Foam Core Sandwich Structures Research Articles

Failure mode maps for glass fibre-reinforced polymer/polyvinyl chloride foam-cored sandwich structures

Purpose This study aims to understand how the facesheet size, orientation and core size influence the analytical failure mechanism mode of glass fibre reinforced polymer (GFRP)/polyvinyl chloride (PVC) sandwich structures subjected to three-point bending. The purpose of this study was to develop failure-mode map of GFRP/PVC sandwich structures. Sandwich structures with different facesheet and core thicknesses were used to develop the failure map. Design/methodology/approach The sandwich structure and facesheet were fabricated using a vacuum-assisted resin infusion method with core sizes of 10, 15 and 20 mm and facesheet thicknesses of 1.5 and 3 mm and were arranged in three different orientations: angle-ply, cross-ply and quasi-isotropic. The key failure modes that occur in sandwich structures were used to predict possible failures in the developed material. Analytical equations were used in MATLAB for each observed failure mode. The probable failure modes, namely, face yielding, core shear and indentation equations, were used to construct the failure maps and were compared with the experimental data. Findings The boundary of the two failure modes shifts with changes in the facesheet and core thicknesses. The theoretical stiffness of sandwich panels was higher than the experimental stiffness. Based on strength-to-weight ratio, specimens E10-4, A15-8 and E20-8 exhibited the best optimum values owing to their shorter distance to the boundary lines. Originality/value In this study, a failure map was used to predict the possible failure modes for different GFRP facesheet orientations and thicknesses and PVC core thickness sandwich structures. Little is known about the prediction of the failure modes of unidirectional GFRP arranged in different orientations and thicknesses and PVC core thicknesses for sandwich structures. Few studies have used failure mode maps with unidirectional GFRP oriented in angle-ply, cross-ply and quasi-isotropic directions as a facesheet for sandwich structures compared to bidirectional mats. This study can serve as a guide for the correct selection of materials during the design process of sandwich structures.

Anti-penetration properties of negative Poisson’s ratio honeycomb sandwich structure filled with foam

The ballistic responses of negative Poisson’s ratio honeycomb sandwich structures filled with aluminum foam are investigated using numerical simulation method. The residual velocity of the projectile, the energy absorption, and the failure forms of the sandwich structures are systematically analyzed. The influence of the impact position and incidence angle of the projectile, wall thickness, and concave angle of honeycomb, as well as the thickness of the front and back sheets of the structure on the penetration resistance is discussed. The results show that the foam-filled honeycomb sandwich structure has a better penetration resistance than the foam core sandwich structure with the same areal density, and the penetration resistance of the foam-filled honeycomb sandwich structure can be improved by designing its geometric parameters.

Mechanical Behaviour of Glass Fibre-Reinforced Polymer/Polyvinyl Chloride Foam Cored Sandwich Structures

This study focuses on the fabrication and analysis of the mechanical behaviour of unidirectional (UD) glass fibre-reinforced polymer (GFRP) facesheet and polyvinyl chloride (PVC) foam core sandwich structures fabricated by a vacuum-assisted resin infusion method (VARIM). These sandwich structures are commonly used in marine and wind turbine blade applications. To date, relatively little knowledge about the functional behaviour of UD GFRP compared to composites reinforced with bidirectional mats is available for day-to-day applications. The effects of the facesheet orientation, facesheet thickness, and core thickness on the mechanical behaviour of the specimens were examined. The UD fibres were oriented in cross-ply (0/90), angle-ply (+45/−45), and quasi-isotropic orientations. Various mechanical properties such as tensile, flexural, flatwise compression, and edgewise compression tests were examined. Characterization of the tensile properties of the facesheet showed that the cross-ply orientation had a higher strength than the angle-ply and quasi-isotropic orientations. The flexural load-carrying capacity of the cross-ply facesheet orientation was superior to the other orientations. The increase in the core thickness changed the flexural failure mode from face yield and core shear to core indentation. Flatwise compression (FWC) was tested to determine the core characteristics of the sandwich structure, and the peak loads of 4.90, 1.81, and 3.90 kN were obtained for 10-, 15-, and 20 mm core thicknesses, respectively. Edgewise compression (EWC) exhibited stable end crushing for thinner facesheet, whereas thicker facesheet showed core crushing and buckling. When the facesheet thickness was increased from 1.5 mm to 3 mm in the EWC, the buckling load increase ranged from 2.53% to 44.83% for core thicknesses 10-, 15-, and 20 mm, respectively.

