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.     

Effect of Interfacial Roughness on the Frictional Stress Measured Using Pushout Tests

A fiber which is partially pushed out of a surrounding matrix and subsequently pushed in the opposite direction exhibits a substantial decrease in sliding friction as it passes through its original position (its "origin"). This is manifest by a decrease in the load required to push the fiber. It is suggested that interfacial roughness causes this phenomenon and that the decrease in load (friction) is associated with the fiber seating back into its original position. The period of the drop has been correlated with the spatial extent of the interfacial surface roughness, and the magnitude of the drop (referred to hereafter as the seating drop) has been correlated with the amplitude of the interfacial roughness. Observation of the seating drop allows separation of the friction associated with interfacial irregularities from that resulting primarily from residual stresses at the interface. Implications for composite design and use are discussed. The effect of abrasion at the sliding interface is also addressed.     

Characterization of Interfacial Properties in Fiber-Reinforced Cementitious Composites

For calculating interface properties from pullout tests, a simple theoretical model is proposed. The model enables calculating of the following material parameters: the parameter of shear stiffness of the fiber–matrix boundary layer, the shear bond strength, the frictional bond strength and the specific interfacial fracture energy. These parameters can be determined from the slope of the load-slip curve, the maximum pullout load and the corresponding slip value. Slip-controlled, multiple-fiber pullout tests were conducted in a closed-loop test system. The effects of embedment length of fibers on the model-predicted material parameters were examined. The model predictions were satisfactorily compared with some previously published test data.     

Analysis of Fiber Frictional Sliding in Fiber Bundle Pushout Test

A simple theoretical model is developed to analyze the fiber frictional sliding resistance in a fiber bundle pushout test. The effect of the radial constraint imposed by the neighboring fibers on the stress transfer and frictional pushout stress is included in this analysis. Comparisons of theoretical results of this study and those of two existing fiber pushout models (i.e., single-fiber pushout and three–cylinder) are also presented. For SiC–RBSN and SiC–glass composites with short embedded fiber lengths less than 1 mm, there is little difference between all these models. However, for larger embedded fiber lengths, the present model gives the highest frictional pushout stress caused by the more realistic radial constraint condition used in the analysis.     

Effect of Interface Mechanical Properties on Pullout in a SiC-Fiber-Reinforced Lithium Aluminum Silicate Glass-Ceramic

The pullout of fibers in the crack wake makes an important contribution to the toughness of ceramic-matrix composites. The pullout is, in turn, influenced by the properties of the fibers and by the sliding resistance of the interface. Basic relationships governing the pullout are developed analytically and investigated experimentally using a lithium aluminum silicate/silicon carbide (LAS/SIC) composite subjected to various heat treatments. The experiments involve determining the strengths of single fibers and then measuring the pullout distributions. The results are used to provide a consistent view of the pullout process and related changes in mechanical properties.     

Melt Infiltration and Reaction at the Fiber/Matrix Interface During the Brazing of a Fiber-Reinforced Ceramic to Metal

The presence of silica in the fiber/matrix interface of SiC-fiber-reinforced alumina facilitates the brazing of these composites with metals. In addition, molten braze infiltrates the interfacial pores to form reaction layers with the matrix and the fiber. The chemical compatibility, wetting, and thermodynamic favorability of the Ti-based braze leads to a potentially useful method of joining a fiber-reinforced ceramic to metal.     

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.     

Residual Stresses in Silicon Carbide–Zircon Composites from Thermal Expansion Measurements and Fiber Pushout Tests

Coefficients of thermal expansion (CTE) in the axial direction of two types of SiC fibers, monolithic zircon, monolithic SiC, and several SiC_f-zircon composites were measured in the temperature range of 50 to 1380C. The measured CTE values of composites were compared with values predicted by the rule-of-mixtures approach, and a small difference in measured and calculated values was ascribed to the nature of interfacial bonding and assumptions implicit in the rule-of-mixtures approach. Fiber pushout tests were performed on these composites and the residual stresses were extracted from the analysis of the load–displacement plots in terms of the shear-lag and progressive debonding models. The radial and axial residual stresses arising from the mismatch in CTE were calculated and compared with values obtained from the fiber pushout tests. The fiber pushout tests in general produced lower values of the residual stresses, but the residual stresses obtained using shear-lag analysis were in good agreement with the calculated values based on the CTE mismatch in composites with lower values of the interfacial shear stress. The influence of anisotropic fiber expansion in the radial and axial directions on the radial and axial residual stresses in composites were also examined.     

