Polyethylene fibers-polyethylene matrix composites: Preparation and physical properties

Drawing on the difference in melting points of UHMPE fiber (150°C) and HDPE matrix (130°C), single-polymer composites were fabricated under various processing conditions. Because of the chemical similarity of the composite components, good bonding at the fiber-matrix interface could be expected. The matrix, the fiber, and unidirectional composite laminae were studied using TMA and DSC analyses, a hot-stage crystallization unit attached to a polarizing microscope, and an universal tensile testing machine. The TMA showed negative thermal expansion of the fiber over the complete temperature range of the experiment. Three regimes of contraction according to the values of the thermal expansion coefficient were detected. DSC analyses of either the fiber or the composite specimens did not show any appreciable changes after various thermal treatments. They also showed no evidence of fiber relaxation during manufacture, probably because of the pressure-related transverse constraint. The tensile strength and modulus values of the composite appeared to be fairly high and close to those reported for other composites reinforced with polyethylene (PE) fibers. An apparent maximum on the temperature dependencies of tensile properties was observed. A study of the matrix microstructure did not give any proof of transcrystalline growth at the fiber-matrix interface even for chemical or plasma surface-treated fibers. © 1993 John Wiley & Sons, Inc.     

Thermal Expansion of Laminated, Woven, Continuous Ceramic Fiber/Chemical-Vapor-Infiltrated Silicon Carbide Matrix Composites

Thermal expansions of three two-dimensional laminate, continuous fiber/chemical-vapor-infiltrated silicon carbide matrix composites reinforced with either FP-Alumina (alumina), Nextel (mullite), or Nicalon (Si-C-O-N) fibers are reported. Experimental thermal expansion coefficients parallel to a primary fiber orientation were comparable to values calculated by the conventional rule-of-mixtures formula, except for the alumina fiber composite. Hysteriesis effects were also observed during repeated thermal cycling of that composite. Those features were attributed to reoccurring fiber/matrix separation related to the micromechanical stresses generated during temperature changes and caused by the large thermal expansion mismatch between the alumina fibers and the silicon carbide matrix.     

Measurement of Fiber/Matrix Interfacial Shear Stress at Elevated Temperatures

An apparatus for measurement of the fiber/matrix interfacial shear stress at temperatures up to 1100° is described. Equipment was used to measure interfacial properties as a function of temperature in two ceramic-matrix composites and one metal-matrix composite. In the composites where the thermal expansion of the matrix was higher than that of the fiber, the interfacial shear stress decreased with temperature. The opposite trend was observed in a system with low matrix thermal expansion. The change of the interfacial shear stress with temperature of all the composites studied can be fully explained by considering the fiber/matrix expansion differences.     

Thermal Expansion Coefficients of Unidirectional Fiber-Reinforced Ceramics

The influence of internal stresses, due to the thermomechanical mismatch between the fiber and the matrix, on the thermal expansion behavior of unidirectional fiber-reinforced ceramics is considered. Using the composite cylinder model, the effective thermal expansion coefficients of the composite are calculated from the total strains, which consist of the strains due to temperature changes and the strains induced by the presence of internal stresses. The results reveal that when the fiber and the matrix have the same elastic constants, the rule of mixtures approach can be used to obtain the thermal expansion coefficients of the composite, as observed in previous analytical solutions. Also, for the case of low volume fractions of fibers with Young's moduli much larger than those of matrices, and the thermal expansion coefficients lower than those of matrices, the transverse thermal expansion coefficient of the composite is higher than that of either the fiber or the matrix. However, unlike previous studies, the present analysis provides a physical basis for this phenomenon in terms of the internal thermal stress state within the composite.     

Dynamic mechanical analysis of fiber reinforced composites

Dynamic mechanical and thermal properties were determined for unidirectional epoxy/glass composites at various fiber orientation angles. Resonant frequency and relative logarithmic decrement were measured as functions of temperature. In low angle and longitudinal specimens, a transition was observed above the resin glass transition temperature which was manifested mechanically as anadditional damping peak and thermally as a change in the coefficient of thermal expansion. The new transition was attributed to a heterogeneous resin matrix induced by the fiber. The temperature span of the glass-rubber relaxation was found to broaden with decreasing orientation angle, reflecting the growth of fiber contribution and exhibiting behavior similar to that of Young's modulus. The change in resonant frequency through the glass transition was greatest for samples of intermediate fiber angle, demonstrating behavior similar to that of the longitudinal shear modulus.     

