Closed-cell Foam Core Research Articles

Curved Polymeric Sandwich Composites Subjected to Air Shock: An Experimental Investigation

BackgroundThe vulnerability of polymeric composite sandwich structures in marine applications to air explosions highlights a significant gap in our understanding of the dynamic behavior of the curved sandwich structures, which is essential for design improvements.ObjectiveThis study aims to explore the dynamic response and failure mechanisms of curved sandwich composite panels subjected to air-blast loading, providing insights into their structural integrity under such conditions.MethodsExperiments were performed using laboratory-simulated air shocks generated by a shock tube, employing high-speed photography and digital image correlation to measure deflections on the back surface of the panels. The panels, made with PVC closed-cell foam cores of two densities (H45 and H130), were tested across three curved geometries (radii of 112 mm, 305 mm, and infinity) under various boundary conditions.ResultsFindings indicate an increase in deformation with a decreased radius of curvature under simple support conditions, a trend that reverses under arrested displacement conditions. Moreover, a reduced radius significantly enhances panel strength and resistance to interfacial damage, with the primary failure mode transitioning from core shear cracking to interfacial debonding as core density increases.ConclusionsThe study reveals that the radius of curvature, boundary conditions, and core density significantly affect curved sandwich panels’ dynamic response and performance. Panels with smaller radii and higher core densities exhibit increased strength, though boundary conditions introduce variable effects on deformation behavior.

Experimental evaluation of OSB-faced structural insulated panels subject to blast loads

This paper reports the results of an experimental program to quantify the blast protection provided by as-built and strengthened structural insulated panels (SIP) subjected to short-duration uniform pressure loads in a blast load simulator facility. SIPs are used in residential and light-commercial buildings. The SIPs used in this study had an overall thickness of 117 mm with 11 mm thick oriented strand board (OSB) facers and a 95-mm thick polyurethane closed-cell foam core. This type of sandwich panel provides improved energy efficiency and reduced construction time. A total of eight panels were tested in four different configurations: (1) as-built SIP design with standard connections, (2) as-built SIP design with strengthened roof connection, (3) flexural strengthened SIP using galvanized steel straps with strengthened roof connections, and (4) flexural strengthened SIP using fiber reinforced polymer (FRP) wallboard with strengthened roof connections. Each panel in this experimental program was 1.2 m by 2.4 m and was tested independently. The panels were subjected to two to five incrementally increasing pressure-impulse combinations. The first was a low pressure shot to verify the elastic performance of the SIP. The magnitude of the pressure-impulse combination for the subsequent shots increased until ultimate failure of the panels. By strengthening the roof connection, the as-built panels withstood a 27% larger impulse. The steel strap and FRP flexural strengthening with the strengthened roof connections led to a three-fold and five-fold increase in impulse resisted, respectively, when compared to the as-built SIP with standard connections. The results of the experimental study were validated using single-degree-of-freedom (SDOF) models. The SDOF analysis included an elastic resistance function for each panel configuration. The SDOF analysis estimated displacements 9% larger than the experimental results on average. The analytical model was more conservative for the lower pressure shots indicating that one-way behavior of SIPs subjected to shortduration uniform pressure loads may be reasonably estimated using fundamental bending mechanics. Overall, the results of the experimental and analytical study indicated simple strengthening methods may be used to significantly improve the blast resistance of SIPs.

Dynamic response of foam core sandwich panel with composite facesheets during low-velocity impact and penetration

This study aims to investigate the dynamic response of foam core sandwich panel with composite facesheets, particularly considering the penetration processes under low-velocity impact. An analytical model based on energy approach is developed to predict contact force, contact time, impactor displacement, energy absorption and failure modes. Contribution of the structural components in energy absorption is further analyzed. The predictions are compared with experimental and numerical results and reasonable agreement is achieved. The present work provides an analytical method to study the mechanisms of impact damage and energy absorption for the composite sandwich panels with closed-cell foam core.

