Role of fibre–fibre and fibre–matrix adhesion in stress transfer in composites made from resin-impregnated paper sheets

Paper-reinforced plastics are gaining increased interest as packaging materials, where mechanical properties are of great importance. Strength and stress transfer in paper sheets are controlled by fibre–fibre bonds. In paper-reinforced plastics, where the sheet is impregnated with a polymer resin, other stress-transfer mechanisms may be more important. The influence of fibre–fibre bonds on the strength of paper-reinforced plastics was therefore investigated. Paper sheets with different degrees of fibre–fibre bonding were manufactured and used as reinforcement in a polymeric matrix. Image analysis tools were used to verify that the difference in the degree of fibre–fibre bonding had been preserved in the composite materials. Strength and stiffness of the composites were experimentally determined and showed no correlation to the degree of fibre–fibre bonding, in contrast to the behaviour of unimpregnated paper sheets. The degree of fibre–fibre bonding is therefore believed to have little importance in this type of material, where stress is mainly transferred through the fibre–matrix interface.     

Microstructural analysis of network-like crack structure formed at Al–B4C interface

The morphology and structure of microcracks extending toward B₄C were investigated from a joint obtained by sandwiching Al foil between two B₄C ceramic plates and heat-treating at 1000 °C in vacuum. Network-like structures of cracks were seen, and Al penetrated the extremely narrow part at the tip. The molten Al penetrated a narrow area within the cracks and filled them. Several compounds were produced in the primary junction area because of reactions between Al and B₄C. However, only pure Al was present within the cracks. Atoms moved easily in a wide joint interface, and a reaction occurred accordingly, whereas in a narrow region inside the micro crack, atoms moved with difficulty even if molten Al penetrated the crack. The fact that the reaction is unlikely to occur inside the crack is consistent with the permeation of Al into the inside of the elongated crack.     

Acid–base interactions and covalent bonding at a fiber–matrix interface: contribution to the work of adhesion and measured adhesion strength

The work of adhesion, W _A, and the practical adhesion in terms of the interfacial shear strength, τ, in some polymer-fiber systems were determined to establish a correlation between these quantities. An attempt was made to analyze the contributions of various interfacial interactions (van der Waals forces, acid-base interaction, covalent bonding) to the 'fundamental' and 'practical' adhesion. The surface free energies of the fibers were altered using different coupling agents. To characterize the strength of an adhesion contact, the ultimate adhesion strength, τ_ult, was determined for the onset of contact failure. The adhesion of non-polar polymers occurs through van der Waals interaction only; therefore, fiber sizing does not affect the adhesion strength. For polar polymers, such as poly(acrylonitrile butadiene styrene) and polystyrene, adhesion is sensitive to fiber treatments: suppression of the acid-base interaction by using an electron-donor sizing agent γ-aminopropyltriethoxysilane results in a decrease of both 'fundamental' and 'practical' adhesion. In the case of epoxy resins, the main contribution to the work of adhesion is made by covalent bonds. Since the process of their formation is irreversible, the work of adhesion determined from micromechanical tests seems to be more reliable than indirect estimations, such as from wetting and inverse gas chromatography techniques. Fiber treatment by sizing agents results in considerable changes in the intensity of adhesional interaction with the epoxy matrix. A correlation between the work of adhesion, the ultimate interfacial shear strength, and the strength of macro-composites has been found.     

Taguchi analysis of bonded composite single-lap joints using a combined interface–adhesive damage model

The mechanical behaviour of bonded composite joints depends on several factors, such as the strength of the composite–adhesive interface, the strength of the adhesive and the strength of the composite itself. In this regard, a finite element model was developed using a combined interface–adhesive damage approach. A cohesive zone model is used to represent the composite–adhesive interface and a continuum damage model for the adhesive bondline. The influence of the composite–adhesive interfacial adhesion and the strength of the adhesive on the performance of a bonded composite single-lap joint was investigated numerically. A Taguchi analysis was conducted to rank the influence of material parameters on the static behaviour of the joint. It was found that the composite–adhesive interfacial fracture energy and the mechanical properties of the adhesive predominantly govern the static performance of the joints. A parametric study was performed by varying the most important material parameters, and a response surface equation is proposed to predict the joint strength. It is shown that the influence of experimental parameter variations, e.g. variation in adhesive curing and surface preparation conditions, can be numerically accommodated to investigate the static behaviour of bonded composite joints by combining finite element and statistical techniques. The methods presented could be used by practicing engineers to describe the failure envelope of adhesively bonded composite joints.     

