Foam Core Sandwich Composites Research Articles

Preparation of thermal expandable epoxy resin and properties of foam sandwich composites formed by thermal expansion molding process

AbstractThermal expansion molding process (TEMP) is a sophisticated molding technique for creating complex sandwich composites. The core key of TEMP is a high‐performance, thermally expandable resin film. In this study, the epoxy resin is used to create a specific crosslinking network that can provide the support force required for the stable molding of the vesicles during the expansion process of the foaming agent. The curing process of the resin was determined by analyzing the change in the exothermic amount of the epoxy resin during the curing process and the flow behavior of the resin. The pore size distribution of the epoxy foam was investigated, and the expansion force of the thermally expandable resin was tested. The TEMP molding process was compared with the vacuum bag press molding (VBPM) process. The TEMP molding technique increased tensile and compressive strength by about 25%, flexural strength by 34.5%, and shear strength by 54.5%. Finally, the section on microanalysis of the comparative materials and the short beam bending experiments of the sandwich structure provides a theoretical foundation for developing complicated sandwich structures.Highlights Preparation of thermally expandable epoxy prepregs with stabilized foaming by means of pre‐curing. The effects of curing temperature, foaming agent content and expansion multiplicity on the thermal expansion force of epoxy prepreg adhesives were investigated. Thermally expandable epoxy prepreg molding composites with different expansion multiplicities and analysis of the mechanical properties of the composites. Preparation of foam core sandwich composites by thermal expansion in‐mold monolithic molding process and their verification by bending experiments.

Curved grid‐scored foam core sandwich composites under low‐velocity impact loads: Effects of curvature, stacking sequence, and core finishing

AbstractThe objective of this paper is to experimentally investigate how material and geometric parameters, including face sheet layup, curvature, and core finishing, affect the contact force‐displacement behavior, absorbed energy, and damage modes of curved sandwich composites with grid‐scored foam under low‐velocity impact loads. An instrumented drop‐weight testing machine with a hemispherical impactor was used to perform low‐velocity impact tests at different energy levels. Angled‐ply stacking had the maximum contact force and impact resistance for low curvature radii. In the event of penetration, the impact energy absorption of all curved sandwich panels increased as the curvature increased. The core finishing influenced the contact forces of the curved and flat sandwich panels but not the penetration energy. The curvature and face sheet layups affected the delamination size and pattern. Angle‐ply and cross‐ply face sheets with the smallest and maximum radius of curvature, respectively, decreased penetration depth. The resin‐filled channels reduced foam cell impact damage and face‐core debonding. LVI penetration resistance was found to be highest for angle‐ply stacking and knife‐cut foam at small curvature radii. The proposed research will also provide naval designers with valuable insight into the impact response of sandwich structures used in curved parts of the hull and deck.Highlights The effect of curvature on the LVI responses varied due to face sheet layups. Core finishing affected contact forces, but not the absorbed energy. Curvature and face sheet layups affected the damage modes. Resin‐filled channels limited damage area and acted as stoppers for debonding.

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.

Mode-III fracture toughness measurements of foam-cored sandwich composites using a constrained Shear-Torsion-Bending specimen

This work presents a constrained version of the Shear-Torsion-Bending (STB) test fixture, specially developed for testing of the mode-III face/core fracture toughness of sandwich specimens. Compared to the traditional STB test fixture, the free end of the specimen is now constrained between two support rollers, restraining the twisting deformation of the specimen resulting from the mode-III shear forces. The constrained STB test fixture was applied for mode-III fracture characterization of sandwich specimens with longitudinal slits and consisting of E-glass/epoxy face sheets and Divinycell PVC foam cores. Three density classes of the foam material were considered and tested, that is, H45, H100, and H160. Furthermore, two core thicknesses were tested for the H160 cored specimens. The experimental results from the constrained STB test method show that stable crack growth is obtained without kinking into the core. However, post-mortem investigations showed that longitudinal 45° tension cracks in the core just below the face/core interface occurred, before interface crack opening. Consequently, an apparent mode-III fracture toughness was computed using a three-dimensional Finite Element (FE) model in conjunction with the Crack Surface Displacement Extrapolation (CSDE) mode-mixity method and an optical Digital Image Correlation (DIC) based crack propagation method. Finally, based on the experimental observations presented in this paper, a general discussion of the existence of pure mode-III face/core interface propagation in foam-cored sandwich composites in relation to core and face/core bond strength is included.

