The effect of matrix composition on fibre/matrix interfacial bond shear strength in fibre-reinforced mortar

The effects of factors associated with the composition of the matrix, i.e. curing conditions and time and mix proportions, on the shear strength of the interfacial bond between steel fibres and a cementitious mortar matrix have been examined experimentally using a single-fibre pull-out test technique. The experimental results indicate that bond shear strength increases significantly with an increase in matrix curing time and, for specimens with the fibre axis perpendicular to the direction of casting and compaction of the matrix, with a decrease in the proportion of water by weight in the matrix mortar. This latter effect is attributed to bleed water gain under the embedded fibre, as it is not observed in specimens with the fibre axis parallel to the direction of casting and compaction of the matrix. Furthermore, the results indicate that there is no correlation between interfacial bond shear strength and matrix mortar compressive strength.     

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.     

The Interfacial Bond Strength in Glass Fibre-Polyester Resin Composite Systems

The interfacial bond strength in glass fibre-polyester resin composites has been investigated using various experimental techniques. These included blocks of resin containing fibre (in which, depending on the geometry of the specimen, failure occurs in either a shear or tensile mode) the pullout of a fibre from a disc of resin and a short beam shear test for interlaminar shear strength determination.

Low power optical microscopy and optical retardation measurements of stress induced birefringence were used to detect the difference between intact and debonded fibre resin interfaces. The shear modulus and shear strength of the resin were obtained from torsion tests on cylindrical rods of the resin.

The single fibre shear debonding specimen and the short beam shear test are shown to be the most viable test methods but interpretation of the results is complicated by the various modes of failure possible and by the different stress states which exist in the area of the specimen where debonding starts. Stress concentration factors obtained by finite element analysis and photoelastic analysis have been applied to the results from these tests and the corrected interfacial bond strengths are in close agreement.

The real interfacial bond strengths of well bonded glass-fibre polyester resin systems is shown to be of the order of 70 MN m^?2.     

Theory and Analysis of Fracture Energy in Fiber Reinforced Composites

A micro-mechanics model is developed to analyze the stress distributions and fracture energies associated with crack propagation and fiber pull-out in reinforced composites. The stress and work mechanisms of interfacial debonding, fiber deformation, and the frictional work of fiber pull-out are considered as semi-independent contributions to fracture toughness. The theoretical expressions of Cottrell for frictional work W_F and Outwater and Murphy for fiber deformational work W_D are obtained as special relations in a general relation for the total work W_T = W_s + W_F + W_D where W_s defines the matrix shear work for interfacial debonding of fiber and matrix. Three dimensional diagrams of fracture energies W_T, W_s, or W_r versus interfacial shear bond strength λ₀ and frictional shear stress λ_f identify regions of optimized fracture energy. The influence of environmental degradation of bond strength upon fracture energy is analyzed in terms of the theory.     

Improvement in interfacial shear strength and fracture toughness for carbon fiber reinforced epoxy composite by fiber sizing

Carbon fiber was sized by a thermoplastic polymer solution mixed with a compatible amine monomer. The effect of sizing agent on tensile strength was studied by single fiber strength testing. Interfacial properties of re‐sized carbon fiber/epoxy composite were investigated, with special emphasis on the improvement in both interfacial shear strength and interfacial fracture toughness. The interfacial fracture toughness of composites was characterized by calculating the effective interphase fracture energy rate through the information obtained from the force–displacement curve in the micro‐bond test. Fracture topography of micro‐bond specimen was observed to discuss the interfacial fracture mechanism. POLYM. COMPOS., 35:482–488, 2014. © 2013 Society of Plastics Engineers     

Bonding behavior of concrete matrix and alkali-activated mortar incorporating nano-SiO2 and polyvinyl alcohol fiber: Theoretical analysis and prediction model