Experimental study of the mechanical behaviour of pin reinforced foam core sandwich materials under shear load

Sandwich structures with a lightweight closed cell hard foam core have the potential to be used in primary structures of commercial aircrafts. Compared to honeycomb core sandwich, the closed cell foam core sandwich overcomes the issue of moisture take up and makes the manufacturing of low priced and highly integrated structures possible. However, lightweight foam core sandwich materials are prone to failure by localised external loads like low velocity impacts. Invisible cracks could grow in the foam core and threaten the integrity of the structure. In order to enhance the out-of-plane properties of foam core sandwich structures and to improve the damage tolerance (DT) dry fibre bundles are inserted in the foam core. The pins are infused with resin and co-cured with the dry fabric face sheets in an out-of-autoclave process. This study presents the results obtained from shear tests following DIN 53294-standard, on flat sandwich panels. All panels were manufactured with pin-reinforcement manufactured with the Tied Foam Core Technology (TFC) developed by Airbus. The effects of pin material (CFRP and GFRP) and pin volume fraction on the shear properties of the sandwich structure and the crack propagation were investigated and compared to a not pinned reference. It has been concluded that the pin volume fraction has a remarkable effect on the shear properties and damage tolerance of the observed structure. Increasing the pin volume fraction makes the effect of crack redirection more obvious and conserves the integrity of the structure after crack occurrence.

Electrostatic adhesion for added functionality of composite structures

Electrostatic adhesion can be used as a means of reversible attachment. The incorporation of electrostatic adhesion into fibre reinforced polymer (FRP) composite structures could provide significant value added functionality. Imparting large potential differences (∼2 kV) across electrodes generates an attractive force, thus providing a means of attachment. This could be used as a reversible latching mechanism or as a means of controllable internal connectivity. Varying the connectivity for discrete elements of a substructure of a given design allows for control of internal load paths and moment of area of the cross section. This could facilitate variable stiffness (both in bending and torsion). Using a combination of existing fabrication techniques, functional electrodes have been integrated within a FRP. Copper polyimide thin film laminate material has been both co-cured with carbon fibre reinforced epoxy and bonded to PVC closed cell foam core material to provide a range of structural configurations with integrated electrodes. The ability of such integrated devices to confer variations in global bending stiffness of basic beam structures is investigated. Through the application of 4 kV across integrated electrostatic adhesive devices, a 112% increase in flexural stiffness has been demonstrated for a composite sandwich structure.

Compressive strength after blast of sandwich composite materials

Composite sandwich materials have yet to be widely adopted in the construction of naval vessels despite their excellent strength-to-weight ratio and low radar return. One barrier to their wider use is our limited understanding of their performance when subjected to air blast. This paper focuses on this problem and specifically the strength remaining after damage caused during an explosion. Carbon-fibre-reinforced polymer (CFRP) composite skins on a styrene–acrylonitrile (SAN) polymer closed-cell foam core are the primary composite system evaluated. Glass-fibre-reinforced polymer (GFRP) composite skins were also included for comparison in a comparable sandwich configuration. Full-scale blast experiments were conducted, where 1.6×1.3 m sized panels were subjected to blast of a Hopkinson–Cranz scaled distance of 3.02 m kg−1/3, 100 kg TNT equivalent at a stand-off distance of 14 m. This explosive blast represents a surface blast threat, where the shockwave propagates in air towards the naval vessel. Hopkinson was the first to investigate the characteristics of this explosive air-blast pulse (Hopkinson 1948 Proc. R. Soc. Lond. A 89, 411–413 (doi:10.1098/rspa.1914.0008)). Further analysis is provided on the performance of the CFRP sandwich panel relative to the GFRP sandwich panel when subjected to blast loading through use of high-speed speckle strain mapping. After the blast events, the residual compressive load-bearing capacity is investigated experimentally, using appropriate loading conditions that an in-service vessel may have to sustain. Residual strength testing is well established for post-impact ballistic assessment, but there has been less research performed on the residual strength of sandwich composites after blast.

Thermal-mechanical behavior of sandwich panels with closed-cell foam core under intensive laser irradiation

Temperature field and thermal deformation of sandwich panels with closed-cell aluminum alloy foam core and heat-protective layer, which are subjected to Gaussian laser beam intensively irradiating, are investigated numerically. In transient heat analysis models, the influence of thermal conductivity, specific heat, and thickness of heat-protective layer on the temperature rise of the sandwich panels is calculated. In stress analysis models, a sequence coupled numerical method is utilized to simulate the thermal stress and deformation of sandwich panels induced by thermal expansion. Simulation results indicate that the temperature at center of sandwich panel increases firstly and then drops gradually with the increase of thermal conductivity of heat-protective layer after laser irradiation, and the critical thermal conductivity is obtained, while it decreases with the increase of specific heat and thickness of heat-protective layer. The thermal stress verifies the ?Cyclo-hoop effect?, i. e. radial stress is compression stress in ?hot zone? and tension stress in ?cold zone?. The max thermal deformation of sandwich panels slightly increases with the increase of thickness of heat-protective layer for given specific heat and thermal conductivity.

