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Users of the microbond test assume that a microbond resin droplet's properties are equivalent to a macroscale specimen. However, there is currently no standardised methodology for determining the cure state of droplet specimens used in the microbond test. In this paper, we present a technique for microbond test users to better understand the properties of thermoset droplet specimens. Utilising a conventional benchtop spectrometer, a novel sample preparation technique involving curing epoxy droplets on thin-steel filaments allowed for high-throughput determination of the microbond droplet cure state. The parity between steel filament and glass fibre microbond samples was confirmed by infrared microspectroscopy. It is shown that cure schedules used in manufacturing composite parts produced microbond droplets with degrees of cure lower than that of bulk matrix specimens subjected to an identical thermal history. Testable microbond droplets could only be prepared for commercial resin systems when introducing a room temperature pre-curing time of at least 2 h. It is concluded that microbond testing should be supported by some droplet cure state characterisation methods to ensure that interfacial effects are not artefacts of droplet sample preparation. [ABSTRACT FROM AUTHOR]

This study improved homemade apparatus for characterizing the interfacial shear strength (IFSS) of carbon-fiber-reinforced polyphenylene sulfide (PPS/CF) composites. The upgraded generation II experimental device includes a newly developed experimental clamp for samples, as well as testing systems. Compared with the initial generation I apparatus and the commercial Toei instrument, the generation II device is easier and more efficient to operate. The average interfacial adhesion values obtained using these devices were consistently approximately 40 MPa, with relatively low data scatter, showing excellent repeatability and applicability during microbond tests. Notably, the generation II experimental device was equipped with an additional high-frequency data-capturing tool to identify the debonding peak force more precisely, which demonstrated a higher interfacial shear strength of 42.81 MPa during testing. Therefore, the new instrument was able to reflect the change in the interfacial stress state during the interface debonding process more accurately and reliably. [ABSTRACT FROM AUTHOR]

Reversible bond exchange reaction gives Vitrimer resin the advantages of being recyclable and reprocessing. So the characterization of the interfacial bonding properties between Vitrimer and carbon fibre is of great interest for aerospace applications. In this study, a kind of thermosetting resin containing bond exchange reaction was configured by adding zinc acetylacetonate catalyst to the conventional thermosetting resin. The force–displacement curve was obtained by microbond tests, and the experimental results showed that the newly configured resin has higher interfacial shear strength. A finite element simulation was established based on cohesive contact model (CZM), and the interface parameters of the CZM were inverted by using finite element update method (FEUM) in combination with the experimental data. And the pull-out force–displacement curves obtained from the numerical simulation are in good agreement with the experimental data. The interfacial stress transfer and debonding failure behavior is further discussed. [ABSTRACT FROM AUTHOR]

Short fiber–reinforced clay mortar is known as an important structural material in traditional and rural buildings because it not only provides superior mechanical properties but can also be easily produced by the rapid, low-cost molding process. The adhesion of the fibers to clay mortar has a significant role in the mechanical characteristics of fiber-reinforced mortar. In this study, interfacial shear stress (ISS) between nine different fibers and clay mortar was investigated by performing the Microbond test. Based on the findings, date palm fiber, hollow polyester, and polypropylene, respectively, demonstrated the highest degree of ISS with clay mortar; nevertheless, the lowest degree belonged to goat hair, acrylic, and wool, respectively. The critical lengths of the fibers to be embedded in the mortar as the short fiber reinforcements were calculated. The highest critical length belonged to glass fibers and date palm, respectively, and the lowest values belonged to acrylic, polyester, polypropylene, and wool, respectively. [ABSTRACT FROM AUTHOR]

The microbond test has been widely adopted by researchers seeking to characterise the fibre-matrix interface in polymer composites. This test requires the preparation of polymer droplets with a mass as little as 20 ng. This brings with it the possibility of scaling issues causing unanticipated significant differences to the properties and performance of these droplets in comparison to macroscale samples. This paper reviews a number of these potential scaling issues in a range of polymer systems which users of the microbond test should be aware of. Some of these phenomena can affect the magnitude of the test results obtained much more than the fibre-matrix interface effects which the test is used to detect. It is concluded that in many cases the structure and properties of the polymer in such microdroplets is not representative of the same polymer, with the same thermal and environmental exposure history, prepared on the macroscale.

