Residual Stresses in Silicon Carbide–Zircon Composites from Thermal Expansion Measurements and Fiber Pushout Tests

Coefficients of thermal expansion (CTE) in the axial direction of two types of SiC fibers, monolithic zircon, monolithic SiC, and several SiC_f-zircon composites were measured in the temperature range of 50 to 1380C. The measured CTE values of composites were compared with values predicted by the rule-of-mixtures approach, and a small difference in measured and calculated values was ascribed to the nature of interfacial bonding and assumptions implicit in the rule-of-mixtures approach. Fiber pushout tests were performed on these composites and the residual stresses were extracted from the analysis of the load–displacement plots in terms of the shear-lag and progressive debonding models. The radial and axial residual stresses arising from the mismatch in CTE were calculated and compared with values obtained from the fiber pushout tests. The fiber pushout tests in general produced lower values of the residual stresses, but the residual stresses obtained using shear-lag analysis were in good agreement with the calculated values based on the CTE mismatch in composites with lower values of the interfacial shear stress. The influence of anisotropic fiber expansion in the radial and axial directions on the radial and axial residual stresses in composites were also examined.     

Effect-of Processing Varcables on Interfacial Properties of an SiG-Fiber-Reinforced Reaction-Bonded Si3N4 Matrix Composite

Fiber/matrix interfacial debonding and frictional sliding stresses were evaluated by single-fiber pushout tests on unidirectional continuous silicon-carbide-fiber-reinforced, reaction-bonded silicon nitride matrix composites. The debonding and maximum pushout loads required to overcome interfacial friction were obtained from load–displacement plots of pushout tests. Interfacial debonding and frictional sliding stresses were evaluated for composites with various fiber contents and fiber surface conditions (coated and uncoated), and after matrix densification by hot isostatic pressing (HIPing). For as-fabricated composites, both debonding and frictional sliding stresses decreased with increasing fiber content. The HIPed composites, however, exhibited higher interfacial debonding and frictional sliding stresses than those of the as-fabricated composites. These results were related to variations in axial and transverse residual stresses on fibers in the composites. A shear-lag model developed for a partially debonded composite, including full residual stress field, was employed to analyze the nonlinear dependence of maximum pushout load on embedded fiber length for as-fabricated and HIPed composites. Interfacial friction coefficients of 0.1–0.16 fitted the experimental data well. The extremely high debonding stress observed in uncoated fibers is believed to be due to strong chemical bonding between fiber and matrix.     

Control of Interfacial Properties through Fiber Coatings: Monazite Coatings in Oxide–Oxide Composites

Fiber pushout tests were used to quantify the effects of fiber coating thickness on the mechanical properties of two model composite systems: a monazite-coated (LaPO₄-coated) alumina (Al₂O₃) fiber in an Al₂O₃ matrix and a LaPO₄-coated yttrium aluminum garnet (YAG) fiber in an Al₂O₃ matrix. Interface properties were quantified using the Liang and Hutchinson (LH) pushout model and mechanistically rationalized by considering the change in residual thermal stresses with changes in the coating thickness. Measures of the pure Mode II interfacial fracture energy, the coefficient of friction, and a radial clamping pressure are extracted by fitting the LH equations to the experimental results. Using the approach that has been developed herein, a methodology is available for measuring the interfacial properties, predicting the effect of coating thickness, and selecting the coating thickness to     

Viability of Oxide Fiber Coatings in Ceramic Composites for Accommodation of Misfit Stresses

The C and BN fiber coatings used in most ceramic composites perform a less obvious but equally essential function, in addition to crack deflection; they accommodate misfit stresses due to interfacial fracture surface roughness. Coatings substituted for them must also perform that function to be effective. However, in general, oxides are much less compliant materials than C and BN, which raises the question of the feasibility of oxide substitutes. The viability of oxide coatings for accommodating misfit stresses in Nicalon fiber/SiC composites was investigated by calculating the maximum misfit stresses as functions of coating properties and geometries. Control of interfacial fracture path was also briefly considered. The implications regarding composite properties were examined by calculating properties for composites with mechanically viable oxide coatings.     

Effect of Interfacial Roughness Parameters on the Fiber Pushout Behavior of a Model Composite

The effect of interfacial roughness on the frictional sliding in composites has been studied using fiber pushout and pushback tests on a model composite of Plexiglas rods in an epoxy matrix. Different extents of roughness were introduced on the Plexiglas rods and the resulting roughness profiles measured. The roughness profiles were characterized using six different roughness parameters. An attempt was made to find a correlation between the sliding resistance and the selected roughness parameters. A parameter defined as the maximum coefficient in the Fourier transform of the roughness profile was found to yield the best correlation. If the roughness introduced is periodic, then the pushout traces exhibit periodic dips, but the magnitude of this periodic dip is significantly smaller than the seating drop obtained from pushback tests.     