As an emerging construction material, alkali-activated mortar is considered as a sustainable alternative to cementitious composites for the repairing and reinforcement of existing defective buildings. Furthermore, the bonding performance of alkali-activated mortar and concrete matrix can be promoted by adding polyvinyl alcohol (PVA) fiber and nano-SiO₂ (NS). In this study, the effects of PVA fiber and NS contents, alkali-activated mortar type, concrete strength grade, and interfacial roughness on the bonding behavior of two-interfaced shear samples were explored. Based on the experimental results, the grey relation analysis was applied to evaluate the significance of each factor on the bond properties of the alkali-activated mortar and concrete matrix. A prediction model of artificial neural network (ANN) was established considering the effects of alkali-activated mortar type, concrete strength grade, and interfacial type on the bond strength of the samples. The relevant factors affecting bond strength derived by grey relation analysis and weight contribution algorithm was compared and analyzed. Results of the two-interfaced shear test showed that the addition of PVA fiber and NS can significantly boost the bonding property of the samples, and the bond strength increases with the increase of concrete strength grade, alkali-activated mortar strength, and interfacial roughness. Grey relation analysis results indicate that the interfacial type has the most noticeable effect on the bond strength of the samples, followed by the concrete strength grades and the alkali-activated mortar types. The optimum bond strength is derived from PN-C40-III, which is alkali-activated mortar with 0.6% PVA fiber and 1.0% NS contents, concrete strength grade of C40, and interface of type III. The prediction results of the ANN indicate that the predicted values of the bond strengths of the samples are consistent with the experimental values (R = 0.982), and the importance of each factor towards the bond behavior derived by the grey relation analysis and weight contribution algorithm is ultimately consistent after normalization.     

Micromechanics prediction of the transverse tensilc strength of carobn fiber/epoxy composites: The influence of the matrix and interface

Currently, there is great interest in understanding and improving the bond between the fibers and matrix in high performcance composite materials. In many recently developed systems, fiber surface treatments have been developed to improve poor bonding. These treatments are often evaluated by measureing their effect on a composite property sensitive to the interfacial bond strength, typically the composite shear strength. This paper presents an evaluation of the influence of the matrix and interface properties on the transverse tensile strength. These effects were quantified by compring transverse flexural experimental data with results from a finite element micromechanics model. The results indicate that the transverse tensile strength is significantly more dependent upon sizing than is the shear strength. Finally, the transvere flexure test appears to provide an additional and complementary test for evaluating interface bond characteristics.     

The Interfacial Bond Strength in Glass Fibre-Polyester Resin Composite Systems

The interfacial bond strength in glass fibre-polyester resin composites has been investigated using various experimental techniques. These included blocks of resin containing fibre (in which, depending on the geometry of the specimen, failure occurs in either a shear or tensile mode) the pullout of a fibre from a disc of resin and a short beam shear test for interlaminar shear strength determination.

Low power optical microscopy and optical retardation measurements of stress induced birefringence were used to detect the difference between intact and debonded fibre resin interfaces. The shear modulus and shear strength of the resin were obtained from torsion tests on cylindrical rods of the resin.

The single fibre shear debonding specimen and the short beam shear test are shown to be the most viable test methods but interpretation of the results is complicated by the various modes of failure possible and by the different stress states which exist in the area of the specimen where debonding starts. Stress concentration factors obtained by finite element analysis and photoelastic analysis have been applied to the results from these tests and the corrected interfacial bond strengths are in close agreement.

The real interfacial bond strengths of well bonded glass-fibre polyester resin systems is shown to be of the order of 70 MN m^-2.     

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.     

Indirect estimation of fiber/polymer bond strength and interfacial friction from maximum load values recorded in the microbond and pull-out tests. Part I: local bond strength

The techniques aimed at adhesion strength measurement between reinforcing fibers and polymer matrices (the pull-out and microbond tests) involve the measurement of the force, F _max, required to pull out a fiber whose end is embedded in the matrix. Then, this maximum force value is used to calculate such interfacial parameters as the apparent bond strength, τ_app, and the local interfacial shear strength (IFSS), τ_d. However, it has been demonstrated that the F _max value is influenced by interfacial friction in already debonded regions, and, therefore, these parameters are not purely 'adhesional' but depend, in an intricate way, on interfacial adhesion and friction. In the last few years, several techniques for separate determination of adhesion and friction in micromechanical tests have been developed, but their experimental realization is rather complicated, because they require an accurate value of the external load at the moment of crack initiation. We have developed a new technique which uses the relationship between the maximum force and the embedded length ('scale factor') to separately measure fiber-matrix interfacial adhesion and friction. Using the equation for the current crack length as a function of the applied load, based on a stress criterion of interfacial debonding, we modeled the pull-out and microbond experiments and obtained the maximum force value versus the embedded length. By varying τ_d and interfacial friction, τ_f, to fit experimental plots, both interfacial parameters were estimated. The micromechanical tests were modeled for three types of specimen geometries (cylindrical specimens, spherical droplets, and matrix hemispheres in the pull-out test) with different levels of residual thermal stresses and interfacial friction. The effect of all these factors on the experimental results is discussed, and the importance of specimen geometry is demonstrated. One of the most interesting results is that the 'ultimate' IFSS (the limiting τ_app as the embedded length tends to zero) is not always equal to the 'local' bond strength.     

