Theoretical and experimental study of foam-filled lattice composite panels under quasi-static compression loading

In this paper, a simple and innovative foam-filled lattice composite panel is proposed to upgrade the peak load and energy absorption capacity. Unlike other foam core sandwich panels, this kind of panels is manufactured through vacuum assisted resin infusion process rather than adhesive bonding. An experimental study was conducted to validate the effectiveness of this panel for increasing the peak strength. The effects of lattice web thickness, lattice web spacing and foam density on initial stiffness, deformability and energy absorbing capacity were also investigated. Test results show that compared to the foam-core composite panels, a maximum of an approximately 1600% increase in the peak strength can be achieved due to the use of lattice webs. Meanwhile, the energy absorption can be enhanced by increasing lattice web thickness and foam density. Furthermore, by using lattice webs, the specimens had higher initial stiffness. A theoretical model was also developed to predict the ultimate peak strength of panels.     

Fluorinated epoxy resin as a low adhesive mould for composite material

The aim of this work is to decrease the adhesion between a cured modified epoxy-based substrate and an in situ cured virgin epoxy-based piece. The effect of perfluorinated additives on the non-adhesion output is investigated through an adapted pull-off test. It appears that additive migration initiates the surface fluorination. Longer the fluorinated chain is, higher the surface fluorination is and weaker the adhesion strength is. The weak chemical affinity between these two epoxy resins is shown to be mainly responsible for these results leading to an adhesive rupture.     

Strain-rate effects in Ni/Al composite metal foams from quasi-static to low-velocity impact behaviour

Metal foams are used as absorbers for kinetic energy but predominantly, they have only been investigated under quasi-static load-conditions. Coating of open-cell metal foams improves the mechanical properties by forming of Ni/Al hybrid foam composites. The properties are governed by the microstructure, the strut material and geometry. In this study, the strain-rate effects in open-cell aluminium foams and new Ni/Al composite foams are investigated by quasi-static compression tests and low-velocity impact. For the first time, drop weight tests are reported on open-cell metal foams, especially Ni/Al composite foams. Furthermore, size-effects were evaluated. The microstructural deformation mechanism was analysed using a high-speed camera and digital image correlation. Whereas pure aluminium foams are only strain-rate sensitive in the plastic collapse stress, Ni/Al foams show a general strain-rate sensitivity based on microinertia effects and the rate-sensitive nano-nickel coating. Ni/Al foams are superior to aluminium foams and to artificial aluminium foams with equal density.     

Quasi-static and high strain rate response of aluminum matrix syntactic foams under compression

Aluminum alloy matrix syntactic foams were produced by inert gas pressure infiltration. Four different alloys and ceramic hollow spheres were applied as matrix and filler material, respectively. The effects of the chemical composition of the matrix and the different heat-treatments are reported at different strain-rates and in compressive loadings. The higher strain rates were performed in a Split-Hopkinson pressure bar system. The results show that, the characteristic properties of the materials strongly depends on the chemical composition of the matrix and its heat-treatment condition. The compressive strength of the investigated foams showed a limited sensitivity to the strain rate, its effect was more pronounced in the case of the structural stiffness and fracture strain. The failure modes of the foams have explicit differences showing barreling and shearing in the case of quasi-static and high strain rate compression respectively.     

Matrix failure in composite laminates under compressive loading

The failure envelope of the matrix in composite laminates under compressive loads has not received much attention in literature. There are very little to no experimental results to show a suitable failure envelope for this constituent found in composites. With increasing popularity in the use of micromechanical analysis to predict progressive damage of composite structures which requires the use of individual failure criteria for the fibre and matrix, it is important that matrix behaviour under compression is modelled correctly.In this study, off-axis compression tests under uniaxial compression loading are used to promote matrix failure. Through the use of micromechanical analysis involving Representative Volume Elements, the authors were able to extract the principal stresses on the matrix at failure. The results indicated that hydrostatic stresses play an important role in the failure of the matrix. Thus, Drucker–Prager failure criterion is recommended when modelling compressive matrix failure in composite structures.     

The compressive properties of expandable microspheres/epoxy foams

Expandable microspheres/epoxy foams with different densities and microstructures were prepared by changing the foaming temperature and the precuring extent. The microstructure of foams reveals a homogeneous distribution of cells at high precuring extent and high foaming temperature, while small cells size at high precuring extent and low foaming temperature. Furthermore, the compressive properties of epoxy foams were investigated. The compressive strength and modulus of the foam exhibited a power-law dependence with respect to density. By optimizing the foaming temperature and the precuring extent, epoxy foams with homogeneous cells and stable compressive property can be obtained. Fracture surface showed that deformed microspheres and less debris were observed at relatively high-density foams.     