The microbond test was used to investigate the interface strength (IFSS) in various glass fibre-vinylester (VE) composite systems and a number of issues related to the sample preparation were identified. Cure schedules that produce well-reacted VE polymers on the macroscale did not result in cured microdroplets. Hence, the microbond test could not be carried out on samples with the same cure history as macroscale composites. Testable microdroplet samples could only be obtained when resin cure was carried out under an inert atmosphere. Higher IFSS values were obtained by raising the final temperature of the cure schedule. Glass fibres with a full sizing gave significantly higher apparent IFSS values comparedf to bare fibres or fibre coated with only silane coupling agent. It was discovered that the measured IFSS of VE compatible glass fibres was approximately doubled when fibres were mounted using epoxy glue instead of cyanoacrylate glue. This phenomenon appears to be related to the deposition of cyanoacrylate vapours onto the surface of the fibres during sample preparation. It is concluded that great care must be taken in ensuring that effects observed using the microbond test are evidence of real material characteristics and not artefacts of sample preparation. [ABSTRACT FROM AUTHOR]

In recent years, natural fibers have received much attention from various industrial applications. As these fibers are lightweight, nonabrasive, low cost, ecofriendly and biodegradable, they can be sometimes considered as alternatives to synthetic fibers in lightweight composite structures. In this work, date palm fibers (DPFs) were treated by oxygen plasma at various plasma discharge power and exposure time. The effects of plasma treatment on tensile strength of DPF and interfacial adhesion between DPF and epoxy were determined by single fiber tensile test and microbond test, respectively. Scanning electron microscopy was used to investigate the surface morphologies of DPFs before and after the plasma treatment. The functional groups on the surface were studied by attenuated total reflectance-Fourier transform infrared spectroscope (ATR-FTIR). Decrease in hemicellulose and lignin content of DPF was indicated in ATR-FTIR spectra of the treated sample with plasma treatment. The results show that plasma treatment cleans the fiber surface and increases the surface roughness by etching effect. Moreover, fiber surface modification significantly improves tensile properties of DPFs and interfacial shear stress (IFSS) of fiber/matrix. However, the effects of plasma power and exposure time on tensile properties and IFSS values of DPFs are not found significant. Moreover, Weibull statistics show that plasma treatment could not decrease the variability in fiber strength due to the nature of fibers.

Typha fibers were chemically retted in 5% sodium hydroxide solution for 1 h, 2 h, 4 h, and 8 h. Changes in chemical compositions of the untreated and treated fibers were monitored with Fourier transmission infrared spectroscopy, while changes in the crystallinity index were studied via Xray diffraction. The FTIR spectra and scanning electron microscope images corroborated the successful removal of amorphous portions from the Typha leaf fibers during alkali treatment, which resulted in an enhanced crystallinity index for alkali-treated fibers. The alkali-treated Typha fiber for 1 h showed the highest water contact angle of 87.5°, while the untreated composite showed the lowest contact angle. Typha fiber treated for 4 h had high tensile strength, Young's modulus, and elongation at break of 158 MPa, 1600 MPa, and 7%, respectively. The results showed that there was a general increase in the interfacial shear strength of Typha fiber with epoxy resin and polyester resin with increased time. Both the mechanical properties and crystallinity index of the Typha leaf fibers increased with increased time of retting until 4 h, after which further alkaline retting resulted in decreased values. The overall results showed that alkaline-extracted Typha leaf fibers are suitable for biodegradable film composites. [ABSTRACT FROM AUTHOR]

The shapes of force–displacement curves recorded in single-fiber pullout and microbond tests are analyzed within the framework of a stress-based model of interfacial debonding. Three characteristic points allowing one to evaluate the local interfacial strength parameters, the local interfacial shear strength (IFSS), and the interfacial frictional stress, using several different methods, can be marked out in each curve. The "alternative" method based on the measured peak force and initial postdebonding force appeared to be more reliable than the "traditional" one using the debond force for calculating the local IFSS in fiber–matrix systems. The effect of specimen geometry on force–displacement curves and on calculated local interfacial strength parameters was investigated. Though the "equivalent cylinder" approximation often yields a good estimate of these parameters, there is a need for a method which would explicitly include the actual specimen geometry. [ABSTRACT FROM AUTHOR]

The interfacial shear strengths between two kinds of mesophase pitch-based carbon fibers and epoxy resin matrix were respectively measured by microbond test. The forces when microdebonding occurring between resin microdroplets and carbon fibers were obtained from the load-displacement curve,and the morphologies of debonded microdroplets and carbon fibers were observed by scanning electron microscopy. Then,the average interfacial shear strength,standard deviation and coefficient of variation for samples were quantitatively calculated. Moreover,the relationships between interfacial shear strength and the lengths, diameters of microdroplets, carbon fiber diameter were analyzed. The results show that interfacial shear strength is proportional to the ratio of length-to-diameter of the resin microdroplet,and inversely proportional to the carbon fiber diameter,to the ratio of the microdroplet length-to-carbon fiber diameter,and to the ratio of the microdroplet diameter-to-carbon fiber diameter.

