Influence of SiC Electron Acceptor–Donor Modification on Thermal and Physical Properties of Carbon Fiber/SiC/Epoxy Composites

In this work, the effects of electron acceptor–donor modification on the surface properties of SiC were investigated in the mechanical interfacial properties of carbon fibers-reinforced SiC-impregnated epoxy matrix composites. The surface properties of the SiC were determined according to acid/base values and FT-IR, and contact angle measurements. The thermal and mechanical interfacial properties of the composites were evaluated using a thermogravimetric analysis, critical strain energy release rate mode II (G _IIC), and impact strength testing. As a result, the electron acceptor-treated SiC had a higher acid value and polar component in surface free energy than did the untreated SiC or the electron donor-treated SiC. The G _IIC and impact strength mechanical interfacial properties of the composites had been improved in the specimens treated by acidic solutions due to the good wetting and a high degree of adhesion with electron donor characteristic epoxy resins.     

Initial damage induced by thermal residual stress and microscopic failure analysis of carbon-fiber reinforced composite under shear loading

In order to study the mechanical properties and the progressive failure process of composite under shear loading, a representative volume element (RVE) of fiber random distribution was established, with two dominant damage mechanisms – matrix plastic deformation and interfacial debonding – included in the simulation by the extended Drucker–Prager model and cohesive zone model, respectively. Also, a temperature-dependent RVE has been set up to analyze the influence of thermal residual stress. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the in-plane shear fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation. The progressive damage process and final failure mode of in-plane shear model which are based on constitute are very consistent with the observed result under scanning electron microscopy of V-notched rail shear test. Also, a transverse shear model was established as contrast in order to comprehensively understand the mechanical properties of composite materials under shear loading, and the progressive damage process and final failure mode of composite under transverse shear loading were researched. Thermal residual stress changes the damage initiation locations and damage evolution path and causes significant decreases in the strength and fracture strain.     

Influence of heat treatment on microstructure and mechanical properties of the interface in an EN AW-6082/1.4310 composite extrusion

The presented paper deals with a unidirectional steel wire reinforced aluminum matrix composite manufactured by composite extrusion. The main objective of this work was to determine the effect of heat treatment, and the influence of long solution annealing times on the composites interface regarding microstructural changes and the resulting interface strength. For evaluation of the microstructure high resolution transmission electron microscope (TEM) investigations accompanied with electron dispersive X-ray spectroscopy (EDX) were performed. It could be shown that diffusion from the steel wire into the aluminum matrix occurs and that the diffusion paths as well as particle formation is influenced by the preceded heat treatment. Diffusion paths in the range of 40–150 nm could be observed for Al, Fe, Cr and Ni. After annealing times over 5 h an extensive growth of an intermetallic reaction layer was found. The mechanical properties of the interface were determined by push-out-tests and tensile tests radial to the interface, which provided the debonding shear strength σ_deb and for the latter experiment the interfacial radial strength σ_IR. It has become apparent that debonding shear strength is highly influenced by matrix properties. In radial tensile tests the failure is predominantly controlled by the chemical bond of the interface. It was shown that interface strength of specimen with small reaction zones of about 3 μm were beneficial for the mechanical behavior in both loading conditions. Longer annealing times showed a drastic decrease of interface shear strength. It was concluded from EDX measurements and in comparison with literature that the reaction zone is dominated by the growth of Al₅Fe₂ (η-phase).     

Assessment of interface deformation and fracture in metal matrix composites under transverse loading conditions

The transverse properties of unidirectional metal matrix composites (MMCs) are dominated by the fiber/matrix interfacial properties, residual stresses and matrix mechanical response. In order to monitor and study, in situ, the failure of interfaces in titanium-based composites subjected to transverse loading conditions, an ultrasonic imaging technique has been developed. The interface was imaged ultrasonically and the change in ultrasonic amplitude with the transverse loading was monitored, indicating the sensitivity of the technique to fracture and deformation of interfaces. This change in amplitude has been explained in terms of the multiple reflection theory of ultrasonic waves. The multiple reflection theory enabled estimation of the interfacial deformation and debonding as a function of loading. The ultrasonic technique was also used in conjunction with finite element modeling in order to quantify the fiber/matrix interfacial transverse strength in situ in MMCs.     

