Fiber Pull-out Length Research Articles

High mechanical performance SiC/SiC composites by NITE process with tailoring of appropriate fabrication temperature to fiber volume fraction

Unidirectional SiC/SiC composites are prepared by nano-powder infiltration and transient eutectic-phase (NITE) process, using pyrolytic carbon (PyC)-coated Tyranno-SA SiC fibers as reinforcement and SiC nano-powder with sintering additives for matrix formation. The effects of two kinds of fiber volume fraction incorporating fabrication temperature were characterized on densification, microstructure and mechanical properties. Densification of the composites with low fiber volume fraction (appropriately 30 vol%) was developed even at lower fabrication temperature of 1800 °C, and then saturated at 3rd stage of matrix densification corresponding to classic liquid phase sintering. Hence, densification of the composites with high volume fraction (above 50 vol%) became restricted because the many fibers retarded the infiltration of SiC nano-powder at lower fabrication temperature of 1800 °C. When fabrication temperature increased by 1900 °C, densification of the composites was effectively enhanced in the intra-fiber-bundles and simultaneously the interaction between PyC interface and matrix was strengthened. SEM observation on the fracture surface revealed that fiber pull-out length was accordingly changed with fabrication temperature as well as fiber volume fraction, which dominated tensile fracture behaviors. Through NITE process, SiC/SiC composites with two fracture types were successfully developed by tailoring of appropriate fabrication temperature to fiber volume fraction as follows: (1) high ductility type and (2) high strength type.

Distribution of fibre pullout length and interface shear strength within a single fibre bundle for an orthogonal 3-D woven Si–Ti–C–O fibre/Si–Ti–C–O matrix composite tested at 1100 °C in air

The distributions of fibre strength, pullout length, and fibre/matrix interface shear strength within a single fibre bundle were investigated for a 3-D woven SiC/SiC composite tensile tested at 1100 °C in air. Fibre pullout lengths were largest at the fibre bundle centre with an embrittled region of approximate width 15 μm at the perimeter. Whereas the fibre strength varied by less than a factor of 2 across the fibre bundle, the fibre/matrix interface shear strength varied by a factor of ∼23 with a minimum (100±16 MPa) at the centre and a maximum (2.25±0.21 GPa) close to the embrittled region. The minimum fibre/matrix interface shear strength required for the transition between pseudo-ductile and brittle behaviour was thus estimated to be 2.25±0.21 GPa for this composite system.

938 SiC/SiC 複合材料の引張強さと引抜け長さの評価

938 SiC/SiC 複合材料の引張強さと引抜け長さの評価

Notched strength of satin-woven Si-Ti-C-O fiber-bonded ceramic composite

The fracture behavior of Si-Ti-C-O fiber-bonded ceramic composite produced by hot-pressing oxidized 8 harness-satin-woven Si-Ti-C-O fibers was investigated by using unnotched and double edge notched tensile test specimens with different width (8 and 40 mm). The main results are summarized as follows. (i) The tensile strength of unnotched specimens for 8 mm width was higher than that for 40 mm width. Such a width-dependence of the unnotched strength could be described fairly well from the viewpoint of effective volume by application of the experimentally estimated Weibull's shape parameter. (ii) The applicability of the fracture toughness criterion (fracture arises when the stress intensity factor reaches the critical value) and net section stress criterion (fracture arises when the strength of the ligament reaches the unnotched strength) to the present composite was examined. The fracture strength of a notched specimen for 8 mm width was described by the net stress criterion. On the other hand, the strength for 40 mm width obeyed the net stress criterion for a small notch length but it shifted toward the fracture toughness criterion for large one. The shift of the fracture criterion from net strength- to fracture toughness-criterion arose around at the relative notch length 0.2 (notch length 8 mm), corresponding to periodical spacing of fiber strands (8 harness). (iii) The fiber pull-out length (0.4 mm on an average) was nearly the same as the half length of the fiber strand whose deformation is not constricted by the other strands in the satin-weave. (iv) The present fiber-bonded ceramic composite is insensitive to notch under the condition where the width of specimen is narrow and the notch length is smaller than 8 mm. This composite could be therefore applicable to industrial objects safely when the objects are designed as to satisfy the notch-insensitive condition.