Analytical study on the dynamic mechanical behaviours of foam-core sandwich plate under repeated impacts

The foam-core sandwich structures are probably subjected to repeated impact loadings in engineering applications, resulting in structural plastic deformation and accumulative damage. A valuable analytical approach is needed to predict the accumulative plastic deformation of foam-core sandwich structures under repeated impacts. In this paper, an analytical model of the dynamic responses of foam-core sandwich plates under repeated impacts is proposed using the circumscribing yield locus and the inscribing yield locus based on the rigid-perfectly plastic theory. By employing the principle of energy dissipation rate balance, the analytical expressions of structure permanent deformation and peak force are obtained from the iteration of initial conditions. In addition, polyvinyl chloride (PVC) foam has steady chemical properties and corrosion resistance, which can better adapt to the practical environment and meet the needs of structural protection. To validate the proposed analytical model, the repeated impact dynamic mechanical behaviours of PVC foam-core sandwich plates were experimentally and numerically studied. Results show that theoretical results such as the permanent deformation, peak force, and deformation modes of foam-core sandwich plates under repeated impacts are in good agreement with experimental and numerical results. With the increase in impact numbers, the global bending deformation and local indentation deformation of facesheets increase, the PVC foam core layer is constantly being compressed, and the bottom facesheet plays an increasingly important part in the repeated impact energy absorption with a maximum value of 29.20 %.

The effects of impactor shape on the compression after impact strength of carbon/epoxy face sheet foam core sandwich structure

This study presents experimental results of compression after impact (CAI) strength testing of foam core sandwich structure with carbon/epoxy face sheets impacted at three different energies with either a blunt or sharp tip impactor. The impact energies used were chosen to span the barely visible impact damage (BVID) thresholds for the sharp (4.1 J) and blunt (12.9 J) impactors with an impact energy approximately halfway between these (8.1 J) also used. While most impact testing on composites utilize a hemispherical (blunt) impactor, actual damage to a part may be due to an object impacting the part that is not blunt, but sharp and the impact response and resulting CAI strength values may be different for a given impact energy level. In this study, with regards to the impact response, the sandwich specimens showed larger transverse displacements (by about a factor of two) during the impact event when impacted by a sharp impactor versus a blunt impactor. The maximum load of impact was larger for the blunt impactor by about a factor of three. The barely visible impact damage (BVID) threshold energy was lower by about a factor of three for the sharp impactor. The absorbed energy of impact was higher for the sharp impactor. The CAI strength results showed that the sharp impactor gave lower average CAI strength values at the lowest impact energy level used, slightly lower average CAI strength values for the medium impact energy used and about the same average CAI strength values for the highest impact energy used.

Improving the structural integrity of foam-core sandwich composites using continuous carbon fiber stitching

This study aims to improve the structural integrity of foam-core sandwich structures by embedding carbon-fiber/epoxy stitched reinforcements with fiber-volume fraction (Vf) as high as 55 %. Sandwich panels with 1.5 mm thick carbon fiber skins and 30 mm thick PVC foam-core were stitched with 0, 12 k and 24 k carbon fiber yarns. X-ray computed tomography (CT) was utilized to determine the “as-manufactured” quality of the stitched panels. Flatwise-, edgewise-compression and 3-point bending tests were conducted with in-situ 3D Digital Image Correlation (DIC) to record the deformation and analyze the structure’s mechanical response. It was observed that compared to non-stitched samples - under flatwise compression loading, the reinforced samples with a stitch Vf of 0.34 and 0.55 showed an increase in stiffness and strength by 130 % and 163 %, and 29 % and 53 %, respectively. For edgewise-compression and 3-point bending loading cases, stiffness and strength of stitched samples showed an increase of ∼20 % over the benchmark. Using DIC, X-ray CT and a fractography analysis, the external and internal damage as well as failure mechanisms were evaluated. Analytical solution was employed to understand the parametric influence of the stitch Vf and stitch density per unit area in pursuit of an optimized sandwich structure.