Electrothermal parameters and useful life of carbon fibre fabricated from viscose fibre by heat treatment at 2200°C under electric load

The electrothermal behavior of carbon fibre fabricated from viscose fibre by heat treatment at 2200°C was investigated. The analytical expressions correlating the linear density with the electrical resistance, heat capacity, thermal conductivity, temperature of the surface of the fibre, predicted useful life, and electric load were obtained. A method was developed for conducting and mathematically interpreting the experiments on determination of the lifetime before combustion of conducting carbon fibre of different linear density as a function of the strength of the electrical current passed through it. The lifetime of the carbon fibre in air with no electric load was equal to 6.3·10¹⁰ sec and decreased exponentially with an increase in the current strength. The specific resistance is approximately 5.2·10⁻⁵ Ω·m at 20°C, the specific heat capacity varied from 0.64 to 0.93 J/(g·K), and thermal conductivity of 83 to 120 W/(m·K) in the 0–100°C temperature range. UVIKOM, Mytishchi. Translated fromKhimicheskie Volokna, No. 1, pp. 55–58, January–February, 2000.     

Photo-conductivity and Hall mobility of holes at polypyrrole–diamond interface

Surface of intrinsic monocrystalline diamoncd is selectively terminated by hydrogen and oxygen atoms to create electrically conductive microscopic square with contact leads in corners. Polypyrrole (PPy) film is then electrochemically deposited onto the H-terminated square. The resulting PPy-diamond system is characterized under a broad-band light illumination by four probe resistivity and Hall mobility measurements and the in-plane transport properties of holes at the PPy-diamond interface are evaluated. We also discuss applicability of these techniques on this specific heterosystem.     

Deformation and damage processes during tensile creep of ceramic-fibre-reinforced ceramic–matrix composites

The tensile creep and creep fracture properties in air at 1300°C are compared for SiC_f/SiC and SiC_f/Al₂O₃ composites, each reinforced with 0.38 volume fractions of interwoven silicon carbide (Nicalon™) fibre bundles aligned parallel and normal to the stress direction. The differing behaviour patterns displayed by these 0/90° woven composites are analysed to identify the processes controlling creep strain accumulation and crack development.     

The role of an SiC interlayer at a graphite–silicon liquid interface in the solution growth of SiC crystals

SiC crystal growth using the top seeded solution growth (TSSG) method involves the precipitation of solid SiC from carbon that is dissolved in a silicon melt. The growth rate of SiC is strongly influenced by the solubility of C in liquid Si, which is quite low. In this study, the dissolution of C from graphite to the Si melt was explored by observing the formation of an SiC interlayer at a graphite – Si liquid interface. The SiC interlayer was observed to become thickened during the several hours needed to reach a certain thickness at 1500 °C. Assuming that the SiC interlayer is a direct C source, a pre-formed SiC layer was coated on the graphite crucible to evaluate its effect on the concentration of C in the Si melt. As a result, the concentration of C in the Si melt increased within a short time, especially at low temperatures. By applying the SiC coated crucible to the TSSG process for SiC crystal growth, we confirmed that the development of a pre-formed SiC layer enhanced the growth rate of SiC crystals, especially at the initial stage of crystal growth at low temperatures.     

Aggregation behavior of a polystyrene–b-poly(phenylsilsesquioxane) H-type copolymer at the air/water interface

A comprehensive Langmuir film balance/atomic force microscope study of the surface aggregation behavior of an H-type block copolymer having four branches of polystyrene (PS) blocks (number average molar mass (M_n) = 5000 Da per each branch) and poly(phenylsilsesquioxane) (PPSQ) (M_n = 40,000 Da) has been performed. Temperature dependent surface pressure isotherms and hysteresis experiments were combined with molecules aggregation number per surface micelle, leading to a self-consistent picture of the surface morphology and aggregation phenomena. In spite of the ladder structure of the PPSQ segment, the micelles mostly exist as spherical aggregates because of strong interaction between PS blocks.     