The use of carbon nanofibers for thermal residual stress reduction in carbon fiber/epoxy laminated composites

Carbon nanofibers (CNFs) were incorporated into an epoxy matrix with three weight fractions of 0.1%, 0.5% and 1% which were then reinforced with unidirectional carbon fibers (CFs) to fabricate laminated composites with cross-ply configuration. Thermomechanical analysis and tensile tests of the specimens were carried out to characterize thermal and mechanical properties of CNF/epoxy composites and compare them with the behavior of the neat resin. Characterization showed that the coefficient of thermal expansion (CTE) of the epoxy matrix is significantly reduced by adding small amounts of CNF. CNFs also moderately increase the Young’s modulus of the epoxy. The slitting method was used for the measurement of residual stresses in cross-ply CF/epoxy and CNF-reinforced CF/epoxy laminates. It involves cutting a narrow slit progressively from one surface of the laminate, and measuring the released strains at the other surface. The results showed that the addition of 0.1%, 0.5% and 1 wt.% CNF leads to 4.4%, 18.8% and 25.1% reductions in residual stress, respectively. These findings confirm that CNFs possess excellent potential to be used as a thermal expansion compensator for the modification of the thermal behavior of the epoxy matrix and the reduction of the thermal residual stresses in the fiber/epoxy laminated composites.     

Interfacial Microstructure and Chemistry of SiC/BN Dual-Coated Nicalon-Fiber-Reinforced Glass-Ceramic Matrix Composites

Glass-ceramic composites with improved high-temperature mechanical properties have been produced by incorporating continuous SiC fibers into a barium magnesium aluminosilicate matrix. Control of the fiber/matrix interface was achieved by a dual-layer coating of SiC/BN(C) applied to the fibers by CVD. The weakly bonded interface resulted in composites with high fracture toughness and strength up to 1100°C, and the composite system was oxidatively stable during long-term exposure to air at high temperatures. Composites with different thermal and mechanical histories were studied, and interfaces were characterized using transmission electron microscopy (TEM), Auger electron spectroscopy, and fiber pushout tests. Observations of interfacial microstructure were correlated with the mechanical properties of the composite and with interface properties determined from fiber push-out tests.     

Interfacial Mechanical Characterization of Nicalon SiC Fiber/Alumina-Based Composites

Interfacial mechanical properties of both Nicalon SiC/aluminum borate and Nicalon SiC/aluminum phosphate with various fiber coatings and heat treatments were evaluated using a commercially-available indenter to induce fiber sliding during load cycling experiments. Varying degrees of sliding due to different coating materials were found. The interfacial characteristics including the shear, the residual axial fiber, and debond stresses were estimated by matching the experimental stress-displacement curves with curves predicted from an existing model. The elastic modulus and hardness of the interphase/interface in ceramic matrix composites were also evaluated. These results provided important insights into the ultimate mechanical performance of fiber-reinforced ceramic-matrix composites.     

Interfacial Mechanical Characterization of Nicalon SiC Fiber/Alumina-Based Composites

Interfacial mechanical properties of both Nicalon SiC/aluminum borate and Nicalon SiC/aluminum phosphate with various fiber coatings and heat treatments were evaluated using a commercially-available indenter to induce fiber sliding during load cycling experiments. Varying degrees of sliding due to different coating materials were found. The interfacial characteristics including the shear, the residual axial fiber, and debond stresses were estimated by matching the experimental stress-displacement curves with curves predicted from an existing model. The elastic modulus and hardness of the interphase/interface in ceramic matrix composites were also evaluated. These results provided important insights into the ultimate mechanical performance of fiber-reinforced ceramic-matrix 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.     

Young's modulus,thermal expansion coefficient and fracture behavior of selected Si–B–C based carbides in the 20–1200 °C temperature range as derived from the behavior of carbon fiber reinforced microcomposites

The Young's modulus, thermal expansion coefficient and fracture behavior of different ceramic phases in the Si–B–C system have been determined from room temperature up to 1200 °C using results of tests performed on matrix-dominated carbon fiber reinforced microcomposites by means of a specific high temperature testing apparatus. Results have shown that the boron-rich materials had higher stresses to failure and thermal expansion coefficients than silicon-rich materials whereas all the boron containing materials exhibited a viscoplastic time-dependant mechanical behavior over 1000 °C. The thermoelastic values of the Si–B–C based carbides thus obtained have been used to compute thermal residual stresses in model composite systems, in view of understanding some results reported in the literature regarding the implantation of layered matrices in ceramic matrix composites.     

