Crack-Wake Debonding and Toughness in Fiber- or Whisker-Reinforced Brittle-Matrix Composites

Solutions are obtained for the mechanics of debonding in the crack wake in fiber- or whisker-reinforced composites for the case where a finite shear traction exists at the fiber/matrix interface in the debonded zone. These solutions are then applied to derive expressions for the steady-state toughness increases obtained in bonded composites wherein the toughness contribution is provided by crack-wake fiber/matrix debonding and crack bridging. The solutions for an unbonded composite containing a frictional fiber/matrix interface can be obtained from the derived equations in the limit of the fiber/matrix interface toughness equal to zero. In this limit, the debond crack length reduces to the slip length and the expressions for the crack opening and the predicted toughness increase reduce to previously derived expressions for unbonded composites. The steady-state toughness is found to depend sensitively on the interface toughness, the fiber fracture strength, and the shear tractions in the debonded zone including other material parameters, such as fiber radius and volume fraction and the moduli of the constituent phases. It is shown that in order to obtain finite toughness increases, the fiber/matrix interface toughness must be less than a critical value dependent on the fiber fracture strength, fiber radius and volume fraction, and fiber and matrix moduli. The predictions of the model are applied to published experimental results from a detailed and complete study of toughness increases in a bonded whisker-reinforced composite.     

Effect of Crack Velocity on Crack Resistance in Brittle-Matrix Composites

Fast- and slow-fracture studies were conducted on a glass matrix–metal particle composite system, using the appliedmoment double-cantilever-beam technique. At 10 vol % metal, the fracture toughness ( K_Ic ) was seen to increase 71% over that of the matrix alone, as compared to an increase of only 40% in the stress intensity ( K_I ) for the same system in slow fracture. This difference was corroborated by fracture surface stereology work, which showed 9.6% metal on the fast-fracture surface as compared to only 5.3% for the slowfracture specimens. This indicates that in slow fracture, the crack "chooses" its path of propagation, thus resulting in a lowering of the expected increase in crack resistance.     

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.     

Modeling the Ultimate Tensile Strength of Unidirectional Glass-Matrix Composites

The ultimate tensile strengths of a unidirectional glass-matrix composite were measured as a function of fiber volume fraction. The results were compared with predictions, using a refined solution of the stress field generated by an axisymmetric damage model, which incorporated the effect of stress concentration in the fiber caused by the presence of a matrix crack both before and after deflection at the fiber/matrix interface. Two possible locations for the fiber failure were considered: (1) at a transverse matrix crack, near a bonded fiber/coating interface and (2) at the tip of a debond, at the fiber/coating interface. At low fiber volume fractions, the measured ultimate tensile strength matched the prediction calculated, assuming no crack deflection. For higher volume fractions, the predictions calculated for a debonded crack matched the observed values. The model results were relatively insensitive to debond length and interfacial shear stress for the range of values in this study. In comparison, the global load-sharing model, which does not account for the stress singularity at the fiber/matrix interface, was found to overpredict the values of the ultimate tensile strength for all fiber volume fractions. An important contribution of the present work was to introduce the use of fiber volume fraction as a parameter for testing theoretical predictions of the mode of fiber failure.     

Cracking and Damage in a Notched Unidirectional Fiber-Reinforced Brittle Matrix Composite

Results of four-point bend tests on notched beams of a laminated unidirectional fiber-reinforced glass matrix composite are presented. The failure sequence has been established through in situ examination. The dominant damage mode is a mixed-mode, split crack that runs parallel to the predominant fiber directions. The crack interacts with and crosses over imperfectly aligned fibers. The resulting bridging tractions are sufficient to cause the critical strain energy release rate to increase substantially as the crack extends. Several other damage modes are also observed. These include mode I (tensile) matrix cracks bridged by fibers, mode II (shear) cracks, and compressive damage at the loading points.     

Interface Design for Oxidation-Resistant Ceramic Composites

Fiber-reinforced ceramic composites achieve high toughness through distributed damage mechanisms. These mechanisms are dependent on matrix cracks deflecting into fiber/matrix interfacial debonding cracks. Oxidation resistance of the fiber coatings often used to enable crack deflection is an important limitation for long-term use in many applications. Research on alternative, mostly oxide, coatings for oxide and non-oxide composites is reviewed. Processing issues, such as fiber coatings and fiber strength degradation, are discussed. Mechanics work related to design of crack deflecting coatings is also reviewed, and implications on the design of coatings and of composite systems using alternative coatings are discussed. Potential topics for further research are identified.     

Fracture Toughness of Graded Metal-Particulate/Brittle-Matrix Composites

A series of 0, 10, and 25 vol% metal-particulate/glass-matrix composites were prepared, with the composite volume fraction both constant and graded across the specimen bulk. Applied-moment double-cantilever-beam (AMDCB) fracture toughness results show, in general, that the graded composites had equivalent values when compared with the constant fraction specimens. This equivalence in toughness occurred despite lower overall volume fraction particulate phase and composite density for the graded composites. Additionally, these results indicate the necessity of a minimum composite layer thicknes for optimization of toughening.     