Kinetic aspects of concentration of solid nanoparticles at the interface in polystyrene–polyethylene mixtures

The effect of several mixing process parameters on the concentration of carbon black particles at the interface between the polymer components in polyethylene–polystyrene mixtures was studied. It was established that the process of particle transport from the polymer phase to the interface happens only under the action of shear deformation, but simultaneously acting shear stresses prevent particle concentration. The nanoparticle accumulation at the interface is determined not only by how favored this process is thermodynamically, but also by the parameters of the mixing process. The filler concentration at the interface is facilitated by a reduction in the shear stress, polymer viscosity, filler particle size, and preliminary injection of filler into the polymer component that is less effective at wetting the filler surface. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48585.     

Synergistic strengthening by load transfer mechanism and grain refinement of CNT/Al–Cu composites

CNT/Al–Cu composites were fabricated by mixing of Al powders and CNT/Cu composite powders which were prepared by molecular level mixing process. The CNT/Al–Cu–Cu composites show a microstructure with a homogeneous dispersion of CNTs in the Al–Cu matrix and had a 3.8 times increase of yield strength and 30% increase of elastic modulus compared to Al–Cu matrix. The strengthening mechanism of CNT/Al–Cu composites was discussed by controlling the aspect ratio of CNTs and it was thought that the CNT/Al–Cu composites were strengthened by both load transfer from the Al matrix to the CNTs and dispersion strengthening of damaged short CNTs. At the same time, the addition of CNTs increases the grain refinement effect of the Al–Cu matrix which results in a grain size strengthening mechanism of the CNT/Al–Cu composites.     

Fiber-stretching test: a new technique for characterizing the fiber–matrix interface using direct observation of crack initiation and propagation

A new micromechanical technique for experimental determination of fiber-matrix interfacial properties is presented. This technique consists in tensile loading of the fiber, with a matrix droplet on it, at both ends, accompanied by continuous direct observation of interfacial crack propagation. In comparison with the well-known microbond test, the new method has two important advantages. First, crack propagation is stable for any embedded fiber length and any relation between adhesion and friction at the interface. Second, compliance of the test equipment does not affect the results, and specimens with long free fiber ends can be successfully tested. A similar result can be reached using the pull-out or microbond test with an 'infinite' (very long) embedded fiber length. An algorithm for separate determination of the interfacial adhesion and friction from experimental relationships between the crack length and applied load is described. The new test was employed to determine the interfacial parameters for composites of glass fibers with polypropylene, polystyrene, and polycarbonate. For each fiber-polymer system investigated, the following parameters were calculated: ultimate interfacial shear strength; critical energy release rate for crack propagation; and adhesional pressure. Our approach to the estimation of the work of adhesion, WA, from micromechanical tests, based on the concept of adhesional pressure, allowed us to calculate the WA values for several thermoplastic matrix-glass fiber pairs and to obtain values consistent with previous estimations made according to other approaches.     

Study on the thermal effects of rubbers during loading–unloading cycles by infrared thermography

In this study, infrared thermography is used to detect the thermal effects of rubber-like materials during loading–unloading cycles. Two salient features of temperature change have been observed: (1) the temperature has a tiny decrease at first and then becomes to increase during the stretching process, which is in agreement with the thermo-elastic inversion effect. (2) Temperature variation is partly reversible in the first cycle and totally reversible in the following cycles. These phenomena are related to complicated deformation mechanisms. Based on the analysis of elastic thermal effects during deformation process, thermodynamics of rubber elasticity has also been investigated by infrared thermography.     

Metal–support interface of a novel Ni–CeO2 catalyst for dry reforming of methane

Simultaneous treatment of both the loaded metal and the ceria support under plasma led to the generation of clean metal–support interface. This novel synthesis route prevents the tendency of Ni atoms migrating into the bulk of the supports, thereby avoiding a diffused interfacial region. The plasma treated sample showed excellent stability and higher catalytic activity than the thermally calcined sample in dry reforming of methane. The high activity of the plasma treated sample is attributed to the clean metal–support interface generated by exposing both the support and the loaded metal to microwave plasma treatment.     