Simulation of Impact Damage in Foam-Based Sandwich Composites

The paper describes the application of a 3D finite element model for prediction of impact induced damage in sandwich composites consisting of laminated skins bonded to a closed cell foam core. The major damage and fracture mechanisms typically developing in transversally loaded sandwich composites were simulated in the model. The model was implemented in the FE package ABAQUS/Explicit and used to predict the impact damage resistance of sandwich panels with different core densities, core thicknesses, and skins layups. Numerical results obtained by FE simulations were compared with experimental data and observations collected through impact tests carried out at various impact energies.

Study on the forming of sandwich shells with closed-cell foam cores

The efficiency and safety of vehicles represent today one of the most important lines of developing in the automotive industry, for example by the introduction of new materials. In fact, the investment in advanced materials represents one of the most important strategies to reduce injury among vehicle occupants in traffic accidents. Associated with the development of safety systems, there is also the possibility of improving efficiency by the introduction of materials that lead to weight reduction, having a direct impact on fuel consumption and carbon dioxide emissions. Metallic foams are one of these materials, due to the excellent ratio between mechanical properties and density. The main goal of this investigation is to study the mechanical behaviour of aluminium sandwich structures, composed by a metallic foam core with two outer layers of metallic sheets. With this work, the authors intend to contribute to a better understanding and consequently to provide design guidelines for the plastic forming of these composites. In order to correctly characterize the mechanical behaviour of the sandwich structure, the foam core and sheets were tested separately. For the aluminium sheet a series of tensile tests were performed, using samples obtained along three different angles to the rolling direction. For the metal foam core, uniaxial compression tests were used. Finally, with the numerical model defined considering isotropic and anisotropic constitutive models, a set of numerical and experimental bulge tests were performed to evaluate the capacity of forming of these panels, using hydroforming processes.

Indentation of metallic foam core sandwich panels with soft aluminium face sheets

A closed-form analytical model is presented for the deformation and fracture behaviour of sandwich panels with ductile foam cores and low yield-strength face sheets when subjected to quasi-static indentation with hemispherical indenters of various diameters and compared with an extensive experimental investigation. The aim is to predict the indentation strength and the influence of the indenter size on it. The face sheet is assumed an isotropic thin sheet undergoing large tensile deformation and bonded to a rigid-plastic foundation. The indentation strength of the sandwich panel is presented as a superposition of the force contributions of the skin, the foam core and the interfacial shear at the skin-foam interface at the point of skin fracture. To validate the model, sandwich panels with 0.32mm thick sheets of AA 1100-O aluminium alloy and aluminium closed-cell foam core are quasi-statically indented with spherical indenters of 10, 15 and 20mm diameters. It is established that failure of the sandwich panels is governed by radial skin fracture, and that the indenter size and the foam resistance greatly influence the indentation response as larger indenters are associated with higher failure loads. A simple parameter for system failure is ascertained, the subtended angle at the indenter-skin contact, which is dependent on the product of the indenter radius and the foam compressive strength. The developed model closely predicts the experimental load-displacement history and the failure loads and may be applied to any thin soft sheets bonded to a rigid-plastic foundation, and thus, to other sandwich constructions of similar constituents.

An advanced centre box of a vertical tail plane with a side panel from CFRP foam-core sandwich structure