The microbond test for natural fibers is difficult to conduct experimentally due to several challenges including controlling the gap distance of the blade, the meniscus shape, and the large data spread. In this study, a finite element simulation was performed to investigate the effects of the bonding characteristics in the interface between the fiber and matrix on the Typha fiber/epoxy microbond test. Our aim was to obtain the accurate mesh and cohesive properties via simulation of the Typha fiber/epoxy microbond test using the cohesive zone model technique. The axisymmetric model was generated to model the microbond test specimen with a cohesive layer between the fiber and matrix. The cohesive parameter and mesh type were varied to determine the appropriate cohesive properties and mesh type. The fine mesh with 61,016 elements and cohesive properties including stiffness coefficients Knn = 2700 N/mm3, Ktt = 2700 N/mm3, and Kss = 2700 N/mm3; fracture energy of 15.15 N/mm; and damage initiation tnn = 270 N/mm2, ttt = 270 N/mm2, and tss = 270 N/mm2 were the most suitable. The cohesive zone model can describe the debonding process in the simulation of the Typha fiber/epoxy microbond test. Therefore, the results of the Typha fiber/epoxy microbond simulation can be used in the simulation of Typha fiber reinforced composites at the macro-scale.

Mechanical properties of ternary composites consisting of fibres, particles and polymer matrix are strongly influenced by the interface combination status of the three components. We investigated the interfacial bond properties of fibre-microballoon-epoxy ternary composites using a microbond test and finite element simulation. The finite element simulation using ANSYS software predicted force-displacement curves and interfacial shear strength (IFSS) consistent with those determined by the microbond test for a wide range of fibre-microballoon-epoxy ternary composites. It is found that fibre diameter has the largest influence on the IFSS of the ternary composites, followed by the volume fraction and size of the microballoon particle. The von Mises and contact friction stress contour plots from the simulation models were used to explain the influences of these parameters on the IFSS of the ternary composites determined by the microbond test. [ABSTRACT FROM AUTHOR]

Two high-performance bacterial cellulose (BC) nanofiber-based hybrid structures were produced using an in situ self-assembly approach, one with microfibrillated cellulose (MFC) and another with sisal fiber, by incorporating them in the fermentation media. The fabricated BC-MFC hybrid and BC-sisal hybrid fibers showed enhanced mechanical properties compared to pure BC and sisal fibers, respectively. Tensile tests indicated BC-MFC hybrid and their nanocomposites fabricated with soy protein isolate (SPI) resin had better tensile properties than corresponding BC and BC-SPI nanocomposites. This was because of the uniform distribution of MFC within the BC nanofiber network structure which reduced the defects such as pores and voids or intersections of the BC nanofibers. BC-sisal hybrid fibrous structures were obtained after BC nanofibers self-assembled on the surface of the sisal fibers during the fermentation. The results of the microbond tests indicated that the BC-sisal hybrid fiber/SPI resin bond strength was higher than the control sisal fiber/SPI resin bond with p value of 0.02 at the significance level of 0.05. Higher bond strength is preferred since it can potentially lead to better tensile properties of the composites. The presented work suggests a novel route to fabricate hybrid nanocomposites with higher functional properties. [ABSTRACT FROM AUTHOR]

Plasma treatment, an environmentally safe method, was applied for surface modification of carbon fibers (CFs) and polyphenylene sulfide (PPS) fibers. The morphology, crystallization property, thermal stability, and chemical properties of CF/PPS composites were determined, respectively, by scanning electron microscopy, differential scanning calorimetry, thermogravimetry analysis, and X-ray photoelectron spectroscopy. Interfacial micromechanical performance of CF/PPS composites was investigated by microbond test. The influence of plasma treatment on the apparent interfacial shear strength (τapp) of CF/PPS micro-composite was systematically evaluated. The plasma treatment of CF reduced the τapp of the micro-composite by 13.7% and that of CFs and PPS fibers by 3.4%. However, the plasma-treated PPS fibers increased the τapp of the micro-composite by 17.1%. The present study demonstrates that matching of polarity between CF and the resin plays a great role in reinforcement of interfacial property. [ABSTRACT FROM AUTHOR]

