Fabrication and properties of SiNO continuous fiber reinforced BN wave-transparent composites

SiNO continuous fiber reinforced boron nitride (BN) wave-transparent composites (SiNO _f /BN) have been fabricated by a precursor infiltration pyrolysis (PIP) method using borazine as the precursor. The densification behavior, microstructures, mechanical properties, and dielectric properties of the composites have been investigated. After four PIP cycles, the density of the composites had increased from 1.1 g·cm^?3 to 1.81 g·cm^?3. A flexural strength of 128.9 MPa and an elastic modulus of 23.5 GPa were achieved. The obtained composites have relatively high density and the fracture faces show distinct fiber pull-out and interface de-bonding features. The dielectric properties of the SiNO _f /BN composites, including the dielectric constant of 3.61 and the dielectric loss angle tangent of 5.7×10^?3, are excellent for application as wave-transparent materials.     

High-temperature oxidation behavior of SiC-coated carbon fiber-reinforced carbon matrix composites

The high-temperature oxidation behavior of bare and SiC-coated carbon fiber-reinforced carbon matrix (C/C) composites was examined in the temperature range 900–3000 K. In the course of the measurements, the main focus was placed upon the effect of oxygen partial pressure on the oxidation behavior and the transition temperature between the passive and active oxidation regimes. To understand the oxidation behavior of SiC-coated C/C composites quantitatively, a morphological characterization of coating cracks was carried out, and then, in the oxygen diffusion controlling temperature range, an analytical model was developed for the prediction of weight loss due to oxidation of SiC-coated C/C composites. The oxidation rates derived from this model were in fairly good agreement with the experiment results.     

Ultimate strength properties of fiber-reinforced composites

The volume effect and stress heterogeneity effect (i.e., the effect of loading type) on the ultimate strength are analyzed for fiber-reinforced composites. The main failure mechanisms are assumed to be fiber breakage and fiber pull-out. Depending on the load redistribution around a broken fiber, two different regimes can be obtained. The results are applied to the prediction of ultimate strengths of SiC fiber-reinforced composites subjected to tension, pure flexure and three-point flexure.     

Nanoscale structure and local mechanical properties of fiber-reinforced composites containing MWCNT-grafted hybrid glass fibers

Carbon nanotubes (CNTs) were grown from the surface of glass fibers by chemical vapor deposition, and these hybrid fibers were individually dispersed in an epoxy matrix to investigate the local composite structure and properties near the fiber surface. High-resolution transmission electron microscopy revealed the influence of infiltration and curing of a liquid epoxy precursor on the morphology of the CNT “forest” region, or region of high CNT density near the fiber surface. Subsequent image analysis highlighted the importance of spatially dependent volume fractions of CNTs in the matrix as a function of distance from the fiber surface, and nanoindentation was used to probe local mechanical properties in the CNT forest region, showing strong correlations between local stiffness and volume fraction. This work represents the first in situ measurements of local mechanical properties of the nano-structured matrix region in hybrid fiber-reinforced composites, providing a means of quantifying the reinforcement provided by the grafted nanofillers.     

Correlation structure in the elasticity tensor for short fiber-reinforced composites

The present work provides a profound analytical treatment and numerical analysis of the material properties of SFRC on the mesoscale as well as the resulting correlation structure taking into account the probabilistic characteristics of the fiber geometry. This is done by calculating the engineering constants using the analytical framework given by Tandon and Weng as well as Halpin and Tsai. The input parameters like fiber length, diameter and orientation are chosen with respect to their probability density function. It is shown, that they are significantly influenced by the fiber length, the fiber orientation and the fiber volume fraction. The verification of the analytically obtained values is done on a numerical basis. Therefore, a two-dimensional microstructure is generated and transferred to a numerical model. The advantage of this procedure is, that there are several fibers with different geometrical properties placed in a preset area. The results of the numerical analysis meet the analytically obtained conclusions. Furthermore, the results of the numerical simulations are independent of the assumption of a plane strain and plane stress state, respectively. Finally, the correlation structure of the elasticity tensor is investigated. Not only the symmetry properties of the elasticity tensor characterize the correlation structure, but also the overall transversely-isotropic material behavior is confirmed. In contrast to the influencing parameters, the correlation functions vary for a plane strain and a plane stress state.     