Temperature-Dependence of Mechanical Properties of Si-Ti-C-O Fiber-Bonded Ceramic.

Temperature-dependence of various mechanical properties of Si-Ti-C-O fiber-bonded ceramic is investigated. The material was produced by hot-pressing the laminae of oxidized satin-woven Si-Ti-C-O fibers, and then showed very high fiber-volume fraction (around 0.85) and very little porosity, compared with ordinary ceramic matrix composites. The in-plain tensile, compressive and inter-laminar shear strengths of the material at room temperature were maintained up to 1400°C, whereas the Young's modulus and the compressive strength of out-plain direction (transverse direction) decreased at 1400°C. The behavior toward the tensile and inter-laminar shear strengths were in good agreement with the changes in fiber pull-out length and delaminating position, respectively. A buckling of the fibers parallel to the stress axis and a large deformation of the specimen were observed in the fracture surface after compression tests of in- and out-plain directions at 1500°C, respectively. From the differences in the mechanical properties and fracture morphologies, it was concluded that the change in the fracture strength at the high temperature was caused by softening of the amorphous SiO2 phase.

Fiber/matrix interface shear strength estimated from fiber pullout length data for Tyranno® Si-Zr-C-O fiber composites with different SiC-based matrices and interfaces

Fiber/matrix interface shear strength estimated from fiber pullout length data for Tyranno® Si-Zr-C-O fiber composites with different SiC-based matrices and interfaces

Mechanical Behavior of Advanced Organic Composites in Space Environmental Condition(Student Poster Session)

We investigate the mechanical behavior of the fiber reinforced organic composites in high vacuum and temperature condition. The material used in this study is short carbon fiber reinforced thermoplastic polyimide composites (SCFR-PI) with various fiber contents. Tension tests were carried out with the aid of AE method. After the tests, we observed the damage propagation in SCFR-PI specimens by scanning electron microscope photographs and optical microscope photographs. The main results are summarized, as follows: (1) Even though tested at the same temperature 298 K (25℃), the SCFR-PI composite specimen kept in high vacuum condition has higher tensile strength and higher equivalent Young's modulus than that in the atmospheric condition. (2) The fiber pull-out length of the specimen in high vacuum condition is shorter than that in the atmospheric condition and the fracture surfaces are not flat. The strength of the interfacial bonding between carbon fibers and the thermoplastic polyimide resin matrix becomes better in high vacuum condition. The degas and/or moisture desorption may change the performance of each materials and microfractures of SCFR-PI composites. (3) It is found that there exists the effect of vacuum condition on the tensile strength and Young's modulus of SCFR-PI composites and the microfractures.

Dynamic effect on strength in SiC f/SiC m composite

Loading rate dependence of mechanical properties of SiC fibre-reinforced SiC composites (SiC f/SiC m) has been experimentally investigated as to the fibre volume fraction and coating materials for SiC fibre. The composites consisting of monolithic SiC and SiC fibre (Hi-Nicalon) coated with Boron-Nitride (BN) or Carbon (C) with fibre volume fractions of 20, 30 and 40% were fabricated by polymer infiltration–pyrolysis (PIP) process. The stress–strain response and strength were measured in tension over a wide range of strain rate,10 −4∼200 s −1. It was shown that the higher volume fraction, the larger tensile strength regardless of the kind of coating and strain rate. The interface friction stress evaluated by the fibre pullout length that is measured through microscopic observations of fractured specimens is larger in dynamic loading than in static loading. The BN-coated fibre gave the composite superior tensile strength to the C-coated fibre. This trend results from the variety of the interface friction stress associated with the coating thickness.