Textile-reinforced composites have become increasingly attractive as protection materials for various applications, including sports. In such applications it is crucial to maintain both strong adhesion at fibre-matrix interface and high interfacial fracture toughness, which influence mechanical performance of composites as well as their energy-absorption capacity. Surface treatment of reinforcing fibres has been widely used to achieve satisfactory fibre-matrix adhesion. However, most studies till date focused on the overall composite performance rather than on the interface properties of a single fibre/epoxy system. In this study, carbon fibreswere treated by mixed acids for different durations, and resulting adhesion strength at the interface between themand epoxy resin as well as their tensile strength were measured in a microbond and microtensile tests, respectively. The interfacial fracture toughness was also analysed. The results show that after an optimum 15-30 min surface treatment, both interfacial shear strength and fracture toughness of the interface were improved alongside with an increased tensile strength of single fibre. However, a prolonged surface treatment resulted in a reduction of both fibre tensile strength and fracture toughness of the interface due to induced surface damage. [ABSTRACT FROM AUTHOR]

The microbond test has been widely adopted by researchers seeking to characterise the fibre-matrix interface in polymer composites. This test requires the preparation of polymer droplets with a mass as little as 20 ng. This brings with it the possibility of scaling issues causing unanticipated significant differences to the properties and performance of these droplets in comparison to macroscale samples. This paper reviews a number of these potential scaling issues in a range of polymer systems which users of the microbond test should be aware of. Some of these phenomena can affect the magnitude of the test results obtained much more than the fibre-matrix interface effects which the test is used to detect. It is concluded that in many cases the structure and properties of the polymer in such microdroplets is not representative of the same polymer, with the same thermal and environmental exposure history, prepared on the macroscale. • Microbond test polymer droplet weighs as little as 20 ng. • Scaling issues in sample preparation identified for many polymers. • Test result often depends on polymer droplet properties more than interface properties. • Testing in uncontrolled atmosphere will quickly result in undefined moisture content of microdroplet. [ABSTRACT FROM AUTHOR]

Interfacial micromechanical performance of carbon fiber (CF)-reinforced polyphenylene sulfide (PPS) composites was studied by the microbond test, which has been well accepted as the most important micromechanical test for evaluating the interfacial properties between the fiber and matrix. The effect of annealing treatment and atmospheric plasma treatment on the interfacial adhesion and micromechanics of PPS/CF composites was investigated. The results showed that the apparent interfacial shear strength (τapp) value of microcomposites increased from 33.5 ± 4.3 to 43.9 ± 2.3 MPa after annealing. Moreover, plasma-modified CF was characterized by scanning electron microscope and X-ray photoelectron spectroscopy. It was shown that the surface roughness was improved; the O/C atomic ratio was increased from 16.2 to 28.5%. Though hydrogen and covalent bondings were induced at fiber/matrix interface, theτappvalues of PPS/CF composites decreased to 36.4 ± 1.0 MPa with atmospheric plasma treatment, due to the remove of the size layer coated on the surface of CF. [ABSTRACT FROM PUBLISHER]

In this work, the interfacial adhesion between the glass fibre and polypropylene–maleic anhydride modified polypropylene copolymer has been studied using a microbond test. The interfacial shear strength of the composite increased from 6.58 to 14.85 MPa, with the increase of the content of maleic anhydride modified polypropylene from 0 to 5% in the matrix composition. Oxygen plasma and γ-aminopropyltrimethoxysilane surface treatments were also introduced to modify the surface chemistry of the glass fibre. Compared with γ-aminopropyltrimethoxysilane surface treatment and the control, the interfacial adhesion of the composites of oxygen plasma was stronger in the same content of maleic anhydride modified polypropylene. This trend matches the law of surface morphology, which was imaged by scanning electron microscope. And there was no transcrystallisation at the interface of fibre and matrix resin, and no crystalline morphology change on the matrix resin by a polarised optical microscope. [ABSTRACT FROM PUBLISHER]

In this work, the interfacial adhesion between a polyamide 66 fibre, and an aqueous polyurethane–acrylic hybrid polymer adhesive was investigated. Silane and air plasma treatments were introduced to modify the surface of the polyamide 66 fibre. The surface chemistry was characterised using X-ray photoelectron spectroscopy (XPS). There were more oxygen-containing functional groups, –OH or –COOH, introduced by air plasma and silane treatments on the surface of polyamide fibre to increase its chemical activity. The microbond test was used to measure the interfacial shear strength (IFSS) between the waterborne polyurethane–acrylic hybrid polymer adhesive and a polyamide fibre. It has been found that air plasma and silane surface treatments can be used to improve interfacial adhesion. IFSS at 8.7 and 5.9 MPa, respectively, were higher than that of the control, 5.0 MPa. After water immersion at 50°C for 48 h, IFSS dropped to 7.0 MPa for air plasma-treated specimen and to 4.4 and 4.1 MPa for silanised and control specimens, respectively. Air plasma surface treatment is more effective than silane treatment to improve the interface adhesion in the polymer fibre–polymer composite. [ABSTRACT FROM PUBLISHER]

