Foam Core Material Research Articles

Foam-filled aramid honeycomb sandwich composites with high acoustic insulation properties prepared using a unique thermal expansion molding process

Contemporary advancements in technology and the rapid development of transportation systems have led to the rise of noise pollution as a major environmental issue globally. Foam sandwich composites, distinguished by their lightweight yet robust attributes such as high-strength functionality, effectively combine the superior noise insulation properties of foam structures with the strength inherent in their surface skins. However, traditional foam core materials exhibit limitations in terms of mechanical durability and suboptimal interfacial bonding with the surface skin. This study addressed these shortcomings by employing a novel approach involving a self-developed thermally expandable epoxy resin and an aramid honeycomb composite. These components were co-cured with a fiber prepreg, culminating in the formation of enhanced sandwich composites. The composites fabricated through this method demonstrate an impressive average sound insulation capacity of 70 dB, alongside a significant enhancement in the mechanical properties of the core material, surpassing the performance of standalone foam structures.

Low-velocity impact behaviors of sandwich composites with different structural configurations of foam core: numerical study and experimental validation

Sandwich composites have very weak properties as well as superior properties. Damages will be invisible and may create huge problems due to the high strength difference of composite components. To investigate sandwich composites, numerical studies’ importance is so high because experimental studies require high labor and are time-consuming. In this study, low-velocity impact properties of the configurations of sandwich composites with E-glass fiber/epoxy and PVC foam core are investigated numerically, and the results are compared with experimental results. Reference sandwich composite, and four different types of configurations are modeled with homogenized properties. Numerical simulations are performed using the material models of rigid impactor, foam core, and composite material models of LS-DYNA software. Numerical and experimental results of these sandwich composites are compared in terms of contact force-time and contact force-displacement curves, core/facesheet debonding areas, peak contact forces, absorbed energy, maximum displacement, and contact time; a good agreement is obtained.

Numerical Study on Free Particle Reinforced Fiber-Metal Composite Sandwiches Under Ballistic Loads

In this study, the ballistic performances of the fiber-metal composite plate with foam core sandwich embedded free particles were numerically investigated. Structures that can change the direction of incoming ammunition in armor designs have always been interesting. Ballistic analyzes of the structures designed with this motivation were carried out with fragment simulated projectile (FSP) under 3 different velocities. The 3D finite element and damage models of the materials were modeled separately, and the analyzes were completed by overcoming the problems encountered in the analysis of the complex structure. The material models of armor system were explained in detail as carbon fiber/epoxy composite plate, aliminum metal plate, PVC foam core material, steel spherical particle and steel fragment simulated projectile. The ballistic behaviors of 2 different types of structures formed according to particle diameter were investigated numerically and the full perforation behavior of the structure formed with small diameter particles was better.

Flexural behavior and microstructural material properties of sandwich foam core under arctic temperature conditions

This study investigates three types of foam core materials used in composite sandwich structures at various densities: H60, H100, F50, F90, PN115, PN200 and PN250. Three-point bending test is conducted to determine relationships between material and flexural properties at both room and low temperature Arctic conditions. X-ray micro-computed tomography is utilized to observe the microstructural relationships between foam density and mechanical properties of the core. This study evaluates Arctic temperature effects on mechanical properties for various types of foam core at varying densities with the intention for future Arctic applications. Although foam core materials become more brittle at a lower temperature, their flexural stiffness and flexural strength are further increased. However, due to the enhanced brittleness, the energy required for fracture is significantly reduced at low temperature conditions. This study utilizes statistical analysis to create contour plots and linear regression equations to predict flexural properties as a function of temperature and foam density. Molecular dynamics simulation is employed to verify experimental results to elucidate the effect of temperature on material behavior. This work provides a deeper understanding of how flexural strength relates to foam density, adding to existing data on foam strength properties under compressive, shear and tensile loads.

