Study on interfacial debonding stress and damage mechanisms of C/SiC composites using acoustic emission

Among ceramic matrix composites (CMCs), carbon fiber-reinforced silicon carbide matrix (C/SiC) composites are widely used in numerous high-temperature structural applications because of their superior properties. The fiber–matrix (FM) interface is a decisive constituent to ensure material integrity and efficient crack deflection. Therefore, there is a critical need to study the mechanical properties of the FM interface in applications of C/SiC composites. In this study, tensile tests were conducted to evaluate the interfacial debonding stress on unidirectional C/SiC composites with fibers oriented perpendicularly to the loading direction in order to perfectly open the interfaces. The characteristics of the material damage behaviors in the tensile tests were successfully detected and distinguished using the acoustic emission (AE) technique. The relationships between the damage behaviors and features of AE signals were investigated. The results showed that there were obviously three damage stages, including the initiation and growth of cracks, FM interfacial debonding, and large-scale development and bridging of cracks, which finally resulted in material failure in the transverse tensile tests of unidirectional C/SiC composites. The frequency components distributed around 92.5 kHz were dominated by matrix damage and failure, and the high-frequency components distributed around 175.5 kHz were dominated by FM interfacial debonding. Based on the stress and strain versus time curves, the average interfacial debonding stress of the unidirectional C/SiC composites was approximately 1.91 MPa. Furthermore, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS) were used to observe the morphologies and analyze the chemical compositions of the fractured surfaces. The results confirmed that the fiber was completely debonded from a matrix on the fractured surface. The damage behaviors of the C/SiC composites were mainly the syntheses of matrix cracking, fiber breakage, and FM interfacial debonding.     

Influence of Interfacial Sliding Stress on Fatigue Behavior of Oxidized Nicalon/Calcium Aluminosilicate Composites

The influence of the interfacial sliding stress on the steady-state tensile fatigue behavior of a Nicalon/calcium aluminosilicate composite has been characterized experimentally. Characterization occurred in an experiment where the microstructure and properties of the carbon interface were altered by oxidation at increasingly greater temperatures. After each oxidation step, the steady-state energy dissipation was quantified. Using a model, changes in the interfacial sliding stress were determined after each oxidation step. The results revealed a minimum in the sliding stress after oxidation in the temperature range of 100°-300°C that corresponded to a maximum in the amount of energy dissipated by frictional sliding.     

Interface Properties in a Porous-Matrix Oxide Composite

This study focuses on the interfacial properties of a family of porous matrix oxide composites with uncoated fibers. Measurements of debond energy and sliding stress are made using a modified version of the established fiber push-in test. Modifications include the following: (i) use of a sphero-conical indenter (not a sharp-tipped one) to produce only elastic deformation of the fibers, and (ii) analysis of the loop width (instead of absolute displacements) to ascertain interface properties. The method obviates the need for indentation tests on reference (non-sliding) fibers. It also mitigates the problems associated with the elastic deformation of the surrounding matrix. The measured debond toughnesses (about 0.05 J/m²) are about two orders of magnitude lower than the fiber toughness. This ensures that debonding will occur when a matrix crack impinges on a fiber. Additionally, the sliding stresses are in the same range as those reported for C-coated Nicalon fibers in glass–ceramic matrices (about 5 MPa). The latter results are qualitatively consistent with the observed damage tolerance in these two seemingly disparate systems, as manifested in the degree of fiber pullout as well as the notch sensitivity of tensile strength.     

Effect of Carbon and Silicon Carbide/Carbon Interlayers on the Mechanical Behavior of Tyranno-SA-Fiber-Reinforced Silicon Carbide-Matrix Composites

Several CVI-SiC/SiC composites were fabricated and the mechanical properties were investigated using unloading–reloading tensile tests. The composites were reinforced with a new Tyranno-SA fiber (2-D, plain-woven). Various carbon and SiC/C layers were deposited as fiber/matrix interlayers by the isothermal CVI process. The Tyranno-SA/SiC composites exhibited high proportional limit stress (∼120 MPa) and relatively small strain-to-failure. The tensile stress/strain curves exhibited features corresponding to strong interfacial shear and sliding resistance, and indicated failures of all the composites before matrix-cracking saturation was achieved. Fiber/matrix debonding and relatively short fiber pullouts were observed on the fracture surfaces. The ultimate tensile strength displayed an increasing trend with increasing carbon layer thickness up to 100 nm. Further improvement of the mechanical properties of Tyranno-SA/SiC composites is expected with more suitable interlayer structures.     