Experimental characterization of impact damage in foam-core sandwich structures using acoustic emission, optical scanning and X-ray computed tomography techniques

Sandwich structures used in transportation and energy industries are commonly subjected to low-velocity impact in the course of manufacturing, service and maintenance, which can lead to the premature degradation of structural stiffness and strength. A single non-destructive evaluation technique is often difficult to provide a comprehensive assessment on impact damage from online to offline and from visible to invisible. In the present study, acoustic emission (AE), three-dimensional (3D) optical scanning and X-ray computed tomography (CT) techniques are combined to provide a systematic approach for damage characterization of thick foam-core sandwich panels under low-velocity impact. In the tests, the mechanical responses such as force-time and force-displacement curves are obtained from the testing system. The AE technique is adopted to monitor damage initiation and evolution in situ; optical scanning and X-ray computed tomography techniques are used for off-line damage identification and quantification. The results show that the AE signal waveform stream, cumulative counts and cumulative energy correlate well with the mechanical responses of the sandwich panel under the low-velocity impact, enabling capture of valuable information for online damage monitoring. Unlike general cases, the AE signals generated from low-velocity impact are found to be presented in strong bursts of nearly coincident events in the short duration. Therefore, the distributions of AE amplitude and peak frequency between adjacent AE counts are extracted to study the different damage modes and damage mechanisms. The X-ray computed tomography images enable to precisely visualize the fracture behaviors of the PET foam-core inside the sandwich panel in a nondestructive fashion, which evidently supports the AE and optical scanning results. The study is anticipated to demonstrate effectiveness and capability of the proposed experimental methodology for damage detection and characterization on sandwiched composite structures subjected to impact loading.

Multiscale modelling of sandwich structured composites using direct FE2

In finite element analyses, composite laminates are often homogenised and modelled using beam and plate elements for computational efficiency. Aside from difficulties in deriving homogenised properties for nonlinear materials and the loss in fine-scale deformation details, such structural elements are notoriously inaccurate for laminates comprising plies of distinctly different in-plane stiffnesses, notably, sandwich structured composites. A multiscale layerwise beam model is proposed to address these shortcomings. The model is based on the Timoshenko-Ehrenfest beam theory with layerwise kinematics incorporated to capture the key deformation modes of bending and transverse shear, and implemented using Direct FE2 – a versatile computational homogenisation scheme. Beyond homogenisation for the structural elements, Direct FE2 also captures deformation mechanisms at the mesoscale. More importantly, it is readily implemented using available functions in commercial FE codes, allowing for ease of adoption. A foam core sandwich structure is modelled to demonstrate that the proposed approach leads to macroscale responses and resolution to the mesoscale deformations that are almost identical to that of finely meshed solid element models, while taking only 23% of the computation time.

Flexural Behaviour of Foam Cored Sandwich Structures with Through-Thickness Reinforcements

Composite sandwich structures are well-suited for applications requiring high bending strength, flexural rigidity, crashworthiness, and light weight. However, skin–core debonding and core failure remain a barrier to optimal structural performance when polymeric foams are used as core materials. Suppressing or compartmentalising these failure modes can enhance the structural integrity of sandwich structures. In this paper, the flexural response of a sandwich structure was improved by adding carbon fibre-reinforced plastic in the form of through-thickness ribs during the manufacturing process. The effect of the position of the ribs was investigated using a quasi-static three-point bend test. A camera was used to capture failure events, while the digital image correlation technique provided the full-strain field at different stages of loading. Improved flexural performance was obtained when a reinforcement was placed on either side of the loading roller. With this configuration, skin–core debonding was restricted to a confined portion of the panel, resulting in a more localised and stable fracture process, which involved enhanced foam crushing and hardening. A simple FEA approach has been adopted in this paper and has proven to be an effective approach for capturing the details of the failure process, including the debonding in the composite foam structures, without the need for complex and computationally expensive interface modelling.