Damping properties of thermoplastic-elastomer interleaved carbon fiber-reinforced epoxy composites

The aim of this study is to characterize the damping properties of carbon fiber-reinforced interleaved epoxy composites. Several types of thermoplastic-elastomer films, such as polyurethane elastomers, polyethylene-based ionomers and polyamide elastomers were used as the interleaving materials. The damping properties of the composite laminates with/without the interleaf films were evaluated by the mechanical impedance method. Also, the effects of the lay-up arrangements of the carbon-fiber prepregs on the damping properties of the interleaved laminates were examined. The viscoelastic properties of interleaved polymer films were reflected in the damping properties of the corresponding interleaved laminates. The loss tangent of the interleaf films at the test temperature played an important roll in the loss factor of the interleaved laminates. Also, the stiffness of the films at the resonant frequency of the laminates was another important parameter that controlled the loss factor of the interleaved laminates.     

BN基复合陶瓷高温力学性能

BN陶瓷因SiO2的引入而使材料的力学性能得到了改善,但依旧存在强度低的不足,为此,本研究将通过在基体中引入高强度、高模量的SiAlON相作为增强相来进一步提高材料的力学性能.以BN、SiO2、AlN为原料,采用热压工艺制备出BN基复合陶瓷.通过高温力学性能测试和扫描电镜(SEM)观察,研究了AlN添加量对复合陶瓷高温力学性能和断裂行为的影响.研究表明:复合陶瓷的高温弯曲强度随着温度的升高而呈先升高后降低的趋势,并在1 300℃达到最大值,而强度的升高主要是由于升温过程中内应力的缓解所致.对于添加体积分数5%AlN的复合陶瓷,在1 300℃时其高温弯曲强度达到376.7 MPa,与室温条件下的弯曲强度相比提升了52.5%,当温度升至1 500℃时其高温弯曲强度为272.0 MPa,具有良好的高温力学性能.     

Micromechanical effective elastic moduli of continuous fiber-reinforced composites with near-field fiber interactions

A higher-order micromechanical framework is presented to predict the overall elastic deformation behavior of continuous fiber-reinforced composites with high-volume fractions and random-fiber distributions. By taking advantage of the probabilistic pair-wise near-field interaction solution, the interacting eigenstrain is analytically derived. Subsequently, by making use of the Eshelby equivalence principle, the perturbed strain within a continuous circular fiber is accounted for. Further, based on the general micromechanical field equations, effective elastic moduli of continuous fiber-reinforced composites are constructed. An advantage of the present framework is that the higher-order effective elastic moduli of composites can be analytically predicted with relative simplicity, requiring only material properties of the matrix and fibers, the fiber?Cvolume fraction and the microstructural parameter ??. Moreover, no Monte Carlo simulation is needed for the proposed methodology. A series of comparisons between the analytical predictions and the available experimental data for isotropic and anisotropic fiber reinforced composites illustrate the predictive capability of the proposed framework.     

Thermal and mechanical properties of waste grass broom fiber-reinforced polyester composites

The main focus of this study is to utilize waste grass broom natural fibers as reinforcement and polyester resin as matrix for making partially biodegradable green composites. Thermal conductivity, specific heat capacity and thermal diffusivity of composites were investigated as a function of fiber content and temperature. The waste grass broom fiber has a tensile strength of 297.58 MPa, modulus of 18.28 GPa, and an effective density of 864 kg/m³. The volume fraction of fibers in the composites was varied from 0.163 to 0.358. Thermal conductivity of unidirectional composites was investigated experimentally by a guarded heat flow meter method. The results show that the thermal conductivity of composite decreased with increase in fiber content and the quite opposite trend was observed with respect to temperature. Moreover, the experimental results of thermal conductivity at different volume fractions were compared with two theoretical models. The specific heat capacity of the composite as measured by differential scanning calorimeter showed similar trend as that of the thermal conductivity. The variation in thermal diffusivity with respect to volume fraction of fiber and temperature was not so significant.The tensile strength and tensile modulus of the composites showed a maximum improvement of 222% and 173%, respectively over pure matrix. The work of fracture of the composites with maximum volume fraction of fibers was found to be 296 Jm⁻¹.     

碳化硅纤维增强磷酸铝基复合材料的制备和性能研究

采用不同的磷酸铝盐基体制备了单向碳化硅纤维增强磷酸铝基复合材料,对其力学性能进行了对比,结果表明：基体对复合材料的力学性能和微观结构有极大的影响,磷酸铝基体随温度的升高脱水、相变残留下一定的气孔,填料的加入可以提高复合材料的整体力学性能;采用磷酸二氢铝基体、α-Al2O3填料制备的复合材料具有最好的性能,其弯曲强度为310MPa、弯曲弹性模量为47GPa,并通过扫描电镜对材料的微观结构形貌进行了分析研究。     

Analysis of oxidation-assisted stress-rupture of continuous fiber-reinforced ceramic matrix composites at intermediate temperatures

A model is presented to estimate the reliability and time-to-failure of an unidirectional continuous fiber-reinforced ceramic composite when subjected to stresses beyond the matrix cracking stress. The particular case of oxidation-assisted stress-rupture at intermediate temperatures is considered. The effects of stress and temperature on the reliability of the model composite are examined. Model predictions are presented for the specific case of CG-Nicalon™/SiC CFCCs with carbonaceous fiber coatings.     

