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.     

Effect of the Fiber Size on the Physicochemical and Mechanical Properties of Composites of Epoxy and Date Palm Tree Fibers

Lignocellulosic fibers from date palm trees were employed to reinforce an epoxy matrix. Two fiber sizes were used, with the length and diameter in the range of 20–30 and 1.5–3 mm, respectively, for the so‐called long fibers, and in the range of 5–15 and 0.25–0.75 mm, respectively, for the so‐called short fibers. The morphologies of the resulting composites, as well as their thermal, mechanical, and water sorption properties were evaluated. Strong interactions between both components and etherification reactions may occur between the hydroxyl groups of the fibers and the epoxy groups of the epoxy‐amine reactive mixture. These effects are emphasized when decreasing the size of the fibers.

     

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.     

单根聚丙烯纤维在植被混凝土中的拉拔特性及临界长度研究

纤维加筋技术在复合土和混凝土等工程加固领域应用广泛,但在生态护坡领域的应用则鲜有报道.通过自主设计的试验装置,研究了单根聚丙烯纤维在不同含水率、不同干密度和不同龄期植被混凝土中的拉拔特性,分析了拉拔试验前后纤维的显微特征,并确定了临界加筋长度经验公式.研究表明:纤维拉力-位移曲线呈显著的多峰特征,主要与植被混凝土中存在水泥水化产物有关;1d试样界面抗剪强度在最优含水率时存在极小值,主要原因是试样初期的筋-土界面锚固力提高值相较于摩擦力减小值要低;随着干密度和龄期的增长,界面抗剪强度和残余抗剪强度都显著提高.通过多元线性回归分析,得出植被混凝土中纤维临界加筋长度的经验公式,对提升植被混凝土在高寒、强降雨、坡面高陡等恶劣环境下的工程耐久性有一定指导意义.     

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.     

聚丙烯纤维对轻骨料混凝土力学性能的影响

采用天然浮石作为粗骨料,同时掺入聚丙烯纤维及聚丙烯纤维和钢纤维混合配制混凝土,对纤维轻骨料混凝土的表观密度、抗压强度、弹性模量、抗折强度以及弯曲韧性进行研究,试验结果表明,掺入聚丙烯纤维,抗压强度有所下降,但不增加轻骨料混凝土的表观密度;聚丙烯纤维和钢纤维混掺可以在不增加表观密度,保证强度的基础上,有效地改善轻骨料混凝土的韧性.     

FRP锚钉锚固长度对FRP加固混凝土构件拉拔性能影响的试验研究

FRP锚钉是能有效阻止FRP加固混凝土构件界面剥离的前沿方法.为了测定FRP锚钉的不同锚固长度对FRP加固混凝土构件粘结性能的影响,采用拉拔试验,参照ACI规范的试验方法,测定了不同锚固长度下FRP加固混凝土构件的拉拔荷载.试验用不安装锚钉的控制试件和安装了15 mm、30 mm、45 mm三组FRP锚钉的试验试件,结果发现,锚固长度不同,FRP加固混凝土构件的破坏模式也各不相同,主要表现为混凝土内部浅层剥离、混凝土锥形破坏、混凝土锥形破坏联合锚钉部分拉拔破坏、混凝土浅层剥离联合锚钉断裂破坏以及FRP加固层内部浅层剥离联合部分混凝土浅层剥离破坏;FRP锚钉的锚固长度越长,加固试件的粘结性能越好;混凝土圆柱体抗压强度设计为30 MPa时,FRP锚钉的锚固长度应不得小于30 mm.试验结论可以为FRP锚钉的设计制作提供参考.     

混杂合成纤维道路混凝土韧性特征与机理分析

根据水泥混凝土路面的实际受力条件,采用弯曲韧性和冲击韧性两个指标对所制备的聚乙烯纤维/聚丙烯粗合成纤维混杂增强混凝土的韧性特征进行全面评价,并结合SEM微结构分析结果对其增韧机理进行了探讨.结果表明,所制备的4组混杂纤维混凝土的弯曲韧性指标均超过理想弹塑性材料的标准,并且在初裂产生后仍具有较高的强度保持能力,试件的裂缝曲折且细微,有效扩散了裂缝尖端的应力集中;混杂纤维混凝土试块的抗冲击韧性大幅提升,吸收的冲击能约是普通混凝土试块的15倍;采用聚乙烯/聚丙烯粗合成纤维可以在几何形状尺寸与力学性能上形成具有一定级配结构的纤维增强材,产生正混杂效应.     

Effects of the glass bead content and the surface treatment on the mechanical properties of polypropylene composites

Polypropylene composites filled with glass beads (GBs) were prepared by means of a twin‐screw extruder. The tensile properties and impact‐fracture strength of the composites were measured at room temperature to identify the effects of the GB content and surface treatment on the mechanical properties. The results show that the relative elastic modulus increased nonlinearly, whereas the tensile strength decreased with increasing GB volume fraction (?_f). The notched impact strength increased with increasing ?_f when ?_f was less than 11%, and then, it decreased; this might have been related to the GB aggregation in the case of higher concentration. The mechanical properties of the composite systems in which the GB surface was treated with silane coupling agent were better than those of the composite systems filled with the untreated GBs under the same conditions. Furthermore, the impact‐fractured surfaces were observed with a scanning electron microscope to understand the interfacial morphology between the inclusion and the matrix and to examine the toughening mechanisms. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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.