Measurement of Interfacial Mechanical Properties in Fiber-Reinforced Ceramic Composites

Properties of the fiber/matrix interface in a SiC/glass-ceramic composite were investigated using an indentation method in Which a pyramidal indenter is used to push on the fibers and cause sliding at the interface. An ultralow-load indentation instrument was used to measure force and displacement continuously during loading, unloading, and load cycling. Frictional sliding and combined debonding/frictional sliding at the interface were analyzed. The analyses enabled the results to be used to provide a measure of the debond fracture energy, the magnitude of the frictional sliding stress, a measure of the uniformity of the frictional stress, and an indication of the sensitivity of the frictional stress to repeated sliding, varying load rate, and exposure to high temperatures.     

粉煤灰对PVA纤维/水泥基体界面作用及复合材料拉伸性能的影响

本文研究了粉煤灰掺量对基体强度、聚乙烯醇(PVA)纤维/水泥基体间界面作用以及无表面修饰PVA纤维应变硬化水泥基复合材料(SHCC)拉伸性能的影响。结果表明,随着粉煤灰掺量的增加,基体的28 d抗压强度在18~93 MPa内呈下降趋势。单轴拉伸试验结果表明,掺入20%(质量分数,下同)和50%粉煤灰对SHCC的影响不明显,随着粉煤灰掺量增至67%和80%,SHCC的多微缝开裂和应变硬化特征呈增强趋势,极限应变值也相应增大,最高达7.2%,并且具有轻质特性。单纤维拔出试验结果显示,高掺量粉煤灰不仅可以降低PVA纤维与基体间的化学黏结作用,还能减弱界面摩擦作用,从而有效抑制了PVA纤维在拔出过程中出现过早断裂,显著提高了无表面修饰PVA纤维SHCC的延展性。     

Preparation and Characterization of Polypyrrole-Modified Henequen Fiber-Reinforced Polymethylmethacrylate Composites

Polypyrrole modified henequen conductive fibers were prepared by insitu chemical polymerization of pyrrole in presence of henequen fiber using FeCl₃ as oxidant. The fibers were initially treated with alkali solution, then modified with polypyrrole and were investigated for their electrical properties, mechanical properties, ATR, SEM and XRD. The resulting material had electrical performance and mechanical properties depending on the thickness of the polypyrrole coating and treatment conditions. Polymethylmethacrylate composites having uniform dispersion of surface treated polypyrrole coated henequen fiber were prepared and characterized. Mechanical properties, thermal properties and electrical properties of polymer composites are reported.     

Interfacial Sliding Friction in Silicon Carbide–Borosilicate Glass Composites: A Comparison of Pullout and Pushout Tests

Interfacial sliding friction stress (τ_f) was assessed using both pushout and pullout tests on SiC-borosilicate glass composite specimens. Single-filament composite specimens were fabricated by heating to 950°C in argon borosilicate glass rods with fine-diameter (250-μm) capillary in which SiC filaments were inserted. The composite specimens prepared in this manner showed only frictional bonding. The maximum frictional sliding loads for pushout and the initial frictional sliding loads in pullout were measured as functions of the embedded length of the filament in the glass rods. The nonlinear variations of the frictional loads were analyzed using shear-lag models that include corrections for the effects of Poisson expansion or contraction on the sliding friction stress. It is shown that under identical conditions of composite fabrication the two tests give nearly identical properties for the interfaces. Pushout tests on hotpressed bulk composite specimens, however, showed both chemical bonding and a higher sliding friction stress relative to the single-filament capillary specimens. The presence of compressive residual stress on the filaments was independently confirmed by evidence of stress-induced birefringence.     

Thermomechanical and Interfacial Properties of Calcium Alginate Fiber-Reinforced Polypropylene Composites

Polypropylene (PP) matrix calcium alginate fiber reinforced unidirectional composites (10% fiber by weight) were fabricated by compression molding. Tensile strength (TS), tensile modulus (TM), bending strength (BS), bending modulus (BM), and impact strength (IS) were found to be 26 MPa, 950 MPa, 38 MPa, 1320 MPa, and 20 kJ/m², respectively. Degradation tests of composites were performed for 6 weeks in soil and it was found that composites retained almost 75% of its original strength. The interfacial properties of the composite were investigated by using single fiber fragmentation test (SFFT) and by scanning electron microscope (SEM).     

