Tailoring interfacial shear strength of SiC fiber-reinforced brittle matrix composites

The interfacial shear strength of Nicalon SiC fiber-reinforced glass-ceramic matrix composites was aimed to be tailored via two methods: (1) varying of the thickness of the carbon-rich interfacial layer between the fiber and the matrix by controlling hot pressing period and (2) formation of the secondary interfacial layer, TaC, at the carbon/matrix boundary by doping the Ta₂O₅ matrix addition. In the series of composites with varying carbon-rich layer thickness, fiber/matrix debonding mostly occurred at the carbon/matrix boundary and hence the increase in the carbon-rich layer thickness did not cause any apparent changes in the interfacial shear strength. In the TaC formed series of composites, the interfacial shear strength was affected considerably by the presence of the TaC phase at carbon/matrix boundary. The Ta₂O₅ addition to control the quantity of the TaC phase has shown to be a useful method to tailor the interfacial shear strength of SiC fiber/glass-ceramic composites.     

Influence of interfacial reactions on the fiber-matrix interfacial shear strength in sapphire fiber-reinforced Nial(Yb) composites

The influence of microstructure of the fiber-matrix interface on the interfacial shear strength, measured using a fiber-pushout technique, has been examined in a sapphire-fiber-reinforced NiAl(Yb) matrix composite under the following conditions: (1) as-fabricated powder metallurgy (PM) composites, (2) PM composites after solid-state heat treatment (HT), and (3) PM com-posites after directional solidification (DS). The fiber-pushout stress-displacement behavior con-sisted of an initial “pseudoelastic” region, wherein the stress increased linearly with displacement, followed by an “inelastic” region, where the slope of the stress-displacement plot decreased until a maximum stress was reached, and the subsequent gradual stress decreased to a “fric-tional” stress. Energy-dispersive spectroscopy (EDS) and X-ray analyses showed that the inter-facial region in the PM NiAl(Yb) composites was comprised of Yb₂O₃,O-rich NiAl and some spinel oxide (Yb₃Al₅O₁₂), whereas the interfacial region in the HT and DS composites was comprised mainly of Yb₃Al₅O₁₂. A reaction mechanism has been proposed to explain the pres-ence of interfacial species observed in the sapphire-NiAl(Yb) composite. The extent of inter-facial chemical reactions and severity of fiber surface degradation increased progressively in this order: PM < HT < DS. Chemical interactions between the fiber and the NiAl(Yb) matrix resulted in chemical bonding and higher interfacial shear strength compared to sapphire-NiAl composites without Yb. Unlike the sapphire-NiAl system, the frictional shear stress in the sap-phire-NiAl(Yb) composites was strongly dependent on the processing conditions. Formerly Research Associate, Department of Chemical Engineering, Cleveland State University     

Fracture resistance of fiber-reinforced ceramic matrix composites

The influence of the properties of the fibers, the matrix and the interface on the mechanical properties of fiber reinforced ceramics is analyzed by a simplified method previously developed by the authors for cohesive materials. The method parts from the assumption that crack displacements are known a priori and furnishes, in a simple and easy way, the fracture resistance curves versus crack length. The numerical results from the model are compared with experimental data from the literature. Finally, the model is used to assess the influence of fiber strength, interface slipping shear stress, fiber radius and fiber defect distribution on the fracture resistance and ductility of fiber-reinforced ceramic composites.     

On determining temperature dependent interfacial shear properties and bulk residual stresses in fibrous composites

The model of the preceding paper is applied to the analysis of interfacial sliding during thermal cycling in three titanium or titanium aluminide alloys reinforced by SiC fibers. The model is found to give an excellent account of the experimental measurements. By fitting the model to the data, values are obtained in all cases for the critical interfacial shear stress, τ₀ at room temperature. In two cases, values are also obtained for the bulk, axial residual stresses at room temperature, and the average of dτ₀/dT over the interval of temperature T between room temperature and the maximum temperature attained in the thermal cycling. The residual stresses are in good agreement with other measurements. In the third case, the residual stresses cannot be determined; but, if values for them are taken from other experiments, then the same average of dτ₀/dT can be determined. The values of τ₀ and the implied coefficient of friction are consistent in all cases with frictional sliding in graphitic layers in the carbon rich coatings on the SiC fibers.     