An Experimental Investigation of the Mechanical Performance of EPS Foam Core Sandwich Composites Used in Surfboard Design.

Surfboard manufacturing has begun to utilise Expanded Polystyrene as a core material; however, surf literature relatively ignores this material. This manuscript investigates the mechanical behaviour of Expanded Polystyrene (EPS) sandwich composites. An epoxy resin matrix was used to manufacture ten sandwich-structured composite panels with varying fabric reinforcements (carbon fibre, glass fibre, PET) and two foam densities. The flexural, shear, fracture, and tensile properties were subsequently compared. Under common flexural loading, all composites failed via compression of the core, which is known in surfing terms as creasing. However, crack propagation tests indicated a sudden brittle failure in the E-glass and carbon fibre facings and progressive plastic deformation for the recycled polyethylene terephthalate facings. Testing showed that higher foam density increased the flex and fracture mechanical properties of composites. Overall, the plain weave carbon fibre presented the highest strength composite facing, while the single layer of E-glass was the lowest strength composite. Interestingly, the double-bias weave carbon fibre with a lower-density foam core presented similar stiffness behaviour to standard E-glass surfboard materials. The double-biased carbon also improved the flexural strength (+17%), material toughness (+107%), and fracture toughness (+156%) of the composite compared to E-glass. These findings indicate surfboard manufacturers can utilise this carbon weave pattern to produce surfboards with equal flex behaviour, lower weight and improved resistance to damage in regular loading.

Preparation and Load-Bearing Capacity of Lattice Cell Warren Truss Slot Resin-Stiffener-Reinforced Foam Sandwich Material

This study optimized and proposed a Warren truss slot-hole structure with a double-sided, square shallow slot and vertical and horizontal corrugated symmetry, achieved with inclined holes based on the stability and a good bearing capacity of an inclined strut truss structure. The tetrahedral truss lattice cells were obverse and reverse-staggered in the central core of the structure. Compared with the double-sided, square shallow groove cylindrical straight hole, the resin consumption of the Warren truss slot holes was similar to that of a vacuum-assisted resin infusion; however, the external flat compression force of the Warren truss slot holes on the resin stiffener structure doubled, and its bending contact force increased by approximately 1.5 times. Furthermore, the resulting Warren truss-slotted resin structure exhibited a late failure time. Compared with the double-sided, square shallow groove cylindrical straight hole foam-core sandwich composite, the Warren truss slot resin-stiffener-reinforced sandwich composite exhibited an increase of 4.7 kN in the flat compression load, an improvement of ~40% in flat compressive strength performance, an increase of ~0.58 kN in the bending load, and an improvement of ~60% in the bending strength, demonstrating its better bearing strength performance.

Experimental study on the behaviour of grid-scored foam-cored sandwich composites under low-velocity and quasi-static punch shear loads

Grid-scored foams are core materials adapted to resin infusion systems used in marine vessels and aerofoil shell structures of wind turbine blades. Perforation resistance is critical in such constructions, and so the goal of this research is to analyse the energy absorption ability of sandwich composites with grid-scored foam core. Grid-scored foam contributed more to the energy absorption capacity of sandwich structures due to causing higher contact force compared with rigid foam under low-speed and quasi-static punch shear loads. At quasi-static punch shear tests, the energy absorption capability of grid-scored foams is more sensitive to test speed. Grid-scored foam improved the elastic energy absorption values of the sandwich structure under quasi-static punch shear tests. The perforation energy results obtained from the quasi-static tests of the grid-scored specimens showed a similar trend to the results obtained from the low-velocity impact tests. Compared with the damaged rigid-cored counterparts, grid-scored foam-cored sandwich specimens demonstrated higher damage tolerance in dynamic and static testing by limiting core crushing and face sheet-foam core delamination between resin channels.