Effects of interfacial adhesion on microdamage and rheological behaviour of glass bead filled nylon 6

Model composites consisting of glass beads dispersed in a matrix of nylon 6 with different degrees of interfacial adhesion were prepared. The effects of interfacial adhesion on damage generation in the glass bead filled nylon 6 were studied by acoustic emission monitoring and scanning electron microscopy, and the critical damage stress was measured. Improvement of the interfacial adhesion enhanced the tensile strength of the composite. The melt rheological behaviour of the glass bead filled nylon 6 was also investigated. It was found that the interfacial adhesion strength significantly affected the rheological behaviour of the glass bead filled nylon 6.     

Analysis of a pull-out test with real specimen geometry. Part II: the effect of meniscus

This paper continues our study on the platelet model of the pull-out specimen, in which the matrix droplet shape is approximated by a set of thin parallel disks with the diameters varying along the embedded fiber. Using this model, the fiber tensile stress and the interfacial shear stress profiles were calculated for real-shaped matrix droplets, including menisci (wetting cones) on the fibers, taking into account residual thermal stresses and interfacial friction. Then, these profiles were used to numerically simulate the processes of crack initiation and propagation in the pull-out test and to obtain theoretical force-displacement curves for specimens with different embedded lengths and wetting cone angles. Our simulations showed that the interfacial crack in real-shaped droplets initiated at very small (practically zero) force applied to the fiber, in contrast to the popular ‘equivalent cylinder’ approximation. As a result, the equivalent cylinder approach underestimated the interfacial shear strength (IFSS) value determined from the pull-out test and at the same time overestimated the interfacial frictional stress; the smaller was the wetting cone angle, the greater the difference. We also investigated the effects of the embedded fiber length and interfacial frictional stress in debonded areas on the calculated IFSS. The simulated force–displacement curves for the real-shaped droplets showed better agreement with experimental curves than those plotted using the equivalent cylinder approach.     

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.     

Effect of Interface Roughness on Fiber Push-Out Stress

A theoretical analysis is given for single-fiber push-out with a rough interface whose asperity amplitude can be expressed by a Fourier series that has a good convergence. Based on a three-cylinder model for the single-fiber push-out test, the solutions of the fiber frictional push-out stress and the relative displacement of the fiber to the matrix at the fiber-matrix interface are obtained. A new asperity wear model is introduced in the analysis to simulate the degradation of the asperity amplitude during the fiber push-out process. The interfacial roughness between the fiber and the matrix is found to have a pronounced influence on the fiber sliding behavior.     

Use of a Crack-Bridging Single-Fiber Pullout Test to Study Steel Fiber/Cementitious Matrix Composites

The fracture process of steel fiber/cementitious matrix composites has been studied using a single-fiber pullout test that permits detailed measurements of the load-crack opening displacement relationship during fiber debonding and unloading. Using a suitable analytical model, the interfacial fracture energy and interfacial sliding friction have been calculated for composites incorporating steel fibers with cement paste or mortar matrices. Comparison of theoretical debonding curves with the experimental data show that the model accurately represents the fiber debonding process, except for a decrease in interfacial sliding friction due to wear of matrix asperities at the interface. Differences between the calculated interfacial properties of several specimens are associated with changes in the interfacial microstructure.     

Interface effect on the mechanical behaviour of rigid particle filled polymer

Glass beads, non‐modified and modified separately with two different coupling agents, were incorporated in high density polyethylene to prepare composite materials with different interfacial adhesion strengths. Tensile tests show that the mechanical behaviour of the materials is sensitive to the strain rates. The strong interfacial adhesion can delay the occurrence of damage and so increase the load‐bearing ability under both monotonic and cyclic loading. In situ tensile tests give damage mechanisms mainly induced by the interfacial debonding. The stronger the interfacial adhesion, the lower the number of glass beads debonded from the matrix under a given stress. The degree of microdamage defined as the percentage of debonded particles is obtained as a function of the applied load. © 2001 Society of Chemical Industry     

Effects of cyclic tensile loading on the rupture behavior of orthogonal 3-D woven SiC fiber/SiC matrix composites at elevated temperatures in air

