Analysis of the tensile behaviour of viscoelastically prestressed polymeric matrix composites

A novel composite material is reported, in which tension, applied to polymeric fibres, is released prior to moulding them into a matrix. Following matrix solidification, compressive stresses imparted by the viscoelastically strained fibres impede crack propagation. Previous Charpy impact studies had demonstrated that these viscoelastically prestressed composites could absorb typically 25–30% more energy than control (unstressed) counterparts and the current study focuses on their tensile behaviour as a function of fibre volume fraction, V_f. Tensile testing was performed on continuous unidirectional nylon 6,6 fibre–epoxy resin samples. Compared with control counterparts, the results showed that viscoelastic prestressing improved tensile properties, the effects being V_f-dependent. Increases in tensile strength, modulus and energy absorbed (to 0.25 strain) exceeded 15%, 30% and 40%, respectively, at an optimum V_f, this being 35–40%. Strain-to-failure was reduced by 10–20%, thereby lowering any improvement in tensile toughness (energy absorbed to fracture) to <10%. Mechanical properties of the fibres themselves were not significantly influenced by the treatment used for generating composite prestress, and we propose that the observed improvements to tensile properties may be attributed to: (i) direct contribution from compressive stress, (ii) attenuation of the dynamic overstress effect on fibre fracture and (iii) improved mechanical integrity through a more collective response from fibres to tensile loads.     

Microstructure and properties of polyurethane nanocomposites reinforced with methylene-bis-ortho-chloroanilline-grafted multi-walled carbon nanotubes

Methylene-bis-ortho-chloroanilline (MOCA), an excellent cross-linker widely used to prepare cured polyurethane (PU) elastomers with high performance, was used to modify a multi-walled carbon nanotube. PU/carbon nanotube (CNT) nanocomposites were prepared by incorporation of the MOCA-grafted CNT into PU matrix. Fourier transform infrared spectra have shown that the modified CNTs have been linked with PU matrix. The microstructure of composites was investigated by Field-Emission Scanning Electron Microscopy. The results of Dynamic Mechanical Thermal Analysis and Differential Scanning Calorimetry have investigated the grafted CNTs as cross-linker in the cured composites. The studies on the thermal and mechanical properties of the composites have indicated that the storage modulus and tensile strength, as well as glass transition temperature and thermal stability are significantly increased with increasing CNT content.     

Electrodeposition of exfoliated graphite nanoplatelets onto carbon fibers and properties of their epoxy composites

Exfoliated graphite nanoplatelet (xGnP)/copper (Cu) coated carbon fibers were fabricated by electrophoretic deposition under different applied voltages. The electrical and mechanical properties of individual fibers and composites made from these fibers and epoxy resin were investigated. The electrical resistivity of xGnP/Cu coated single carbon fiber is lower than that of the uncoated control sample and decreases with increase in the applied voltage. The xGnP and metallic Cu were simultaneously deposited on the carbon fiber surface as a result of the electrochemical cell configuration. The interfacial shear strength decreases with applied voltage up to 30 V but increases with applied voltage of over 30 V. The interfacial shear strength for the coated samples except the 50 V treated sample is lower than that of control sample. The flexural strength and modulus of xGnP/Cu coated carbon/epoxy composites is higher than those of control sample due to the reinforcing effect of xGnP/Cu coated on the carbon fibers.     

Structure evolution of conductive polymer composites revealed by solvent vapor induced time-dependent percolation

The authors of this paper synthesized a series of amphiphilic triblock copolymers of polystyrene-b-poly(ethylene glycol)-b-polystyrene (PS-PEG-PS) having different PEG/PS ratios with nearly identical molecular weights of the entire copolymers. The interfacial interactions in the composites consisting of carbon black and the copolymers can thus be tailored. When these conducting composites are exposed to certain solvent vapors, their electrical resistances greatly increase, showing the gas sensitivity. The present work indicated that this switching behavior is controlled by the structural relaxation of the composites because matrix swelling acts as the main mechanism. The response time has been correlated with absolute temperature by Arrhenius equation, and the estimated activation energy reflects mobility of the fillers involved in the solvent induced expansion of the surrounding polymer. Therefore, by using the gas sensibility of the conductive composites, the structure evolution of the composite materials in solid state and the effect of filler/matrix interfacial interaction on the relaxation property of the matrix polymer has been inspected. It was found that lower activation energy represents stronger interfacial interaction in case good solvent of the matrix was used for the test.     