Evaluation on the interfacial fracture toughness of fiber-reinforced titanium matrix composites by push out test

The models for single-fiber push out test are developed to evaluate the fracture toughness G_IIc of the fiber/matrix interface in titanium alloys reinforced by SiC monofilaments. The models are based on fracture mechanics, taking into consideration of the free-end surface and Poisson expansion. Theoretical solutions to G_IIc are obtained, and the effects of several key factors such as the initial crack length, crack length, friction coefficient, and interfacial frictional shear stress are discussed. The predictions by the models are compared with the previous finite element analysis results for the interfacial toughness of the composites including Sigma1240/Ti-6-4, SCS/Ti-6-4, SCS/Timetal 834, and SCS/Timetal 21s. The results show that the models can reliably predict the interfacial toughness of the titanium matrix composites, in which interfacial debonding usually occurs at the bottom of the samples.     

Interphase sizing temperature effect of LaRC PETI-5 on the dynamic mechanical thermal properties of carbon fiber/BMI composites

The dynamic mechanical thermal properties of carbon fiber-reinforced bismaleimide (BMI) composites processed using polyacrylonitrile(PAN)-based carbon fibers unsized and sized with LaRC PETI-5 amic acid oligomer as interphase material at 150°C, 250°C, and 350°C were investigated by means of dynamic mechanical thermal analysis. It was found that the storage modulus, loss modulus, tan δ and the peak temperature significantly depend on the sizing temperature as well as on the presence and absence of LaRC PETI-5 sizing interphase. The result showed that the carbon fiber/BMI composite sized at 150°C had the highest storage modulus at a measuring temperature of 250°C. The storage modulus decreased with increasing sizing temperature from 150°C to 350°C, being influenced by interdiffusion and co-reaction between the LaRC PETI-5 interphase and the BMI matrix resin. The present result is quite consistent with the interfacial result reported earlier in term of interfacial shear strength and interlaminar shear strength of carbon fiber/BMI composites. It is addressed that in the present composite system the sizing temperature of LaRC PETI-5 interphase critically influences not only the interfacial properties but also the dynamic mechanical thermal properties and its control is also important.     

Long-Term Hydrothermal Aging Behavior and Aging Mechanism of Glass Fibre Reinforced Polyamide 6 Composites

Long-term hydrothermal aging of polyamide 6 (PA6)/glass fibre (GF) composites was conducted and the effects of the GF on variations of structure and properties of the composites with aging time were investigated. It was found that the first stage of aging was a Fickian process and corresponded to the physical absorption of water until equilibrium, resulting in a slight change of reduced viscosity and chemical structure of the PA6. The water diffusing process was slowed down slightly by addition of the GF. The second stage of aging was the initiation process of hydrolytic degradation of PA6, resulting in a rapid decrease of reduced viscosity and an increase of end group content. In the final stage of aging, the relative weight gain (W_r) dropped, the reduced viscosity decreased and the end groups increased slowly. The degradation rate and carbonyl index of PA6 increased with increasing GF content, and the increasing rate of end groups concentration of the composites was higher than that of pure PA6 during the aging process, indicating addition of GF accelerated the hydrolysis degradation and oxidative aging of PA6. In mechanical property tests, compared with unaged samples of the composites which underwent matrix rupture around the matrix-fiber interfacial layer, for aged samples several smooth fibres without coatings were pulled out and the interfacial debonding was the main failure mode, causing severe deterioration in mechanical properties. The hydrolytic degradation activation energy (E_a) was calculated through a method based on the Arrhenius model by considering both temperature and humidity as environment factors; with increasing GF content, E_a decreased, indicating that the addition of GF made PA6 easier to degrade.     