Abstract: Adhesion of a water-based polyurethane adhesive to a human hair has been studied using the microbond test. Adhesion promotion using a silane coupling agent and air plasma treatment was compared. The results show that after air plasma treatment, the interfacial shear strength (IFSS) between the polyurethane and human hair was increased from 3.9 to 8.1MPa. After immersion in an artificial sweat solution at 50°C for 48h, the retained interfacial shear strength was 6.4MPa, and the control remained at 3.7MPa. γ-Aminopropyltrimethoxysilane (APS), a silane coupling agent was not very effective at modifying the surface of the human hair for adhesion because the interfacial shear strength was at 4.5MPa. Furthermore after ageing of in synthetic sweat, the interfacial shear strength fell to 3.5MPa. [Copyright &y& Elsevier]

Abstract: An improved microbond method, with a corresponding testing device, was developed to measure the interfacial shear strength (IFSS) between carbon fibers and epoxy resin. Compared to other methods, this proposed approach is both highly efficient and easy to operate. As a case study for this new method, we measured the IFSS between carbon fibers and epoxy resin. Although the average IFSS obtained was only 7.08 MPa, which is much lower than values documented in several previous studies, the displacement-load curves demonstrate the strong reliability of this method. The lower IFSS could be explained by the highly inert surface of the carbon fibers, which was highly graphitized and had no sizing treatment. Therefore, this method has high potential in applications for screening the sizing agents of carbon fibers or optimizing the surface sizing processes. [Copyright &y& Elsevier]

Glycidoxypropyltrimethoxysilane (GPS) and γ-aminopropyltrimethoxysilane (APS) were used to modify the surface chemistry of polyamide fibre. The surface chemistry was characterised using X-ray photoelectron spectroscopy. The silanol functional group was designed to be introduced on the surface of polyamide fibre to increase its chemical activity by N-alkylation of GPS and hydrolysis of APS, and to improve the poor interfacial adhesion between a polyamide 66 fibre and an aqueous polyurethane polymer adhesive. The microbond test was used to measure the interfacial shear strength between the waterborne PU adhesive and the polyamide fibre. It has been found that APS hydrolysis and GPS-alkylated fibre surface can be used to improve the interfacial adhesion of polyamide fibre to PU. The IFSS can be improved by N-alkylation of GPS from 5.0 to 8.4 MPa. After water immersion at 50 °C for 48 h, then drying, the IFSS increased to 8.8 MPa due to the plasticisation of PU in water. Better interfacial adhesion was also observed by the hydrolysis of APS, but not significantly improved by this method due to the relatively weak hydrogen bond at the interface between APS and polyamide fibre. [ABSTRACT FROM PUBLISHER]

Abstract: Interfacial micromechanical performance of carbon fiber (CF) reinforced polyphenylene sulfide (PPS) composite was studied by the microbond test. A novel homemade apparatus which is operated simply was employed and a different microsample preparation method was introduced. Comparative tests at various loading speeds were made to investigate the influence of testing rate on the interfacial shear strength of CF/PPS micro-composite. It was found that the interfacial shear strength increased with increasing fiber embedded length at speeds below 0.02 mm/s and above 0.04 mm/s, but it was constant between 0.02 and 0.04 mm/s. It is recommended to perform the microbond test in the test rate range of 0.02–0.04 mm/s, where the interfacial shear strength does not change with the embedded length and no significant deviations are observed, thus could better reflect the true interface strength of the composite. [Copyright &y& Elsevier]

Abstract: The microbond test has been investigated with respect to test accuracy and repeatability. It has been concluded that the interfacial shear strength deduced from this method is uncertain due to the droplet shape, which can, however, be reduced by modifying the geometry. In the new method it was proposed that the droplet is replaced by a cylinder. Comparative tests proved that the accuracy improved due to the modifications. The effects of test speed and geometrical parameters (height/diameter of the cylinder) on the interfacial shear strength were investigated. It was found that the test results are sensitive to these parameters and the related changes are discussed. It is recommended to perform the cylinder test in the pull-out range of 10–100 mm/min where the interfacial shear strength does not change with testing speed. [Copyright &y& Elsevier]