Blast resistance of graded aluminium foam core sandwich structure against blast loading

Blast resistance of graded aluminium foam core sandwich structure against blast loading

Optimization of Pin Position and Angle for Z-Pin-Reinforced Foam Core Sandwich Structures

Sandwich panels (SP) are very promising components for structures as they ally high levels of specific stiffness and strength. Civil, marine and automotive industries are some examples of the sectors that use SPs frequently. This work demonstrates the potential of manufacturing Z-pin-reinforced foam core SPs, using a design strategy that indicated optimal values for both pin position and angle, keeping the same pin diameter as determined in a previous study. A simple search algorithm was applied to optimize each design, ensuring maximum flexural stiffness. Designs using optimal pin position, optimal pin angle and optimal values for both parameters are herein investigated using numerical and experimental approaches. The optimal pin position yielded an increase in flexural stiffness of around 8.0% when compared to the non-optimized design. In this same comparison, the optimal pin angle by itself increased the flexural stiffness by about 63.0%. Besides, the highest increase in the maximum load was found for those composites, molded with optimized levels of pin position and pin angle, which synergistically contributed to this result. All results were demonstrated with numerical and experimental results and there was a good agreement between them.

The influence of pin on the low-velocity impact performance of foam sandwich structure

In this study, experimental and computational approaches are used to explore the influence of relative position parameter (ratio of the axial distance between the impactor and the pin to the sum of their radius) and dimensional parameter (ratio of impactor diameter to pin diameter) of pin and impactor on foam core sandwich structure under the low-velocity impact. First, sandwiches with various specifications were subjected to low-velocity impact tests with an energy of 34 J. Then reinforcing effect of the pins and the specimen damage condition were thoroughly evaluated using finite element analysis (FEA). The findings revealed that the enhancement impact was substantial when position parameters were below a value of one and was negligible when position parameters were greater than three. In addition, the deformation and damage patterns of the pins gradually transitioned from crushing to large curvature bending, micro-buckling, and negligible deformation were observed with increasing values of position parameters. To establish the law associating position and dimensional parameters with impact performance (including maximum contact force, resistance to impact damage, perforation threshold energy and dent depth), experiments were designed by the central composite design (CCD) method and simulated through FEA. The analysis of variance revealed that the position parameter have higher significance on the maximum contact force, damage resistance and perforation threshold energy, while the dimensional parameter have higher significance on the dent depth. Finally, a parameter optimization design approach was proposed that increased the sandwich's perforation resistance and damage resistance.

EXPERIMENTAL INVESTIGATION OF CRUSH ENERGY ABSORPTION AND STRENGTH PROPERTIES OF SANDWICH PLATES WITH ALUMINUM FACESHEET/ EXPANDED POLYPROPYLENE FOAM CORE

Due to the developing electric vehicle industry in the last decade, weight reduction studies on vehicle bodies have gained great importance. Foam core sandwich structures stand out as the most ideal materials in terms of providing both weight reduction and strength conditions in the bodies of electric individual and public transportation vehicles. In this study, EPP foams with two different densities were placed between aluminum plates and sandwich structures were obtained by combining the two structures with an EVA-based adhesive. Compression and bending behaviors of the produced sandwich structures were investigated under quasi-static and dynamic loading conditions. With the tests carried out, the strength of the sandwich structures and the amount of energy they absorb were calculated and compared experimentally. According to the results obtained, it was observed that the denser D2 foam exhibited approximately 1.4 to 2.05 times more strength than the lower density D1 foam in all tests. In terms of the energy they absorb, the D2 foam absorbs 1.25 to 2.5 times more energy than the other foam. Contrary to this situation, only the dynamic compression test occurred in the tests performed. When the post-damage behavior of the sandwich structures was examined, it was also observed that the D2 foam returned to a very similar dimensions to its original size, giving more of the deformation after the damage at the end of 72 hours.