Evaluation of Interfacial Properties of Fiber-Reinforced Ceramic Composites Using a Mechanical Properties Microprobe

The application of a mechanical properties microprobe to evaluate the interfacial properties of fiber-reinforced ceramic composites is addressed. The stress–displacement relation of the embedded fiber, which is subjected to an axial loading–unloading cycle, is analyzed. The interfacial bonding, Coulomb friction at the debonded interface, Poisson's effect of the loaded fiber, and residual stresses are included in the analysis, and closed-form analytical solutions are obtained. Based on the analytical solutions, a methodology is established to extract the interfacial properties from experimental stress–displacement curves. The roles of interfacial bonding, Poisson's effect, and residual axial stresses on the residual fiber displacement after complete unloading are also addressed in the present study.     

Carbon-Coated-Glass-Fiber-Reinforced Cement Composites: I, Fiber Pushout and Interfacial Properties

Interfacial mechanical properties of carbon-coated-S-glass-fiber-reinforced cement were characterized by a fiber pushout technique. The pushout experiments were conducted on model composites, where the S-glass monofilaments with and without carbon coating were unidirectionally embedded in ordinary portland cement. Interfacial properties, including bonding strength, frictional stress, residual stress, and fracture energy, were extracted from the previously developed progressive debonding model. The composite with a carbon interface exhibited a weaker interfacial bonding strength and frictional stress than did the composite without a carbon interface. The interfacial fracture energy of the composite with a carbon interface was 7.9 J/m², as compared to 47.6 J/m² for the composite without a carbon interface. The composite with the carbon interface exhibited a smaller residual clamping stress (18 MPa), in comparison to that for the composite without a carbon interface (69 MPa). Scanning electron microscopy observations indicated that the filament without a carbon coating was significantly attacked by the alkaline environment and was strongly bonded onto the matrix, whereas the filament with a carbon coating remained intact under the same curing conditions. These studies suggest that carbon coating provides the glass fiber with significantly improved corrosion resistance to alkali in the cement environment.     

Thermoformed glass fiber reinforced polypropylene: Microstructure,mechanical properties and residual stresses

The objective of this work was to characterize the microstructure, mechanical properties and residual stresses in glass fiber reinforced polypropylene (PP) composites with respect to the thermoforming parameters and as a function of the fiber-matrix interface quality. First, differential scanning calorimetry (DSC) was used to investigate the crystallization behavior of the PP matrix. Second, short beam shear tests and tensile tests in the ±45° directions have been conducted to characterize respectively the interfacial strength and the matrix properties in the composites. Finally, residual stresses were measured via the curvatures of unsymmetric cross-plied laminates. The cooling rate was found to be a critical parameter of the molding process since the matrix crystallization temperature, the interfacial strength as well as the residual stresses showed large variations with various cooling rates. At slow cooling, the crystallization process initiates at higher temperatures and covers longer time periods resulting in more spherulitical matrix structures. In this case, the composites becomes stiffer but also fragile indicating a decrease in the stress transfer efficiency at the interface level. This effect was also observed in the improved interface system, suggesting that the fiber-matrix interaction operates through the amorphous phase surrounding the fibers. The fiber-matrix interface improvement was accompanied by an increase in residual stresses, possibly due to the inhibition of some stress relief mechanism.     

Effects of Interfacial Films on Thermal Stresses in Whisker-Reinforced Ceramics

Toughening of whisker-reinforced (or fiber-reinforced) ceramics by whisker pullout requires debonding at the whisker/matrix interface. Compressive clamping stresses, which would inhibit interface debonding and/or pullout, are expected in composites where the matrix has a higher thermal expansion coefficient than the whisker. Because such mismatch in thermomechanical properties can result in brittle composites, it is important to explore approaches to modify the thermal stresses in composites. As a result, the effects of a film at the whisker/matrix interface on the stresses due to thermal contraction mismatch upon cooling are considered in this study. Analysis of various properties of the film are considered for the whisker/matrix systems, in particular for SiC/Al₂O₃, SiC/cordierite, and SiC/mullite composites. Reduction of thermomechanical stresses is shown to occur when the interfacial film has a low Young's modulus. Also, when the whisker has a lower thermal expansion coefficient than the matrix (e.g., SiC/Al₂O₃), the interfacial stresses generated during cooling decrease as the thermal expansion coefficient of the film increases.     

