Single Fiber Fragmentation Test Research Articles

Enhanced interfacial adhesion via interfacial crystallization between sisal fiber and isotactic polypropylene: direct evidence from single-fiber fragmentation testing

Bioresource natural sisal fiber (SF) was used to prepare single fiber-reinforced isotactic polypropylene (iPP) composites. Three kinds of interfacial crystalline morphologies, spherulites, medium nuclei density transcrystallinity (MD-TC) and high nuclei density transcrystallinity (HD-TC), were obtained in the single fiber-reinforced composites by implementing quiescent or dynamic shear-enhanced crystallization and by modulating the compatibility interaction between SF and iPP. The development of interfacial shear strength (IFSS) during the interfacial crystallization process was demonstrated for the first time using a combination of single-fiber fragmentation testing and optical microscope observation. A close correlation between IFSS and morphological characteristics of interfacial crystallization was well elucidated. The increases in IFSS were very different for spherulitic, MD-TC and HD-TC morphologies. The highest IFSS obtained was 28 MPa, after the formation of HD-TC, which was about 62% of the tensile strength of neat iPP (45 MPa). These results offer powerful and direct evidence that interfacial crystallization could play an important role in the enhancement of interfacial adhesion of real SF/iPP composites. © 2013 Society of Chemical Industry

Role of Surface Chemistry in Adhesion between ZnO Nanowires and Carbon Fibers in Hybrid Composites

Low interface strength is a persistent problem in composite materials and cascades to limit a variety of bulk material properties such as lamina shear strength. Whiskerization has long been pursued as a method to reinforce the interphase and improve both the single fiber interface strength as well as the bulk properties. Recent developments have shown that ZnO nanowire whiskerization can effectively improve the properties of a bulk composite without requiring the high temperatures that previous deposition processes needed. Although the efficacy of a ZnO nanowire interphase has been established, the mechanism for adhesion of the interphase to the fiber has not been identified. Specifically, the addition of the ZnO nanowires to the surface of the fibers requires that the ZnO nanowires have strong chemical adhesion to the fiber surface. This work will create a variety of chemical environments on the surface of the fibers through new and common chemical functionalization procedures and quantify the surface chemistry through X-ray photoelectron spectroscopy. The effect of fiber surface chemistry on the adhesion of the ZnO is assessed through single fiber fragmentation testing. The interface strength is found to strongly correlate with the concentration of ketone groups on the surface of the fibers. Following the experimental observations, liftoff of a ZnO crystal from a graphene surface was simulated with a variety of surface functionalizations. The computational models confirm the preference for ketone groups in promoting adhesion between ZnO and graphite.

Silica nanoparticles as coupling agents for polypropylene/glass composites

Abstract An evaluation of the fibre/matrix interfacial shear strength was performed by the single-fibre fragmentation tests on polypropylene–glass fibre microcomposites filled with various types and amounts of silica nanoparticles. In particular, both non-functionalized and dimethyldichlorosilane-functionalized silica nanoparticles were added up to a weight content of 7%. Moreover, the effect of various amounts of maleic anhydride modified polypropylene (PPgMA) on the fibre/matrix adhesion was also investigated, including some selected formulations containing both PPgMA and silica nanoparticles. Interfacial shear strength was found to remarkably increase (up to a factor of about 5 for a 7 wt% content of surface treated nanoparticles) with respect to the case of neat polypropylene matrix. The observed effect was explained by considering that silica nanoparticles increase the work of adhesion of polypropylene with respect to glass, as proven by contact angle measurements in different liquids. In addition, silica nanoparticles promoted a remarkable enhancement of both elastic modulus and creep stability of the selected polypropylene matrix.

Interfacial strength and mechanical properties of biocomposites based on ramie fibers and poly(butylene succinate)

In this work, a single ramie fiber was firstly treated with various surface modification agents including alkali, silane, maleic anhydride and acetic anhydride. Then single fiber fragmentation was carried out to evaluate the interfacial interaction between the fiber and poly(butylene succinate) (PBS) matrix by means of interfacial shear strength (IFSS). The highest IFSS was found for the fiber treated with alkali. In this simple way, alkali was easily selected as a good surface modification agent for the preparation of PBS/ramie fibers biocomposites. In order to verify that the single fiber fragmentation is an effective method for the selection of a good surface modification agent, the interfacial interaction and mechanical properties of the prepared PBS/alkali treated ramie fibers composites were investigated. It was found that alkali treated ramie fibers could be well dispersed and had better interfacial adhesion with the matrix than untreated fibers, as indicated by scanning electron microscope (SEM) observation and dynamic mechanical analysis (DMA). The tensile strength and modulus of PBS were greatly increased by adding the alkali treated fibers, while only a slight increase of tensile strength and modulus was observed for those compounded with untreated fibers. These results indicate that alkali is indeed a good coupling agent for the improvement of the interfacial strength of the composites, which is well consistent with that obtained via the IFSS measurement. Therefore, single fiber fragmentation test can be used as an effective and simple method to evaluate the surface modification and interfacial enhancement as well.