Characterization of Fracture in Fiber-Reinforced Ceramic Composites under Shear loading

This study developed a test technique for mode II fracture testing of a fiber-reinforced ceramic composite. This method employed a small, straight-notched, precracked, end-notched flexure specimen subjected to three-point bending. This method was demonstrated by measuring the mode II critical strain energy release rate, G _{II c} of a fiber-reinforced glass-ceramic composite at room temperature.     

Mechanical Behavior of Fiber-Reinforced Cement-Based Composites

The use of fibers in a cement-based matrix can fundamentally improve its mechanical properties. Such improvement may lead to a new class of cement-based materials. Further developments depend on an understanding of the interaction between different fibers and cement-based matrices. The current knowledge on the mechanical behavior of fiber-reinforced cement-based composites is summarized. Toughening mechanisms, interface properties, and tensile response of fiber-reinforced cement-based composites are presented. Various theoretical approaches used to describe the mechanical behavior of fiber-reinforced composites are reviewed.     

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.     

Fabrication and Characterization of H. sabdariffa Fiber-Reinforced Green Polymer Composites

Present work is concerned with the evaluation of the mechanical properties of compression molded Hibiscus sabdariffa (HS) fibre-reinforced Urea-Formaldehyde (UF) matrix based green polymer composites. Reinforcing of the Urea-Formaldehyde (UF) resin with Hibiscus sabdariffa (HS) fibre was accomplished in the form of short fibre (3 mm). It has been observed that mechanical properties of UF matrix increases with fibre loading and then decreases for higher loading (beyond 30%). Morphological and thermal studies of the matrix, fibre and short fibre-reinforced (SF-Rnf) green composites have also been carried out.     

Debonding Properties of Residually Stressed Brittle-Matrix Composites

Trends in interface debonding have been calculated during fiber pullout for composites with interfaces subject to residual tension. The debond behavior is shown to depend sensitively on the thermal expansion mismatch. The results are used as the basis for designing a pullout test specimen suitable for measuring the mixed-mode fracture energy of bimaterial interfaces. The solutions also provide the background needed to assess the role of debonding in the toughening of ceramics by fibers.     

Mode I Fracture Resistance of a Laminated Fiber-Reinforced Ceramic

The mode I fracture resistance of a ceramic matrix composite has been measured. Simultaneous observations have revealed that the resistance is dominated by frictional dissipation upon the pullout of fibers that fracture in the crack wake off the crack plane. Numerical and analytical crack growth simulations have been compared with the experimental results. One important feature in this comparison concerns the occurrence of large-scale bridging. With these effects taken into account, the simulations and the experiments are found to be in good correspondence for acceptable magnitudes of the interface sliding stress.     

Influence of Fiber and Interfacial Properties on Fracture Behavior of Fiber-Reinforced Ceramic Composites

The fracture and fracture resistance behaviors of zirconmatrix composites uniaxially reinforced with either uncoated or BN-coated silicon carbide fibers are studied by performing experiments in three-point flexure and by analyzing results analytically using a cohesive crack model that incorporates crack bridging and fiber pullout mechanisms. A comparison of experimental results with the model predictions demonstrates good agreement. This analytical approach is then used in a parametrical study to demonstrate the role of fiber and fiber-matrix interfacial properties on the mechanical behavior of fiber-reinforced ceramic-matrix composites. Material parameters that enhance ultimate strength and ductility or toughness are elucidated.     

Interfacial Characterization of Silicon Carbide Fiber/Lithia-Alumina-Silica Glass Matrix Composites

The development of the carbon-rich interphase in Nicalon SiC fiber/Li₂O-Al₂O₃–SiO₂ glass matrix composites was examined as a function of processing parameters with the use of high-resolution scanning electron microscopy and Auger electron spetroscopy. Specifically, hot-pressing temperatures (1000°, 1100°, and 1200°C) and times (15, 30, 60, and 240 min) were systematically varied in such a manner so as to fabricate dense composites suitable for evaluation of reaction kinetics. Carbon-rich interphase thickness, which ranged from 1400 to 5400 Å (140 to 540 nm), was observed to increase with either increasing times at constant temperature or increasing temperatures at constant time. The kinetics of formation of the carbon-rich interphase followed a diffusion-controlled model, with an activation energy of 25.4 kcal/mol.     

