Oxidation and Temperature Effects on the Interfacial Shear Strength in SCS-6 Fiber-Reinforced Reaction-Bonded Silicon Nitride

The fiber/matrix interfacial shear strength of Textron SCS-6 SiC-fiber-reinforced reaction-bonded Si₃N₄ (RBSN) was studied as a function of temperature after oxidation for 24 h at 600°C. Fiber push-out experiments were conducted using a diamond indenter in a high-temperature micro-hardness tester under vacuum. The interfacial shear strength increased with temperature because of the relief of residual tensile stresses arising from the difference in thermal expansion coefficients between the fiber and the matrix. Most of sublayer 2 of the fiber outer coating, which mainly consisted of carbon in the form of BSU (basic structure unit) aggregates, had disappeared after the heat treatment of the composite. Oxidation resulted in severe changes in the fiber outer coating and caused a lower interfacial shear strength with respect to that of the unoxidized composite.     

Effects of short fiber tip geometry and inhomogeneous interphase on the stress distribution of rubber matrix sealing composites

The normal and interfacial shear stress distributions with flat fiber tip of short‐fiber‐reinforced rubber matrix sealing composites (SFRC) compared with the shear lag model were investigated by using the finite element method (FEM). The results indicate that stress values do not agree with those calculated by the shear lag model. The effect of different geometrical shapes of fiber tip on the stress distributions of SFRC was also investigated. The geometrical shapes of fiber tip under present investigation are flat, semi‐elliptical, hemispherical, and circular cone, respectively. The results show that the hemispherical fiber tip transfers the load with less stress concentration and is contributed to controlling the interface debonding failure more effectively than other shapes of fiber tip. Further study on the effect of the inhomogeneous interphase properties on the normal and interfacial shear stresses of hemispherical fiber tip was also conducted. The results indicate that the normal stress increases with the increase of the interphase thickness and interfacial shear stress remains unchanged, and the normal stress values of SFRC with interphase are higher than those without interphase. The interphase elastic modulus has no influence on the stress distributions along the direction to the fiber axis. The stress distributions along the radial direction in the interphase end are largely dependent on the interphase elastic modulus, and the interfacial shear stress is larger than the normal stress, which reveals that a significant part of the external load is transferred from the fiber to the matrix through shear stresses within the interphase. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41638.     

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.     

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.     

Effects of imperfect adhesion on thermal micro-residual stresses in polymer matrix composites

This paper investigates the influences of imperfect bonding between the fiber and matrix on thermal micro-residual stress fields in polymer matrix composites. For this purpose, a representative volume element consisting of a three-phase composite material subjected to a uniform temperature change is considered. Based on the energy method, a three-dimensional closed-form solution for micro-residual stresses is obtained. Besides, a finite element model is developed and the results are compared with the analytical solution. Both the energy method and finite element analysis show similar trend for thermal stress distribution along the fiber length, while due to the stress singularity, the interfacial shear stress from the finite element solution cannot satisfy the stress-free condition at the fiber end. The analysis shows that the magnitude of thermal stresses and their distribution mainly depend on the bonding efficiency parameter. An increase in thermal and elastic properties bonding efficiencies leads to a considerable decrease in composite axial and shear residual stresses, while the Poisson's ratio bonding efficiency does not affect the thermal stress fields. The interfacial radial residual stress distribution is approximately independent of the bonding conditions. Inefficient bonding may result in higher residual stresses in comparison with the perfect bonding condition. It means that in cases of low bonding efficiency conditions, the ability of composites to sustain and transmit load decreases drastically. Thermal stress concentration occurs at the vicinity of the fiber ends, although peak values depend on the bonding efficiency value.     

Interfacial Peeling Moments and Shear Forces at Free Edges of Multilayers Subjected to Thermal Stresses

Stresses normal to interfaces, i.e., interfacial peeling stresses and interfacial shear stresses, exist locally at edges of multilayers because of both the thermal mismatch between layers and the free-edge effect. These peeling and shear stresses can result in modes I and II edge delamination, respectively. However, because of the complexity of the problem, exact closed-form solutions for these stresses are very difficult if not impossible to derive even for bilayered systems. Hence, instead of the detailed stress field at edges, both the interfacial peeling moment resulting from the localized peeling stresses and the interfacial shear force resulting from the localized shear stresses are analyzed here. Exact closed-form solutions for the peeling moment and the shear force at each interface in elastic multilayered systems are derived. To illustrate the application of present closed-form solutions, specific results are calculated for five-layered thermal barrier coating systems, and a finite-element analysis is also performed to confirm the analytical results.     