The combined effect of temperature and seawater on the compression properties of carbon fiber vinyl ester composites for sandwich structures

For naval applications, composite sandwich structures are of significant interest, and are often manufactured using thin (2 to 4 mm) composite facings made from carbon or glass fiber reinforcement, attached to a thick (25 to 50 mm) section of PVC cellular foam or balsa wood-based core materials using a suitable polymeric resin. In the present study, we focus on the hygrothermal effect on the fiber-dominated compression properties of carbon fiber reinforced vinyl ester resin based polymeric composite (CF/VE), used as “skin” for a polymeric composite sandwich material. Hygrothermal conditioning is achieved by saturating samples in simulated seawater at 40°C. Compression properties are evaluated for coupons extracted along warp and fill undergoing- no conditioning, conditioning till saturation (up to 6 months), and long-term conditioning (2 years). Sea-water saturation yields in up to 12% drop in compression strength with a further 3–4% drop resulting from long-term conditioning. No statistically significant modulus degradation is noticed due to short or long-term hygrothermal exposure. The failure mechanism of the warp extracted coupon, which fails in a splitting failure mode originating due to the delamination between the 0/90 interface, or the fill extracted coupon, which fails due to the instability caused by tow micro-buckling, remains unchanged due to combined exposure (short or long-term) of seawater and temperature. The loss in strength is attributed to the degradation of the fiber-matrix interface, which is validated via conducting single fiber push-in tests with a nominal diameter of 7 micron for conditioned and unconditioned coupons.

The Mechanical Behavior and Enhancement Mechanism of Short Carbon Fiber Reinforced AFS Interface

The aluminum foam sandwich (AFS), which perfectly combines the excellent merits of an aluminum foam core and face sheet materials, has extensive and reliable applications in many fields, such as aerospace, military equipment, transportation, and so on. Adhesive bonding is one of the most widely used methods to produce AFS due to its general applicability, simple process, and low cost, however, the bonding interface is known as the weak link and may cause a serious accident. To overcome the shortcomings of a bonded AFS interface, short carbon fiber as a reinforcement phase was introduced to epoxy resin to reinforce the interface adhesion strength of AFS. Single lap shear tests and three-point bending tests were conducted to study the mechanical behavior of the reinforced interface and AFS, respectively. The failure mechanism was studied through a macro- and microanalysis. The result showed that after the reinforcement of carbon fiber, the tangential shear strength of the interface increased by 73.65%. The effective displacement of AFS prepared by the reinforced epoxy resin is 125.95% more than the AFS prepared by the unreinforced epoxy resin. The flexure behavior of the reinforced AFS can be compared with AFS made through a metallurgical method. Three categories of reinforcement mechanisms were discovered: (a) the pull off and pull mechanism: when the modified carbon fiber performed as the bridge, the bonding strength improved because of the pull off and pull out of fibers; (b) adhesion effect: the carbon fiber gathered in the hole edge resulted in epoxy resins being gathered in there too, which increased the effective bonding area of the interface; (c) mechanical self-locking effect: the carbon fiber enhanced the adhesive filling performance of aluminum foam holes, which improved the mechanical self-locking effect of the bonding interface.

Low velocity impact (LVI) and flexure-after-impact (FAI) behaviours of rotationally moulded sandwich structures

There is limited academic knowledge and industrial understanding available on low velocity impact (LVI) properties, impact damage propagation mechanism and post impact residual strength of rotationally moulded skin-foam-skin sandwich structures. In this work, two roto-moulded sandwich structures (sandwich-1 and sandwich-2) were manufactured at the same thickness and skin layers configurations, with two different commercially available foam core material types and densities. They were tested using an instrumented drop weight impact tester at 25 J, 35 J and 45 J energy levels to analyse the force–time, force–deflections, and energy–time properties. The damage mechanism was investigated with a high-resolution X-ray micro Computed Tomography (μ-CT) technique which correlated the measured impact properties for both type of sandwich structures. Flexure-after-impact (FAI) test was carried out to characterise the effects of impact induced damage on the residual strength of impacted sandwich specimens for the first time in this study. In comparison of two sandwich structures, the lower density foam material manufactured sandwich structures (sandwich-2) showed a better impact property, damage resistance and FAI strength compared to higher density foam material sandwich structures (sandwich-1). This result was not industrially expected and could be related to the incomplete decomposition of blowing agents or forming of immature foam cells in the foaming process of higher density foam material in sandwich-1 leading to its less impact and FAI test load bearing capacities.

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.

The effect of boron carbide additive on the low‐velocity impact properties of low‐density foam core composite sandwich structures

AbstractSandwich structures with carbon fiber‐epoxy face sheets and polyvinyl chloride foam core material are known for their high strength and flexural stiffness despite their low weight. However, the structural response, in terms of crush strength, of the particles added sandwich structures are not very well known under impact loading conditions. In this study, the impact resistance and damage characteristics of particle added low weight composite sandwich structures were investigated with a low‐velocity drop weight impact test device. Boron carbide (B4C) particles, which had excellent hardness, thermoelectric, and radiation absorbing characteristics, were used as an additive for the epoxy matrix. For this purpose, 2%, 5%, and 10% by weight additives were mixed into the epoxy matrix and sandwich structures were produced with hand lay‐up followed by vacuum bagging method. All configurations were subjected to low‐velocity drop weight impact test at three different energy levels (10, 17.50, and 25 J). The results obtained from the experiments and the images of the post‐impact damage of the sandwich structures were presented comparatively. According to the test results, configurations containing 10% boron carbide (B4C) additive has shown the best performance in terms of resistance to impact load.