This study examined the rupture mechanisms of an orthogonal 3D woven SiC fiber/BN interface/SiC matrix composite under combination of constant and cyclic tensile loading at elevated temperature in air. Monotonic tensile testing, constant tensile load testing, and tension–tension fatigue testing were conducted at 1100 °C. A rectangular waveform was used for fatigue testing to assess effects of unloading on the damage and failure behavior. Microscopic observation and single-fiber push-out tests were conducted to reveal the rupture mechanisms. Results show that both oxidative matrix crack propagation attributable to oxidation of the fiber–matrix interface and the decrease in the interfacial shear stress (IFSS) at the fiber–matrix interface significantly affect the lifetime of the SiC/SiC composites. A rupture strength degradation model was proposed using the combination of the oxidative matrix crack growth model and the IFSS degradation model. The prediction roughly agreed with the experimentally obtained results.     

Fatigue behavior and damage evolution of SiC/SiC composites under high-temperature anaerobic cyclic loading

In the present study, the fatigue behavior and damage evolution of SiC/SiC minicomposites at elevated temperatures in oxygen-free environment are investigated which are important for their application and are still unclear. The high-temperature fatigue test platform is developed and the fatigue stress-life curve and the stress-strain response are obtained. The test result shows that the life of the material at elevated temperature is shorter than that at room temperature under the same stress level. Moreover, the hysteresis loop width and the residual strain increase with the increasing of the cycles while the hysteresis modulus decreases during the fatigue cycling. The evolution process of matrix cracks is observed using the real-time remote detection system. It is found that matrix cracking is insensitive to the cyclic loading which is similar to room temperature and is due to that the degeneration of the interfacial shear stress reduces the area of high stress in matrix. The fiber/matrix interfacial shear stress under different cycles is determined based on the fatigue modulus of each hysteresis loop. The result shows a fatigue enhancement phenomenon for the interface which is not observed at room temperature.     

Temperature Dependence of Tensile Strength for a Woven Boron-Nitride-Coated Hi-Nicalon™ SiC Fiber-Reinforced Silicon-Carbide-Matrix Composite

The temperature dependence of tensile fracture behavior and tensile strength of a two-dimensional woven BN-coated Hi-Nicalon™ SiC fiber-reinforced SiC matrix composite fabricated by polymer infiltration pyrolysis (PIP) were studied. A tensile test of the composite was conducted in air at temperatures of 298 (room temperature), 1200, 1400, and 1600 K. The composite showed a nonlinear behavior for all the test temperatures; however, a large decrease in tensile strength was observed above 1200 K. Young's modulus was estimated from the initial linear regime of the tensile stress–strain curves at room and elevated temperatures, and a decrease in Young's modulus became significant above 1200 K. The multiple transverse cracking that occurred was independent of temperature, and the transverse crack density was measured from fractographic observations of the tested specimens at room and elevated temperatures. The temperature dependence of the effective interfacial shear stress was estimated from the measurements of the transverse crack density. The temperature dependence of in situ fiber strength properties was determined from fracture mirror size on the fracture surfaces of fibers. The decrease in the tensile strength of the composite up to 1400 K was attributed to the degradation in the strength properties of in situ fibers, and to the damage behavior exception of the fiber properties for 1600 K.     

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.     

The pull-out and failure of a fiber bundle in a carbon fiber reinforced carbon matrix composite

The interfacial failure is examined for a unidirectionally reinforced carbon fiber/carbon matrix composite. A novel tensile test is conducted which realizes the processes of interfacial debonding and subsequent pull-out of a fiber bundle from the surrounding composite medium. The critical stress at the onset of delamination cracking is related to the fracture energy (the critical energy release rate for mode II cracking). A force-balance equation of a fiber bundle, which is quasi-statically pulled-out of the composite socket, is formulated in terms of the inter- and intra-laminar shear strengths of the composite. This equation is successfully used to estimate the delamination crack length along the debonded fiber bundle, as a function of the stress applied to the bundle.     