Temperature Dependence ofin situConstituent Properties of Polymer-infiltration-pyrolysis-processed Nicalon™ SiC Fiber-reinforced SiC Matrix Composite

Temperature dependence ofin situfiber strength, effective interface shear stress, Young's modulus of matrix, and matrix fracture energy in a polymer-infiltrationpyrolysis (PIP)-processed two-dimensional plain-woven fabric carbon-coated Nicalon™ SiC fiber-reinforced SiC matrix composite was studied through a tensile test in air at 298 (room temperature), 800, and 1200 K.In situfiber strength and effective interface shear stress were determined by fracture mirror size and fiber pullout length measurements, respectively. The fiber strength was insensitive to test temperature up to 800 K but dropped significantly at 1200 K. Conversely, the interface shear stress showed a strong temperature dependence, decreasing at 800 K and drastically increasing at 1200 K. The temperature dependence of both values was reasonably explained. Temperature dependence of Young's modulus of matrix was derived from Young's modulus of the composite and fiber and ranged from ≈40 to ≈38 GPa. Matrix fracture energy was also determined from the transverse matrix cracking stress and ranged from ≈16 to ≈5.5 J/m2. Both Young's modulus of matrix and the matrix fracture energy showed only slight temperature dependence up to 800 K; however, both values decreased significantly at 1200 K.

Comparison of fibre/matrix interface strength for a 3D woven SiC/SiC composite

The fibre/matrix interface shear strength, τ P, was determined by analysis of fibre pullout length distributions for a 3D woven SiC/SiC-based composite that had undergone tensile testing between room temperature and 1300°C in vacuum and air. Data was compared with the fibre/matrix interface shear strength, τ S, obtained previously for this system by analysis of in situ fibre strength distributions. τ P was found to follow the same general trend as that of τ S and this was explained in terms of the carbon-rich fibre/matrix interface region. However, τ P was smaller than τ S by a factor of 3-4 for all cases, but the reason for this remains unclear although several tentative suggestions have been put forward.

Prediction of the fracture toughness of fibrous composites

A statistical/micromechanical model is developed for the prediction of the fracture toughness of fibrous composites. The fracture resistance of the material is assumed to be related to the statistical distribution of the fiber pull-out length. The distribution of the fiber pull-out length is derived from the fiber strength distribution. The R-curve behavior of the fibrous composite is predicted and interpreted based on the present model. The limiting fracture toughness is predicted to be proportional to the square root of the ineffective length, or proportional to the square root of the fiber length if the fiber length is less than the ineffective length.

Fibre and interfacial properties measured in situ for a 3D woven SiC/SiC-based composite with glass sealant

Abstract In situ fibre strength Weibull parameters and fibre pullout length distributions were investigated for a 3D woven SiC/SiC-based composite with glass sealant oxidation protection system after tensile testing at 1000°C and 1200°C in air. Results were similar to those previously observed for unsealed specimens tested in vacuum (1200–1380°C), but significantly different from the case of unsealed specimens tested in air (1100–1200°C). Overall, it was concluded that the glass sealant provided excellent oxidation protection for the composite at the test conditions employed, i.e. short-term exposure up to 1200°C in air.

Modeling brittle and tough stress–strain behavior in unidirectional ceramic matrix composites

A new simple model for predicting the uniaxial stress–strain behavior of a unidirectional ceramic matrix composite, including stochastic matrix crack evolution, stochastic fiber damage and ultimate failure, is presented. The model demonstrates an important transition in composite behavior. “Brittle” (low failure strain) behavior occurs when the matrix cracking stresses are sufficiently high; the composite fails during the matrix cracking regime of deformation and at a strain that is controlled by the matrix flaw population and elastic properties. “Tough” (high failure strain) behavior occurs when the matrix cracking stresses are lower; matrix cracking is completed prior to failure and the failure strain of the composite is controlled by the fibers. In both cases, the failure strength is fiber-controlled. The model is applied to study SiC/SiC 500-fiber minicomposite deformation, using data recently obtained by Lissart and Lamon on two material types, “B” and “C”. Parameters for the matrix flaw population are used to fit the experimental stress–strain data but the failure is controlled by the measured fiber strength statistics. Excellent agreement is found for the “C” materials, which are in the transition regime between the brittle and tough limits and variations in fiber strength are postulated to be responsible for the wide range of behaviors found in the “B” materials. The fitted matrix flaw parameters are then used to predict the fiber/matrix interfacial sliding resistance and the values obtained are in excellent agreement with independent values determined from both unload/reload hysteresis loops and fiber pullout lengths. The new model provides a useful tool for understanding the interplay matrix and fiber flaw distributions and the overall dependence of stress–strain behavior on all the underlying constituent material properties.