The energy-absorbing characteristics of composite tube-reinforced foam structures

This paper reports the findings of a research study investigating the energy-absorbing characteristics of polymer foams reinforced with small carbon fibre reinforced epoxy tubes. Initial attention focuses on establishing the influence of tube diameter on the specific energy absorption (SEA) characteristics of the chamfered CFRP tubes. Here, it is shown that the SEA of the tubes increases rapidly with decreasing diameter/thickness ratio, with the highest values being close to 93 kJ/kg. Similar tests were conducted at dynamic rates of strain, where it was observed that the measured values of SEA were lower than the corresponding quasi-static data, possibly due to rate-sensitive effects in the delamination resistance of the composite material. In the next stage of the investigation, the composite tubes were embedded in a range of polymer foams in order to establish the influence of both tube arrangement and foam density on the crush behaviour of these lightweight structures. In addition, a limited number of blast tests have been undertaken on structures based on these core materials. Here, extensive crushing of the composite tubes was again observed, suggesting that these structures should be capable of absorbing significant energy when subjected to this severe loading condition. Finally, the results of these tests are compared with previously-published data from studies on a range of different cores materials. Here, it has been shown that the energy-absorbing characteristics of these systems exceed values associated with other core materials, such as aluminium honeycombs, polymer honeycombs and metal foams.     

Quasi-static and dynamic mechanical responses of hybrid laminated composites based on high-density flexible polyurethane foam

Hybrid laminated composites were fabricated based on high-density flexible polyurethane foam and reinforced with inter/intra-ply hybrid laminates. Transient responses of hybrid composites under quasi-static and dynamic loadings with various thicknesses and expansion factors were comparatively investigated. Experimental results revealed that foam cell collapse and hybrid laminates rupture were dominant mechanisms of energy absorption. Interlaminar stress and composite tensile strength determined the compressive potential energy and double-peak behavior. Quasi-static bursting and puncture resistances exhibited totally different relationships to various constructions and expansion factors. Energy dissipation capacity is influenced more significantly by the constant rate of transverse (CRT) puncture than dynamic puncture process. CRT puncture resistance is superior to the corresponding dynamic puncture resistance for all constructions. The hybrid laminated composites contributes to eliminate more than 95% of the incident force in the drop weight impact test. Compared with non-laminated panel, the hybrid laminated composites exhibited higher resistance to static and dynamic loadings.     

Compressive mechanical properties of closed-cell aluminum foam–polymer composites

The compressive mechanical properties of two kinds of closed-cell aluminum foam–polymer composites (aluminum–epoxy, aluminum–polyurethane) were studied. The nonhomogeneous deformation features of the composites are presented based on the deformation distributions measured by the digital image correlation (DIC) method. The strain fluctuations rapidly grow with an increase in the compressive load. The uneven level of the deformation for the aluminum–polyurethane composite is lower than that for the aluminum–epoxy composite. The region of the preferentially fractured aluminum cell wall can be predicted by the strain distributions in two directions. The mechanical properties of the composites are investigated and compared to those of the aluminum foams. The enhancement effect of the epoxy resin on the Young’s modulus, the Poisson’s ratio and the compressive strength of the aluminum foams is greater than that of the polyurethane resin.     

Effect of fabric compaction and yarn waviness on 3D woven composite compressive properties

3D-woven fabrics incorporate through-thickness reinforcement and can exhibit remarkable inter-laminar properties that aid damage suppression and delay crack propagation. However, distortions in the internal architecture such as yarn waviness can reduce in-plane properties, especially in compression. The degree of yarn waviness present in a 3D woven fabric can be affected by a range of factors including weave parameters and manufacturing-induced distortions such as fabric compaction. This paper presents a thorough analysis of the effect of fabric compaction and yarn waviness on the mechanical properties and failure mechanisms of an angel-interlock fabric in compression. Tests were conducted on coupons moulded to different volume fractions and data compared to previous measurements of local yarn angle. Major findings show the importance of yarn straightness on compressive strength and how this can be affected by optimising moulding thickness. Failure initiation was also found to be heavily influenced by weave style and yarn interlacing.     

Mechanical,thermal and morphological properties of a bio-based composite derived from banana plant source

This paper focuses on the synthesis and testing of a novel bio-based composite structure in which banana fibres was infused with resin made from banana sap. The mechanical, thermal, morphological and biodegradation properties of the bio-composite were characterized and it was found that the material was suitable for general non-functional components. Mechanical tests indicated 15% increase in tensile strength, 12% improvement in tensile modulus and a 25% improvement in flexural modulus when compared to structures produced without banana sap. At elevated temperatures a decrease in the moduli was observed. The thermal stability of the biocomposite composite improved and this corresponded with an increase in the glass transition temperature. Morphological studies using scanning electron microscopy revealed improved compatibility between the fibre and banana sap matrix. This resulted in improved dynamic modulus values and low damping values. Finally, degradation tests revealed increased microbial activity on the banana sap composite. This was indicative of improved biodegradation rates.     