Investigation on the Micromechanical Behavior of fiber Reinforced Epoxy with a Semi-automatic Phase Shifting Method

The computer aided prediction of the effective macroscopic material behavior is a goal of research to benefit from the potential of fiber reinforced composite materials. Beside the mechanical characteristic of the material components, an extensive knowledge of the mechanical interaction between the components is necessary. To get experimental information in this field, a novel semiautomatic measurement system was developed. It is able to provide a combination of statistical results on the damage evolution and analytical results to interpret the photoelasticity during the measurement process. This measurement system is applied to the single fiber fragmentation test. The experimental setup enables the microscopic observation of gradual increasing fragmentation of the single embedded fiber and the stress rearrangement at the matrix material at low strain rates. To get information on the stress field out of the dynamic image scene without contact, a four image phase shifting method is used. This presentation describes the developed measurement system and the test setup. Methods to extract characteristic features by applying image processing methods are introduced and the former utilization to obtain comparable mechanic values is explained.

Correlation of temperature dependences of macro- and micro-interfacial properties in carbon fiber/epoxy resin composite

The short beam shear test and single fiber fragmentation test with an energy-based model were conducted to characterize the interfacial properties of two types of carbon fiber–reinforced epoxy resin composites. The temperature dependences of interlaminar shear strength and interfacial fracture energy ( Γi) from room temperature to 130°C were investigated. The correlation between interlaminar shear strength and the micro-interfacial property at elevated temperature was discussed. The scanning electron microscope images of fractographic cross-sections of the composite laminates were used to analyze the failure mode transition as the temperature rising. The results show that the interfacial properties of the two carbon fiber composites with the same matrix are obviously different, especially at high temperature. The changes of interlaminar shear strength and interfacial fracture energy are different at elevated temperature, mainly ascribed to the different treating ways of thermal residual stress and matrix property in the two methods. The similar temperature sensitivities in the changes of Γi and the amount of matrix adhering on carbon fibers in the short beam shear failure images suggest the variations of the micro-interfacial properties in the short beam test as increasing temperature.

The interfacial strength and fracture characteristics of ethanol and polymer modified carbon nanotube fibers in their epoxy composites

Carbon nanotube (CNT) fibers spun from CNT arrays were used as the reinforcement for epoxy composites, and the interfacial shear strength (IFSS) and fracture behavior were investigated by a single fiber fragmentation test. The IFSS between the CNT fiber and matrix strongly depended on the types of liquid introduced within the fiber. The IFSS of ethanol infiltrated CNT fiber/epoxy varied from 8.32 to 26.64MPa among different spinning conditions. When long-molecule chain or cross-linked polymers were introduced, besides the increased fiber strength, the adhesion between the polymer modified fiber and the epoxy matrix was also significantly improved. Above all, the IFSS can be up to 120.32MPa for a polyimide modified CNT fiber, one order of magnitude higher than that of ethanol infiltrated CNT fiber composites, and higher than those of typical carbon fiber/epoxy composites (e.g. 60–90MPa). Moreover, the composite IFSS is proportional to the tensile strength and modulus of the CNT fiber, and decreases with increasing fiber diameter. The results demonstrate that the interfacial strength of the CNT fiber/epoxy can be significantly tuned by controlling the fiber structure and introducing polymer to optimize the tube–tube interactions within the fiber.