Mechanical and thermal properties of sisal fiber-reinforced rubber seed oil-based polyurethane composites

The development of high-performance composite materials from locally sourced and renewable materials was investigated. Rubber seed oil polyurethane resin synthesized using rubber seed monoglyceride derived from glycerolysis of the oil was used as matrix in the composite samples. Rubber seed oil-based polyurethane composite reinforced with unidirectional sisal fibers were prepared and characterized. Results showed that the properties of unidirectional fiber-reinforced rubber seed oil-based polyurethane composites gave good thermal and mechanical properties. Also, the values of tensile strengths and flexural moduli of the polyurethane composites were more than tenfold and about twofold higher than un-reinforced rubber seed oil-based polyurethane. The improved thermal stability and the scanning electron micrographs of the fracture surface of the composites were attributed to good fiber–matrix interaction. These results indicate that high-performance “all natural products” composite materials can be developed from resources that are readily available locally.     

Thermoreversible and remendable glass-polymer interface for fiber-reinforced composites

Adhesion of the reinforcement to the polymer matrix is essential for load transfer from the polymer matrix to the reinforcement material in fiber-reinforced composites. The reversible Diels-Alder reaction between a furan-functionalized epoxy-amine thermosetting matrix with a maleimide-functionalized glass fiber was used to impart remendability at the polymer-glass interface for potential application in glass fiber-reinforced composites. At room temperature the Diels-Alder adduct is formed spontaneously and above 90 °C the adduct breaks apart to reform the original furan and maleimide moieties. Healing of the interface was investigated with single fiber microdroplet pull-out testing. Following complete failure of this interface, significant healing was observed, with some specimens recovering over 100% of the initial properties. Healing efficiency was not affected by the distance of displacement, with an overall average of 41% healing efficiency. Up to five healing cycles were successfully achieved. It is expected that a glass fiber-reinforced composite of maleimide-sized glass within a furan-functionalized network will demonstrate extension of fatigue life.     

Thermal conductivity and mechanical properties of various cross-section types carbon fiber-reinforced composites

In this work, to study the characteristics of carbon fiber-reinforced composites with different fiber cross-section types, such as round, C, and hollow-shape, the thermal conductivity and mechanical properties were investigated and compared. The thermal conductivity was measured by means of steady-state method to the parallel and perpendicular direction of reinforcing fibers. The mechanical properties were evaluated by a variety of test methods i.e., flexural, interlaminar shear strength, and impact strength. As a result, it was found that the thermal conductivity was greatly depended on the cross-section type of the reinforcing fibers, as well as, the reinforcing orientation. Especially, the anisotropy factor (k _///k ) and the thermal diffusivity factor (_///) of C and hollow-type carbon fiber-reinforced composites showed about two times higher values than those of round-type one. Also, the mechanical results showed that C and hollow-type carbon fibers-reinforced composites had higher values than those of round-type one in all mechanical tested. These results were probably due to the basic properties of non-circular (C and hollow-type) carbon fiber which can improve interfacial binding forces and widen interfacial contact area between reinforcement and matrix, resulting in effectively transferring the applied stress.     