Measurement of fiber/matrix interface properties of C/C composites by single fiber and fiber bundle push-out methods

Interfacial fracture stresses of carbon/carbon composites were measured by indentation methods. Two types of test methods, namely, single fiber push-out, and bundle fiber push-out tests were conducted. Both methods successfully gave fiber/matrix interface mechanical properties, especially debonding behavior. However, when the interface was strong, the single fiber push-out test encountered technical difficulty in processing the extremely thin specimen required to realize the fiber push out. On the other hand, the bundle fiber push-out test gave a good estimation of interfacial fracture stresses.     

Viscoelasticity and morphology of an organic hybrid of chlorinated polyethylene and N,N′-dicyclohexyl-2-benzothiazolyl sulfenamide

The viscoelasticity and morphology of an organic hybrid of chlorinated polyethylene (CPE) and N,N′-dicyclohexyl-2-benzothiazolyl sulfenamide (DBS) were studied by means of tensile and shear complex modulus and differential scanning calorimetry (DSC) analysis. Tensile and shear loss modulus (E″ and G″), which are shown as indexes of vibration damping performance, showed one peak corresponding to the glass transition. The peak maximum values (E″_max and G″_max)increased in proportion to DBS content ( _DBS) and the slope of E″_max against _DBS became steep above a certain DBS content, i.e. the critical DBS content ( _c). A high damping material was obtained by the addition of DBS, especially when DBS content was higher than _c. These increases in loss moduli below and above _c are caused by the interaction between CPE and DBS molecules and the friction between DBS molecules, respectively. It was found that CPE/DBS is a compatible blend at all DBS contents from the analysis of the glass transition temperature with DSC. Furthermore, the influence of chlorine content in CPE on those characteristics was investigated. Higher chlorine content led to lower _c, a decrease in E″ below _c and an increase in E″ above _c. These results are due to the increase in the number of dichloromethylene units (CCl₂), which reduces the α-hydrogen atom in CPE.     

Effect of matrix yield properties on fragmentation behavior of single fiber composites

Experimental and theoretical investigations have been conducted to study the dependence of fiber fragmentation behavior on matrix yielding properties. The cured Epikote 828 resins with two types of curing agents have almost similar elastic moduli, but different tensile yield strengths. The interfacial chemistry between fiber and epoxy resin is unchanged due to the same constituent of the epoxy resin. The experimental results indicate that the fragmentation behavior of the fibers embedded in the matrix is significantly different for the tested glass fiber treated by γ-glycidoxypropyltrimethoxysilane. The average fragment length decreased with increasing tensile yield strength of resin, which suggests that the interfacial shear strength determined in the fragmentation test should be different depending on the tensile yield strength of resin used. The important phenomenon observed is the transition of the micro-damage mode from matrix crack to interfacial debonding. An elastoplastic shear-lag model was used to calculate the shear stress and fiber tensile stress distributions considering different plastic behaviors of the matrices. The theoretical results indicate that the plastic behavior of the matrix has a large influence on stress transfer. Based on elastic and plastic properties of the matrix, the fiber fragmentation behavior in the matrix is predicted. Experimental and theoretical results are favorably compared.     

Study of elastic properties of CeO2 by statistical moment method

The purpose of the present paper is to investigate the temperature and pressure dependences of the elastic properties of cerium dioxide using the statistical moment method (SMM). The equation of states of bulk CeO₂ is derived from the Helmholtz free energy, and the pressure dependences of the elastic moduli like the bulk modulus, B_T, shear modulus, G, Young’s modulus, E, and elastic constants (C11, C12, and C44) are presented taking into account the anharmonicity effects of the thermal lattice vibrations. In the present study, the influence of temperature and pressure on the elastic moduli and elastic constants of CeO₂ has also been studied, using three different interatomic potentials. We compare the results of the present calculations with those of the previous theoretical calculations as well as with the available experiments.     