Measurement of Fiber/Matrix Interfacial Shear Stress at Elevated Temperatures

An apparatus for measurement of the fiber/matrix interfacial shear stress at temperatures up to 1100° is described. Equipment was used to measure interfacial properties as a function of temperature in two ceramic-matrix composites and one metal-matrix composite. In the composites where the thermal expansion of the matrix was higher than that of the fiber, the interfacial shear stress decreased with temperature. The opposite trend was observed in a system with low matrix thermal expansion. The change of the interfacial shear stress with temperature of all the composites studied can be fully explained by considering the fiber/matrix expansion differences.     

Fiber Bundle Pushout: A Technique for the Measurement of Interfacial Sliding Properties

An experimental technique for the measurement of interfacial sliding properties in fiber-reinforced materials is presented. The technique involves pushing a bundle of fibers simultaneously through a thin section of the composite. Using this technique, sample averages are immediately available, eliminating the need for repeated single-fiber tests. The technique is particularly useful for composites that contain small-diameter fibers. Salient features of the technique are demonstrated through tests on a CAS/SiC composite.     

Evolution of Impedance Spectra during Debonding and Pullout of Single Steel Fibers from Cement

Impedance measurements were made during the debonding and pullout of a fully embedded, crack-bridging single steel fiber from a cement matrix. Nyquist plots gave evidence of two bulk arcs, and the cusp between them proved to be sensitive to both debonding and pullout of the embedded fiber. Physical simulations that used a steel wire in tap water were applied to interpret the debonding and pull-out results. The cusp resistance from impedance spectroscopy provided quantitative information about the extent of pullout and, at least qualitatively, correlated with the debond length before pullout. Impedance measurements on both sides of the matrix crack showed that crack deflection and debonding occurred on both sides symmetrically.     

Determination of Individual Fiber Failure in Fiber Bundles

The onset of fiber failure signals the commencement of catastrophic structural failure in fiber bundle systems, a process that is important in standalone bundles and in other material structures. In this work, we have demonstrated a technique by which the onset of fiber failure can accurately be identified by monitoring the electrical continuity and resistance of filaments and bundles during loading. The dual sensor/loading functionality of the fibers differs from most prior approaches wherein separate sensors were added to the load-bearing fiber array. Individual fiber failure and bundle onset failures in 50 fiber bundles were reliably and reproducibly detected.     

In Situ Observations of Toughening Processes in Alumina Reinforced with Silicon Carbide Whiskers

An in situ study is made of crack interfaces in composites of alumina reinforced with silicon carbide whiskers. Both qualitative observations of the whisker-bridging micro-mechanisms and quantitative measurements of the crack profile are made to assess the specific role of the whiskers on the toughness curve ( T -curve or R -curve). At small crackwall separations the whiskers act as elastic restraints to the point of rupture. In some cases the whiskers remain in frictional contact with the alumina matrix over large pullout distances (more than 1 μm) corresponding to a bridging zone approaching 1 mm. The results are discussed in relation to existing models of whisker reinforcement and published long-crack T -curve data.     

In Situ Measurements of Bridged Crack Interfaces in the Scanning Electron Microscope

A device for in situ SEM examination of crack propagation during loading of compact tension specimens is described, with a specific demonstration on an alumina ceramic. The device facilitates direct qualitative observations of the inception and subsequent frictional pullout of grain-localized bridges at the crack interface. Quantitative data on the bridging mechanism are obtained from measurements of the crack-opening displacements behind the crack tip. The crack profile is found to be closer to linear than parabolic at the bridged interface. Deconvolution of these crack-opening data allow for an evaluation of the closure tractions operative at the crack walls within the bridging zone, and thence the R -curve.     