Test and Evaluation Procedure of Foam Core Materials for Composite Ships

Sandwich structures are general-purpose structures that can reduce the structural weight of composite ships. Core materials are essential for these structures, with polyvinyl chloride (PVC) foams being the most popular. These foam core materials are subjected to various tests in the development process, and must satisfy the performance requirements of several ISO and ASTM standards. Therefore, a procedure for evaluating the performance of foam core materials was proposed in this paper. In addition, prototypes were fabricated using a commercial PVC foam core product in accordance with the structural design of an 11 m fiber-reinforced plastic yacht. Then, a case study was conducted on the proposed evaluation procedure. The proposed procedure facilitates the understanding of the performance requirements and evaluation of core materials used in composite ships and is expected to be utilized in developing core materials for marine structures.

Radar-absorbing foam-based sandwich composite with electroless nickel-plated glass fabric

This study presents a broadband radar-absorbing foam-based sandwich structure (RAS) composed of a foam core, glass fiber-reinforced polymer (GFRP), and electroless nickel-plated glass fabric. The nickel-coated fiber (NCF) for electromagnetic wave absorption is prepared using electroless nickel plating on glass fabric with discussing the detailed electromagnetic wave absorption mechanisms relationship between the plating thickness and the skin depth in terms of lossy dielectric properties. To design the broadband NCF RAS, transmission line theory linked with a genetic algorithm is used to optimize the absorption performance. The EM wave absorption performance of fabricated broadband NCF RAS was measured using by using a free-space measurement system operating from 5.8 GHz to 18 GHz. From the measurement and the tolerance analysis, the proposed NCF RAS works well in the designed frequency range and is very robust for the fabrication tolerance. In addition, a flatwise tensile test was performed according to ASTM C297 to check the influence between the NCF and the foam core materials in terms of interfacial behavior. It has been indicated that they have no significant effect on interfacial properties.

Tension and compression of sandwich composites with weft-knit fabric cores

The manufacturing techniques of sandwich composites containing core layers of weft-knit glass fabric (WG) and weft-knit carbon fabric (WC) with carbon fabric skin layers are discussed herein. The core layers of the sandwich composites were fabricated with WG-reinforced epoxy (E) resin, WC-reinforced epoxy resin, and polyurethane foam (F). The core layer was then stacked with two pieces of carbon fabric on the top and bottom surfaces to fabricate the sandwich composites. Three sandwich composites [plain carbon fabric sandwich composite with a WG core layer (C/E/WG), plain carbon fabric sandwich composite with a WC core layer (C/E/WC), and plain carbon fabric sandwich composite with an F core layer (C/E/F)] were developed in this study. A two-step manufacturing procedure was developed to achieve sufficient adhesiveness between the skin and core layers. The tensile, flatwise compressive, and longitudinal compressive properties of these sandwich composites were measured according to referred ASTM standards on a materials test system (MTS 810). Experimental results revealed that the WC core materials displayed excellent resistance to a flatwise compressive force and the foam core material show weak resistance. Under longitudinal compression, the skin and core layer of the C/E/F specimen separated, indicating that the C/E/F specimen could not withstand longitudinal force. Moreover, the C/E/WG and C/E/WC specimens both bend at the end of the same test.

Impact penetration and perforation performance of square sandwich panels with EPS foam core

This paper addresses the impact penetration and perforation behaviour of sandwich panels having a low density core expanded polystyrene foam (EPS) bonded to two aluminum (6061-T6) face-sheets. The effects of foam and plate thicknesses on the impact energy absorption of sandwich panels were also investigated. The dynamic response of panels was analyzed using the explicit finite element method. The foam core material was modeled as a crushable foam material with ductile damage, and the metal face-sheets as Johnson-Cook (JC) material. The cohesive response of the adhesive interface was considered using cohesive zone model. The analyses showed that the impact energy, face-sheet and foam core thickness affected significantly failure modes, contact force levels and histories. The low-speed impact tests of the sandwich panels with different face-sheet and foam core thicknesses showed similar contact force histories, energy absorption ability and deformation modes to those of finite element analyses.