Interface Debonding and Fiber Cracking in Brittle Matrix Composites

Analyses of debonding along interfaces and of the kinking of interface cracks into a fiber have been used to define the role of debonding in fiber-reinforced, brittle matrix composites. The results reveal that, for fibers aligned with the tensile stress axis, debonding requires an interface fracture energy, Γ_i, less than about one-fourth that for the fiber, Γ_f. Further-more, once this condition is satisfied, it is shown that fiber failure does not normally occur by deflection of the debond through the fiber. Instead, fiber failure is governed by weakest-link statistics. The debonding of fibers inclined to the stress axis occurs more readily, such that debonds at acutely inclined fibers can deflect into the fiber, whereupon the failure of fibers is dominated by their toughness.     

Crack Deflection Process for Hot-Pressed Whisker-Reinforced Ceramic Composites

A crack deflection model for two-dimensional randomly arranged rods has been derived by making an appropriate modification to the model by Faber and Evans. This model predicts more effective toughening in certain directions than that for three-dimensional randomly arranged rods. The theoretical predictions are compared with experimentally measured fracture toughness data of hot-pressed whiskerreinforced ceramic-matrix composites.     

Prediction of Crack Paths in Particulate Composites Using Electrical Analog

An electrical analog technique was used to simulate crack paths in the vicinity of elastic inhomogeneities in ceramic composites. The technique makes use of the analogy that exists between electrical potential differences between clamped ends of a conducting foil with electrical inhomogeneities (second phases of different electrical conductivity) and the load-point displacements of a mechanically loaded plate with elastic inhomogeneities (second phases of different elastic modulus). In crack problems, this analogy is exact for mode III loading. The specific problems investigated were the interaction between a crack and a well-bonded second phase of higher elastic modulus (simulating a fiber or a second-phase particle) and the interaction between a crack and a pore. Crack paths were incrementally determined by maximizing the end-point potential differences (corresponding to the maximum energy release rate criterion in the mechanical system) with respect to the direction of crack growth. Comparable mechanical tests were carried out using precracked Plexiglas * plates with either circular holes or elastic inclusions. Good agreements were obtained between the electrically simulated crack paths and the crack paths observed in the mechanical tests despite the fact that the mechanical tests were carried out in mode I loading and the analogy is not exact.     

国内外植物纤维增强水泥基复合材料的研究

自然界广泛存在的植物纤维的形态具有长径比大,比强度高,比表面积大等优点。纤维在抑制混凝土裂缝发展中具有重要的作用,研究开发植物纤维增强水泥基复合材料不仅能降低混凝土的造价,而且有利于环保和可持续发展,具有深远的意义。文章综述了植物纤维的性能与增强作用、在混凝土应用中的研究进展、发展前景和存在的一些问题。     

Rate of Strength Decrease of Fiber-Reinforced Ceramic-Matrix Composites during Fatigue

An experimental investigation was performed to study the rate at which strength-controlling fatigue damage evolves in a ceramic-matrix composite. Tensile specimens of a unidirectional SiC-fiber-reinforced calcium aluminosilicate matrix composite were cycled to failure or to a preselected number of cycles under similar loading histories. The residual strength of the precycled specimens was found to be similar to that of virgin specimens. Microstructural investigations showed that the fracture surfaces of the specimens cycled to failure had a central region where fiber pullout was negligible. It is proposed that frictional heating (due to interfacial sliding) is the cause of fatigue failure. High interfacial temperatures are assumed to cause the formation of a strong interface bond, leading to internal embrittlement.     

Crack opening behavior in ceramic matrix composites

The evolution of matrix cracks in a melt‐infiltrated SiC/SiC ceramic matrix composite (CMC) under uniaxial tension was examined using scanning electron microscopy (SEM) combined with digital image correlation (DIC) and manual crack opening displacement (COD) measurements. CMC modeling and life prediction strongly depend a thorough understanding of when matrix cracks occur, the extent of cracking for given conditions (time‐temperature‐environment‐stress), and the interactions of matrix cracks with fibers and interfaces. In this work, strain relaxation due to matrix cracking, the relationship between CODs and applied stress, and damage evolution at stresses below the proportional limit were assessed. Direct experimental observation of strain relaxation adjacent to regions of matrix cracking is presented and discussed. Additionally, crack openings were found to increase linearly with increasing applied stress, and no crack was found to pass fully through the gage cross‐section. This calls into question the modeling assumption of through‐cracks for all loading conditions and fiber architectures, which can obscure oxidation mechanisms that are active in realistic cracking conditions. Finally, the combination of SEM with DIC is demonstrated throughout to be a powerful means for damage identification and quantification in CMCs at stresses well below the proportional limit.     