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.

Analytical study on the low-velocity impact penetration of the fully-clamped foam-core composite sandwich panels

An analytical model based on Hamilton's principle is proposed to predict the load/displacement history, failure characteristics and energy partition of composite sandwich panels subjected to low-velocity impact of a projectile. Lagrangian functions based on the five-stage penetration process is developed, in which failure of each constituent is considered to demonstrate the progressive failure process. After the analytical model was validated against the experimental results, it was used to investigate the effect of core density and face-sheet stacking sequence on the impact response. Discussions showed that the impact resistance of the panel correlates strongly to the core density, while stacking sequence has minor effect on sandwich panel under low energy impact. Energy contribution of all components during impact penetration under different energy were studied to discuss the optimum configuration of the structure. The analytical model may provide a quick and reliable preliminary assessment in structure design and optimization.

A new crushable foam model for polymer-foam core sandwich structures

An elastic-plastic, viscoelastic damage model to accurately describe the crushing and energy absorption of polymer foams in sandwich panels was implemented in ABAQUS Explicit. Good agreement was found between ABAQUS predictions and experimental results with Divinycell H100 foams under triaxial compression, triaxial compression-tension and triaxial compression-shear. The elastic-plastic viscoelastic damage model was used to simulate the response of a doubly curved foam core composite sandwich shell subjected to direct pressure pulse and water pressure pulse loading. Results using the new crushable foam model were compared to those using the ABAQUS built-in isotropic crushable foam model. It was found that sandwich panels with the more commonly used, isotropic crushable foam model overestimated the stiffness, strength and energy dissipation of sandwich shells with Divinycell H100 foam cores. The proposed elastic-plastic-viscoelastic-damage model would provide more accurate results and offer better protection of composite sandwich structures than the isotropic crushable foam model.

Characterization Shear Properties of PVC Foams Instrumented by Optical Fiber under Flexural Loading

Abstract The bending behavior of foam core sandwich composites has increasingly attracted attention and application in industries such as shipbuilding, aircraft, and wind turbine industries. The main objective of this research work is the assessment of shear strain in a foam core beam by means of optical fiber sensors during a bending test. Experimental studies were conducted on a polyvinyl chloride (PVC) foam beam in which three optical fibers were embedded in a longitudinal plane across the thickness of the foam core; straight optical fibers measure strains due to the tension/compression load, whereas the sinusoidal fibers catch strains due to the shear load. A finite element model was used to predict strain levels in order to validate and explain optical fiber sensor measurements from three- and four-point bending tests. The concordance of the shear properties identified by optical fiber sensor results and obtained by finite element simulation was evaluated to validate the newly developed technique of characterization. Results shows good agreement between the experimental and numerical responses.

A novel test fixture for mode III fracture characterization of monolithic laminates and composite sandwich specimens

This work presents a novel test fixture for mode III fracture characterization of delaminations in monolithic laminates and face-core debonds in foam core composite sandwich specimens. The test fixture is configured as an extension of the already existing shear-torsion-bending (STB) test designed for monolithic laminates. The specimen sizing, lay-up configuration and the manufacturing process are presented. Accordingly, an overview of the test fixture is provided along with the data reduction method employed to compute the energy release rate. The results from representable fracture characterization tests are presented as force vs. displacement curves, where different definitions of the critical load for crack propagation can be defined. Thus, the critical value of the energy release rate is computed using analytically based equations for the different definitions given for the critical loads. The results show a stable crack growth for monolithic laminate specimens. However, a highly non-linear response of the sandwich specimens, before the onset of crack propagation, is observed. A non-linear numerical analysis and destructive specimen inspections are carried out in order to identify the source of the non-linear behaviour observed in the experimental results.