Theoretical and finite element studies of interfacial stresses in reinforced concrete beams strengthened by externally FRP laminates plate

The paper presents the results of an analytical and numerical solution for interfacial stresses in carbon fiber reinforced plastic (CFRP)–reinforced concrete (RC) hybrid beams studied by the finite element method. The analytical analysis is based on the deformation compatibility approach where both the shear and normal stresses are assumed to be invariant across the adhesive layer thickness. The adherend shear deformations are taken into account by assuming a parabolic shear stress through the thickness of both the concrete beam and the bonded plate. In numerical analysis, the mesh sensitivity test shows that the finite element results for interfacial stresses are not sensitive to the finite element mesh. The finite element analysis then is used to calculate the interfacial stress distribution and evaluate the effect of the structural parameters on the interfacial behavior. It is shown that both the normal and shear stresses at the interface are influenced by the material and geometry parameters of the composite beam. Numerical results from the present analysis are presented both to demonstrate the advantages of the present solution over existing ones and to illustrate the main characteristics of interfacial stress distributions. We can conclude that this research is helpful for the understanding the mechanical behavior of the interface and design of the FRP–RC hybrid structures.     

Mapping of stress distribution in woven‐fabric composites

Mapping of the stress distribution in composite materials, both at the fiber/matrix interface and in the composite constituents, is important to understand the material mechanical response. Stress mapping can help predict composite behavior under certain stresses especially failure or delamination. In this work, two analytical models were proposed to map the stress distribution at fiber, matrix and fiber/matrix interface utilizing the concept of stress superposition. The first model dealt with the fiber in the longitudinal direction considering axisymmetric conditions. The second model addressed the fiber stress distribution in the transverse direction. Experimental data from four‐point flexural tests of woven fabric composites was processed using the Graphical Integrated Numerical Analysis (pcGINA) to obtain the maximum stress in the target laminate and this value was used as the input for the two analytical models. The value for the maximum interfacial shear stress was calculated using the models and results were compared to pull‐out fiber test values obtained from literature. Good agreement was observed between the model calculations and the literature data. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Stress analysis of the thermal barrier coating system near a cooling hole considering the free-edge effect

Due to the thermal mismatch between layers and the free-edge effect, interfacial peeling and shear stresses are generated locally around the edges of cooling holes in a thermal barrier coating (TBC)–film cooling system. These interfacial peeling and shear stresses may lead to modes I and II edge delamination, resulting in TBC spallation around the cooling hole. In this study, analytical and numerical models were built to study the stress and interfacial cracking behaviors of TBCs near the cooling hole. Analytical solutions for interfacial peeling moment and shear force at each layer were obtained to analyze the free-edge effect on the stress distributions in TBCs, and they were verified by the finite element calculations. The results showed that interfacial peeling moment and shear force were functions of the hole radius and thicknesses of top coat and oxide layer. The increase of interfacial peeling moment and shear force raised the likelihood of edge cracking around the hole. Derived by the local stresses, the interfacial cracks in TBCs initiated and propagated from the hole edge upon cooling.     

Analytical Evaluation of Interfacial Shear Strength for Fiber-Reinforced Ceramic Composites

A methodology is described to evaluate the shear strength at the fiber–coating interface. Stress analyses are performed for the following system: a fiber with a uniform coating in its central portion which is subjected to an axial tensile stress through its bare ends. The results show that the axial tensile stress in the coating decreases with the increase in the thickness of the coating and that the interfacial shear stress exhibits the opposite trend. Hence, a critical thickness of the coating can be defined depending on the properties of the material, such that when the coating thickness is less than the critical value, failure of the composites initiates from segmentation of the coating during tensile loading. Conversely, when the coating thickness is greater than the critical value, debonding of the interface occurs. Furthermore, the interfacial shear strength can be related to this critical thickness and the tensile strength of the coating.     