Abstract: The microbond test was used to measure the interfacial shear strength (IFSS) between a polyamide fibre and a water-based polyurethane. Due to the low viscosity of the water based polyurethane, the droplet could not be formed using a conventional preparation method. A disc shaped microbond droplet forms when an aqueous-based colloidal polymer adhesive was deposited onto the pin hole in a mounting card with a vertical polyamide fibre in the middle after drying and curing at an elevated temperature. Since the droplets were formed with a disc shape, which differs from the conventional ellipsoid, a finite element analysis of the stress distribution at the interface for these contrastingly shaped droplets were calculated and compared. The stress analysis showed that the interfacial shear stress profiles were well matched for both providing confidence that the disc-shaped droplets could be used for interfacial analysis. The microbond test using a disc-shaped droplet was used to study the influences of silane treatment and plasma treatment on the interfacial shear strength between a polyamide fibre and an aqueous deposited polyurethane. The interfacial shear strength of the fibre after plasma treatment was 10.3MPa, much higher than that of the control and the silanised fibres, 5.2MPa and 5.4MPa respectively. The results showed that the microbond test could be used to investigate the interfacial properties of the polyamide fibre and water-based polymer adhesive. [Copyright &y& Elsevier]

Carbon fibers were treated in a HF glow discharge in tetrafluoroethylene and octafluorocyclobutane in order to improve their adhesion to poly(tetrafluoroethylene) matrix. As the result of the plasma treatment, a thin (20-140 nm) fluoropolymer coating was deposited onto the fiber surface. The structure of this coating was studied by means of IR spectroscopy, XPS, AFM and SEM techniques. The coating material appeared to be similar to PTFE in its chemical composition but distinguished by branched, partially crosslinked, amorphous structure and included unsaturated chemical bonds. The coating thickness of 70 nm was sufficient to effectively screen the field of molecular forces of the initial substrate, thus, decreasing the surface energy of the fibers and improving their compatibility with the PTFE matrix. The adhesive strength in the PTFE-carbon fiber systems, measured by means of the microbond test, more than doubled upon the plasma treatment (the local interfacial shear strength increased from 10.7 to 29.7 MPa, apparent IFSS from 4.3 to 7.8 MPa), and the interfacial frictional stress increased by 70%. The new composite material consisting of 20% short coated carbon fibers in the PTFE matrix showed better mechanical, thermal and tribological characteristics as compared with the composite reinforced with untreated fibers. [ABSTRACT FROM AUTHOR]

Abstract: Discrete element method (DEM) was used to simulate the dynamic process of microbond test of fiber reinforced composites, in which the fiber and matrix were modeled as elastic and elastic–plastic materials, respectively. The interface between fiber and matrix was represented by a bilinear contact softening model. Plastic deformation and progressive cracking of the matrix were observed in the simulation with comparable similarity to the existing experimental results. The initiation and propagation of interfacial debonding were also captured by the DEM simulations, whereas it is very difficult to achieve this by other numerical methods. Vertical and inclined vises with two different vise angles were tested in the simulations. It was found that both vise geometry and vise angle had significant effect on the damages of the material, and the inclined vise was sensitive to the vise angle in terms of large variation of cutting force. The developed DEM model can also be applied to predict material damages in other more complicated fiber reinforce composite system. [Copyright &y& Elsevier]

Abstract: The present study has focused on achieving a micromechanical understanding of the microbond test, which involves pulling a fiber out of a bead of matrix (i.e. droplet) through a knife-edge, in order to quantify the interfacial fracture properties of fiber-reinforced composites. According to the microbond test results for carbon-fiber and epoxy-resin system, matrix cracking occurred during the fiber pullout, in addition to the debonding at the fiber–matrix interface. Therefore, in evaluating the fracture properties of the fiber–matrix interface, we should pay attention to the coupling effects of matrix failure and interfacial debonding on the test results. Then, we discuss how to best extract the interfacial properties while excluding the influence of matrix plasticity and cracking, using numerical simulations. The key mechanism demonstrated here is that the pullout force, in the cases where the influence of matrix cracking is negligible, appears as the upper limit among the experimental data of the pullout force for a constant initial embedded length of the fiber in the matrix. For this reason, the upper-limit data all over the range of embedded fiber length in experiments can be reasonably evaluated by the simulation focusing on the debonding process with matrix plasticity. This evaluation technique is effective as a way of extracting interfacial properties appropriately from microbond test results. [Copyright &y& Elsevier]