Interfacial adhesion and micro-failure phenomena in multi-fiber micro-composites using fragmentation test

Statistical fragments and micro-failure modes in the multi-fiber-reinforced micro-composites were investigated by fragmentation test. The specimen consisted of three fibers using carbon fibers (CFs) and glass fibers (GFs), embedded in the epoxy resin with three-dimensional arrangement. Fracture morphology and micro-failure behavior from the progressive fragmentation of fibers and fiber/matrix interfacial adhesion were observed via polarized-light microscope. The interfacial shear strength of CF/epoxy micro-composites is higher than that of the GF/epoxy micro-composites measured by the single fiber fragmentation test. The results show that fragment number on monofilament demonstrates obvious differences between multi-fiber and single fiber systems, and the location of the breakpoint is determined by the CFs that fracture firstly, indicating clustering fracture modes. This is because stress concentration around the breakpoints influences the stress redistribution on the adjacent fibers. Distinct micro-failure modes were observed in three-fiber and the hybrid systems, where matrix cracks around the CFs and interfacial debonding occurs around the GFs. The mixture of CFs and GFs demonstrates distinctive hybrid effect by the changes of the fragment number and initial fracture strain of fibers in hybrid systems.

Application of continuously-monitored single fiber fragmentation tests to carbon nanotube/carbon microfiber hybrid composites

Abstract To assess the effect of carbon nanotube (CNT) grafting on interfacial stress transfer in fiber composites, CNTs were grown upon individual carbon T-300 fibers by chemical vapor deposition. Continuously-monitored single fiber composite (SFC) fragmentation tests were performed on both pristine and CNT-decorated fibers embedded in epoxy. The critical fragment length, fiber tensile strength at critical length, and interfacial shear strength were evaluated. Despite the fiber strength degradation resulting from the harsh CNT growth conditions, the CNT-modified fibers lead to a twofold increase in interfacial shear strength which correlates with the nearly threefold increase in apparent fiber diameter resulting from CNT grafting. These observations corroborate recently published studies with other CNT-grafted fibers. An analysis of the relative contributions to the interfacial strength of the fiber diameter and strength due to surface treatment is presented. It is concluded that the common view whereby an experimentally observed shorter average fragment length leads to a stronger interfacial adhesion is not necessarily correct, if the treatment has changed the fiber tensile strength or its diameter.

Carbon fiber/epoxy composite property enhancement through incorporation of carbon nanotubes at the fiber–matrix interphase – Part I: The development of carbon nanotube coated carbon fibers and the evaluation of their adhesion

A new method to realize the uniform coating of carbon nanotubes (CNTs) to carbon fibers (CFs) has been developed, which enables the scalable fabrication of CNT containing CF/epoxy composites. In this method, CNTs are treated by cationic polymers, then, the CNTs are coated to CFs by immersion into a CNT/water suspension. Good dispersion is achieved by repulsive force between positively charged CNTs and uniform coating of the CNTs is achieved by attractive forces between positively charged CNTs and negatively charged CFs. It is found that the use of specific cationic polymers including polyethyleneimine (PEI) results in stable CNT/water suspensions, and uniform coating of the CNTs. Single fiber fragmentation tests of the CF/epoxy composites were conducted to evaluate the strength of interface and interphase under shear loading. The results show that the combination of epoxy resin sizing and PEI treated CNT coating to CFs results in high interfacial shear strength.

Comparative experimental studies of phosphate glass fiber/polypropylene and phosphate glass fiber/natural rubber composites

Phosphate glass fiber (PGF)-reinforced polypropylene (PP)/natural rubber (NR)-based unidirectional composites with four levels of fiber loading (5, 10, 15, and 20 wt%) were fabricated by compression molding. The influence of the addition of fiber loadings on the mechanical properties such as tensile strength, bending strength, tensile modulus, and percentage elongation at break of the composites was investigated. Based on the fiber loading, 20% PGF-reinforced PP composites yielded better mechanical properties compared with the PGF/NR composites. Degradation tests of the composites were performed in aqueous and soil medium, and it was found that PGF/NR composites lost much of its original strength and modulus compared with that of the PGF/PP composites. Interfacial shear strength (IFSS) of PGF/PP and PGF/NR systems was investigated using the single-fiber fragmentation tests. The IFSS of PGF/PP and PGF/NR systems appeared to be 5.5 and 1.1 MPa, respectively.

Interfacial shear behavior of 3D composites reinforced with CNT-grafted carbon fibers

This paper presents the Molecular Dynamic (MD) models and the simulation results for the shear deformation process of an interface representative cell to develop an understanding of the roles of multi-walled carbon nanotube (MWNT) in enhancing fiber–matrix interfacial properties such as shear modulus and strength. Based on the MD results and the two simple formulae of rule of mixture, the shear modulus and strength of the carbon nanotube (CNT) grafted interface can be predicted. It is shown that there exists a good agreement between the predicted fiber–matrix interfacial shear strength with and without grafted CNTs and these measured from single-fiber micro bond test and single-fiber fragmentation test. The MD simulation also shows the MWNTs’ shear stress cross-section distribution is similar to that of a circular beam in shear with a non-zero shear stress on the neutral plane.