Experimental Investigation of Aluminum/Epoxy Interfacial Properties in Shear and Tension

This paper presents the results of comprehensive testing to characterize the effect of several different surface treatments on shear and tensile bond strength between 7075-T6 aluminum and two epoxy systems: EPON 815/V40 and EPON 828/Z. A rod pull-out test was used to determine interfacial shear strength, modeled after similar tests on reinforced concrete. The tensile bond strength was characterized using a tension test fixture designed in this study. Overall, the interfacial shear strengths were higher than the tension strengths. Surface knurling gave the highest interfacial shear strength, representing a 72% increase over untreated specimens. Phosphoric acid anodization (PAA) was also quite effective in shear. In tension, the highest strength was obtained from specimens treated with the PAA process along with a silane coupling agent. These specimens showed an increase in interfacial tensile strength by a factor of 5.6.     

Plasma surface modification of carbon fibers: a review

The properties of the fiber/matrix interface in carbon fiber-reinforced composites play a dominant role in governing the overall performance of the composite materials. Understanding the surface characteristics of carbon fibers is a requirement for optimizing the fiber-matrix interfacial bond and for modifying fiber surfaces properly. Therefore, a variety of techniques for the surface treatment of carbon fibers have been developed to improve fiber-matrix adhesion as well as to enhance the processability and handling of these fibers. Many research groups have studied the effects of plasma treatments, correlating changes in surface chemistry with the interfacial shear strength. This article reviews the recent developments relative to the plasma surface modification of carbon fibers.     

Interfacial bond property of UHMWPE composite

The ultrahigh molecular weight polyethylene (UHMWPE)/hydrocarbon (PCH) composite was prepared by selecting a PCH resin as the matrix, which has the similar structure to UHMWPE fiber. The interfacial bond property between the PCH resin and UHMWPE fiber was investigated by macromechanics, micromechanics, and contact angle. The results show that the PCH resin has good wettability with the UHMWPE fiber surface. The UHMWPE/PCH composite has excellent transverse tensile strength, interlaminar shear strength, and the pull-out strength together with the outstanding interfacial bond property.     

Effect of compounding on the properties of short fiber reinforced injection moldable thermoplastic composites

The mechanical properties of short-fiber-reinforced thermoplastic composites depend on the degree of interfacial bond strength between the fibers and polymer matrix. This interfacial bond strength can be increased by appropriate coupling agents. This study shows, for example, that an amino silane coupling agent improves the bond strength of nylon-aluminum fiber composites, but not polycarbonate-aluminum fiber composites. For cases where appropriate coupling agents are not available it is important to maintain as high a fiber aspect ratio as possible in a molded part. This study shows that a single screw compounder does less damage to glass or carbon fibers than a twin screw compounder under similar processing conditions when the polymer is in the form of pellets. When the polymer is supplied as a powder, satisfactory dry blends can be produced and the twin screw compounder does less damage to the fibers. In both cases, however, fibers initially 6 mm long are reduced to an average length less than 0.5 mm. The greatest degree of fiber size retention was observed when extrusion coated fiber pellets were used in the injection molding machine. The relationship between a fiber's tensile strength and the interfacial shear strength between a fiber and matrix yields a critical fiber aspect ratio below which the maximum reinforcing capability of the fibers are not being utilized. For the polymers investigated in this program, the critical aspect ratio for carbon fibers was found to be between 16 and 25 to 1. The polymers investigated include flame-retardant grades of acrylonitrile-butadiene-styrene (ABS) and poly(phenylene oxide)/polystyrene blend, nylon 6/6 and poly(phenylene sulfide).     

