Interfacial properties of kevlar-49 fiber-reinforced thermoplastics

A single-filament pull-out test was used to study adhesion of Kevlar-49 fibers to thermoplastic polymers. The test involved pulling a partially embedded fiber out of a polymer film. Kevlar-49 fibers with three different surface treatments were used with five thermoplastic materials. The test resulted in the measurement of two properties, an interfacial bond strength and a frictional shear strength. The interfacial bond strength is an essential factor in determining the critical aspect ratio of discontinuous fibers in a composite. The frictional shear strength was found to correlate with the tensile strength of discontinuous fiber composites which fail by fiber pull-out. Scanning electron microscopy was used to examine the fiber pull-out specimens after testing. Observations of the fiber showed that the failure mode at the fiber–matrix interface was complex. The predominant failure mode was fracture at the interface (or in some weak boundary layer). In some cases, cohesive failure of the fiber surface was observed, with the result that strips of material were torn from the fiber surface.     

A new method for study of the fiber-matrix interface in composites: single fiber pull-out from a microcomposite

—A new method, single fiber pull-out from a microcomposite (SFPOM), was developed to study the fiber/matrix interface in composites. By pulling a fiber out of a seven-fiber microcomposite, the SFPOM test provides the real feeling of a fiber pulled out of an environment similar to that in a real composite. Interfacial shear strength decreased as the fiber volume fraction increased in the fiber-matrix system tested in the experiment. Three factors were suggested to be responsible for the phenomenon: (1) poor bonding between fibers when close to each other; (2) shear stress concentration in the matrix between neighboring fibers; and (3) possible change in matrix properties, thus altering the failure mechanism from interfacial debonding to a mixture of interfacial debonding and matrix fracture.     

Wettability and Adhesion Characteristics of Plasma-Treated Carbon Fibers

The surface free energy (γ_s) of modified carbon fibers was determined by tensiometry and effects of CF₄-O₂ plasma treatment were evaluated. The treatment with the gas mixture in which oxygen was above 40% accelerated preferentially the oxidation of fiber surfaces and the nondispersive component of the surface free energy, γ^P_S, increased to about three times that of the untreated fiber. On the other hand, the treatment with the gas containing CF₄ above 80% induced fluorination and surface species such as - CF, - CF₂, or - CF₃ were formed. The γ^P_S values decreased to almost zero and the dispersive component became about 18 mJ/m². The calculated work of adhesion between various fibers and the epoxy resin was well correlated with the interfacial shear strength of the composites formed with these materials.     

Interfacial Debonding Behavior of Mullite/SiC Continuous Fiber Composite

Mullite/SiC continuous fiber composites were fabricated by hot-pressing under different processing conditions. The interfacial shear strength was measured during the pull-out test, and the effect of fabrication conditions on interfacial debonding behavior was discussed. The debonding length during the pull-out test was quantitatively evaluated using acoustic emission. The interfacial shear strength was evaluated by stress analysis. The control of interfacial shear strength was achieved by controlling the hot-press temperature. An increase of load was found during the pull-out process after complete debonding. In order to explain the increased load, a new model is presented.     

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.     

Calendar of events

—A fracture mechanics approach to the problem of single fiber pull-out is formulated in terms of the critical energy release rate. The solution is more general than that of Outwater and Murphy [1] since the matrix compressive and shear rigidities are more realistic and stress distribution is taken into consideration. The model also takes into account frictional forces between the filament and the matrix along the debonded surface. The shear-lag method is used for stress analysis. A relationship between the filament pull-out force, the critical value of the energy release rate and the position of the tip of the cylindrical crack is established. Numerical examples show the effects of fiber and matrix rigidities and diameters on the apparent average debonding stress predicted by the model. The results of a computer study on the relationship between the pull-out force and the notch size are presented along with the analysis of the model sensitivity to different parameters.     