Phenolic rigid organic filler/isotactic polypropylene composites. II. Tensile properties

A novel phenolic rigid organic filler (KT) was used to modify isotactic polypropylene (iPP). The influence of KT particles on the tensile properties of PP/KT microcomposites was studied by uniaxial tensile test and the morphological structures of the stretched specimens were observed by scanning electron microscopy (SEM) and polarized optical microscopy (POM). We found that the Young’s modulus of PP/KT specimens increased with filler content, while the yield and break of the specimens are related to the filler particles size. The yield stress, the breaking stress and the ultimate elongation of PP/KT specimens were close to those of unfilled iPP specimens when the maximal filler particles size is less than a critical value, which is 7 ?m at a crosshead speed of 10 mm/min and 3 ?m at 200 mm/min, close to that of glass bead but far more than those of other rigid inorganic filler particles. The interfacial interaction was further estimated from yield stress, indicating that KT particles have a moderate interfacial interaction with iPP matrix. Thus, the incorporation of small KT particles can reinforce iPP matrix and simultaneously cause few detrimental effects on the other excellent tensile properties of iPP matrix, due to their organic nature, higher specific area, solid true-spherical shape and the homogenous dispersion of the ROF particles in microcomposites.     

Shear-Lag Analysis of Fiber Push-Out (Indentation) Tests for Estimating Interfacial Friction Stress in Ceramic-Matrix Composites

A shear-lag analysis is presented for estimating sliding friction stress at fiber-matrix interfaces in ceramic-matrix composites using the single-fiber push-out test. The analysis includes an approximate correction for the increased interfacial compression and, therefore, the interfacial friction stress arising from the transverse (Poisson) expansion of the fibers subjected to the compressive load. An exponential decrease of the interfacial shear stress along the fiber length is predicted. This result is similar to the results of a finite-element analysis reported in the literature. The analysis also provides a basis for the experimental determination of a coefficient of interfacial friction (μ) and a residual interfacial compression (σ₀). It is shown that the sliding friction stress (τ_f=μσ₀) can be overestimated if the transverse expansion of the fibers is not taken into account.     

Phenolic rigid organic filler/isotactic polypropylene composites. II. Tensile properties

A novel phenolic rigid organic filler (KT) was used to modify isotactic polypropylene (iPP). The influence of KT particles on the tensile properties of PP/KT microcomposites was studied by uniaxial tensile test and the morphological structures of the stretched specimens were observed by scanning electron microscopy (SEM) and polarized optical microscopy (POM). We found that the Young’s modulus of PP/KT specimens increased with filler content, while the yield and break of the specimens are related to the filler particles size. The yield stress, the breaking stress and the ultimate elongation of PP/KT specimens were close to those of unfilled iPP specimens when the maximal filler particles size is less than a critical value, which is 7 μm at a crosshead speed of 10 mm/min and 3 μm at 200 mm/min, close to that of glass bead but far more than those of other rigid inorganic filler particles. The interfacial interaction was further estimated from yield stress, indicating that KT particles have a moderate interfacial interaction with iPP matrix. Thus, the incorporation of small KT particles can reinforce iPP matrix and simultaneously cause few detrimental effects on the other excellent tensile properties of iPP matrix, due to their organic nature, higher specific area, solid true-spherical shape and the homogenous dispersion of the ROF particles in microcomposites.     

界面破坏对割口单向叠层板的应力集中及强度的影响

基于剪滞理论,建立一种计及界面损伤的分层剪滞模型,分析了含割口的单向叠层板在拉伸荷载作用下的应力重新分布问题,据此可求得界面损伤区长度和割口前缘完整纤维的应力集中因子.在此基础上,采用细观统计破坏理论,研究了割口单向叠层板的拉伸破坏机理和强度,定量获得了残余拉伸强度与界面剪切强度的关系,所得结果与现有实验吻合较好.结果表明,应力集中和强度与割口长度及界面剪切强度有关;适宜的界面黏结,具有较高的残余拉伸强度.     

Effect of water absorption on the behavior of E-glass fiber/nylon-6 composites

The effect of water absorption on the stress transferability across E-glass fiber/nylon 6 interface has been studied using the embedded single fiber composite technique. The behavior of silane coated fiber and untreated fiber composites after periods of water immersion were compared. The silane coating provided both higher interfacial shear stress transferability and protection from permanent water damage in the interphase region. It was found that water absorption in the nylon matrix reduced the shear stress transferability through plasticization of the matrix, weakening of the interface, and the development of tensile swelling stresses at the phase boundaries. In untreated materials the shear stress transferability was limited by decoupling of the matrix from the broken fiber ends by either interface slippage or local plane strain fracture in the interphase region near the fiber end. In the silane treated materials the shear stress transferability was limited by constrained yielding of the polysiloxane/nylon interphase at the fiber end, thus indicating plasticization of the matrix was the primary factor. After 20 days of water immersion, there was permanent deterioration of stress transferability in the untreated samples, but very little permanent damage in the treated samples.