光纤缠绕用正交各向异性碳纤维复合材料线轴的研究

本文以四官能团氨基环氧树脂为基体,碳纤维T700为增强材料,缠绕制备了光纤缠绕用正交各向异性碳纤维复合材料线轴。结果表明：正交各向异性碳纤维复合材料线轴的径向热膨胀系数为0.4×10-6/℃,轴向热膨胀系数主要取决于树脂基体,灌注层越厚,线轴的热膨胀系数越大。     

Crack Initiation in Borosilicate Glass-SiC Fiber Composites

The initiation of matrix microcracking was investigated in unidirectional glass matrix composites having controlled fiber spacing. Observations were taken from composites consisting of regular arrays of TiB₂-coated SIGMA 1240 and carbon-coated SCS-6 monofilament SiC fibers in a series of borosilicate glasses. The thermal expansion mismatch between the fibers and glass matrix was varied such that the resulting radial stresses after processing ranged from tensile to compressive. The glass strongly bonds to the TiB₂-coated SIGMA 1240 fiber but weakly bonds to the carbon coating of the SCS-6 fiber, allowing the investigation of the effects of bonding at the fiber/matrix interface. The observed crack initiation stresses of the various composites are compared to predictions based on a previously developed semiempirical model and used to study the influence of the volume fraction of fibers, residual stress state and interface strength.     

An analytical approach to evaluate the coefficients of thermal expansion of textile composite materials

An analytical approach is developed to evaluate the coefficients of thermal expansion (CTE) of textile reinforced composites. At the micro level, a cylindrical composite model is employed to model the fiber/matrix thermal and mechanical interactions. The effects of voids and fiber coating on the thermal expansion coefficients of composites are considered at this level. The cylindrical model was then embedded in a macro hybrid finite element solutio structure to calculate the value of the CTE for textile composites. AS‐4/epoxy balanced plain weave textile composites were manufactured. Five different fiber volume fractions were tested for CTE. Evaluatio of the thermal expansion coefficients using the current model was compared to experimental data for in‐plane and out‐of‐plane directions.     

A multiscale modeling for predicting the thermal expansion behaviors of 3D C/SiC composites considering porosity and fiber volume fraction

Coefficients of thermal expansion (CTEs) are an essential design criterion of the three-dimensional carbon fiber reinforced SiC matrix composites (3D C/SiC). Representative volume element (RVE) models of microscale, void/matrix, and mesoscale developed in this work were used to investigate the CTEs of these composites. A coupled temp-displacement steady-state analysis step was created for assessing the thermal expansions behaviors of the composites by applying periodic displacement and temperature boundary conditions. Three RVE models of cuboid, hexagonal and fiber random distribution were respectively established to comparatively study the influence of fiber package pattern on the CTEs at microscale. Similarly, the effects of different void size, locations, and shapes on the CTEs of the matrix are comparatively analyzed by the void/matrix models. The prediction results at mesoscale corresponded closely to the experimental results. The effect of the porosities on the CTEs was studied by the void/matrix RVE models. The voids were effective in lowering the CTE of the 3D C/SiC composites. Furthermore, the effect of fiber volume fractions on the CTE were also taken into consideration. Equal in-plane and out-of-plane CTEs were realized by selecting appropriate fiber volume fractions for the different directions. The multiscale models developed in this work can be used to predict the thermal expansion behaviors of other complex structure composites.     

Preparation of C/SiC Composites by Hot Pressing, Using Different C Fiber Content as Reinforcement

Unidirectional C/SiC composites were successfully prepared by hot pressing at 1850°C under 20 MPa, using different fiber volume fractions (from 28 vol% to 55 vol%) as reinforcement. The densification process of the composites became increasingly difficult with increasing fiber volume fraction, and some small pores were still distributed in the intrabundle regions of the composites. The cracks, resulting from the residual thermal stress in the composites due to the mismatch of the thermal expansion coefficient of the matrix and the fiber, were distributed in the matrix. With the increase of fiber content, the mechanical properties of the composites could be improved and the composites exhibited an obvious noncatastrophic fracture behavior due to a decrease in the thermal residual stress and an increase in the fiber pull outs.     

Low temperature mechanical behavior of 1D-Cf/SiO2 composite

Unidirectional carbon fiber reinforced fused silica (1D-C_f/SiO₂) composite was prepared by slurry infiltration and hot-pressing. The flexural strength and the coefficient of thermal expansion (CTE) at room and liquid nitrogen temperature (77 K) were investigated. The flexural strength of the composite tested at 77 K was 878 MPa, higher than that 667 MPa at room temperature. Moreover, the CTE of the composite at 77 K was higher than that at room temperature. Due to the difference of CTE between the matrix and fiber, gaps appeared at the fiber/matrix interface of as-prepared specimens. However, they may be healed up because of the thermal expansion of carbon fiber at 77 K. It led to a higher interfacial sliding resistance and changed the weak fiber/matrix interfacial bonding. Thus, it was helpful for the load transfer from matrix to fiber.     