Investigation of load transfer between the fiber and the matrix in pull-out tests with fibers having different diameters

Single-fiber pull-out tests were used for investigation of the interfacial bond strength or toughness and load transfer between polymeric matrices and glass fibers having different diameters. The interfacial bond strength was well characterized by an ultimate interfacial shear strength (τ_ult) whose values were nearly independent of the fiber diameter. The same experiments were also analyzed by fracture mechanics methods to determine the interfacial toughness (G_ic). The critical energy release rate (G_ic) was a good material property for constant fiber diameter, but G_ic for initiation of debonding typically became smaller as the fiber diameter became larger. It was also possible to measure an effective shear-lag parameter, β, characterizing the load transfer efficiency between the fiber and the matrix. β decreased considerably with the fiber radius; this decrease scaled roughly as expected from elasticity theory. The measured results for β were used to calculate the radius of matrix material surrounding the fiber that was significantly affected by the presence of the fiber. The ratio of this radius to the fiber radius (R_m/r _f) was a function of the fiber diameter.     

Rate constants, activation free enthalpies and activation entropies of the electrochemical reduction of 1,4-diazines in DMF

The standard exchange current densities of nine 1,4-diazines at gold electrodes were measured in DMF in the temperature range of +25 up to −59°C. From the resulting free enthalphy of activation of pyrazine as a function of temperature an activation entropy of −1.5 k is calculated. It is compared with the activation entropy of −1.1 k following from the theory of Marcus. The relative change of the free enthalpy of activation with temperature is only a property of the temperature dependences of refractive index and dielectric constant of the solvent. It is used to obtain the free enthalpies of activation for the standard temperature of 298 K.

The experimental values for the compounds are compared with theoretical values, calculated from the inner and outer reorganization energies λ_i and λ₀. For λ_i the bond lengths and force constants were obtained from Hückel calculations, for λ_o different approximations according to Marcus and Peover were used. The calculated and the measured values coincide within less than 10%.

The relation between the free solvation enthalpy and the outer reorganization energy is discussed.     

Interfacial Bonding and Environmental Stability of Polymer Matrix Composites

Recently developed adsorption-interdiffusion (A-I) theory of adhesion is employed to isolate the (London) dispersion γ_i,j^d and (Keesom) polar γ_i,j^p components of the excess interfacial free energy γ_i,j=γ_i,j^d+γ_i,j^p at the fiber-matrix interface in polymer matrix composites. For adsorption bonded interfaces the theory defines a new method of mapping the surface energy effects of an immersion phase upon the Griffith fracture energy γ_G. The stability of interfacial bonding between graphite fiber-epoxy matrix is defined in terms of the theoretical model and experimentally evaluated by accelerated aging studies which monitor changes in fracture energy for crack propagation perpendicular to the fiber axis. Applications of the model to control fiber surface treatments and select matrix components for optimized bond strength and environmental resistance is discussed.     

Interfacial Bonding and Friction in Silicon Carbide [Filament]-Reinforced Ceramic- and Glass-Matrix Composites

Interfacial shear strength and interfacial sliding friction stress were assessed in unidirectional SiC-filament-reinforced reaction-bonded silicon nitride (RBSN) and borosilicate glass composites and 0/90 cross-ply reinforced borosilicate glass composite using a fiber pushout test technique. The interface debonding load and the maximum sliding friction load were measured for varying lengths of the embedded fibers by continuously monitoring the load during debonding and pushout of single fibers in finite-thickness specimens. The dependences of the debonding load and the maximum sliding friction load on the initial embedded lengths of the fibers were in agreement with nonlinear shear-lag models. An iterative regression procedure was used to evaluate the interfacial properties, shear debond strength (T _d ), and sliding friction stress (T _f ), from the embedded fiber length dependences of the debonding load and the maximum frictional sliding load, respectively. The shear-lag model and the analysis of sliding friction permit explicity evaluation of a coefficient of sliding friction (μ) and a residual compressive stress on the interface (σ₀). The cross-ply composite showed a significantly higher coefficient of interfacial friction as compared to the unidirectional composites.     

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.     