Measurement of Interfacial Bonding Force between Foam and CFRP in Foam-Cored CFRP Sandwich Composites

Measurement of Interfacial Bonding Force between Foam and CFRP in Foam-Cored CFRP Sandwich Composites

Improvement of Interfacial Bonding Force between PMI Foam and CFRP in PMI Foam-Cored CFRP Sandwich Composites

In the polymethacrylimide (PMI) foam-cored carbon fiber reinforced polymer (CFRP) sandwich composite, PMI foam is the core material and CFRP is the surface material. The interfacial bonding force between the foam core and the skin layer is an important factor that determines the mechanical properties of the foam-cored sandwich composite. This study improved the interfacial bonding force between the PMI foam core and the CFRP skin layer in the PMI foam-cored sandwich CFRP composite. This was achieved by fabricating holes of a constant volume on the surface of the PMI foam; in addition, an epoxy, the matrix of CFRP, was filled to increase the bonding area of the interface between the PMI foam and the CFRP skin layer. Then by processing holes with constant distance, it was confirmed that the interfacial bonding force according to the intervals of the holes. As a result, the interfacial bonding force of samples with holes increased on average up to 63.5 % compared with the samples without the processed holes.

Elucidating the mechanisms of damage in foam core sandwich composites under impact loading and low temperatures

Recent interest in Arctic exploration has brought new challenges concerning the mechanical behavior of lightweight materials for offshore structures. Exposure to seawater and cold temperatures are known to degrade the mechanical properties of several materials, thus, compromising the safety of personnel and structures. This study aims to investigate the low-velocity impact behavior of woven carbon/vinyl ester sandwich composites with Polyvinyl chloride (PVC) foam core at low temperatures for marine applications. The tests were performed in a drop tower impact system with an in-built environmental chamber. Impact responses, such as the contact force, displacement and absorbed energy, at four impact energies of 7.5 J, 15 J, 30 J, and 60 J were determined at four in-situ temperatures of 25°C, 0°C, −25°C and −50°C. Results showed that temperature has a significant influence on the dynamic impact behavior of sandwich composites. The sandwich composites were rendered stiff and brittle as the temperature decreased, which has a detrimental effect on their residual strength and durability. At 7.5 J at all temperatures, the samples experienced matrix cracking, fiber fracture, and delamination at the top face sheet. The samples impacted at 15 J at all temperatures experienced fiber fracture, matrix cracking, and delamination at the top facesheet and localized core crushing/fracture. At 30 J for all the temperatures, the samples exhibited perforation of the top facesheet and penetration into the core. As the temperature decreased, the penetration of the striker into the core increased. At 60 J for all temperatures, the samples experienced perforation of the top facesheet and core, and the back facesheet exhibited varying extent of damage. At −25°C and −50°C, the sandwich composite samples were almost completely perforated. In general, low temperatures rendered the sandwich composites stiff and brittle, resulting in an increase in the degree of damage and more pronounced damage modes. At all impact energies, the sandwich composites were rendered stiff and brittle as the temperature decreased, which has a detrimental effect on their residual strength and durability.

Impact damage tolerance of energy storage composite structures containing lithium-ion polymer batteries

Multifunctional composite structures that combine high load-bearing properties with electrical energy storage capacity have potential application in electric and hybrid powered cars, and therefore must be impact resistant in the event of collision. This paper examines the effect of low velocity impact on the damage, compression properties and energy storage capacity of composite laminates and sandwich composites containing an embedded lithium-ion polymer (LiPo) battery. The impact responses of a carbon fibre-reinforced epoxy (CFRP) laminate and a polymer foam core sandwich composite are investigated. The materials were impacted at different incident energy levels at the location immediately above the embedded battery. The battery at the impact location increased the impact energy absorption of both the laminate and sandwich composite due to its capacity to yield and plastically deform. Impact-induced damage reduced the compressive properties of the composite laminate and sandwich composite in part due to deformation, cracking and debonding of the battery. Low impact energy events (≤4 J) had negligible effect on the residual energy storage capacity of the LiPo battery, although higher energies (≥6 J) caused an internal short circuit due to excessive plastic deformation and crushing.