Pressure-induced structural phase transition and elastic properties of rare earth Pr chalcogenides and pnictides

Pressure-induced structural aspects and elastic properties of NaCl-type (B1) to CsCl-type (B2) structure in praseodymium chalcogenides and pnictides are presented. Ground-state properties are numerically computed by considering long-range Coulomb interactions, Hafemeister and Flygare type short-range overlap repulsion, and van der Waals interaction in the interionic potential. From the elastic constants, Poisson's ratio ν, the ratio R_G/B of G (shear modulus) over B (bulk modulus), anisotropy parameter, shear and Young's moduli, Lamé's constant, Kleinman parameter, elastic wave velocity and thermodynamical property such as Debye temperature are calculated. Poisson's ratio ν and the ratio R_G/B indicate that PrX and PrY are brittle in B1 phase and ductile in B2 phase. To our knowledge, this is the first quantitative theoretical prediction of the ductile (brittle) nature of praseodymium chalcogenides and pnictides and still awaits experimental confirmation.     

First-principles study of elastic and electronic properties of layered ternary nitride SrZrN2 under pressure

The structural, elastic, and electronic properties of SrZrN₂ under pressure up to 100?GPa have been carried out with first-principles calculations based on density functional theory. The calculated lattice parameters at 0?GPa and 0?K by using the GGA-PW91-ultrasoft method are in good agreement with the available experimental data and other previous theoretical calculations. The pressure dependence of the elastic constants and the elastic-dependent properties of SrZrN₂, such as bulk modulus B, shear modulus G, Young's modulus E, Debye temperature Θ, shear and longitudinal wave velocity V_S and V_L, are also successfully obtained. It is found that all elastic constants increase monotonically with pressure. When the pressure increases up to 140?GPa, the obtained elastic constants do not satisfy the mechanical stability criteria and a phase transition might has occurred. Moreover, the anisotropy of the directional-dependent Young's modulus and the linear compressibility under different pressures are analysed for the first time. Finally, the pressure dependence of the total and partial densities of states and the bonding property of SrZrN₂ are also investigated.     

Effects of residual stress and interfacial frictional shear stress on interfacial debonding at notch-tip in two-dimensional unidirectional composite

A calculation method based on the shear lag approach was presented to get an approximate estimate of influences of residual stresses and frictional shear stress at the debonded interface on the interfacial debonding behavior at the notch-tip along fiber direction in two-dimensional unidirectional double-edge-notched composites. With this method, the energy release rate for initiation and growth of debonding as a function of composite stress were calculated for some examples. The calculation results showed in outline how much the tensile and compressive residual stresses in the matrix and fiber along fiber direction, respectively, act to hasten the initiation and growth of the debonding when the final cut element in the notch is matrix, while they act to retard them when the final cut element is fiber, and how much the frictional shear stress at the debonded interface reduces the growth rate of the debonding.     

Structural properties,phase stability,elastic properties and electronic structures of Cu–Ti intermetallics

The structural properties, phase stabilities, anisotropic elastic properties and electronic structures of Cu–Ti intermetallics have been systematically investigated using first principles based on the density functional theory. The calculated equilibrium structural parameters agree well with available experimental data. The ground-state convex hull of formation enthalpies as a function of Cu content is slightly symmetrical at CuTi with a minimal formation enthalpy (–13.861 kJ/mol of atoms), which indicates that CuTi is the most stable phase. The mechanical properties, including elastic constants, polycrystalline moduli and anisotropic indexes, were evaluated. G/B is more pertinent to hardness than to the shear modulus G due to the high power indexes of 1.137 for G/B. The mechanical anisotropy was also characterized by describing the three-dimensional (3D) surface constructions. The order of elastic anisotropy is Cu₄Ti₃ > Cu₃Ti₂ > α-Cu₄Ti > Cu₂Ti > CuTi > β-Cu₄Ti > CuTi₂. Finally, the electronic structures were discussed and Cu₂Ti is a semiconductor.     