Fiber-reinforced composite foam from expandable PVC microspheres

The properties of composite foam based on PVC expandable microspheres reinforced with continuous aramid fibers are described. The foam was fabricated by infiltrating low-density non-woven fiber webbing with PVC microspheres. The assembly was subsequently heated to expand the foam. The resulting composite foam consisted of 10 wt% aramid fibers and had a density of 100 kg/m³. Mechanical properties, crack propagation, and microstructure of composite foams were evaluated and compared with properties of similar unreinforced foam and with commercial PVC foam of comparable density. The influence of fiber concentration, fiber architecture and bonding was investigated also. Properties were measured in tension, shear, compression, and flexure using standard ASTM test methods. The composite foam performance equaled or surpassed the performance of most thermoplastic foams commercially available. The tensile strength and modulus of the composite foam increased by factors of 6 and 8, respectively, and the shear strength and modulus increased by factors of 1.8 and 2.4. The composite foam also exhibited improved strain energy density and damage tolerance, and reduced notch sensitivity.     

Effect of carbon nanofibers on tensile and compressive characteristics of hollow particle filled composites

The effect of presence of carbon nanofibers on the tensile and compressive properties of hollow particle filled composites is studied. Such composites, called syntactic foams, are known to have high specific modulus and low moisture absorption capabilities and are finding applications as core materials in aerospace and marine sandwich structures. The results of this study show that addition of 0.25 wt.% carbon nanofibers results in improvement in tensile modulus and strength compared to similar syntactic foam compositions that did not contain nanofibers. Compressive modulus decreased and strength remained largely unchanged for most compositions. Tensile and compressive failure features are analyzed using scanning electron microscopy.     

Moisture diffusion in composite sandwich structures

The mechanical properties of polymer core materials in sandwich structures are often degraded by moisture that is absorbed during storage. To date, there is no reliable model to predict the amount of moisture that is present in these sandwich core materials. A multi-layer diffusion model applicable to these sandwich structures is described in this report. Inputs to this model are: (1) diffusivities of core and face sheet materials as functions of temperature, (2) moisture saturation data as a function of relative humidity, and (3) sandwich structure exposure history. The output is a prediction of the amounts of moisture in the core material and face sheets as a function of time.

In order to validate this model, moisture diffusion experiments were performed on a sandwich material consisting of graphite–epoxy face sheets and a core of Rohacell^® polymethacrylimide 200WF foam. Samples of this material were dried, and then hydrated at either 32 °C or 65 °C at either 83% or 100% relative humidity. The face sheets were separated from the core and each component was weighed, dried, and weighed again in order to determine the moisture distribution in the sandwich structure. The results were then compared with the model predictions.     

Effect of multi-walled carbon nanotubes on mechanical,thermal and electrical properties of phenolic foam via in-situ polymerization

In this study, phenolic foam (PF)/multi-walled carbon nanotubes (MWCNTs) composites were fabricated by in-situ polymerization, and carbonized foams based on these PF foams were prepared and the electrical property was investigated. TEM results indicated excellent dispersion of MWCNTs in the phenolic resin matrix. Scanning electron microscope results indicated that PF composites exhibited smaller cell size, thicker cell wall thickness, and higher cell density, compared with pure PF. The incorporating of MWCNTs significantly improved the mechanical properties of PF. All PF composites showed a lower thermal conductivity versus pure PF. Moreover, the carbonized pure and composites PF exhibited open-cell three-dimensional skeleton carbon structure and the MWCNTs were well-dispersed on the surface of the skeletons. It is noteworthy that the introduction of MWCNTs significantly improved the electrical performances of foams and carbonized foams by construction of conductive MWCNTs network.     

The effect of temperature on the bending properties and failure mechanism of composite truss core sandwich structures

The effects of temperature on the bending properties and failure mechanism of carbon fiber reinforced polymer composite sandwich structure with pyramidal truss cores were investigated and presented in this paper. The three-point bending tests of composite sandwich structures were performed at seven different temperatures, and the scanning electron microscope was used to examine the fiber-matrix interface properties in order to understand the deformation and failure mechanism. Then the effects of temperature on deformation modes, failure mechanism and bending failure load were studied and analyzed. The results showed that the temperature has visible impact on the deformation modes, failure mechanism, and bending failure load. The bending failure load decreased as temperature increased, which was caused by the degradation of the matrix properties and fiber-matrix interface properties at high temperature. The analytical formulae were also presented to predict the bending stiffness and failure load of composite sandwich structures at different temperatures.     