Influence of Residual Stresses, Interface Roughness, and Fiber Coatings on Interfacial Properties in Ceramic Composites

Fiber pushout tests are performed on zircon-matrix composites especially fabricated with a variety of silicon carbide reinforcing fibers and fiber coatings in order to create samples with different interfacial properties, surface roughness, and possibly in different states of residual stress to demonstrate their role on the interfacial and mechanical properties of fiber-reinforced composites. The data obtained from fiber pushout tests are analyzed using linear, shear-lag, and progressive debonding models to extract important interfacial properties, residual stresses, and surface roughness. The nature and magnitude of residual stresses in composites are independently characterized by measuring the coefficient of thermal expansion of the fiber, the matrix, and the composite for comparison with similar values measured using the fiber pushout tests. These results are then compared for self-consistency among different ways of analyzing data and with independently measured and calculated values. The results have shown that independent and complementary methods of data acquisition and analysis are required to fully understand interfacial properties in ceramic composites. In particular, independent measures of the coefficient of thermal expansion, residual stresses, and surface roughness are required to confidently interpret interfacial properties obtained by different analytical approaches and then relate them to the overall mechanical response of composites. It is also shown that composites with optimum mechanical response can be created by suitably engineering the interface using multiple fiber coatings.     

Multiple Cracking of Unidirectional and Cross-PlyCeramic Matrix Composites

This paper examines the multiple cracking behavior of unidirectional and cross-ply ceramic matrix composites. For unidirectional composites, a model of concentric cylinders with finite crack spacing and debonding length is introduced. Stresses in the fiber and matrix are found and then applied to predict the composite moduli. Using an energy balance method, critical stresses for matrix cracking initiation are predicted. Effects of interfacial shear stress, debonding length and bonding energy on the critical stress are studied. All the three composite systems examined show that the critical stress for the completely debonded case is lower than that for the perfectly bonded case. For cross-ply composites, an extensive study has been made for the transverse cracking in 90° plies and the matrix cracking in 0° plies. One transverse cracking and four matrix cracking modes are studied, and closed-form solutions of the critical stresses are obtained. The results indicate that the case of combined matrix and transverse crackings with associated fiber/matrix interfacial sliding in the 0° plies gives the lowest critical stress for matrix cracking. The theoretical predictions are compared with experimental data of SiC/CAS cross-ply composites; both results demonstrated that an increase in the transverse ply thickness reduces the critical stress for matrix cracking in the longitudinal plies. The effects of fiber volume fraction and fiber modulus on the critical stress have been quantified. Thermal residual stresses are included in the analysis.     

Nanoscale toughening of carbon fiber‐reinforced epoxy composites through different surface treatments

Interests in improving poor interfacial adhesion in carbon fiber‐reinforced polymer (CFRP) composites has always been a hotspot. In this work, four physicochemical surface treatments for enhancing fiber/matrix adhesion are conducted on carbon fibers (CFs) including acid oxidation, sizing coating, silane coupling, and graphene oxide (GO) deposition. The surface characteristics of CFs are investigated by Fourier transform infrared spectroscopy, X‐ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, interfacial shear strength, and interlaminar shear strength. The results showed that GO deposition can remarkably promote fiber/matrix bonding due to improved surface reactivity and irregularity. In comparison, epoxy sizing and acid oxidation afford enhancement of IFSS owing to effective molecular chemical contact and interlocking forces between the fiber and the matrix. Besides, limited covalent bonds between silane coupling and epoxy matrix cannot make up for the negative effects of excessive smoothness of modified CFs, endowing them inferior mechanical properties. Based on these results, three micro‐strengthening mechanisms are proposed to broadly categorize the interphase micro‐configuration of CFRP composite, namely, “Etching” “Coating”, and “Grafting” modifications, demonstrating that proper treatments should be chosen for combining optimum interfacial properties in CFRP composites. POLYM. ENG. SCI., 59:625–632, 2019. © 2018 Society of Plastics Engineers     

Effect of shear deformation on interfacial stresses in plated beams subjected to arbitrary loading

Bonding a fibre reinforced polymer (FRP) composite or metallic plate to the soffit of a reinforced concrete (RC), timber or metallic beam can significantly increase its strength and other aspects of structural performance. These hybrid beams are often found to fail due to premature debonding of the plate from the original beam in a brittle manner. This has led to the development of many analytical solutions over the last two decades to quantify the interfacial shear and normal stresses between the adherends. The adherends are subjected to axial, bending and shear deformations. However, most analytical solutions have neglected the influence of shear deformation of the adherends. For the few solutions which consider this effect in an approximate manner, their applicability is limited to one or two specific load cases. This paper presents a general analytical solution for the interfacial stresses in plated beams under an arbitrary loading with the shear deformation of the adherends duly considered. The shear stress distribution is assumed to be parabolic through the depth of the adherends in predicting the interfacial shear stress and Timoshenko's beam theory is adopted in predicting interfacial normal stress to account for the shear deformation. The solution is applicable to a beam of arbitrary prismatic cross-section bonded symmetrically or asymmetrically with a thin or thick plate, both having linear elastic material properties. The effect of shear deformation is illustrated through an example beam. The influence of material and geometric parameters of the adherends and adhesive on the interfacial stress concentrations at the plate end is discussed.     