Residual strength of a fiber reinforced metal matrix composite with a crack

The residual strength of a cracked unidirectional fiver reinforced metal matrix composite is studied. We propose a bridging model based on the Dugdale strip yielding zones in the matrix ahead of the crack tips that accounts for ductile deformations of the matrix and fiber debonding and pull-out in the strip yielding zone. The bridging model is used to study the fracture of an anisotropic material and its residual strength is calculated numerically. The predicted results for a SiC/titanium composite agree well with the existing experimental data. It is found that a higher fiber bridging stress and a larger fiber pull-out length significantly contribute to the composite's residual strength. The composite's strength may be more notch-insensitive than the corresponding matrix material's strength depending on several factors such as fiber-matrix interface properties and the ratio of the matrix modulus to an ‘effective modulus’ of the composite.

Effect of Loading Rate on the Monotonic Tensile Behavior of a Continuous‐Fiber‐Reinforced Glass‐Ceramic Matrix Composite

The stress‐strain behavior of a continuous‐fiber‐reinforced ceramic matrix composite has been measured over a wide range of loading rates (0.01 to 500 MPa/s). It was found that the loading rate has a strong effect on almost every feature of the stress‐strain curve: The proportionality stress, the composite strength and failure strain increase with increasing loading rate. The microstructural damage varies also with the loading rate; with increasing loading rate, the average matrix crack spacing increases and the average fiber pullout length decreases. Using simple models, it is suggested that these phenomena are caused partly by time‐dependent matrix cracking (due to stress corrosion) and partly by an increasing interfacial shear stress with loading rate.

The Influence of Fiber/Matrix Interface on the Mechanical Behavior of Nicalon SiC Fiber Reinforced Glass-Ceramic Composites

ABSTRACTThe mechanical properties of unidirectional Nicalon SiC fiber reinforced calcium aluminosilicate (CAS/SiC) and magnesium aluminosilicate (MAS/SiC) glass-ceramic composites have been investigated by tensile testing and a nondestructive laser-ultrasound technique. The barium-stuffed MAS was either undoped or doped with 5% borosilicate glass. The degradation of the elastic stiffness constant Cu in the transverse direction due to interface damage was monitored in-situ by measuring the laser-generated ultrasound wave velocity. The three composite materials show distinctly different macroscopic deformation characteristics, which are correlated strongly to the interface degradation. A stronger reduction trend of the elastic constant ?? is associated with a larger degree of inelastic deformation. Observations of the fracture surfaces also reveal the close relation between fiber pullout length and interfacial characteristics. Interfaces of these composites have been studied by TEM, and their influence on inhibiting and deflecting matrix cracks is discussed.

Fatigue crack growth of SM-1240/TIMETAL-21S metal matrix composites at elevated temperatures

A series of high-temperature fatigue crack growth experiments was conducted on a continuous-fiberreinforced SM1240/TIMETAL-21S composite using three different temperatures, room temperature (24 °C), 500 °C, and 650 °C, and three loading frequencies, 10, 0.1, and 0.02 Hz. In all the tests, the cracking process concentrated along a single mode I crack for which the principal damage mechanism was crack bridging and fiber/matrix debonding. The matrix transgranular fracture mode was not significantly influenced by temperature or loading frequency. The fiber debonding length in the crack bridging region was estimated based on the knowledge of the fiber pullout lengths measured along the fracture surfaces of the test specimens. The average pullout length was correlated with both temperature and loading frequency. Furthermore, the increase in the temperature was found to lead to a decrease in the crack growth rate. The mechanism responsible for this behavior is discussed in relation to the interaction of a number of temperature-dependent factors acting along the bridged fiber/matrix debonded zone. These factors include the frictional stress, the radial stress, and the debonding length of the fiber/matrix interface. In addition, the crack growth speed was found to depend proportionally on the loading frequency. This relationship, particularly at low frequencies, is interpreted in terms of the development of a crack tip closure induced by the relaxation of the compressive residual stresses developed in the matrix phase in regions ahead of the crack tip during the time-dependent loading process.