Despite its superior specific stiffness, the sandwich design is currently applied in commercial aviation mainly in secondary structures. A major reason for this is its low resistance to a concentrated out-of-plane load, such as impact loading by a foreign object. In this scenario, the resulting damage is also known as foreign object damage and may penetrate further into the sandwich core. An improvement of impact resistance shall be achieved to introduce sandwich into the primary structures. Within the German aeronautics research programme, a primary sandwich structure, i.e. the centre box for a next generation vertical tail plane, was developed. The geometrical characteristics of the sandwich shell of the box, which has a unique lens shaped or lenticular cross section, lead to a noticeable improvement of its impact resistance. The considered sandwich configuration consists of carbon-fibre reinforced plastic skins and a closed cell foam core in between, which allows manufacturing using liquid composite moulding, such as a vacuum assisted infusion process. Embedded core reinforcements and damage arresters, respectively, can be manufactured within the same process step. Furthermore, the developed novel air-coupled ultrasonic scanning can detect the manufacturing flaws contact-freely as well as impact damages and thus enhances the inspection and damage tolerance capability of the sandwich structure. For the proof of structural integrity, a cutout in the critical area of the sandwich panel was defined and modified, such that it has no significant eccentricity and fits into an available shear-compression test facility. Loads are increased stepwise to explore all structural responses including interactions. The results show excellent structural stability with respect to the damage tolerance, even after multiple impact loadings up to 50 J, which agree with previously conducted non-linear structural analysis. A cyclic loading scenario with 8,000 simulated flight cycles in the context of research induces no significant growth of the impact damages and no deteriorating of the structural behaviour, respectively. Finally, the sandwich panel was used to investigate expected thermo-mechanical loadings during the takeoff and landing phases of the reference aircraft.

STRUCTURAL RESPONSE OF ALUMINIUM FOAM HYBRID SANDWICH PANELS UNDER THREE-POINT BENDING LOADING

The present study focuses on the structural response of sandwich panels consisting of a commercial closed-cell foam core and thin aluminium sheet skins under static three-point bending loading. Panels of different thicknesses and span lengths were tested, and the influence of the foam density, core thickness and skin type on the response was revealed. The failure modes in bending were greatly dependent on the span length but independent on the foam thickness. For short spans, the deformed shape at failure was asymmetric, as opposed to a symmetric mode for long spans. The density and thickness of the foam core, the presence of reinforcing face sheets and the beam span determined the failure load and bending strength of the sandwich panels.

Failure modes of composite sandwich beams

A thorough investigation of failure behavior of composite sandwich beams under three-and four-point bending was undertaken. The beams were made of unidirectional carbon/epoxy facings and a PVC closed-cell foam core. The constituent materials were fully characterized and in the case of the foam core, failure envelopes were developed for general two-dimensional states of stress. Various failure modes including facing wrinkling, indentation failure and core failure were observed and compared with analytical predictions. The initiation, propagation and interaction of failure modes depend on the type of loading, constituent material properties and geometrical dimensions.

An Experimental Investigation on Composites Through-The-Thickness Stitched Foam Core Sandwich Panels

This study explored the feasibility and potential benefits provided by the addition of through-the-thickness reinforcement to foam core sandwich structures. Through-the-thickness stitching is proposed to increase the interlaminar strength and damage tolerance of composite sandwich structures. A low-cost, out-of-autoclave processing method was developed to produce composite sandwich panels with carbon fiber face sheets, a closed-cell foam core, and through-the-thickness Kevlar stitching. The sandwich panels were stitched in a dry preform state, vacuum bagged, and infiltrated using Vacuum Assisted Resin Transfer Molding (VARTM) processing. For comparison purposes, unstitched sandwich panels were produced using the same materials and manufacturing methodology. Five types of mechanical tests were performed: flexural testing, flatwise tensile and compression testing, core shear testing, and edgewise compression testing. Drop-weight impact testing followed by specimen sectioning was performed to characterize the damage resistance of stitched sandwich panels. Compression after impact (CAI) testing was performed to evaluate the damage tolerance of the sandwich panels. Results show significant increases in the flexural stiffness and strength, out-of-plane tensile strength, core shear strength, edgewise compression strength, and compression-after-impact strength of stitched sandwich structures.

Wave Propagation and Dispersion in Sandwich Plates Subjected to Blast Loads

This paper investigates through-thickness propagation of compression waves induced by blast loads on the facesheet of a sandwich plate and how it is affected by the underlying material properties. A conventional sandwich design, which utilizes a closed cell foam core, is considered together with a modified design, which includes a thin polyurea interlayer between the outer (loaded side) facesheet and the foam core. The latter design is known to perform exceptionally well under impulse loads due to the significant stiffening of polyurea under high strain rates. This property reduces the amplitude and velocity of compression waves traveling into the foam core and prevents its sudden, excessive crushing. The problem was modeled with the explicit LS-Dyna finite element code. The results compare the short term behavior of the conventional and modified sandwich designs using a one-dimensional model for through-thickness wave propagation, and illustrate the long term effects on a multi-span sandwich plate subjected to a blast load. It is shown that the polyurea interlayer retards blast-induced compression waves propagating into the foam core in the initial blast period, and this reduces the core compression and leads to an improved long term behavior.