Flexural response of 3D printed sandwich composite

Among many lightweight materials used in marine applications, sandwich structures with syntacticfoam core arepromising because of lower water uptake in foam core amid face-sheets damage. HDPE (high-density polyethylene) filament is used to 3D print sandwich skin, and glass microballoon (GMB) reinforced HDPE syntactic foam filaments are used for the core. The optimized parameters are used to prepare blends of 20, 40, and 60 vol% of GMB in HDPE. These foamed blends are extruded in filament form to be subsequently used in commercially available fused filament fabrication (FFF) based 3D printers. The defect-free syntactic foam core sandwich composites are 3D printed all at once (skin-core-skin printing in sequence at once) using optimized printing parameters and characterized for the flexural behavior. The result reveals that the addition of GMB increases both specific modulus and strength in sandwich composites and is highest for the sandwich having a core with 60 vol% of GMB. The measured properties of sandwiches are compared with a respective core to study the effect of the sandwich. It is observed that flexural strength, fracture strength, and strain of foam core sandwiches registered superior response than their respective cores. The experimental results are found in good agreement with theoretical predictions. Finally, the failure mode of the printed sandwich is also discussed, and it is observed that none of the 3D printed syntactic foam core sandwiches fractured due to shear failure.

Compression after impact performance of carbon-fiber foam-core sandwich composites in low temperature arctic conditions

This study investigates the compression after impact (CAI) behavior of carbon-fiber foam-core sandwich composites in low temperature arctic conditions. Low-velocity impact and CAI tests are experimentally conducted across temperature from −70 °C to 23 °C. Results show higher compressive strength for non-impacted specimens at low temperature. However, upon impact, severe damage and complex failure modes at low temperature lead to significant reduction in CAI strength by 60%. X-ray micro-computed tomography reveal additional CAI damage mechanisms, such as auxiliary facesheet matrix cracking, localized back facesheet delamination-induced buckling, and front facesheet debonding. The types of CAI damage modes and failure locations are apparently different for room and low temperature conditions. The findings of this study are supported by a newly proposed analytical model that predicts CAI performance using fracture mechanics based concepts. This work evidently demonstrates that the damage tolerance of sandwich composites is severely reduced at low temperature conditions.

An improved analysis of a STB specimen for fracture characterization of laminates and foam-cored sandwich composites under mode III loads

This work presents the development of an improved data reduction method for the shear-torsion-bending (STB) test designed for monolithic laminates. The analytical derivation extends the applicability of the STB rig to composite sandwich specimens subjected to an out-of-plane shear loading. The data reduction method consists of an analytical model that expresses the global energy release rate in terms of applied loads, specimen geometry and material properties. The mathematical derivation of the energy release rate relies on first order shear deformation theory, Vlasov theory for non-uniform torsion of beams and near tip effects are also taken into consideration by the analytical model. Face sheets and core are modelled as homogeneous, linear elastic and orthotropic materials. The analytical expression is verified using the energy release rate extracted from a high-fidelity 3D FE based fracture mechanics model of the specimen. A compliance based method is used to generate global predictions, while local predictions are extracted using a displacement-based mode separation method. Local predictions are used to discuss accuracy and limitations of the approximate analytical model.

Thermal stability and flammability characteristics of phenolic syntactic foam core sandwich composites

Polymeric foam core sandwich composites are the nascent materials used in marine and aerospace structural applications for its low-density characteristics. A special class of foam called syntactic foam is one of the promising core material having high specific properties. However, these polymeric foam core sandwich composite structures may encounter problems in correlation with temperature and fire. Damages that happen due to the variation in temperature and by the catch of fire are sometimes imperceptible which may lead to the deterioration of load carrying ability or catastrophic failure of these composite structures. Present investigation is focused on the possibilities of reducing the extent of damages due to variation in temperature and by the catch of fire by enhancing its thermal stability and flame resistance characteristics. This was achieved from the development of syntactic foam by embedding hollow micro-spherical particles in phenolic resin for fire containment or fire isolation. Result of the experimentation reveals that the phenolic syntactic foam core was thermally more stable than glass/epoxy face skins up to a temperature of 450°C. The minimum concentration of oxygen required for burning was found to be 30%, in which phenolic syntactic foam core helps in flame isolation, whereas E-glass/epoxy face skins contribute to flame spread in the event of burning of sandwich composites. Improved thermal stability and fire resistance characteristics of developed sandwich composites are attributed to the phenolic syntactic foam core and by its orientation.