Constituent damage mechanisms in metal matrix composites under fatigue loading, and their effects on fatigue life

Load controlled fatigue experiments were performed on 8-ply unidirectional ([0] 8) SCS-6-Ti-15-3 metal matrix composites (MMCs) at different temperatures, and the results were interpreted in terms of the overall three-regime framework of fatigue. The emphasis was on understanding the mechanisms and mechanics of constituent damage evolution, and their effects on fatigue life. Most tests were performed at an R-ratio of 0.1, but limited fully-reversed ( R = −1) tests were conducted. In regime 1, damage was fiber failure dominated, but the exact mechanisms were different at room and elevated temperatures. In regime 2, observation of matrix cracks and persistent slip bands provided convincing evidence of matrix dominated damage. Weak fiber-matrix interfaces contributed to crack bridging. However, fiber fracture also played an important role in regime 2; tension-tension and tension-compression tests showed similar lives on a maximum fiber stress basis, although the strain range, which primarily controls matrix crack growth, was almost double for R = −1 compared with R = 0 or 0.1. Good agreement was obtained from the different R-ratio tests, between the MMC and matrix data, and data at room and elevated temperatures, when compared based on the strain range in the tension part of a cycle. Analyses and observations of fiber pull-out lengths and fiber fractures in the matrix crack wake provided evidence of fiber damage; the analyses also helped to explain increased fiber bridging with fiber volume fraction. Issues of fatigue life prediction are briefly discussed.

Effect of strain rate on the fracture of ceramic fibre reinforced glass matrix composites

The effect of strain rate on the fracture behaviour of two ceramic fibre reinforced glass matrix composites was studied. Increasing the strain rate was found to enhance catastrophic failure in both of these composites. This was attributed to the crack deflection and changes in the fibre pullout length as a function of strain rate. Enhanced strain rates were found to decrease the strength, static toughness and fracture energy of the composites. This effect was more pronounced in the case of the coated fibre composites as compared to the uncoated fibre composites. This is because of fibre/matrix isolation, obtained as a result of the coating.

Interface engineering in alumina/glass composites

Oxide-fiber/oxide-matrix composites, such as alumina-fiber/glass-matrix composites, represent an important class of ceramic matrix composites because of their inherent stability in air at high temperatures. Alumina and glass, however, form a very strong chemical bond, which is undesirable from a toughness point of view. We present an interface engineering approach, which involves the incorporation of an interphase between the matrix and the fiber in order to produce energy dissipating processes such as interface debonding, crack deflection, and fiber pull-out in this system. We first examined the efficacy of tin dioxide as a barrier coating between alumina and glass bars. We corfirmed by microprobe analysis that alumina and tin dioxide were mutually insoluble but there was some solubility between silica and tin dioxide. This was followed by coating continuous PRD-166 (alumina+15 wt.% zirconia) fiber with SnO 2 and analyzing the microstructure and mechanical behavior of coated fiber composites. We observed that although the SnO 2 coating provided the intended diffusion barrier and the thermal stress distribution was of the desirable kind, a neat and clean fiber pull-out was absent because of the roughness of the PRD-166-SnO 2 interface. Some fiber/matrix debonding, crack deflection, and crack bridging occurred. The roughness-induced radial clamping stress was too large to allow fiber pull-out. To reduce this radial clamping effect, we then used a relatively smooth fiber, Saphikon, a single-crystal alumina fiber. As expected, the SnO 2-coated-Saphikon-fiber/glass composite showed a much larger fiber pull-out length than the coated-PRD-166-fiber/glass composite. Thus, a judicious interplay of thermal stress distribution and interfacial roughness in a ceramic matrix composite with an interphase can result in the deformation micromechanisms required for enhanced toughness.