Effect of moisture absorption on the micromechanical behavior of carbon fiber/epoxy matrix composites

Carbon fiber/epoxy material in the form of a single fiber unidirectional composite was subjected to controlled humidity environments. Moisture uptake in polymer composites has significant effects on the mechanical properties of the matrix as well as on the final performance of the composite material. Diminishing of the mechanical properties of the matrix is attributed to a decrease of its glass transition temperature (Tg). The quality of the fiber–matrix interphase was assessed using the single fiber fragmentation test and the fiber-fragment length, considered as an indicator of interfacial quality. In order to measure the fiber fragment lengths and indentify failure mechanism at the interface optical observation and acoustic emission technique were used. The speed of propagation of an acoustic wave in the material was also determined. A comparison is made of interfacial shear strength values determined by acoustic emission and optical techniques. Excellent agreement between the two techniques was obtained. By means of a micromechanical model, it was possible to determine from the fragmentation lengths a measure of the interfacial shear strength between the fiber and the matrix. The role of moisture uptake swelling of the matrix on the residual stresses is considered to be important when considering the effect deterioration of interfacial shear properties. Both the contribution of the radial stresses and the mechanical component of fiber–matrix adhesion are seen to decrease rapidly for higher moisture contents in the matrix and/or interface.

Study on the mechanical, degradation, and interfacial properties of calcium alginate fiber-reinforced polyethylene oxide composites

Calcium alginate fibers (CAFs) were prepared from sodium alginate, which is a natural polymer extracted from brown seaweeds, by extruding aqueous sodium alginate solution (4% by weight) into a calcium chloride (2% by weight) bath. Water uptake of CAF was determined in deionized water at room temperature (25°C) and it was found that the fibers absorbed 49% of water within a minute and indicated strong hydrophilic nature. Polyethylene oxide (PEO) based CAF (as a filler/reinforcing agent) reinforced unidirectional composites (10% fiber by weight) were fabricated by compression molding. Tensile strength, tensile modulus, bending strength, bending modulus, and impact strength of the PEO/CAF composite were found to be 11 MPa, 320 MPa, 18 MPa, 565 MPa, and 12 kJ/m2, respectively. Degradation tests of the PEO/CAF composites were performed for 8 weeks in soil medium and it was found that composites retained almost 50% of its original strength. The interfacial shear strength (IFSS) of the PEO/CAF composites was also measured by single fiber fragmentation test. The IFSS was found to be 0.47 MPa that indicated good fiber–matrix adhesion.

The role of matrix cracks and fibre/matrix debonding on the stress transfer between fibre and matrix in a single fibre fragmentation test

The single fibre fragmentation test is commonly used to characterise the fibre/matrix interface. During fragmentation, the stored energy is released resulting in matrix cracking and/or fibre/matrix debonding. Axisymmetric finite element models were formulated to study the impact of matrix cracks and fibre/matrix debonding on the effective stress transfer efficiency (EST) and stress transfer length (STL). At high strains, plastic deformation in the matrix dominated the stress transfer mechanism. The combination of matrix cracking and plasticity reduced the EST and increased STL. For experimental validation, three resins were formulated and the fragmentation of an unsized and uncoupled E-glass fibre examined as a function of matrix properties. Fibre failure was always accompanied by matrix cracking and debonding. With the stiff resin, debonding, transverse matrix cracking and conical crack initiation were observed. With a lower modulus and lower yield strength resin the transverse matrix crack length decreased while that of the conical crack increased.

Influence of flaw distribution on single fibre fragmentation

The single fibre fragmentation (SFF) test is an important technique used to characterise interface and fibre properties in polymer matrix composite materials. This paper describes a Monte Carlo simulation technique to investigate the influence of flaw size characteristics (i.e. flaw size probability distribution parameters and flaw number) on the number of fragments resulting from the SFF test. The main result suggested that a minimum number of flaws was required in order to provide valid results – the actual number varied between approximately 100 and 1000 depending on the parameters being varied. Above this threshold value, the number of fragments generally increased as a power law with respect to flaw number. In addition, the main parameter influencing the number of fibre fragments was concluded to be the portion of unshielded fibre at the point of saturation.