Role of Interfacial Carbon layer in the Thermal Diffusivity/Conductivity of Silicon Carbide Fiber-Reinforced Reaction-Bonded Silicon Nitride Matrix Composites

The role of an interfacial carbon coating in the heat conduction behavior of a uniaxial silicon carbide nitride was investigated. For such a composite without an interfacial carbon coating the values for the thermal conductivity transverse to the fiber direction agreed very well with the values calculated from composite theory using experimental data parallel to the fiber direction, regardless of the ambient atmosphere. However, for a composite made with carbon-coated fibers the experimental values for the thermal conductivity transverse to the fiber direction under vacuum at room temperature were about a factor of 2 lower than those calculated from composite theory assuming perfect interfacial thermal contact. This discrepancy was attributed to the formation of an interfacial gap, resulting from the thermal expansion mismatch between the fibers and the matrix in combination with the low adhesive strength of the carbon coating. In nitrogen or helium the thermal conductivity was found to be higher because of the contribution of gaseous conduction across the interfacial gap. On switching from vacuum to nitrogen a transient effect in the thermal diffusivity was observed, attributed to the diffusion-limited entry of the gas phase into the interfacial gap. These effects decreased with increasing temperature, due to gap closure, to be virtually absent at 1000°C.     

Effect of Silane Coupling Agent on the Tensile Properties of Carbon Fiber-Reinforced Thermoplastic Polyimide Composites

Silane coupling agent SG-Si900 (SGS) modification and air-oxidation methods were used to improve the interfacial adhesion of the carbon fiber-reinforced polyimide (CF/PI) composite. The interfacial characteristics of the composites reinforced by the carbon fibers treated with different surface modification methods were comparatively investigated. Results showed that both SGS modification and air-oxidation method improved the adhesion between the reinforcement and matrix and SGS modification method was superior to air-oxidation method. For CF/PI composite the optimum interfacial adhesion was obtained at 0.3 wt% SGS concentration. The fracture surfaces of samples were investigated by scanning electronic microscopy (SEM) to analyze the effects of different surface treatment methods.     

Mechanical Properties of Hemp‐Fibre‐Reinforced Euphorbia Composites

A composite material consisting of hydroxide‐modified hemp fibres and euphorbia resin was produced. The composites were tested in tension, short‐beam interlaminar shear stress and in impact. There was an increase in the tensile strength and modulus for resins with high‐hydroxyl‐group based composites. Similar results were obtained for interlaminar shear stress while low‐hydroxyl group euphorbia resin based composites exhibited high impact strength. The euphorbia resin with high hydroxyl content yielded composites with high stiffness. The use of euphorbia‐based resins in composite manufacture increases the value of the euphorbia oil as well as creating a new route of composite manufacturing.

     

Short Palm Tree Fibers Polyolefin Composites: Effect of Filler Content and Coupling Agent on Physical Properties

Short date palm tree lignocellulosic fibers have been used as a reinforcing phase in commodity thermoplastic matrices [poly(propylene) and low density polyethylene]. Compatibilization of the fibers was carried out with the use of maleic anhydride copolymers. The morphology, thermal and mechanical properties of the resulting composites were characterized using SEM, DSC and tensile tests. The reinforcing capability of the unmodified fibers was found to depend on the nature of the matrix and the main parameter governing the composite behavior was the degree of crystallinity of the matrix. Compatibilization was reported to enhance the mechanical performances for both sets of composites up to a critical amount of compatibilizer beyond which the degree of crystallinity of the matrix decreases.

     

Theoretical Analysis of the Fiber Pullout and Pushout Tests

The fiber pullout and pushout tests have been analyzed to predict the load-displacement behavior in terms of fiber/matrix interface parameters. The effects of residual axial strain in the fiber and fiber surface topography were included. The residual axial strain was found to be a significant parameter. It is shown that the interface failure can be progressive or catastrophic. In the case of a progressive failure of the interface, the load-displacement curve is nonlinear. The portion of the curve from above the first nonlinearity to near the peak load can be predicted in terms of parameters of the interface, viz., the friction coefficient, the radial stress at the interface, the fracture toughness of the interface, and the residual axial strain in the fiber. Values for these parameters can be obtained from a single loaddeflection curve. The peak load and load drop, which are usually reported, are found not to be directly relatable to any interface property, since the length of the last portion of the fiber to debond is influenced by end effects and hence not easily predicted. However, for data which describe the peak load as a function of initial embedded length, that factor can be eliminated and the data reduced to yield the relevant interface parameters. In pullout, the peak and friction loads saturate with large specimen thickness. Catastrophic failure is favored when the debond initiation load is high or when residual stress is low. Finally, a methodology to extract interface parameters from experimental data is suggested.