Enhancement of ablative and interfacial bonding properties of EPDM composites by incorporating epoxy phenolic resin

Effects of epoxy phenolic resin (EPR) on ablative and interfacial bonding properties of EPDM composites were evaluated. Ablative properties of EPDM composites were enhanced by two folds with incorporating 10 phr EPR. This significant enhancement was attributed to positive effect of EPR on thermal stability and thermal insulating properties of EPDM composites as well as formation of compact char layer onto composites. Furthermore, interfacial shear strength of EPDM composites with carbon fiber/epoxy (CF/EP) composites was increased by 55.6% with incorporating 10 phr EPR, due to interfacial chemical reaction of epoxide groups of EPR molecule from EPDM composites with amine group of hardener from CF/EP composites.     

Effect of alumina,silk and ceria short fibers in reinforcement of Bis-GMA/TEGDMA dental resin

The present study is focused to investigate influence of short fibers such as Alumina Microfibers (AMFs), Silk Microfibers (SMFs) and Ceria Nanofibers (CNFs) as reinforcements in Bis-GMA/TEGDMA resin towards development of composite dental filler. Morphologies of AMFs, SMFs, CNFs and their representative fracture surfaces of the reinforced dental resins/composites were examined by SEM. X-ray Diffraction Analysis was done to analyse the phase of the fibers used in this study and degree-of-conversion of the fiber incorporated base resin was studied by FTIR. Viscosity study of fiber resin mixture, depth of cure and mass change behaviour of the fibers resin composites in artificial saliva were done to analyse the flow ability and physical properties of the fiber resin composites. Mechanical properties of the composites were tested by a universal testing machine. This study demonstrated that incorporation of 10% AMFs, 5% SMFs, and 3.33% CNFs individually in Bis-GMA/TEGDMA dental resin resulted in similar degree of conversion compared to the control. Also the fiber reinforced composites (10% AMFs, 5% SMFs, and 3.33% CNFs) demonstrated significant improvement in mechanical properties compared to Bis-GMA/TEGDMA resin (Control). However, depth of cure was significantly reduced due to incorporation of fibers in the resin. The reinforcement effect of AMFs, SMFs in dental resin was superior due to their uniform distribution and good interfacial bonding between fibers and resin matrix. In case of CNFs, rapid increase in viscosity during mixing of fibers with resin and inhomogeneous mixing were the major problem encountered during formulation, which was mainly associated with high surface to volume ration of the nanofibers. The resultant composite containing CNFs had less improvement in mechanical properties which may be due to less fiber content, formation of agglomerates and improper distribution of the fibers in the composite which subsequently resulted in reduction of adhesive strength.     

Nanoparticles with various surface modifications as functionalized cross-linking agents for composite resin materials

The performance of epoxy resins used for carbon fibre reinforced plastics can be significantly improved by the incorporation of nanoparticles. It is well known that the effect of material altering depends on many factors as filler material, particle distribution, particle size and shape. This paper investigates the hypothesis that particle surface modifications lead to a further improvement of the mechanical properties. Results of nanocomposites filled with four different surface modified boehmite particles are presented. The material was tested with different filler contents and analysed for chemical bonding, viscosity, thermal properties and bending performance. Surprising results show a strong influence of the surface modification on the viscosity, but no significant changes in the other material characteristics. The change of filler content in contrast has an influence on all tested performances of the nanocomposites. The results show a contrary effect of network interruption due to sterical hindrance by the particles and reinforcement due to the stiff ceramic fillers. For different filler contents these two effects have a varying influence on the material characteristics. From these results a model for the mechanism of the particle reinforcement in thermosets is concluded, which helps to understand the effectiveness of nanoparticles as reinforcement of epoxy resins.     

A review: Starch-based composite foams

The large quantities of the petroleum-based foam materials used have raised concern due to their negative effects on the environment, predominantly single-use articles in packaging applications. Thus, considerable efforts have been put forth to develop environmentally friendly alternatives and, in particular, starch foams. Many techniques including extrusion, hot-mold baking/compression, microwave heating, freeze-drying/solvent exchange, and supercritical fluid extrusion can be used to produce starch foams with different cellular structures and properties. Starch by itself is, however, rather weak and water sensitive. To improve microstructure, mechanical and thermal properties, moldability, water resistance, lightness and other properties of starch-based foams, many approaches, e.g., chemical modification of starches, blending with various biodegradable polymers, incorporation of natural fibers, and addition of nanofillers, have been attempted and are intensively reviewed in this article.