A theory for creep by interfacial flaw growth in ceramics and ceramic composites

The nucleation and growth of flaws along grain boundaries and interfaces are known to cause significant reductions in elastic moduli and to play an important role in determining the deformation characteristics of ceramic materials at elevated temperatures. This paper presents an analysis of the creep behavior of deteriorating elastic solids where the principal mechanism of deformation is the growth of intergranular or interfacial flaws. The changes in elastic moduli induced by the growth of internal damage are used to derive the stress exponent in the power-law creep regime. When the flaws advance at a rate which is proportional to the local normal stress or normal strain, a power-law creep exponent of 2 is predicted for short time, steady-state creep for a population of aligned slit cracks and randomly oriented penny-shaped cracks. For long-time creep, the variation of nonsteady state creep strain rate as a function of the far-field stress and time is explicitly determined. General solutions for creep strain rates are also presented for situations where the microcrack growth rate has a power-law dependence on the local normal stress or stress intensity factor. The predicted dependence of creep strain rate on the far-field stress, the progression of damage and the consequent reduction in elastic moduli, overall creep ductility, and implications pertaining to microstructural and temperature effects on creep are found to be in accord with a wide variety of experimental observations for ceramics and ceramic composites. The temperature, stress and material conditions for which the proposed mechanism is applicable are discussed and a general theory of creep damage in progressively microfracturing elastic brittle solids is developed.     

Interface characterization of ceramic fiber-reinforced ti alloy composites manufactured by infrared processing

Interfacial reactions between several ceramic fibers (SCS-0, SCS-6, and carbon fibers) and a liquid titanium-nickel-copper alloy were investigated using electron microscopic analysis. Composite spec-imens were produced using a rapid infrared manufacturing (RIM) process. In SCS-O/Ti alloy com-posites, SiC dissolved in the alloy. The main reaction product was discontinuous agglomerates of titanium carbide which formed from the reaction between dissolved carbon and titanium. Polygonal precipitates of Ti₅Si₃, which are believed to have formed during cooling, were also noticed. Two distinct interface morphologies were observed in these composites: uniform fronts caused by iso-thermal dissolution and scalloped fronts formed as a result of an accelerated dissolution mechanism caused by localized heating. The presence of the accelerated dissolution mechanism suggests that SiC fibers cannot be infiltrated with liquid titanium alloys without applying a coating. In the C/Ti system, carbon fibers reacted with the liquid alloy to form a continuous layer of Ti_xC_1-x. Further growth of this layer occurred by the diffusion of carbon atoms across the reaction product. In SCS-6/Ti alloy composites, free carbon present in the coating formed a discontinuous layer of Ti^C,^, whereas SiC particles dissolved in the alloy. Due to channeled dissolution in the coating, the accel-erated dissolution mechanism was not observed in these composites. As a result, the presence of the carbon-rich coating prevented degradation of the fibers. Although the coating present on SCS-6 fibers moderately retarded reactions in the SiC/Ti alloy composite system during infrared liquid infiltration, it is recommended that the fibers be coated with pure carbon to effectively limit the attack of the fiber by molten titanium. Formaly Postdoctoral Fellow, Department of Materials Science and Engineering, University of Cincinnati     

Effects of composite processing on the strength of sapphire fiber-reinforced composites

The current interest in tough, high-temperature materials has motivated fiber coating development for sapphire fiber-reinforced alumina composites. For this system, it has been demonstrated that the interfacial properties can be controlled with coatings which can be eliminated from the interface subsequent to composite consilidation. However, these fugitive coatings can contribute to the high temperature strength degradation of sapphire fibers. Such degredation,which compromises the composite strength and toughness, is the focus of the current investigation. It has been observed that, in some cases, by selecting appropriate composite processing conditions, such effects can be minimized. But overcoming fiber strength loss remains an important issue.     

Influence of Cr and W alloying on the fiber-matrix interfacial shear strength in cast and directionally solidified sapphire nial composites

Sapphire-reinforced NiAl matrix composites with chromium or tungsten as alloying additions were synthesized using casting and zone directional solidification (DS) techniques and characterized by a fiber pushout test as well as by microhardness measurements. The sapphire-NiAl(Cr) specimens exhibited an interlayer of Cr rich eutectic at the fiber-matrix interface and a higher interfacial shear strength compared to unalloyed sapphire-NiAl specimens processed under identical conditions. In contrast, the sapphire-NiAl(W) specimens did not show interfacial excess of tungsten rich phases, although the interfacial shear strength was high and comparable to that of sapphire-NiAl(Cr). The postdebond sliding stress was higher in sapphire-NiAl(Cr) than in sapphire-NiAl(W) due to interface enrichment with chromium particles. The matrix microhardness progressively decreased with increasing distance from the interface in both DS NiAl and NiAl(Cr) specimens. The study highlights the potential of casting and DS techniques to improve the toughness and strength of NiAl by designing dual-phase microstructures in NiAl alloys reinforced with sapphire fibers. R. TIWARI, formerly Research Associate, Department of Chemical Engineering, Cleveland State University     

Analysis of shear stress distribution in pushout process of fiber-reinforced ceramics