Deformation and Failure of Composite Sandwich Structures

An investigation was conducted of failure modes and criteria for their occurrence in composite sandwich columns and beams. The initiation of the various failure modes depends on the material properties of the constituents (facings and core), geometric dimensions and type of loading. The loading type or condition determines the state of stress throughout the sandwich structure, which controls the location and mode of failure. The appropriate failure criteria at any point of the structure account for the biaxiality or triaxiality of the state of stress. The specimens were made of unidirectional carbon-epoxy facings and aluminum honeycomb and PVC closed-cell foam cores. The constituent materials were fully characterized and, in the case of the foam core, failure envelopes were developed for general two-dimensional states of stress. Sandwich specimens were loaded under bending moment, shear and axial loading and failure modes were observed and compared with analytical predictions. The failure modes investigated are face sheet compressive failure, adhesive bond failure, indentation failure, core failure and facing wrinkling. The transition from one failure mode to another for varying loading or state of stress was discussed. Experimental results are compared with analytical predictions.

Failure Modes of Composite Sandwich Beams

An investigation was conducted of failure modes and criteria for their occurrence in composite sandwich beams. The initiation of the various failure modes depends on the material properties of the constituents (facings and core), geometric dimensions and type of loading. The beams were made of unidirectional carbon/epoxy facings and aluminum honeycomb and PVC closed-cell foam cores. The constituent materials were fully characterized and in the case of the foam core, failure envelopes were developed for general two-dimensional states of stress. Sandwich beams were loaded under bending moment and shear and failure modes were observed and compared with analytical predictions. The failure modes investigated are face sheet compressive failure, adhesive bond failure, indentation failure, core failure andfacing wrinkling.

Failure of Composite Sandwich Beams

A thorough investigation of failure behaviour of composite sandwich beams under three- and four-point bending was undertaken. The beams were made of unidirectional carbon/epoxy facings and a PVC closed-cell foam core. The constituent materials were fully characterised and in the case of the foam core, failure envelopes were developed for general two-dimensional states of stress. Various failure modes including facing wrinkling, indentation failure and core failure were observed and compared with analytical predictions. The initiation, propagation and interaction of failure modes depend on the type of loading, constituent material properties and geometrical dimensions.

Nonlinear behavior of composite sandwich beams in three-point bending

The load-deflection behavior of a composite sandwich beam in three-point bending was investigated. The beam was made of unidirectional carbon/epoxy facings and a polyvinyl chloride closed-cell foam core. The load-deflection curves were plotted up to the point of failure initiation. They consist of an initial linear part followed by a nonlinear portion. A nonlinear mechanics of materials analysis that accounts for the combined effect of the nonlinear behavior of the facings and core materials (material nonlinearity) and the large deflections of the beam (geometric nonlinearity) was developed. The theoretical predictions were in good agreement with the experimental results. It was found that the effect of material nonlinearity on the deflection of the beam is more pronounced for shear-dominated core failures in the case of short span lengths. It is due to the nonlinear shear stress-strain behavior of the core. For long span lengths, the observed nonlinearity is small and is attributed to the combined effect of the facings nonlinear stress-strain behavior and the large deflections of the beam.

Modelling face-core bonding in sandwich manufacturing: Thermoplastic faces and rigid closed-cell foam core

A model to predict the bond strength between thermoplastic faces and rigid, closed-cell foam cores in structural sandwich components is presented. The model is based on the assumption that the bond strength is proportional to the amount of resin that flows during manufacturing into the open surface cells of the core to bond the constituents to each other. Following model formulation, a numerical example considering bonding of glass reinforced polyamide 12 faces to an expanded polymethacrylimide core is used to illustrate the relative influence of the independent processing parameters. The numerical example is compared to a preliminary experimental investigation carried out with the same material system and a favourable, albeit preliminary, correlation between model predictions and experimentally determined bond strengths is found.