The effect of electric charge, present at a glass fiber–polymer interface during the composite material formation, on the adhesive pressure between the components was studied. Additional electric charge was delivered to the interface by means of the deposition of charged polymer particles onto the fiber surface from a fluidized bed in an electric field. The adhesive pressure was calculated using the results of the variational mechanics analysis by Scheer and Nairn from the data obtained with a single fiber microbond test. Our experiments showed that the adhesive pressure increased by 15–20% in the case of charged polymer matrices. This can be attributed to both intensification of van der Waals forces due to extra interfacial pressure and, for polar polymers, the increase of the surface concentration of local adhesive bonds. The mechanical and kinetic models have been proposed to describe the observed behavior of the adhesive pressure; theoretical curves for the adhesive pressure as a function of the time and temperature of the contact formation, including the effect of electric factors, have been obtained. From the kinetic model, the activation energies for the process of adhesive contact formation were determined. The effect of acceleration of adhesive bonding in the presence of electric charge can be explained in terms of a decrease of the activation energy for local bond formation in an electric field. [ABSTRACT FROM AUTHOR]

Abstract: The effect of geometry on interfacial micromechanical behavior of fiber/matrix microdroplets is investigated by means of the combination of microbond test and Micro-Raman spectroscopy in this paper. Microbond test is usually employed to measure the interfacial properties of fiber/matrix composites. Since the existing method was realized through traditional microscopy technique, the test results may not satisfy those accurate analyses such as geometry influence on interfacial micromechanical behavior of composites. To overcome this problem, Micro-Raman spectroscopy is introduced into the microbond test to detect the distributions of micromechanical properties including fiber axial stress, residual stress, interfacial shear stress and stress transfer length along the interfaces between Kevlar-29 aramid fiber and epoxy resin matrix microdroplets. The obtained experimental results show that axial stress transfer will accelerate and then the shear stress concentration will be enhanced along with an increase in the interfacial edge angle. The study indicates that the geometrical characteristics can affect significantly the stress distributions in the fiber/matrix microdroplets. [Copyright &y& Elsevier]

We have derived the equations which explicitly express the peak force, Fmax, and the apparent interfacial shear strength, τapp, measured in the pull-out and microbond tests, as functions of the embedded length. Three types of test geometries were considered: (1) a fiber embedded in a cylindrical block of the matrix material; (2) microbond test with spherical matrix droplets; and (3) pull-out test in which the matrix droplet had the shape of a hemisphere. Our equations include the local interfacial shear strength (IFSS), τd, and the frictional interfacial stress, τf, as parameters; the effect of specimen geometry appeared in the form of dependency of the effective fiber volume fraction on the embedded length. The values of τd and τf were determined by fitting our theoretical curves to experimental Fmax (le) plots by using the least squares method. Our analysis showed how the local IFSS and the frictional interfacial stress affected the measured Fmax and τapp values. In particular, it was revealed that intervals of embedded lengths could exist in which frictional interfacial stress had no effect on Fmax and τapp, even if the τf value was high. We also derived an equation relating the scatter in the interfacial strength parameters (τd and τf) to the scatter in τapp, which is experimentally measurable, and proposed a procedure to determine the standard deviations of τd and τf from experimental pull-out and/or microbond test data. [ABSTRACT FROM AUTHOR]

A direct observation of crack propagation in the microbond test was carried out for five different fiber/polymer matrix systems. This technique appeared to be a very effective tool for interface characterization. Experimental plots of the force required for further crack propagation as a function of debond length were analyzed using both energy-based and stress-based models of debonding. The fracture mechanics analysis was used to construct families of crack resistance or R-curves which showed the variation of energy release rate, G, with the debond length, and included the effect of interfacial friction in debonded regions. For the first time, analogs of the R-curves were created within the scope of the stress-based model to present the local shear stress near the crack tip, τ, as a function of crack length. In both models, the behavior of the interfacial parameter (G or τ) strongly depends on the assumed value of the interfacial frictional stress (τf). However, for each matrix/fiber system there exists such a τf value for which the investigated parameter is nearly constant over the whole region of stable crack propagation (70–90% of the embedded length). Moreover, these best-fit τf values for each specimen appeared to be practically the same for both energy-based and stress-based approaches. Thus, both interfacial toughness, Gic, and local interfacial shear strength, τd, adequately characterize the strength of a fiber/matrix interface. Extrapolation of R-curves and their analogs to zero crack length allows measurement of the interfacial parameters with good accuracy. [ABSTRACT FROM AUTHOR]

