial velocity and temperature field are studied as a function of Reynolds number and twist ratio. The variation of friction factor and Nusselt number with Reynolds number for different twist ratios is also presented. The heat transfer enhancement due to insertion of twisted tape mainly comes from the tangential and radial components of velocities, which are regarded as secondary fluid motion. It is evident from the results that with increase in Reynolds number the axial convection increases. However, with the decrease in the twist ratio, the tangential and radial convection increases, leading to increased heat transfer. The secondary flow affects the thermal boundary layer inside the tube and increases the cross-flow mixing, which increases the heat transfer. The correlations for prediction of friction factor and Nusselt number based on the numerical data are also proposed.     

Robust modeling of RC structures with an “event-by-event” strategy

The sequentially linear analysis is a robust alternative to nonlinear finite element analysis of structures when bifurcation, snap-back or divergence problems arise. The load–displacement response is captured by a series of linear analyses as a sequence of ‘events’. Every ‘event’ is a scaled critical state corresponding to the reaching of some peak of some saw-tooth for some softening element. Now, the approach is extended with a rippled saw-tooth curve which applies to any stress–strain diagram, including compression nonlinearity and yielding of reinforcement. Several RC structural examples demonstrate that both sharp snap-backs as well as ductile failures can be handled correctly.     

Optoelectronic property modeling of carbon nanotubes grafted with gold nanoparticles

A three-dimensional (3D) electrodynamic model is built using the finite-difference time-domain (FDTD) method to investigate the optical response of carbon nanotubes grafted with gold nanoparticles. Theoretical characterizations suggest an anisotropic response, in line with previously observed absorption peaks of such systems in the optical range. An investigation of geometric and wavelength dependences is conducted, predicting the ability to tune the sub-wavelength intensity enhancement for efficient localization and propagation. The support of electric field enhancement along the nanotube walls raises the possibility of utilizing such systems as plasmon generators and waveguides for optical signal propagation.     

Forming performance and biodegradability of woodfibre–Biopol™ composites

A Taguchi approach to experimental design has been used to analyse the hotpressing and vee-bending of woodfibre–Biopol™ composites. Analysis of the hotpressing process clearly shows that platen temperature is the parameter with the most influence on tensile performance of the composite sheet produced. In bending (a common manufacturing situation), geometric conformance is maximised when forming time is 60 s, forming rate is 250 mm/min and forming radius/thickness ratio is 2 for the composite sheets studied in this paper. A study of the influence of fibre volume fraction on the biodegradability of these sheets show that these composites are highly biodegradable, often degrading at a rate greater than that of pure Biopol™. The results also suggest that a woodfibre mass fraction of ∼15% maximises the degradation of the woodfibre–Biopol™ composites.