Effect of Interfacial Roughness Parameters on the Fiber Pushout Behavior of a Model Composite

The effect of interfacial roughness on the frictional sliding in composites has been studied using fiber pushout and pushback tests on a model composite of Plexiglas rods in an epoxy matrix. Different extents of roughness were introduced on the Plexiglas rods and the resulting roughness profiles measured. The roughness profiles were characterized using six different roughness parameters. An attempt was made to find a correlation between the sliding resistance and the selected roughness parameters. A parameter defined as the maximum coefficient in the Fourier transform of the roughness profile was found to yield the best correlation. If the roughness introduced is periodic, then the pushout traces exhibit periodic dips, but the magnitude of this periodic dip is significantly smaller than the seating drop obtained from pushback tests.     

聚丙烯腈纤维预氧化工艺条件对其结构和性能的影响

密度、氧质量分数和芳构化指数是判断预氧化程度的参数,借助密度分析、X射线衍射、元素分析等技术,研究预氧化工艺条件对密度、氧质量分数和芳构化指数的影响。研究结果表明,随着预氧化温度的升高,密度、氧质量分数和芳构化指数均增大;随预氧化时间的增长,密度、氧质量分数也增加,牵伸和原丝纤度对氧质量分数影响不大。     

Mullite‐glass and mullite‐mullite interfaces: Analysis by molecular dynamics (MD) simulation and high‐resolution TEM

The properties of mullite‐glass and mullite‐mullite interfaces have been investigated at 1800 K by molecular dynamics (MD) simulation and high‐resolution TEM. The simulation showed that mullite‐glass interfaces typically have much lower interfacial energies than mullite‐mullite interfaces, which results from the structural flexibility of the glass and associated accommodation of interfacial mismatch. The (110)‐glass interface has the lowest energy of all interfaces studied, which is consistent with the observed dominance of this interface in experimental mullite‐glass samples examined by TEM. The simulation shows that the interfacial energies of the (100)‐glass and (010)‐glass interfaces are higher than that those of the (001)‐glass interface, so [100] and [010] would be expected to be the dominant growth directions. However, the growth of mullite in glass occurs predominantly in the [001] direction. This apparent discrepancy can be explained by the fact that growth in the [100] and [010] directions is limited by the slow growth of (110) plane (i.e., [110] direction), which facilitates [001] growth, which is confirmed by the TEM data.     

Adhesion and Hydrolytic Stability of the Interface Between High-Modulus Polyethylene Fibers and Acrylic Resins

The effects of oxyen plasma treatment on the surface chemistry of Spectra 1000® high modulus polyethylene fibers and on the mechanical properties of fiber-reinforced composites of the fibers in a Bis-GMA based acrylic resin have been studied. X-ray photoelectron spectroscopy and diffuse reflectance FTIR spectroscopy have been used to show that the majority of oxygen on the fiber surface exists mostly in the form of ether and/or epoxy linkages, with carbonyl-, carboxylic- and ester-containing compounds accounting for less than 10 percent of the total. While the untreated and plasma-treated fibers have similar chemical compositions, the surfaces of the plasma-treated fibers are more polar and the oxygen is chemically bonded instead of being merely physisorbed. The interfacial shear strength between the fibers and the acrylic resin is increased by a factor of 2.3 by the plasma treatment indicating the presence of a weak boundary layer on the surface of the untreated fibers. The hydrolytic stability of the composite interfaces was investigated for fibers sized with several Bis-GMA-based adhesives. Maximum stability was attained by sizing with Bis-GMA containing a peroxide catalyst or an amine accelerator. The flexural properties of composites utilizing plasma-treated and untreated fibers were compared in three-point bending. The ultimate bending loads for composites using treated fibers were much higher than those for composites with untreated fibers, but only a fraction of that for glass or Kevlar®-reinforced materials.     

Correlating the Mechanical Properties of a Continuous Fiber-Reinforced Ceramic-Matrix Composite to the Sliding Resistance of the Fibers

The sliding resistance of NICALON fibers (SiC) in a lithium aluminosilicate (LAS III) glass-ceramic matrix has been correlated with the mechanical properties of the composite. Push-down measurements of the sliding resistance for individual fibers show an order of magnitude increase when the ceramic composite is annealed in air for 4 h at 800°C. Such increases are correlated with a loss of toughness, a reduction in the ultimate strength, and an increase in the matrix cracking stress of the composite, as measured using tension tests and bend tests of bulk samples.