Comparative Studies of Mechanical and Interfacial Properties between Jute and Bamboo Fiber-Reinforced Polypropylene-Based Composites

Jute and bamboo fiber-reinforced polypropylene (PP) based composites (50 wt% fiber) were fabricated by compression molding. Tensile strength (TS), bending strength (BS), tensile modulus (TM), and bending modulus (BM) of the jute-reinforced PP composite were found to be 48, 56, 900, and 1500 MPa, respectively. Then, bamboo fiber-reinforced PP-based composites (50 wt% fiber) were fabricated and the mechanical properties evaluated. The TS, BS, TM, and BM of bamboo-reinforced PP composites were found to be 60, 76, 4210, and 6210 MPa, respectively. It was revealed that bamboo fiber-based composites had higher TS, BS, TM, and BM compared to jute-based composites. Degradation tests of the composites (jute fiber/PP and bamboo fiber/PP) were performed in soil at ambient conditions for up to 24 weeks. It was revealed that bamboo fiber/PP composite retained its original mechanical properties higher than that of jute fiber/PP composite. The interfacial shear strength of the jute and bamboo fiber-based composites was investigated using the single-fiber fragmentation test and it was found to be 2.14 and 4.91 MPa, respectively. Fracture sides of the composites were studied by scanning electron microscope, and the results revealed poor fiber matrix adhesion for jute fiber-based composites compared to that of the bamboo fiber-based composites.

Fabrication and Mechanical Characterization of Calcium Alginate Fiber-Reinforced Polyvinyl Alcohol Based Composites

Calcium alginate fibers were prepared from sodium alginate by extruding aqueous sodium alginate solution (4% by weight) into a calcium chloride (2% by weight) bath. Water uptake and mechanical properties of the calcium alginate fiber were investigated. Water uptake tests of calcium alginate showed that it absorbed 50% of water within a minute and indicated strong hydrophilic nature. Polyvinyl alcohol (PVA)-based calcium alginate fiber reinforced unidirectional composites (10% fiber by weight) were fabricated by compression molding. Tensile strength (TS), tensile modulus (TM), bending strength (BS), bending modulus (BM) and impact strength (IS) of the PVA matrix and the composite were evaluated. TS, BS, TM, and BM of the PVA matrix were found 10, 18, 320 and 532 MPa, respectively. TS and BS of the PVA based composite were found to be 16 and 27 MPa, respectively, which were 60 and 50% higher than that of the PVA matrix. TM and BM of the composite were found to be 620 and 1056 MPa, respectively, which were improved by 94 and 98% over the matrix material. Degradation tests of the composites were performed for up to 2 months in soil medium and found that composites lost almost 50% of its original mechanical properties. The interfacial properties of the composite were also investigated by using the single fiber fragmentation test (SFFT).

A Comparative Study on the Mechanical, Degradation and Interfacial Properties of Jute/LLDPE and Jute/Natural Rubber Composites

Jute fabric (hessian cloth) reinforced low-density polyethylene (LLDPE) composites (40 wt%) and solid natural rubber-(NR) based composites (40 wt%) were fabricated by compression molding. Tensile strength (TS), tensile modulus (TM) and percentage elongation at break (Eb) of jute/LLDPE composites were found to be 29, 680 MPa and 20%, and for jute/NR-based composites were also found to be 15, 122 MPa and 94%, respectively. Interfacial shear strength (IFSS) of the jute/LLDPE and jute/NR systems was investigated by using the single fiber fragmentation test (SFFT). Scanning electron microscopy (SEM) and aqueous degradation tests were also performed.

Mechanical, Degradation and Interfacial Properties of Chitosan Fiber-Reinforced Polypropylene Composites

Polypropylene (PP) matrix natural degradable chitosan fiber reinforced unidirectional composites (5% fiber by weight) were fabricated by compression molding. It was found that tensile strength (TS), tensile modulus (TM), elongation at break (%), bending strength (BS), bending modulus (BM) and impact strength (IS) were found to be 30 MPa, 771 MPa, 11%, 35 MPa, 2360 MPa and 6.5 kJ/m2 respectively. Degradation tests of the fibers and composites were performed for 2 weeks and 6 months, respectively in aqueous medium at room temperature (25°C). After six months, the mechanical properties of the composites retained almost 65% of their original properties. The interfacial shear strength (IFSS) of the composites was also measured by single fiber fragmentation test (SFFT). The IFSS of the composite system was found to be 0.44 MPa.