Strength reduction of foam-core sandwich structures with core mismatches

Discontinuities in foam-core composite sandwich panels are challenging to assess, in part due to shortcomings in the fundamental understanding of failure processes occurring in the foam. Extensive testing and analysis are frequently required to overcome this lack of understanding when evaluating large-scale structures. This integrated experimental and analytical study investigates the effect of core thickness mismatches on the load carrying capabilities of composite sandwich structures. These mismatches are commonly found in large composite components in aerospace, marine, automotive and other industries and can cause major reductions in sandwich shear strength. Mismatches result from joints between separate pieces of foam core within the sandwich, becoming potential failure initiation sites. Quantifying the reduction in shear strength is difficult due to the complex interaction between the facesheets, butt-joints and core. Sandwich structures containing butt joints are investigated through a research effort that included fabrication, testing, analysis, and nondestructive evaluations. The strength reduction of sandwich beams due to the presence of butt joints is quantified via three-point bend (3PTB) testing for various types of mismatches such as height and thickness of the butt joint. Key mechanical properties of the foam and the lamina are measured through testing and incorporated into analytical models. A stress area approach is proposed as a failure criterion, as test results show this to be promising in predicting the strength reductions measured in the 3PTB testing and the failure initiation site.

Strength of asymmetric tapered composite foam core sandwich structures subjected to thermomechanical loading

An experimental test program and companion analytical study were designed and conducted to gain understanding of the tensile failure mechanisms in asymmetrical tapered sandwich core structures at cold temperatures. Representative test coupons were subjected to a tensile load in an environmental chamber to induce foam tensile failure in the core. Finite-element modeling with a proposed maximum principal stress criterion was used to predict the failure loads of the tapered test coupon design at the test temperatures. The predicted failure loads were in good agreement with test results. An important finding is that the cure stresses in the foam are significant and should not be ignored. Additionally, the analysis correctly predicted the failure initiation location, which was verified using high speed photography during the tensile test. The study identified the critical failure regions of asymmetrical tapered sandwich core designs and the failure load dependence on the temperature gradient in the structure.

3D printed continuous carbon fiber reinforced thermoplastic composite sandwich structure with corrugated core for high stiffness/load capability

In this work, sandwich structure with trapezoidal corrugated cores was 3D printed by fused deposition modeling (FDM) method applying to short and continuous carbon fiber (CF) reinforced nylon filament. The split core of the sandwich structure was designed and fabricated. To investigate the bending characteristics for the sandwich structures, the three-point bending (3 PB) tests were conducted for the different core types. Consequently, it was found that the mechanical behavior and failure modes were significantly changed by core materials and the split number of cores. Also, the dramatic improvements in the mechanical performances (4.1 times higher specific flexural stiffness, 2.6 times larger specific load capability and 8.6 times larger energy absorption capability than those of PET foam core sandwich structure) could be obtained in corrugated sandwich structure which was 3D printed using continuous carbon fiber. This result comes from the precise fiber alignment of continuous carbon fiber along the core, which can be realized only by continuous CF 3D printing technology.

Impact behavior of natural rubber based syntactic foam core sandwich structures

Abstract In this study, the impact behavior of sandwich panels of natural rubber-based syntactic foam cores with aluminum face sheets was investigated experimentally and with the help of finite element analysis (FEA). Syntactic foam cores were produced byadding glass bubbles (GB) to the natural rubber (NR). Natural rubber was dissolved at room temperature with chemical solvents mixed with glass bubbles at 10, 20, and 30 weight percentages. Very low density (~0.8 g × cm-3) and high compressible foams were obtained depending on the GB weight percentages. Aluminum face sheets and the NR/GB syntactic foam core developed were joined by adhesive bonding to produce sandwich beam specimens. The sandwich beams manufactured in this way were subjected to impact loading under three-point bending boundary conditions experimentally. The experimental results were compared with finite element simulation results under the same loading and boundary conditions. The damage mechanism of the sandwich panels devised were analyzed. According to the results, natural rubber containing an additive of 20 wt.-% GBs showed better impact resistance than the others.