Strength and Toughness of Continuous-Alumina-Fiber-Reinforced Glass-Matrix Composites

Unidirectional, continuous-fiber composites were fabricated using polycrystalline alumina fibers and four different silicate glass matrices of differing thermal expansion. Fracture toughness measurements, strength measurements, and fractographic analysis of failed specimens are used to identify the failure mechanism. Results show that the elastic modulus mismatch between the matrix and the fibers shields the reinforcing fibers from matrix crack extension, thereby increasing composite toughness without fiber pullout. Fractographic analysis shows that fiber shielding leads to fiber failure ahead of matrix crack. Composite toughness increases linearly with increases in the residual compressive stress in the matrix phase. Ultimate composite strengths are dependent upon thermal-expansion-induced residual stresses and fiber strength.     

Thermophysical properties of carbon fiber reinforced multilayered (PyC–SiC)n matrix composites

Three kinds of carbon fiber reinforced multilayered (PyC–SiC)_n matrix (C/(PyC–SiC)_n) composites (n = 1, 2 and 4) were prepared by means of layer-by-layer deposition of PyC and SiC via chemical vapor infiltration. Thermal expansion behaviors in the temperature range of 800–2500 °C and thermal conductivity from room temperature to 1900 °C of C/(PyC–SiC)_n composites with various microstructures were investigated. The results show that with increasing PyC–SiC sequences number (n), the coefficients of thermal expansion of the composites decrease due to the increase of interfacial delamination, providing room for thermal expansion. The thermal diffusivity and thermal conductivity also decrease with the increase of sequences number, which are attributed to the enhancement of phonon-interface scattering resulted from the increasing number of interfaces. Modified parallel and series models considering the interfacial thermal resistance are proposed to elaborate thermal conductivity of the composites, which is in accordance with the experimental results.     

In-plane thermal expansion behavior of dense ceramic matrix composites containing SiBC matrix

In order to reveal the effect of matrix cracks resulted from thermal residual stresses (TRS) on the thermal expansion behavior of ceramic matrix composites, SiBC matrix was introduced into C_f/SiC and SiC_f/SiC by liquid silicon infiltration. The TRS in both two composites were enlarged with incorporating SiBC matrix which has higher coefficients of thermal expansion (CTEs) than SiC matrix. Due to the relatively high TRS, matrix cracks and fiber/matrix (f/m) debonding exist in C_f/SiC-SiBC, which would provide the space for the expansion of matrix with higher CTEs. For SiC_f/SiC, no matrix cracking and f/m debonding took place due to the close CTEs between fiber and matrix. Accordingly, with the incorporation of SiBC matrix, the in-plane CTE of C_f/SiC between room temperature to 1100 °C decreases from 3.65 × 10⁻⁶ to 3.19 × 10⁻⁶ K^-1, while the in-plane CTE of SiC_f/SiC between room temperature to 1100 °C increases slightly from 4.97 × 10⁻⁶ to 5.03 × 10⁻⁶ K^-1.     

Thermal expansion of organic and inorganic matrix composites: A Review of theoretical and experimental studies

This paper reviews theoretical and experimental studies conducted in organic, ceramic, and metal matrix composites containing cylindrical, lamellar, and spheroidal inclusions as reinforcements. Mathematical formulations proposed to predict thermal expansion coefficients of fiber, disc, and sphere reinforced organic and inorganic matrix composites have been reviewed. Experimental studies undertaken to confirm theoretical predictions of thermal expansion coefficients of a variety of reinforcement geometry composites have also been discussed.     

Effect of polypyrrole deposition of carbon fiber on the thermal expansion of carbon fiber-epoxy composites

Polypyrrole (PPy) was deposited onto carbon fibers via continuous electrochemical deposition (ECD). Composites of PPy-deposited carbon fiber and epoxy were prepared. The thermal expansion coefficients of these materials were determined using either a thermal mechanical analyzer or an imbeded strain gauge. The results show that PPy has a negative thermal expansion coefficient while carbon fiber and epoxy have positive thermal expansion coefficients. The resulting composite has a smaller thermal expansion coefficient, higher interlaminar shear stress and a smaller critical fiber length than the composite using untreated carbon fiber. This suggests that the deposition of PPy can effect an improvement in the fiber-matrix interfacial bonding of the composite.     