Silane-modified MWCNT/PMMA composites – Preparation, electrical resistivity, thermal conductivity and thermal stability

Multiwalled carbon nanotubes (MWCNT) were modified using 3-isocyanato- propyltriethoxysilane (IPTES). Crosslinkable PMMA was prepared from MMA monomer and Vinyltriethoxysilane (VTES) (PMMA–VTES). The IPTES-modified MWCNT (Si-MWCNT) was mixed with the PMMA–VTES copolymer and crosslinked with catalyst to form Si-MWCNT/PMMA–VTES composites. The degree of condensation of tri-distribution structure of the Si-MWCNT/PMMA–VTES composites decreases as the Si-MWCNT content increases. The morphology of the Si-MWCNT/PMMA–VTES composites was analyzed by SEM and TEM. The MWCNTs were well dispersed in the PMMA–VTES matrix. Surface and volume electrical resistivity decreased as the MWCNT content increased. The thermal conductivity of the PMMA–VTES composites increased by 87.5% when 0.99 wt% Si-MWCNT content was added to neat PMMA–VTES. The thermal stability of the PMMA–VTES in nitrogen and air increased significantly even when a small quantity (0.5 wt%) of Si-MWCNT was added.     

Characterisation of proton irradiated Ba0.65Sr0.35TiO3/P(VDF-TrFE) ceramic–polymer composites

Barium strontium titanate/poly (vinylidene fluoride-trifluoroethylene)70/30 (Ba_0.65Sr_0.35TiO₃/P(VDF-TrFE) 70/30) composite with high dielectric permittivity was developed by integrating high dielectric permittivity ceramic powder with proton irradiated polymer matrix. The composite after irradiation behaves as a relaxor ferroelectric material and this behaviour is similar to that of irradiated P(VDF-TrFE) 70/30 co-polymer. Due to the irradiation, dielectric peaks broadened and moved towards the lower temperature, creating high relative permittivity values in a broad temperature range. Ba_0.65Sr_0.35TiO₃/P(VDF-TrFE) composite with 0.5 ceramic volume fraction with a dosage of 80 Mrad can reach a relative permittivity of 160 at room temperature (at 1 kHz), which is about 14 times higher than that of pure copolymer. Polarization-electric field hysteresis loops of composites are strongly depended on the ceramic powder volume fraction and the effects of irradiation is less apparent in composites with higher ceramic powder volume fraction.     

Micro- and nano-scale deformation behavior of nylon 66-based binary and ternary nanocomposites

The primary aim of this paper is to provide an insight on the effect of the location of organoclay on the micro- and nano-scale deformation processes in melt-compounded nylon 66/organoclay/SEBS-g-MA ternary nanocomposites prepared by different blending sequences. In addition, the deformation processes of the ternary nanocomposites were compared to the binary nanocomposites (nylon 66/organoclay and nylon 66/SEBS-g-MA) and neat nylon 66. The incorporation of SEBS-g-MA particles toughened nylon 66 markedly; but the flexural modulus and strength were both reduced. Conversely, the use of organoclay increased the modulus but decreased the fracture toughness of nylon 66. Nylon 66/SEBS-g-MA/organoclay ternary nanocomposites exhibited balanced elastic stiffness and toughness. Stress-whitening studies of the fracture surfaces in terms of gray level were also performed and an attempt was made to correlate the optical reflectivity characteristics with fracture toughness. It was concluded that the capability of SEBS-g-MA particles to cavitate was decreased by the presence of organoclay in the SEBS-g-MA phase, resulting in reduced toughening efficiency. The best micro-structure for toughness and other mechanical properties is thus to maximize the amount of exfoliated organoclay in the nylon 66 matrix rather than to have it embedded in the finely dispersed SEBS-g-MA particles.     

Mechanical hinge system for delamination tests in beam-type composite specimens

During the experimental study of composite delaminations external loads are usually applied by means of steel or aluminium parts bonded to the surface of beam-type specimens. The bonded joints between the metallic parts and the composite specimen might fail, especially when the tests are carried out under extreme temperatures or fatigue conditions. In addition, the point of application of the external load does not coincide with the neutral axis of the specimen beam, inducing non-linear effects that can lead, for example, to incorrect estimations of fracture toughness. In this paper, the relative importance of the non-linear effects in delamination tests is evaluated and the corresponding correction factors discussed. Next, the design of an improved mechanical hinge that avoids non-linear effects, eliminates bonded joints and can be adapted to different specimen thicknesses is introduced.     