The dependence of interfacial shear strength on matrix and interphase properties

Experiments were conducted to determine the dependence of the interfacial shear strength on the bulk material matrix properties using model compounds based on epoxy/amine chemistry. AS4, carbon fibers were used as the subject for these measurements with both a difunctional epoxy (DGEBA) system as well as a tetrafunctional epoxy (MY720) system. Amine curing agents were carefully chosen to produce matrices which produced a range of matrix properties from brittle, elastic to ductile, plastic. The fiber-matrix interfacial chemistry was constant throughout this study by always using a stoichiometric amount of curing agent. The results indicate that, for both the difunctional as well as the tetrafunctional epoxy system, the interfacial shear strength (as determined by the fragmentation test) decreases nonlinearly with decreasing modulus of the matrix. Linear elastic analysis yields a nearly linear relationship, for both systems, between the interfacial shear strength and the product of strain to final break and the square root of the matrix shear modulus. A linear relationship is also found between the difference in test temperature and glass transition temperature of the cured matrix and the interfacial shear strength. Additionally, the failure mode is seen to remain interfacial as the ductility of the matrix changes.     

Development of a thin film technique for an amorphous metallic ribbon/thermoplastic matrix composite system

A thin film method is developed for the fabrication of an amorphous metallic ribbon/PP (polypropylene) matrix composite system. The interfacial shear strength of composites fabricated with thin film is 0.52 ± 0.09 MPa (process B), which is comparable to that of the sandwich method of composite fabrication reported recently. The optical micrographs of the interfacial zone appear to be defect free and there are no visible voids, cracks, or air entrapment during fabrication. This qualitative analysis of the ribbon/matrix interface suggests that thin film can provide a better interfacial bond, a result which is supported by the results of pullouts tests. The composite fabrication time using the thin film method is short, and this method therefore has the potential to produce composites at a high volume fraction of ribbon reinforcements as compared to the sandwich method.     

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.     

The Effect of Sodium Ions on the Stability of the Interphase Region of Glass Fibre Reinforced Composites

The single embedded filament fragmentation and the short beam shear strength tests together with angle-resolved X-ray Photoelectron Spectroscopy (XPS) have been used to investigate the interfacial region of vinyl ester composites reinforced with sized AR-glass fibres, with and without amino and vinyl functional adhesion promoters.

The 7-aminopropyltriethoxysilane (APS) deposit on AR-glass is susceptible to a thermal degradation during post-cure, which has been attributed to a base catalysed equilibration of the siloxane bonds. The functional groups of APS required for resin compatibility were buried beneath the surface layers, contributing to a low bond strength, furthermore, mobile sodium ions existed within the interfacial region. Aqueous extraction prior to fabrication enhanced the composite bond strength by removing the soluble silane oligomers, the sodium ions, and exposing the organo-functional groups for co-reaction with the matrix.

The silane deposit on AR-glass is made hygroscopic by the presence of sodium ions. This increased the equilibrium moisture content of AR-glass composites, and diminished their retained short beam shear strength in contrast to the E-glass control which retained its properties after redrying.     

沸水对PI/CF复合材料界面作用机理的研究

研究了聚酰亚胺(PI)／碳纤维(CF)复合材料界面在沸水中的稳定性。结果表明,经沸水浸泡后的复合材料层间剪切强度和界面剪切强度均有所提高,且随水煮时间的延长而增大;试样断面观察表明,水没有对复合材料界面产生破坏作用,力学性能的变化与基体／纤维界面粘结的湿热稳定性有直接关系;沸水对界面的作用机理是,在PI／CF复合材料的界面区,树脂的水解使氢键的数量增加,形成了防水层,阻碍了水沿界面的侵入,同时水松驰了界面局部应力。     

Evaluation of the bond strength of hard coatings by the contact fatigue test

The conventional contact fatigue test was applied to evaluate the bond strength between hard coatings and substrates. It was designated as an interfacial fatigue test for coated samples and was conducted with either cylindrical or spherical rolling. A shear stress range at the interface was derived based on the mechanics of elasticity and used as a measure of the interfacial fatigue strength between the coating and the substrate. It was found that the coating exfoliated after the coated specimen was subjected to a number of cycles under cyclic contact loading, but it would be intact when the cyclic load was decreased to a critical value. The shear stress range corresponding to this critical load was defined as the bond strength which would reflect the change in the physical and/or chemical state in the interfacial region. In the calculation of the shear stress range, the effects of the thickness and Young's modulus of the coatings were taken into account.