连续纤维增强陶瓷基复合材料界面研究进展

在陶瓷基复合材料中引入高强陶瓷纤维的目的是为了增强陶瓷的断裂韧性,纤维与基体的界面是决定CMC韧性的关键因素。国内外许多专家和机构研究重点主要集中于连续纤维增强陶瓷基复合材料的界面,包括纤维与基体的化学相容性和热物理相容性,以及用TEM、HRTEM、SADP、AEM、声学显微法、EDX等微观测试手段研究不同体系的界面形成机理。本文对上述界面研究概况进行了综述,并简述了界面设计原则和近年来计算机技术在界面研究中的应用情况。指出,连续纤维增强陶瓷基复合材料界面研究将一直是复合陶瓷基复合材料界研究的重点和难点。     

Crack Growth and Acoustic Emission in Ceramics During Thermal Shock

It is recognized that microcracks may contribute to failure of ceramics undergoing thermal shock, although there is little direct experimental evidence. The combination of acoustic emission (AE) and SEM observations provides such evidence. In this work AE data were analyzed after rapid quenching of samples in silicone oil. A thermal shock resistant material, alumina, and a material resistant to thermal damage, advanced zirconia refractory, have been examined. Cracks were detected by analysis of AE amplitudes and durations and their growth was monitored by systematic SEM observations as thermal shocks of increasing severity were applied. Three-point bending strengths were determined in air after quenching. For the first time SEM images are presented showing early stages of crack initiation for temperature differences less than Δ T_crit , i.e., where the fracture was not believed to occur. Further development of the cracks leads to abrupt strength reduction in alumina and controlled strength loss in zirconia, although AE data did not indicate any particular pattern of catastrophic crack propagation when substantial loss of strength occurred.     

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.     

Does a True Fatigue Limit Exist for Continuous Fiber-Reinforced Ceramic Matrix Composites?

The high-cycle high-frequency fatigue behavior of a Nicalon-fiber-reinforced calcium aluminosilicate ceramic composite was investigated. A key goal of the room-temperature fatigue experiments was to determine if a true fatigue limit or endurance limit existed for this ceramic matrix composite. Although no fatigue failures occurred beyond 10⁷ cycles, the stress–strain hysteresis modulus and frictional heating continued to change up to 10⁸ cycles, at which point the 200 Hz experiments were terminated. This suggests that fatigue damage continued to evolve and that a true fatigue limit may not exist in ceramic matrix composites that have undergone interfacial frictional sliding.     

Chemically Extracted Cornhusk Fibers as Reinforcement in Light‐Weight Poly(propylene) Composites

Flexural, impact resistance, tensile, and sound absorption properties of composites from cornhusk fiber (CHF) and PP have been investigated. The effect of holding temperature, CHF length, CHF concentration, and enzyme treatment of CHF on mechanical properties and the effect of the latter two on sound absorption have been studied. Compared with jute/PP composites, CHF/PP composites have similar impact resistance, 33% higher flexural strength, 71% lower flexural modulus, 43% higher tensile strength, 54% lower tensile modulus, and slightly higher noise reduction coefficient. Enzyme treatment of CHF results in increased mechanical and sound absorption properties.

     

A Review on Wear and Friction Performance of Carbon–Carbon Composites at High Temperature

Carbon–carbon (C/C) composite is one of the best ceramic matrix composite due to its high mechanical properties and applications at control environments in various sectors. Carbon–carbon composite is made of woven carbon fibers; carbonaceous polymers and hydrocarbons are used as matrix precursors. These composites generally have densities <2.0 g/cm³ even after densification. C/C composites have good frictional properties and thermal conductivity at high temperature. Also C/C composite can be used as brake pads in high‐speed vehicles. In spite of various applications, C/C composites are very much prone to oxidation at high temperature. Therefore, C/C composites must be protected from oxidation for the use at high temperature.     

Micromechanical Stress Concentrations in Two-Phase Brittle-Matrix Ceramic Composites

A quantitative investigation was conducted on the effect of micromechanical stress concentrations on the strength of two-phase brittle-matrix ceramic systems. The materials consisted of a continuous brittle matrix containing dispersions with elastic properties different from those of the matrix. A soda borosilicate glass was used as the matrix and the dispersions consisted of spherical alumina particles 60μ in diameter and spherical pores 60μ in diameter. Stress concentrations were varied by measuring the strength of the composite under uniaxial and biaxial tensile stress conditions. The experimental results showed that micromechanical stress concentrations strongly affect the macroscopic strength of the composite. Under biaxial tensile stress, additions of either alumina microspheres or spherical porosity to the glass matrix resulted in a decrease in strength equal to the maximum calculated stress concentration factor. Under uniaxial tensile stress conditions, however, the reduction in strength for the glass-alumina system was negligible. The glass-porosity system gave a reduction in uniaxial strength which was not equal to the maximum calculated stress concentration factor. Experimental results suggest that differences in strength of brittle multi-component systems under uniaxial and biaxial stress states can be attributed in part to micro-structural features. On the basis of the experimental work, a hypothesis is developed relating the relative size of the region in the glass matrix over which stress concentrations act to the size of the Griffith flaws responsible for failure. This hypothesis is extended to the effect of porosity on the strength of polycrystalline brittle ceramic materials.