Pyrolysis and combustion characteristics and reaction kinetics of carbon fiber/epoxy composites

The pyrolysis and combustion characteristics and reaction kinetics of epoxy resin matrix and carbon fiber/epoxy composites based on epoxy resin were studied by cone calorimetry and thermogravimetry. The results show that with the increase in thermal radiation intensity (increase from 25 kW m−2 to 55 kW m−2), the average ignition time of the experimental samples decreases, the peak heat release rate increases, and the peak appears earlier. Carbon fiber can inhibit the pyrolysis and combustion of epoxy resin. It can effectively inhibit the droplet and splash phenomenon during the combustion process. The time of ignition, heat release, and the peak value of the heat release rate are delayed. The ignition temperature of foam core materials in carbon fiber/epoxy foam laminates is low, which makes the average ignition time and peak heat release rate appear earlier. The theoretical critical heat fluxes of four experimental samples (epoxy resin matrix, carbon fiber/epoxy bidirectional woven fabric, carbon fiber/epoxy prepreg, and carbon fiber/epoxy foam laminate) were obtained by calculation. The theoretical critical heat fluxes are 12.12 kW m−2, 13.21 kW m−2, 11.12 kW m−2, and 0.93 kW m−2, respectively. The pyrolysis of carbon fiber/epoxy bidirectional woven fabric can be divided into three stages: two stages of epoxy resin matrix decomposition and carbon fiber decomposition. The heating rate has a significant influence on the pyrolysis process. With the increase in heating rate, the maximum weight loss rate temperature moves toward high temperature. The Kissinger method and Flynn-Wall-Ozawa method were used to analyze the pyrolysis kinetics. The apparent activation energy and pre-exponential factors were obtained at different heating rates. The results obtained by the two methods are basically the same. When the pyrolysis temperature of the three composites reached 450 °C, the mass loss was 20%, 17%, and 28%, respectively. This shows that the thermal stability of carbon fiber/epoxy prepreg is the best, the thermal stability of the two-way woven fabric of carbon fiber is better, and the thermal stability of the carbon fiber sandwich board is the weakest. It is consistent with this rule at any temperature.

Bending Behavior of Sandwich Structures with Different Fiber Facing Types and Extremely Low-Density Foam Cores

Sandwich structures offer innovative alternative solutions in many weight critical industrial fields due to their lightweight and very high flexural rigidity compared to conventional materials. A vast number of sandwich configurations can be produced considering different type of materials for face sheets and core as well as matrix. Although there are many sandwich structures available in the literature to obtain the desired mechanical and physical properties, the usage of very low-density core materials is very limited. In this study, carbon and glass fiber fabrics having woven plain and ±45 fiber orientations and industrial PVC foam core having extremely low density of 40 kg.m-3 and 48 kg.m-3 were used to manufacture the sandwich panels. The fabrications of eight different configurations were performed by hand lay-up followed by vacuum bagging method. According to ASTM C393/C393M Standard, the sandwiches were subjected to three-point bending (TPB) tests. After performing the TPB tests, the composite sandwich specimens were examined under a stereomicroscope to determine the failure modes. The primary failure modes under quasi-static bending loading are found to be top face sheet failures due to fiber and matrix cracks and delamination, and core shear failures due to core crushing just below the top facing and core fractures. In addition, the consistency of the test results were verified and the effects of parameters were investigated by using statistical variance (ANOVA) and regression analysis. The study provides valuable contribution to the literature for sandwich structures having extremely low-density foam core materials, and may contribute to the material universe by introducing strong, stiff and lightweight sandwich composites. It gives a comprehensive comparison by considering the effect of different fiber types, fabric fiber orientations and core densities.

Experimental and numerical investigation on enhancing the structural integrity of composite sandwich structure

Composite sandwich structure possesses unique characteristics in which the facings’ material provides a high in-plane stiffness/strength, while the core material keeps the facings a part to enhance the flexural stiffness/strength. However, sandwich structure is susceptible to shear failure due to the weak bond between the facings and the core materials. This article addresses this issue and provides an efficient semi-circular shear keys concept to be inserted in the interfacial surface between the sandwich structural elements to improve the facings–core interaction with marginal sacrifice in the strength-to-weight ratio. Composite sandwich panels composed of glass fibre–reinforced polymer skins and polyurethane foam core material have been investigated in this study. Mechanical tension, compression and shear tests were performed on the facings, while compression tests were conducted on the core materials to define the elasto-plastic response of the constituent’s materials. Four-point bending tests were performed on the conventional composite sandwich panel and panels incorporated with shear keys concept. Considerable improvement in the flexural stiffness and strength along with detouring the crack path was noticed for the samples incorporated with shear keys. A non-linear finite element analysis was established to verify the experimental results. Comparisons between the experimental and finite element results were presented and showed good agreement between both results which justified the parameters used in the finite element models and their capabilities to capture the mechanical behaviour and the damage mode of the investigated models.