Effective Fiber Properties to Incorporate Coating Thermoelastic Effects in Fiber/Matrix Composite Models

A method to incorporate the thermoelastic effects of fiber coatings into models of fiber/matrix composites was deter-mined. The coated fiber was replaced by an "effective" transversely isotropic fiber so that the properties of this effective fiber could be used in composite models. This ap-proach was used to determine the magnitude of errors re-sulting from neglect of the coatings in modeling fiber/matrix debonding and sliding and in reduction of data from real composites. The effects of carbon and BN coatings in the Nicalon/SiC system were found to be significant. It was found that significant errors could be expected from fitting models to experimental data if the compliance and coeffi-cient of thermal expansion of the coatings were ignored, even when the coatings were thin. Wide use of the approach required revision of composite models to allow inclusion of a transversely isotropic fiber. Such a revision was de-rived for a popular model of matrix cracking stress, and significant effects again were found to result from neglect of coatings.     

Mechanical and thermal shock properties of Cf/SiBCN composite: Effect of sintering densification and fiber coating

In this work, resin-derived carbon coating was prepared on carbon fibers by polymer impregnation pyrolysis method, then silicoboron carbonitride powder was prepared by mechanical alloying, and finally carbon fiber-reinforced silicoboron carbonitride composites were prepared by hot-pressing process. The effects of sintering densification and fiber coating on microstructure, mechanical properties, thermal shock resistance, and failure mechanisms of the composites were studied. Fiber bridging hinders the sintering densification, causing more defects in fiber-dense area and lower strength. However, higher sintering temperature (1800–2000°C) can improve mechanical properties significantly, including bending strength, vickers hardness, and elastic module, because further sintering densification enhances matrix strength and fiber/matrix bonding strength, while the change of fracture toughness is not obvious (2.24–2.38 MPa·m^1/2) due to counteraction of higher debonding resistance and less pull-out length. However, fiber coating improves fracture toughness greatly via protecting carbon fibers from chemical corrosion and damage of thermal stress and external stress. Due to lower coefficient of thermal expansion, lower fiber loading ratio, less stress concentration at the fiber/matrix interface, and better defect healing effect, lower sintering temperature favors thermal shock resistance of composites, and thermal shock recession mechanisms are the damage of interface.     

Interfacial adhesion study on UHMWPE fiber-reinforced composites

Ultrahigh molecular weight polyethylene (UHMWPE) fiber has many outstanding properties. However, poor interfacial adhesion of the UHMWPE fiber/polymer matrix interface limits its applications as reinforcement in high performance polymer matrix composites. Therefore, a new thermosetting resin system, named PCH, which is only composed of carbon and hydrogen elements, has been developed according to law of similar mutual solubility and the structural characteristics of UHMWPE fiber. The adhesion property was investigated by mechanical properties test, thermal performance test, and polymer solution properties test. Test results show that a strong interaction occurs between UHMWPE fiber and the PCH matrix due to the structural and polar similarity. In the case of slight difference between solubility parameters of UHMWPE fiber and cured PCH resin, it is found that the wettability of PCH resin on surface of the fiber can be improved and the difference between the coefficients of thermal expansion of the matrix and the fiber decreases with the increase of styrene added into the PCH. An optimal interfacial adhesion can be obtained as the ratio of PCH/styrene is approximately 55/45.     

Thermal insulation behaviour of Ethylene propylene diene monomer rubber/kevlar fiber based hybrid composites containing Nanosilica for solid rocket motor insulation

The current research discusses the properties of an elastomeric heat-shielding material, based on nano-silica (NS) filled ethylene propylene diene monomer (EPDM) rubber/Kevlar fiber (KF) hybrid composites. The developed elastomeric insulating material consists of an aromatic polyamide fiber (KF) and silica nanoparticles. An in-depth analysis of mechanical properties, density, coefficient of thermal expansion, thermal conductivity, thermogravimetric analysis, and heat release rate of the insulating materials -was performed. TEM micrograph represents an excellent distribution of nanoparticles in the EPDM matrix. The improvement in the mechanical and the flame retardancy of the NS filled EPDM/KF hybrid composite insulations is based on the fiber/matrix adhesion. Maleic anhydride grafting confers polarity to the nonpolar rubber matrix. The char residues of the insulations inspected by scanning electron microscopy and energy dispersive spectroscopy are depicting a rigid and rough surface by the optimal composites, which can aid in better insulation. The optimal formulation of the hybrid composites exhibited a 220% enhancement in char residue with improved thermal stability and mechanical properties.