Interfacial Bonding and Friction in Silicon Carbide [Filament]-Reinforced Ceramic- and Glass-Matrix Composites

Interfacial shear strength and interfacial sliding friction stress were assessed in unidirectional SiC-filament-reinforced reaction-bonded silicon nitride (RBSN) and borosilicate glass composites and 0/90 cross-ply reinforced borosilicate glass composite using a fiber pushout test technique. The interface debonding load and the maximum sliding friction load were measured for varying lengths of the embedded fibers by continuously monitoring the load during debonding and pushout of single fibers in finite-thickness specimens. The dependences of the debonding load and the maximum sliding friction load on the initial embedded lengths of the fibers were in agreement with nonlinear shear-lag models. An iterative regression procedure was used to evaluate the interfacial properties, shear debond strength (T _d ), and sliding friction stress (T _f ), from the embedded fiber length dependences of the debonding load and the maximum frictional sliding load, respectively. The shear-lag model and the analysis of sliding friction permit explicity evaluation of a coefficient of sliding friction (μ) and a residual compressive stress on the interface (σ₀). The cross-ply composite showed a significantly higher coefficient of interfacial friction as compared to the unidirectional composites.     

纤维增强热固性复合材料构件的固化变形研究进展

热固性复合材料的固化是一个热性能、化学性能和力学性能同时发生变化的复杂过程,也是固化变形和残余应力产生的过程。引起复合材料变形的因素主要包括构件的结构形式、树脂含量、铺层方式、基体树脂的特性、固化工艺参数及模具因素等。其中,复合材料固化过程中树脂的热收缩、化学收缩以及模具材料与复合材料间热膨胀系数的差异是引起复合材料发生固化变形的根本原因。     

Influence of surface treatment of carbon fibers on interfacial adhesion strength and mechanical properties of PLA‐based composites

In the present study C/PLA composites with different fiber surface conditions (untreated and with nitric acid oxidation for 4 h and 8 h) were prepared to determine the influence of surface treatment on the interfacial adhesion strength and mechanical properties of the composites. A chemical reaction at the fiber–matrix interfaces was confirmed by XPS studies. Nitric acid treatment was found to improve the amount of oxygen‐containing functional groups (particularly the carboxylic group, —COOH) on carbon fiber surfaces and to increase the surface roughness because of the formation of longitudinal crevices. The treated composites exhibited stronger interface adhesion and better mechanical properties in comparison to their untreated counterparts. There was a greater percentage of improvement in interfacial adhesion strength than in the mechanical properties. The strengthened interfaces and improved mechanical performance have been mainly attributed to the greater extent of the chemical reaction between the PLA matrix and the carbon fibers. The increased surface roughness also has had a slight contribution. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 367–376, 2001     

Microstructure,residual stresses,and mechanical performance of surface crystallized translucent glass-ceramics

Mechanical properties of glasses can be significantly increased by inducing surface crystallization of a low coefficient of thermal expansion phase. In this work, we produced surface crystallized lithia-alumina-silica glass-ceramics with different crystallized layer thicknesses and analysed the resulting residual stresses and their effect on mechanical properties. The residual stress magnitude was estimated by analytical and experimental methods, as well as numerical modeling. The surface compressive stress reached 390 MPa and 490 MPa, as given by the analytical and experimental determination, respectively. These stresses prevented radial cracking in microhardness and scratch tests. The best glass-ceramic achieved a Vickers hardness of 7.5 GPa and fracture strength of 680 ± 50 MPa in a ball-on-three-ball test. These glass-ceramics are translucent, providing 50–60% transmittance over the visible wavelength spectrum (1.3 mm-thick-sample). This study unveiled the causes of improved mechanical properties and validates the concept that surface crystallization is a valuable technique for developing high strength glass-ceramics.     

Predicted Effects of Interfacial Roughness on the Behavior of Selected Ceramic Composites

Potential effects of interfacial roughness in ceramic composites were studied using a model that included the progressively increasing contribution of roughness with relative fiber/matrix displacement during debonding of the fiber/matrix interface. A parametric approach was used to study interfacial roughness in conjunction with other parameters such as the strength, radius, and volume fraction of the fiber. The progressive roughness contribution during initial fiber/matrix sliding caused a high effective coefficient of friction, as well as an increased clamping stress, which led to rapidly changing friction with increasing debond length. Calculated effects implied a potentially significant contribution to the behavior of real composite systems and the necessity for explicit consideration in the interpretation of experimental data to understand composite behavior correctly. In a tension test, the Poisson's contraction of the fiber may negate the effects of roughness, allowing an "effective constant shear stress" (tau) approximation. This was evaluated using a piecewise linear approximation to the progressive roughness model in an analysis of composite stress-strain behavior; for the Nicalon/SiC system, the effective tau value was lower than the values that would be obtained from fiber pushout tests and/or matrix crack spacings.