Double cantilever beam testing of multidirectional laminates

Several difficulties in the double cantilever beam (DCB) tests of multidirectional laminates often prevent valid measurements of the mode I critical strain energy release rate G_Ic. In this paper, several DCB specimens were analysed with 3D finite element models. The results showed that the undesired effects of residual stresses and of mode-mixity can be minimised. An interlaminar stress based fracture criterion predicts that the G_Ic of multidirectional specimens is typically 10–40% higher than the G_Ic of unidirectional [0°]_n laminates. This agrees with the few valid experimental data available.     

Water absorption and solubility of PHBHV/HA nanocomposites

A thorough understanding of the influence of bioceramic phase on the water absorption and solubility of biomaterial is of importance in tailoring its degradation and the formation of bone-like apatite for clinical implant applications in a wet environment. The objective of this study was to characterize and quantify the water transport properties and solubility of biomaterial incorporating poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBHV) and nano-hydroxyapatite (nano-HA) modified with a silane coupling agent. Solubility and transport parameters such as diffusion, permeability, and sorption coefficients were determined at three different temperatures using the weighing method. When the environmental temperature reached 60 °C, the water uptake of the nanocomposite reaches equilibrium after a normally fast absorption process, and then decreases as the immersion time is prolonged due to the solubility of the material. Moreover, this phenomenon becomes more significant with increasing the volume fraction of nano-HA. Compared to those for the base resin, the diffusion coefficients for the nanocomposite decrease, whereas the sorption coefficients and the solubility show an opposite tendency. All of the transportation parameters are temperature sensitive and obey the Arrhenius or the van’t Hoff relationship. Results from thermodynamic analysis imply that when using a high filler loading level (20 vol%) the sorption of the nanocomposite is mainly dominated by a Langmuir sorption mode giving an exothermic process.     

Temperature dependence of fracture toughness of silica/epoxy composites: Related to microstructure of nano- and micro-particles packing

Temperature dependence of the fracture toughness of epoxy composites reinforced with nano- and micro-silica particles was evaluated. Epoxy composites containing varied composition ratios Φ_SP of spherical nano- and micro-silica particles, 240 nm and 1.56 μm, were prepared at a fixed volume fraction (V_P = 0.30). The thermo-viscoelasticity and fracture toughness of the composites and neat epoxy were measured at 143 K, 185 K, 228 K, 296 K, 363 K, and 399 K. Experimental results revealed that fracture toughness strongly depended on the microstructure of nano- and micro-particles bidispersion as well as its interactions with the matrix at all temperature, but depended on toughened matrix due to increase in mobility of matrix at the relaxation temperatures.     

Poly(vinyl alcohol) reinforced with large-diameter carbon nanotubes via spray winding

For practical application of carbon nanotube (CNT)/polymer composites, it is critical to produce the composites at high speed and large scale. In this study, multi-walled carbon nanotubes (MWNTs) with large diameter (∼45 nm) and polyvinyl alcohol (PVA) were used to increase the processing speed of a recently developed spraying winding technique. The effect of the different winding speed and sprayed solution concentration to the performance of the composite films were investigated. The CNT/PVA composites exhibit tensile strength of up to 1 GPa, and modulus of up to 70 GPa, with a CNT weight fraction of 53%. In addition, an electrical conductivity of 747 S/cm was obtained for the CNT/PVA composites. The good mechanical and electrical properties are attributed to the uniform CNTs and PVA matrix integration and the high degree of tube alignment.     

Ternary nano-CaCO3/poly(ethylene terephthalate) fiber/polypropylene composites: Increased impact strength and reinforcing mechanism

The binary nano-CaCO₃/polypropylene (PP), poly(ethylene terephthalate) (PET) fibers/PP and ternary nano-CaCO₃/PET fibers/polypropylene composites were prepared by melt blending method, and their structure and mechanical properties were investigated. The results show that the ternary nano-CaCO₃/PET fibers/PP composite displays significantly enhanced mechanical properties compared with the binary PET fibers/PP and nano-CaCO₃/PP composites, and neat PP. The X-ray diffraction, dynamic mechanical analysis, scanning electron microscopy and analysis of the non-isothermal crystallization kinetics were used to investigate the reinforcement mechanism of composites. The results indicate that the interfacial action and compatibility between PET fiber and PP are obviously enhanced by the addition of modified nano-CaCO₃ particles in the ternary composites and the mechanical property enhancement in the ternary system may be mainly originated from the formation of β-form crystallites of PP induced by the synergistic effect between PET fibers and nano-CaCO_3.