Protein–polymer interaction: Transfer loading at interfacial region of PES-based membrane and BSA

The adsorption of proteins onto polymeric surfaces is encountered in many natural and industrial processes and is a prerequisite to their efficient identification, separation, and purification by methods such as chromatography, and filtration. Nevertheless, the exact nature of the adsorption mechanisms and interfacial interactions is not easy to identify for a given protein–polymer system. Here, we aim to document the adsorption mechanism of a protein–polymer system by investigating the adsorption as well as desorption phenomenon of a protein [bovine serum albumin (BSA)] from the polymeric surface [polyethersulfone (PES)]. The analyses performed to document the adsorption mechanism of the BSA–PES system include scanning electron microscope (SEM), attenuated total reflection-Fourier transform infrared (FTIR), contact angle, zeta potential, surface charge density measurement, and Derjaguin–Landau–Verwey–Overbeek (DLVO). Here, SEM and FTIR identified the physical and chemical properties of pure PES and PES–BSA membranes. The low water contact angle of the PES–BSA membrane confirms its applicability for tissue engineering applications. Further, the zeta potential, surface charge density measurement, and DLVO analyses were performed to document the adsorption mechanism. The adsorption of BSA particles on the PES surface was carried out for pH values that ranged from 4 to 10 for contact times that ranged from 1 to 3 days. A monotonic increase in the zeta potential of the PES–BSA system indicated considerable adsorption of BSA particles on the PES surface. Further, BSA adsorption was very strong for pH values greater than 4.7 which confirms to strong electrostatic interactions between BSA and PES. The strong electrostatic interaction is also collaborated by low desorption rate, which was only ∼22% for pH 10 after 3 days of contact. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47931.     

Mechanical and thermal properties of hybrid carbon fibre–phenolic matrix composites containing graphene nanoplatelets and graphite powder

Carbon fibre–phenolic matrix (CF–P) composites containing graphene nanoplatelets (GNPs) were manufactured for improved mechanical and thermal properties. For comparison, micrometer-size pyrolytic graphite powder (GP) was also incorporated in CF–P composites. The loading of carbon fibres was kept constant at 60?wt-% while the quantity of GNPs was varied from 0.1?wt-% to 0.3?wt-% and GP from 1.0?wt-% to 3.0?wt-%. Only GNPs were functionalised by ultraviolet-ozone treatment to improve their dispersion in the matrix while all the composites were manufactured by hand layup method and characterised by scanning electron microscopy, impact, flexural, thermogravimetry and ablation tests. The composite containing 0.3?wt-% GNPs showed considerable improvement in ablation, flexural and impact testing as compared to CF-P composites containing GP. Finally, the ablation mechanisms of post-ablated composites were discussed in the light of available data in the literature.     

Oxidation mechanisms of SiC-fiber–reinforced Si eutectic alloy matrix composites at elevated temperatures

SiC-fiber–reinforced binary Si eutectic alloy composites have been developed for aerospace applications using the melt infiltration method. In this study, the oxidation mechanisms of various binary Si eutectic alloys were evaluated at elevated temperatures. We suggest that the oxidation resistance of eutectic alloys could be predicted using the Gibbs energy change for the oxidation reaction. Based on these calculations, eutectic alloys of Si-16at%Ti, Si-17at%Cr, Si-22at%Co, Si-38at%Co, and Si-27at%Fe were prepared. These alloys produced uniform SiO₂ layers and showed the same oxidation resistance as Si at 1000°C under humid conditions. Therefore, SiC composites using Si alloys with excellent oxidation resistance can be predicted using thermodynamic calculations.     

Superposition of microconvective and macroconvective mass transfer at gas-evolving electrodes—a theoretical attempt

For mass transfer at gas-evolving electrodes with superimposed electrolyte flow through the interelectrode gap a mathematical model is developed and discussed. The resulting equation for the combined mass transfer coefficient takes account of the individual mass transfer coefficients. The new equation can be satisfactorily approximated by an older empirical design equation.