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.     

纤维树脂基复合材料微观界面性能表征方法的进展

纤维树脂基复合材料中界面微观结构和性质对材料的宏观性能起着关键性作用。为了能够准确地测试纤维树脂间的界面粘结力,表征纤维与树脂之间界面剪切应力传递效率,人们一直试图找到一种方法能够真实全面反映界面性能,以及纤维断裂、界面脱粘、树脂横向断裂等因素对界面性能的影响。为此作者对现有微观界面表征方法进行介绍与分析,包括:微珠试验、单纤维拉出试验、压入试验、单纤维断裂试验、激光拉曼(或荧光)色谱法、光弹分析法,全面剖析它们的适用范围。     

Control of Interfacial Properties through Fiber Coatings: Monazite Coatings in Oxide–Oxide Composites

Fiber pushout tests were used to quantify the effects of fiber coating thickness on the mechanical properties of two model composite systems: a monazite-coated (LaPO₄-coated) alumina (Al₂O₃) fiber in an Al₂O₃ matrix and a LaPO₄-coated yttrium aluminum garnet (YAG) fiber in an Al₂O₃ matrix. Interface properties were quantified using the Liang and Hutchinson (LH) pushout model and mechanistically rationalized by considering the change in residual thermal stresses with changes in the coating thickness. Measures of the pure Mode II interfacial fracture energy, the coefficient of friction, and a radial clamping pressure are extracted by fitting the LH equations to the experimental results. Using the approach that has been developed herein, a methodology is available for measuring the interfacial properties, predicting the effect of coating thickness, and selecting the coating thickness to     

Physical interactions at carbon nanotube-polymer interface

Mechanical properties of carbon nanotube (CNT) reinforced polystyrene rod and CNT reinforced epoxy thin film were studied and the CNT-polymer interface in these composites was examined. Transmission and scanning electron microscopy examinations of CNT/polystyrene (PS) and CNT/epoxy composite showed that these polymers adhered well to CNT at the nanometer scale. Molecular mechanics simulations and elasticity calculations were used to quantify some of the important interfacial characteristics that critically control the performance of a composite material. In the absence of chemical bonding between CNT and the matrix, it is found that the non-bond interactions, consist of electrostatic and van der Waals forces, result in CNT-polymer interfacial shear stress (at 0 K) of about 138 and 186 MPa, respectively, for CNT/epoxy and CNT/PS. The high interfacial shear stress calculated, about an order of magnitude higher than micro fiber reinforced composites, is believed attributed to intimate contact between the two solid phases at the molecular scale. Simulations and calculations also showed that local non-uniformity of CNT and mismatch of the coefficients of thermal expansions between CNT and polymer matrix also promote the stress transfer ability between the two.     

Interfacial Mechanical Characterization of Nicalon SiC Fiber/Alumina-Based Composites

Interfacial mechanical properties of both Nicalon SiC/aluminum borate and Nicalon SiC/aluminum phosphate with various fiber coatings and heat treatments were evaluated using a commercially-available indenter to induce fiber sliding during load cycling experiments. Varying degrees of sliding due to different coating materials were found. The interfacial characteristics including the shear, the residual axial fiber, and debond stresses were estimated by matching the experimental stress-displacement curves with curves predicted from an existing model. The elastic modulus and hardness of the interphase/interface in ceramic matrix composites were also evaluated. These results provided important insights into the ultimate mechanical performance of fiber-reinforced ceramic-matrix composites.     

Interfacial Mechanical Characterization of Nicalon SiC Fiber/Alumina-Based Composites

Interfacial mechanical properties of both Nicalon SiC/aluminum borate and Nicalon SiC/aluminum phosphate with various fiber coatings and heat treatments were evaluated using a commercially-available indenter to induce fiber sliding during load cycling experiments. Varying degrees of sliding due to different coating materials were found. The interfacial characteristics including the shear, the residual axial fiber, and debond stresses were estimated by matching the experimental stress-displacement curves with curves predicted from an existing model. The elastic modulus and hardness of the interphase/interface in ceramic matrix composites were also evaluated. These results provided important insights into the ultimate mechanical performance of fiber-reinforced ceramic-matrix composites.