Fabrication and Mechanical Properties of POSS Coated CNTs Reinforced Expancel Foam Core Sandwich Structures

Background: Sandwich structures are progressively being used in various engineering applications due to the superior bending-stiffness-to-weight ratio of these structures. We adapted a novel technique to incorporate carbon nanotubes (CNTs) and polyhedral oligomeric silsesquioxanes (POSS) into a sandwich composite structure utilizing a sonochemical and high temperature vacuum assisted resin transfer molding technique. Objective: The objective of this work was to create a sandwich composite structure comprising of a nanophased foam core and reinforced nanophased face sheets, and to examine the thermal and mechanical properties of the structure. To prepare the sandwich structure, POSS nanoparticles were sonochemically attached to CNTs and dispersed in a high temperature resin system to make the face sheet materials and also coated on expandable thermoplastic microspheres for the fabrication of foam core materials. Methods: The nanophased foam core was fabricated with POSS infused thermoplastic microspheres (Expancel) using a Tetrahedron MTP-14 programmable compression molder. The reinforced nanophased face sheet was fabricated by infusing POSS coated CNT in epoxy resin and then curing into a compression stainless steel mold. Result: Thermal analysis of POSS-infused thermoplastic microspheres foam (TMF) showed an increase in thermal stability in both nitrogen and oxygen atmospheres, 19% increase in thermal residue were observed for 4 wt% GI-POSS TMF compared to neat TMF. Quasi-static compression results indicated significant increases (73%) in compressive modulus, and an increase (5%) in compressive strength for the 1 wt% EC-POSS/CNTs resin system. The nanophased sandwich structure constructed from the above resin system and the foam core system displayed an increase (9%) in modulus over the neat sandwich structure. Conclusion: The incorporation of POSS-nanofillier in the foam core and POSS-coated nanotubes in the face sheet significantly improved the thermal and mechanical properties of sandwich structure. Furthermore, the sandwich structure that was constructed from nanophased resin system showed an increase in modulus, with buckling in the foam core but no visible cracking.

Fracture behaviour of foam core sandwich structures with manufacturing defects using phase-field modelling

This study investigates fracture behaviour of sandwich structures with foam core slits. These slits are typically machined in the foam core materials to improve manufacturability but inevitably lead to material discontinuities such as resin-starving regions or voids. Using the phase-field method, which does not require the exact location of crack initiation or the crack path known as a prior, the complex fracture process of sandwich structures with different resin-filling slits in the foam core materials is numerically reproduced. We examine the effective stiffness, the peak force, the displacement at crack initiation, and the dissipated energy during fracture of the sandwich structure under shear loads following the ASTMC273 test standard. It is found that the sandwich structure with fully or partly resin filled slits in the foam core exhibits better fracture resistance than the ones with the intact foam and with unfilled slits. The sandwich structure with partly resin filled slits also shows good ductility due to the presence of voids. The effects of the number of slits, slit spacing and foam core density on the load-carrying capacity and fracture resistance are also examined, providing insights into fracture behaviour and damage tolerance of foam core sandwich structures with manufacturing defects.

Experimental and numerical assessment of high-velocity impact behavior of syntactic foam core sandwich structures

In this study, the high-velocity impact behavior of new designed sandwich structures with syntactic foam core and FML skins is investigated by experimental and numerical approaches. An appropriate numerical model is developed in order to modelling of the impact behavior of sandwich structures using the finite element software ABAQUS/Explicit. In this model, two VUMAT subroutine are presented to simulate the constitutive models of GFRP composite and syntactic foam. In addition, a multiscale finite element procedure is employed to predict the effective mechanical properties of woven orthogonal composite from known properties of its constituents. This procedure is implemented the numerical homogenization techniques using Python scripting. High-velocity impact tests are performed using a gas-gun and steel conical projectiles to assess the accuracy of the numerical model. To this end, the numerical model is validated by comparing the high-velocity impact behavior of sandwich structures to that obtained from experimental study and demonstrated to be effective and accurate. The presented numerical model can predict the residual velocity and perforation energy with 88.8% and 97.4% accuracies, respectively. Then the validated numerical model is employed to investigate the effect of projectile shape, projectile mass, and stacking sequence of skins on the impact behavior of the sandwich structure.