Indirect estimation of fiber/polymer bond strength and interfacial friction from maximum load values recorded in the microbond and pull-out tests. Part II: Critical energy release rate

A new approach to experimental data treatment in the pull-out and microbond tests has been developed. It uses the relationship between the maximum force recorded in these tests and the embedded length ('scale factor') to separately determine adhesional interfacial parameters (critical energy release rate, local bond strength) and interfacial friction in debonded regions. The new method does not require the measurement of the debond force, which corresponds to interfacial crack initiation, and is, therefore, much more convenient and simpler than 'direct' techniques involving continuous monitoring of crack growth. Using the equation for the current crack length as a function of the load applied to the fiber, based on a fracture mechanics analysis of interfacial debonding, we modeled the pull-out and microbond experiments and obtained the maximum force versus the embedded length. By varying the critical energy release rate and interfacial frictional stress to fit experimental plots, both interfacial parameters were determined for several fiber-polymer pairs. Effects of specimen geometry, residual thermal stresses, and interfacial friction on the measured values are discussed. The results are compared with those obtained with our similar stress-based approach. The energy criterion works when the embedded length is not very short, and in this range of embedded length it is better than the stress criterion. Both criteria can be complementarily used for interface characterization.     

Stress Transfer in Single Fiber/ Resin Tensile Tests

Microscale (25 mm gauge length) “dogbone” resin specimens with single carbon fibers embedded through the length of the specimen have been studied as a method for determining the fiber-resin interphase strength. The specimens are pulled in tension until the fiber fragments to a critical length, l_c. Evidence is presented here, based primarily on the relaxation of stress birefringence around the fiber fragment, that this test may not be an unambiguous measure of fiber-resin adhesion. Data obtained for various production lots of AS-4, AS-6, and IM-6 fibers indicate an increase in l_cd with laminate tensile strength. Although there is theoretical justification for this correlation, it requires that the interphase shear strength is relatively constant.

In those instances where interfacial adhesion was expected to be low, i.e., surface contamination or unsurface treated fiber, there was a significant increase in l_c/d and usually a distinct difference in stress birefringence compared to “good” adhesion. However, the distinction in stress birefringence was not always clear cut.     

Relation of interfacial adhesion in Kevlar®/epoxy systems to surface characterization and composite performance

A central problem in composite materials is the poorly understood relation between the nature of the surfaces at the fiber/matrix interface, the actual interfacial bond strength, and interface-sensitive composite properties, in this study on the Kevlar®/epoxy composite system, the interface was varied chemically by fiber sizings. The sized and unsized fiber surfaces and the cured matrix surface were characterized by contact angle measurements. The interfacial shear strength was directly measured by single-filament pull-out tests of sized and unsized fibers in epoxy matrix. The shear strengths of the composites made with sized and unsized fibers were measured. The results from surface analysis, interfacial shear tests, and composite shear tests were consitent. This suggests that surface-contact-angle analysis and single-filament pull-out tests may be helpful in screening strength of the composite.     

Finite element analysis of load transfer at a fibre–matrix interface during pull-out loading

Load transfer ability of the fibre–matrix interface is well known to mainly control the mechanical behaviour of fibre-reinforced materials. This load transfer phenomenon is of great importance in dentistry when a post is used for fixing a ceramic crown on the tooth. The pull-out test has been well accepted as the most important micromechanical test for evaluating the interaction properties between the fibre and matrix. In this study, a finite element model is developed to analyse the pull-out process of a steel fibre from an epoxy matrix. Based on the pull-out force–displacement curves, developed in our previous experimental work, specific load transfer laws at the fibre–matrix interface have been proposed for each stage of the pull-out process, i.e., before and after fibre–matrix debonding. Predicted initial extraction forces for different implantation lengths were fitted to experimental values and an initial interference fit of 4 μm was determined. An interfacial shear strength of 21 MPa was then determined by fitting the predicted debonding forces for different implantation lengths to the experimental values. According to the load transfer laws considered, analysis of the interfacial shear stress indicates that fibre–matrix debonding initiates simultaneously at both the lower and upper extremities of the interface.     