A mechanical model for fiber-reinforced and particulate composites

Summary A set of displacement equations of motion suitable for describing the dynamic behavior of composite materials is derived. The composite material may be a fiber-reinforced composite or a particulate composite. The composite is represented by a lattice model in which the continuous mass distribution of the actual body is replaced by a system of springs attached to small rigid masses. Effective values are determined for the masses of the rigid bodies and the stiffnesses of the springs. The kinetic and strain energies stored in an extended composite medium are expressed in terms of the displacements at the positions of the mass points. By employing Taylor series expansions, and by replacing summations by integrations, representative kinetic and strain energy densities are obtained. Thus, transition to a continuum is achieved. Subsequent application of Hamilton's principle gives the displacement equations of motion which are of the same form as those of a first strain-gradient theory of linear elasticity. The proposed set of equations yield dispersion of plane harmonic waves propagating in an infinite composite medium, and dispersion relations are of the same type as those of crystal lattice theories. The dispersion curve for longitudinal waves propagating in the direction normal to the fibers is plotted together with the dispersion curve obtained from experimental investigations, and good agreement is found between the two.Zusammenfassung Ein Satz von Bewegungsleichungen für die Verschiebungen, zum Beschreiben des dynamischen Verhaltens von Verbundwerkstoffen, wird hergeleitet. Der Verbundwerkstoff kann faser-oder teilchenverstärkt sein. Er ist dargestellt als ein Gitter-Modell, in welchem die stetige Massenverteilung des wirklichen Körpers durch ein System von Punkt-Massen, verbunden mit Federn, ersetzt ist. Die effektiven Werte der Masse der Punkt-Massen und der Federkonstanten der Federn werden bestimmt. Die kinetische Energie und die Verzerrungsenergie, die in einem Körper gespeichert sind, werden durch die Verschiebungen der Punktmassen ausgedrückt. Mit Hife der Taylor-Reihen-Entwicklung und durch Ersetzen der Summen durch Integrale werden die Ausdrücke, für die kinetische Energie und die Verzerrungsenergie abgeleitet. Auf diese Weise geht man zum Kontinuum über. Die Anwendung des Hamilton-Prinzips gibt die Bewegunsgleichungen der Verschiebungen, welche von der gleichen Art wie die Gleichungen der ersten Verzerrungsgradient-Theorie der linearen Elastizitätstheorie sind. Der abgeleitete Satz von Gleichungen ergibt die Dispersion von ebenen, harmonischen, sich im unendlichen Verbundwerkstoff fortpflanzenden Wellen. Die Dispersionsgleichungen sind vom gleichen Typ wie diejenigen der Kristall-Gitter-Theorie. Die Dispersionskurve für longitudinale Wellen, die sich senkrecht zur Faser fortpflanzen, ist zusammen mit der Dispersionskurve der experimentellen Untersuchung dargestellt, und man erkennt, daß die beiden Ergebnisse gut übereinstimmen.

---

Ein mechanisches Modell für faser- und teilchenverstärkte Verbundwerkstoffe.

---

With 5 Figures

---

Presented at the 14th International Congress of Theoretical and Applied Mechanics, Delft, The Netherlands, August 30 to September 4, 1976.     

Microstructure and properties of BN/Ni-Cu composites fabricated by powder technology

Microsize Powders of Ni and Cu were prepared by water atomization technique to fabricate metal matrix composites containing various percentages of nanosized boron nitride particles (1, 2, 3, 4, 5 wt. % of BN in a matrix containing (20 wt. %Ni and 80 wt. %Cu). The prepared mixtures were cold compacted under 400 MPa, and sintered for 2 h at 1000 °C in a controlled atmosphere of 3:2 N₂/H₂ gas mixtures. The microstructure and the chemical composition of the prepared powders as well as the consolidated composites were investigated by X-ray diffraction as well as field emission scanning electron microscope (FESEM) equipped with an energy dispersive spectrometer (EDS). The produced Cu and Ni powders have spheroid shape of size less than 100 microns, but the investigated BN has an equiaxed particle shape and particle size of ～ 500 nm. It has been also observed that BN and Ni particles were homogeneously distributed in the Cu matrix of the present BN/Ni-Cu composites. The density, electrical resistivity, saturation magnetization and hardness of the composites were measured. It was observed that, by increasing BN content, the relative density was decreased, while the saturation magnetization, electrical resistivity and hardness were increased.     

Experimental description of thermomechanical properties of carbon fiber-reinforced TiC matrix composites

Titanium carbide ceramic is a good potential material used in high temperature environment for its good strength, erosion resistance and thermal stability. Unfortunately, the low thermal shock resistance and low fracture toughness are the well-known impediments to its application as high temperature structure components. In order to extend the application of TiC ceramics at high temperature, 20 vol.% short carbon fiber was added into TiC matrix to improve the thermomechanical properties. With the incorporation of carbon fiber, the thermal expansion coefficient of TiC composites was decreased and the thermal conductivity was increased slightly below 900 °C. The flexural strength was improved from 471 MPa for monolithic TiC to 593 MPa for TiC composites, and the strengthening effect of carbon fiber became more prominent at high temperatures. The addition of fiber decreased the elastic modulus of TiC composite. The elastic modulus of the composite decreased with increasing temperature. The improvement of high temperature strength and thermal conductivity and the decrease of thermal expansion will benefit the application of TiC composites in high temperature environment where the temperature usually varies.