Phase stability,anisotropic elastic properties and electronic structures of C15-type Laves phases ZrM2 (M = Cr,Mo and W) from first-principles calculations

Abstract

Systematic investigations of phase stability and mechanical properties of C15-type ZrM₂ (M = Cr, Mo and W) Laves phases were performed using first-principles calculations. The formation enthalpies of ZrM₂ are in good agreement with the theoretical and experimental values. The elastic properties, including elastic constants and moduli, Poisson’s ratio and B/G, were discussed. The elastic anisotropy was also investigated via the anisotropy indexes (A^U, A^Z, A_shear and A_comp), the anisotropy of shear modulus and the 3D construction of bulk and Young’s moduli. The elastic anisotropy of ZrM₂ is in order of ZrCr₂ < ZrMo₂ < ZrW₂. The variations in the shear modulus and hardness show similar trends with increasing values from ZrCr₂ to ZrW₂. The electronic structures for these C15-type Laves phases were analysed to obtain deeper understanding of chemical bonds and phase stabilities. Finally, the sound velocities and Debye temperatures were also investigated.     

Growth of interfacial debonding in notched two-dimensional unidirectional composite under stress and displacement controls

A simplified calculation method for study of the growth of interfacial debonding between elastic fiber and elastic matrix ahead of the notch-tip in composites under displacement and stress controlled conditions was presented based on the shear lag approach in which the influences of residual stress and frictional shear stress at the debonded interface were incorporated. The calculation method was applied to a model two-dimensional composite. An outline is given of the difference and similarity in the growing behavior of the debonding between the displacement and stress controls, and of the influences of the residual stresses, frictional shear stress, the nature of the final cut component (fiber or matrix) and sample length on the debonding behavior.     

Elastic moduli of η‐Ta2N3, a tough self‐healing material,via laser ultrasonics

The elastic moduli of the dense polycrystalline oxygen‐bearing η‐Ta₂N₃, a novel hard and tough high‐pressure (HP) material, were measured using the laser ultrasonic technique. The bulk modulus was determined to be B₀ = 281(15) GPa which is only ～11% below that from HP compression measurements. Our value of the shear modulus G₀ = 123(2) GPa is below those ones predicted theoretically for model structures. The discrepancies in G₀ could be due to a substitution of an‐ ions and the formation of cation vacancies in η‐Ta₂N₃. Self‐healing behaviour of η‐Ta₂N₃ by mechanical polishing was observed and confirmed by two independent experimental methods. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Anisotropic elastic properties of MB (M = Cr,Mo, W) monoborides: a first-principles investigation

First principles calculations were performed to systematically investigate structure properties, phase stability and mechanical properties of MB (M = Cr, Mo, W) monoborides in orthorhombic and tetragonal structures. The results of equilibrium structures are in good agreement with other available theoretical and experimental data. The elastic properties, including bulk modulus B, shear modulus G, Young’s modulus E and Poisson’s ratio ν were calculated by the Voigt-Reuss-Hill approximation. All considered monoborides are mechanically stable. The results of elastic anisotropies show that elastic anisotropy of orthorhombic structure is larger than that of tetragonal structure. Moreover, the minimum thermal conductivities were also estimated using the Cahill’s model, and the results indicate that the minimum thermal conductivities show a dependence on directions.     

Enhanced interface interaction between modified carbon nanotubes and magnesium matrix

Carbon nanotube (CNT)/metal interface interaction is critical to the mechanical properties of CNT-reinforced metal matrix composites (MMCs). In this paper, in order to realize the chemical modification of the interface interaction between CNTs and Mg matrix, different types of defects (monovacancy, carbon and oxygen adatoms, as well as p-type boron and n-type nitrogen substitution) are introduced in CNTs to investigate the effect of the defects on the interface interaction (E_ib) between CNT and Mg (0 0 0 1) surface. Moreover, two models (adsorption model and interface model) are compared and validated to investigate the interface interaction. It is revealed that the CNT with the carbon adatom has the highest E_ib with the Mg (0 0 0 1), and the effect of boron doping on E_ib is superior to the intermediate oxygen which has already been proved experimentally in the enhancement of the interface interaction in MMCs. In terms of the electronic structure analysis, we reveal the micro-mechanism of the increase of E_ib under the action of different types of defects, and propose that the presence of holes (boron dopant) and the unsaturated electrons in CNTs can generate the chemical interaction between CNT and Mg matrix effectively. Our results are of great scientific importance to the realization of robust interfacial bonding between CNTs and Mg matrix via the reinforcement modification, so as to enhance the mechanical properties of CNTs reinforced Mg matrix composites.