Bending behavior of sandwich structures with different fiber facing types and extremely low-density foam cores

Abstract Sandwich structures offer innovative alternative solutions to many weight-critical industrial fields due to their lightweight and very high flexural rigidity compared to conventional materials. A vast number of sandwich configurations can be produced from a variety of materials for use as face sheets and core as well as matrix. Although there are many sandwich structures available in the literature to obtain the desired mechanical and physical properties, the usage of very low-density core materials is very limited. In this study, carbon and glass fiber fabrics having woven plain and ±45 ° fiber orientations and industrial PVC foam core having extremely low density of 40 kg × m−3 and 48 kg × m−3 were used for manufacturing the sandwich panels. Eight different configurations were constructed by hand lay-up followed by vacuum bagging. According to ASTM C393/C393M standard, the sandwiches were subjected to three-point bending (TPB) tests. After performing the TPB tests, the composite sandwich specimens were examined under a stereomicroscope to determine failure modes. The primary failure modes under quasi-static bending loading were found to be top face sheet failures due to fiber and matrix cracks and delamination, and core shear failures due to core crushing just below the top facing and core fractures. In addition, the consistency of the test results were verified and the effects of parameters were investigated by using statistical variance (ANOVA) and regression analysis. The study provides a valuable contribution to the literature regarding sandwich structures having extremely low-density foam core materials and may contribute to the material universe by introducing strong, stiff and lightweight sandwich composites. It provides a comprehensive comparison by considering the effect of different fiber types, fabric fiber orientations and core densities.

Low speed impact behaviour of adhesively bonded foam-core sandwich T-joints

This paper investigates the effects of foam core density and aluminum skin plates on the low speed impact behaviour of adhesively bonded sandwich T-joints having a PVC foam core and aluminum face-sheets. The dynamic response of adhesively bonded sandwich T-joints was analyzed by the explicit finite element method. Two different material models were implemented to the foam core material: a hyperelastic model and a crushable foam material with ductile damage whereas the aluminum face-sheets were modelled as an elasto-plastic material. The cohesive response of adhesive interfaces was included using three dimensional cohesive element based on cohesive zone model. Adhesively bonded sandwich T-joint specimens were manufactured and tested to validate the numerical model. A very good agreement between the experimental and FE results were achieved. The density of the foam core material of adhesively bonded sandwich T-joint played important role on the joint failure mechanism. The joint having a stiffer foam core experienced more damage in both stiffener panel and adhesive layers.

Experimental investigation of thermal properties and fire behavior of carbon/epoxy laminate and its foam core sandwich composite

According to structural characteristics, composites are classified as laminated structure and sandwich structure. Carbon/epoxy laminate and foam core sandwich composite are the most commonly used laminate and sandwich structure material in the aircraft industry. The flammability of epoxy resins and foam core material is an inherent hazard. Many previous studies focused primarily on their mechanical properties, while the studies on the thermal and fire properties of carbon/epoxy laminate and its foam core sandwich composite have rarely conducted. Therefore, to characterize their thermal and fire properties, a comprehensive experimental investigation and theoretical analysis were carried out in this work using thermogravimetric analysis, cone calorimeter, vertical/horizontal burning tests, limiting oxygen index and scanning electron microscope tests. Several typical characteristic parameters were obtained and analyzed, such as pyrolysis temperature, heat release rate, mass loss, flaming spread rate and limiting oxygen index. These experimental data coupled with theoretical analysis can provide support for fire risk assessment and fire protection design in aircrafts. The carbon/epoxy laminate and foam core sandwich composite are both characterized as the thermally thick materials. The ignition models and mass loss rate models were obtained. Foam core material negatively affects most of the thermal and fire properties of sandwich composite, but the foam core sandwich composite has self-extinguishing behavior during horizontal burning tests, whose LOI is higher than that of carbon/epoxy laminate. Thus, an important conclusion was reached that the ignition position and flame spread direction have critical effect on the fire behavior of foam core material.