An analysis has been made of the tensile strength of sisal fibres and the interfacial adhesion between fibres and polyester resin droplets. Density and microscopy methods were used to determine the cross-sectional area of the sisal fibres. The average tensile strength of treated sisal fibres decreased by a modest amount following treatment with 0.06 M NaOH. However, this treatment resulted in a substantial increase in the interfacial shear strength at the sisal fibre to polyester resin interface. Weibull analysis has been used successfully to analyse variability in tensile strengths and interfacial shear strength using probability of failure plots. Scanning electron microscopy has revealed the shape of resin droplets on the surface of treated and untreated sisal fibres and contact angles are much lower for droplets on treated fibres. Damage to the surface of fibres has been examined following shear testing. Weibull analysis is an effective tool for characterising highly variable fibre properties and evaluating the level of adhesion between polymer resin and the fibre surface. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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[sub 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[sub app], and the local interfacial shear strength (IFSS), τ[sub d]. However, it has been demonstrated that the F[sub 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[sub d] and interfacial friction, τ[sub 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[sub app] as the embedded length tends to zero) is not always equal to the 'local' bond strength. [ABSTRACT FROM AUTHOR]

In micromechanical tests for estimating fiber-matrix interfacial properties, such as the pull-out and microbond tests, fiber debonding from a matrix is often accompanied by friction in debonded areas. In the present study, force-displacement curves, which are usually recorded in these tests, were modeled with taking interfacial friction into consideration. The friction stress was assumed, as a first approximation, to be constant across the interface. Two different approaches to interfacial failure were used: the shear-lag approach with a stress-based debonding criterion (the ultimate interfacial shear strength) and the linear elastic fracture mechanics approach using the critical energy release rate as a condition for crack propagation. The force-displacement curves derived from both models are in good agreement with each other and with experimental micromechanical data. It was shown that any pull-out and microbond experiment comprises four stages: (1) linear loading up to the point where debonding starts; (2) stable crack propagation with friction-controlled debonding; (3) catastrophic debonding; and (4) post-debonding friction. Stable crack propagation was shown to be controlled by both friction and release of residual thermal stresses. An algorithm for estimating both fiber-matrix adhesion and interfacial friction from the microbond and pull-out tests data has been proposed. [ABSTRACT FROM AUTHOR]

A series of novel aqueously soluble semi‐aromatic (co)polyamides, poly[(ethylenedioxy)bis(ethylene) terephthalamide]‐ran‐poly[(ethylenedioxy)bis(ethylene) (2,5‐dihydroxy‐3,6‐diallyl)terephthalamide] (CRPAOT‐x), were prepared via the solid‐state Claisen rearrangement reaction of poly[(ethylenedioxy)bis(ethylene) terephthalamide]‐ran‐poly[(ethylenedioxy)bis(ethylene) 2,5‐bis(allyloxy)terephthalamide] (APAOT‐x), where x (= 0, 13, 25, 50, 75, and 100) represents the molar fraction of allyloxy containing unit. APAOT‐x was obtained through the interfacial copolymerization of terephthaloyl chloride, 2,5‐bis(allyloxy)terephthaloyl chloride, and 2,2′‐(ethylenedioxy)bis(ethylamine). The reaction conditions including reactant concentration, temperature, and solvent were optimized. The chemical structures of the target (co)polyamides and their precursors were characterized by proton nuclear magnetic resonance (1H NMR) and Fourier transform infrared spectroscopies. The incorporation of hydrophilic (ethylenedioxy)bis(ethylene) units greatly improved the polymer solubility. CRPAOT‐x dissolved not only in polar organic solvents such as dimethylformamide, dimethyl sulfoxide, and N‐methyl pyrrolidone but also in ammonia water. Due to the presence of hydroxyl groups and oxygen‐rich aliphatic units, CRPAOT‐x showed excellent affinity toward epoxy resins. The microbond test indicated that the aramid fibers sized by these dual‐functional polymers exhibited remarkably enhanced interfacial shear strength in epoxy matrices. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47132. A series of semi‐aromatic (co)polyamides with much improved solubility in both organic and basic aqueous solvents were prepared. The factors influencing the interfacial polymerization were carefully studied. The potential application of CRPAOT‐x serving as the sizing agents was evaluated by microbond test and inter‐laminar shear test, showing a maximum increase of 101% in IFSS. The aqueous sizing condition provided a promising perspective for the future industrial achievement. [ABSTRACT FROM AUTHOR]