The single fiber pull-out test analysis: influence of a compliant coating on the stresses at bonded interfaces

The mechanical properties at the fiber/matrix interface play a significant role in controlling the fracture resistance of fiber-reinforced composites. By coating the fiber with sizing and coupling agents, these interfacial properties can be modified. The aim of the present analysis was to examine the effects of the coating thickness and modulus on the stresses at the bonded interfaces between the fiber, coating, and matrix. Using the fiber pull-out test as the analytical model, the stresses are first obtained by minimizing the total complementary energy in the coated fiber/matrix composite. The analytical results show that the interfacial shear stress between the fiber and the coating is higher than that between the matrix and the coating. Also, a thin and compliant coating reduces substantially the peak interfacial shear stress but not the interfacial radial stress due to Poisson's effect on the fiber. Furthermore, the shear stress transfer from the fiber to the matrix across the coating layer is found to be more uniform. The implications of these findings are discussed.     

不同龄期钢纤维增强水泥砂浆纤维拉拔试验与模拟研究

钢纤维增强水泥基复合材料作为一种多相复合材料,其增强增韧效果的发挥依赖于钢纤维与基体之间的界面粘结性能。通过开展不同龄期的钢纤维增强水泥基复合材料单根纤维拉拔试验及数值模拟研究,分析了龄期对钢纤维增强水泥砂浆界面粘结性能的影响,建立了不同龄期的单根纤维拉拔细观模型,通过将模拟结果与试验结果进行对比验证模型的有效性。根据所建立的细观模型分别对不同龄期钢纤维增强水泥砂浆纤维-基体间的界面粘结作用机理及纤维-基体间粘结表面在纤维拔出过程中的应力变化进行了分析。结果表明:所建立细观模型模拟得到的纤维最大拉拔力及荷载-滑移曲线与试验结果吻合较好,钢纤维的最大拉拔力及钢纤维-水泥砂浆基体的界面粘结强度均随着龄期的增加而增加;在7 d龄期内界面粘结强度的增长速度较快,7 d龄期后增长速度放缓;随着龄期的增加,不同龄期段的界面粘结强度的增长率逐渐减小并趋于稳定。采用拟合得到的粘结表面材料参数能够有效模拟各龄期下单根钢纤维从水泥砂浆中的拔出过程。     

Temperature Dependence of Interfacial Shear Strength in SiC-Fiber-Reinforced Reaction-Bonded Silicon Nitride

The interfacial shear strength of AVCO SCS-6 SiC-fiber-reinforced reaction-bonded Si₃N₄ (RBSN) composites was studied as a function of temperature. Fiber "push-through" experiments were conducted with a diamond indenter and a high-temperature microhardness tester. The interfacial shear strength was variable and depended mostly on interfacial bonding at low temperatures (5 to 18 MPa at room temperature) and frictional forces at high temperatures (12 to 32 MPa at 1300°C). The frictional component is attributed to the surface roughness of the fibers. The interfacial shear strength increased with temperature, because of the relief of residual stresses arising from the thermal expansion mismatch between fiber and matrix. Because of the composite nature of these fibers, a number of interfaces were tested in each experiment. The interface which debonded and slid was not always the same. Interfacial fracture took place either between the two outermost carbon layers of the SCS-6 fibers, or between the SiC core and the innermost of the two outer carbon layers. The outermost carbon layer of the fiber always stayed bonded to the Si₃N₄ matrix.     

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.     

短玻纤增强聚丙烯的研究进展

综述了近年来有关短玻纤增强聚丙烯复合材料的力学性能、变形机理和断裂韧性等方面的研究工作。短玻纤取向后的复合材料注射样的力学性能是各向异性的 ,复合材料在取向方向上具有更高的拉伸强度。玻纤与树脂基体间界面结合力的强弱对材料的力学性能同样起着至关重要的作用。良好的界面结合力保证了应力有效地从基体向玻纤传递 ,从而提高了复合材料的强度。由于短玻纤的分布既不均匀又不规则 ,在受到负荷时的变形过程很复杂 ,包括玻纤 -基体的界面脱黏、脱黏后的摩擦、基体的塑性变形、玻纤的塑性变形、玻纤断裂、基体断裂和玻纤抽出等