The interfacial shear stress distribution of a thin specimen of SiC fiber-reinforced glass matrix composite (fiber volume fraction of 0.1, 0.5 and 0.7) during a fiber pushout process was subjected to finite element analysis using a three concentric axisymmetrical model which consisted of fiber, matrix, and composite. A stress criterion was used to determine interface debonding. Effects of thermally-induced stress and a post debond sliding process at the interface were also included in the analysis. The analytical result showed that shear stress near the specimen surface was introduced during the specimen preparation process. Before the interfacial debonding, the distribution of shear stress during the pushout test was affected by the existence of thermally-induced stress in the specimen. The interfacial shear debonding initiated ≈ 30 μm below the pushing surface and the sliding at the debonded interface proceeded in the direction of both the pushing surface and back surface from the peak shear position; the debonding from the back surface initiated just before the complete debonding of the interface. The pushout load-displacement curve near the origin was straight, however, after the existence of interface sliding at the debonded interface, the curve exhibited non-linearity with the increase in applied load up to the complete debonding at the interface. This debonding process was essentially independent of the fiber volume fraction. The results indicate that the total of thermally-induced stress in the specimen and shear stress distribution generated by applied load are important for the initiation of debonding and the frictional sliding process of the thin specimen pushout test.     

Interfacial shear strength of cast and directionally solidified NiAl-sapphire fiber composites

The feasibility of fabricating intermetallic NiAl-sapphire fiber composites by casting and zone directional solidification has been examined. The fiber-matrix interfacial shear strengths measured using a fiber push-out technique in both cast and directionally solidified composites are greater than the strengths reported for composites fabricated by powder cloth process using organic binders. Microscopic examination of fibers extracted from cast, directionally solidified (DS), and thermally cycled composites, and the high values of interfacial shear strengths suggest that the fiber-matrix interface does not degrade due to casting and directional solidification. Sapphire fibers do not pin grain boundaries during directional solidification, suggesting that this technique can be used to fabricate sapphire fiber reinforced NiAl composites with single crystal matrices.     

考虑残余应力的船舶强度校核分析方法

船舶进行结构设计时,通常是按照外载荷计算结构强度,并按照规范许用应力校核结构的安全性,并未考虑船舶各个构件在加工成形、焊接、装配等过程中产生的残余应力的影响.为此,对一艘VLCC油船在常见工况下加入大小不同残余应力的强度校核方法进行研究,得到各主要构件应力分布受残余应力影响的规律.结果表明,有必要在船体强度校核中考虑工艺残余应力的影响.     

Plastic relaxation of thermoelastic stress in aluminum/ceramic composites

The dislocation generation due to a thermoelastic stress in 2024 Al/ceramic (SiC or TiC) composites was studied using transmission electron microscopy Composites containing different ceramic particulates, ceramic volume fraction, and particle size were investigated. Dislocation density profiles were measured as a function of the distance from an Al/ceramic interface and compared with those calculated from an elastoplasticity model which accounts for the volume fraction of the ceramic particles. The intensity of dislocation generation showed a strong particle size dependence: as the ceramic particle size became of the order of a micron, the intensity of dislocation generation increased significantly. With an increase in the volume fraction of the ceramic particles, the dislocation density also increased, and the dislocation structure became a more tangled arrangement. If heat dissipation was taken into account as part of the plastic work, the predicted dislocation densities of the elastoplasticity model were found to be in reasonable agreement with the measured dislocation densities of 10⁹ to 10¹⁰ cm⁻².     

Effect of residual stresses on the strength of ceramic materials (Review)

The causes and laws of the appearance of residual stresses in ceramic materials during technological operations (sintering, annealing, neutron irradiation deformation processing, chemical interaction with a medium) are analyzed using our results and the data obtained by other researchers. The effect of residual stresses on the strength during mechanical static and cyclic loading, the thermal resistance, and the cracking resistance is considered.     

The role of metal plasticity and interfacial strength in the cracking of metal/ceramic laminates

Two models based on elastic-plastic fracture mechanics and fiber bridging are developed to study the role of plastic yielding in metals and the interfacial strength of metal/ceramic laminates. There are two types of damage observed in metal/ceramic laminates: multiple cracking and macroscopic crack propagation. The former occurs around the macroscopic crack tip and thus distributes the damage and enhances the composite's toughness. The present models establish that there exists a critical metal/ceramic layer thickness ratio above which multiple cracking dominates and that this ratio decreases (hence increasing the possibility of multiple cracking) as the ratios of metal yield stress over ceramic strength, metal modulus over ceramic modulus, and metal/ceramic interfacial strength over ceramic strength increase. Good agreement between the present models and experimental results is observed for both damage modes, i.e. multiple cracking vs macroscopic crack propagation, and for critical stress intensity factors. The elastic-plastic fracture mechanics and fiber-bridging models predict that multiple cracking is ensured if the metal layer thickness is 2.5 times larger than the ceramic layer thickness, regardless of the metal/ceramic properties.