Structure and morphology of small diameter electrospun aramid fibers

Very small diameter fibers of poly(p-phenylene terephthalamide) (PPTA) ranging in diameter from 40nm to a few hundreds of nanometers were made by the electrospinning process. The fibers have a circular cross-section, are birefringent and stable at temperatures greater than 400°C. Electron diffraction patterns obtained from both as-spun and annealed fibers show meridional and equatorial reflections demonstrating order in the material. The crystal structure and morphology of the fibers were characterized using bright and dark field electron microscopy, electron diffraction and atomic force microscopy.     

Statistical aspects of tensile strength of aramid fibers and unidirectional composites

The effect of gage length and temperature on tensile strength of aramid monofilaments, yarns, and strands has been studied in view of statistical models of the theory of strength. The tensile strength of monofilaments and yarns is found to decrease as the gage length is increased. The strength of strands remains constant. The temperature resistance of strands is determined by the glass transition temperature of a polymeric matrix.     

Transcrystalline morphology of nylon 6 on the surface of aramid fibers

In this paper, the isothermal crystallization of nylon 6 in the presence of Kevlar 129 fiber was investigated by polarized optical microscopy (POM). The formation of a transcrystalline domain was found to be mainly controlled by crystallization conditions, such as the temperature of the isothermal crystallization, residual time at melting temperature and the cooling rate of the melt. The nucleation rate of nylon 6 on the fibers was mainly affected by the crystallization temperature. The interfacial transcrystallinity of nylon 6 occurred on the surface of Kevlar 129 fiber in the temperature range 130–190 °C. The reason for the formation of interfacial transcrystalline morphology is discussed from the molecular level, based on the understanding of the packing mode of nylon 6 chains around fibers and the interaction between matrix and fibers. It was found that the lattice matching and hydrogen‐bonding between nylon 6 and poly(p‐phenylene terephthalamide) (PPTA) crystals play an important role in the epitaxial crystallization. Copyright © 2004 Society of Chemical Industry     

Long‐term mechanical behavior of aramid fibers in seawater

Aramid fibers are today proposed in ropes and cables for marine applications. As these highly crystalline materials are loaded in tension for a longer period in seawater, their long‐term mechanical behavior has to be understood. However, the response is time‐dependent and exhibits a nonlinear effect with stress. In this study, two types of aramid fibers are studied: Twaron and Technora. Mechanical properties are measured using static tensile tests and creep‐recovery tests. A nonlinear viscoelastic–viscoplastic model, based on the Schapery formulation, allows discriminating between the instantaneous and the time‐dependent response as well as the reversible and nonreversible phenomena (plasticity). First, this procedure allows the overall mechanical behavior of the fibers to be compared, considering creep rate, plasticity, and instantaneous moduli. Then, using these parameters, the effect of the testing condition, air or seawater is studied. Finally, the effect of aging in seawater is quantified for both fibers. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

芳纶的应用和发展

芳纶是一种耐高温的高强高模特种纤维。已工业化的芳纶主要是芳纶1313（聚间苯二甲酰间苯二胺纤维）和芳纶1414（聚对苯二甲酰对苯二胺纤维）两大类。根据其特性,阐述了芳纶在航空航天、橡胶、电子通讯、汽车、运动器材等工业方面的广泛应用,并简述了它们在国内外的应用现状和发展趋势。     

芳纶表面改性研究进展

简单回顾了芳纶的发展历史,阐述了表面涂层法、化学改性、物理改性等几种芳纶表面改性方法的研究现状,同时介绍了3种常用来表征纤维复合材料界面结合强度的方法,最后指出芳纶表面改性技术的发展方向。     

Fracture behavior and morphology of spun collagen fibers

The influence of gultaraldehyde (GA) crosslinking, basic chromium sulfate (Cr) tanning, and thermal treatments on the fracture behavior and morphology of spun collagen fibers has been studied. The fracture morphology of the fibers is characterized by longitudinal splitting along the fiber axis. Although the essential fracture morphology was not influenced by GA crosslinking, Cr tanning, and thermal treatments, the process of splitting depended on the kind of crosslink. Noncrosslinked and Cr-tanned fibers were split into fibrils, but GA-crosslinked fiber was split without fibrillation. The thermal treatments have two effects: One is decrease in number of defects and/or flaws; the other is gelatinization. In the thermal treatment above 140°C, the gelatinization plays a more important role for the tensile properties than does the effect of a decrease of a defect. Gelatinization results in the enhancements of slippage and separation of macrofibrils and/or fibrils after the yield point. © 1996 John Wiley & Sons, Inc.     

Debond behavior of copper fibers in thermoset matrices and their effect on fracture toughening

Single fiber pullout experiments were conducted to determine the adhesion quality, debond behavior and subsequent matrix fracture behavior for a variety of end-modified copper fibers. The matrices were: two different epoxy resins, polyester and polyurethane; the end-modified copper fibers were: straight, flat end-impacted, flat end-impacted with release agent applied and straight end-oxidized. The goal was to determine how the bonding and debonding behavior as well as the pullout behavior of the various fiber-matrix combinations affected the composite fracture toughness increment (ΔG). Results indicate that the greatest improvement in the calculated ΔG occurred with a fiber-matrix combination that had a moderate interface bond strength with an interfacial bond failure, minor matrix damage during fiber pullout and moderate post-debond interface friction. Selective oxidation of the fiber end was performed to determine if chemical anchoring of the fiber end could be as effective as mechanical (end-shaping) anchoring of the fiber into the matrix. Improvement in the adhesion bond strength as a result of the chemical anchoring resulted in a significantly lower ΔG compared to the end-impacted fibers because interfacial failure was not possible. This indicates that for the materials tested, mechanical anchoring of the fiber was better than chemical anchoring in improving ΔG. To decrease the adhesion bond strength and allow the fibers to debond, a release agent was applied to the flat end-impacted fiber prior to embedment into the matrix. This resulted in a significantly lower ΔG compared to straight and flat end-impacted fibers for all matrices tested, because the resulting debonding force and friction were significantly reduced. Pullout curves showed that with release agent applied, the end-shape did not effectively anchor the fiber into the matrix. The reduction in the pullout work indicates that the friction at the fiber-matrix interface plays a crucial role in actively anchoring the end-shaped fiber into the matrix after debonding.     

Comparison of tensile fracture morphologies among various polyacrylonitrile-based carbon fibers

Summary Tensile fracture morphologies of three kinds of polyacrylonitrile-based carbon fibers were investigated by scanning electron microscope and high resolution transmission electron microscope. Homogeneous and granular texture could be observed in cross sections of all these three carbon fibers. There is a void up to 1 μm in diameter in fracture surface of T1, which limits its compactness and tensile strength. Apart from homogeneous texture, linear and fan-shaped morphology are found on surface of T300 and T700, respectively. No core-sheath structure was found in these specimens. Misoriented crystallites are also found from HRTEM images in the transverse sections of T1, T300 and T700, which initiate fiber tensile failure. The tensile fracture mechanism and the implication of the fiber structure under tensile force were also elucidated. T700 has the smallest interplanar spacing, the highest degree of graphitization and the highest crystallinity among three carbon fibers.     

Hybridization effects on tensile and bending behavior of aramid/basalt fiber reinforced epoxy composites

In this study, the mechanical properties of aramid/basalt hybrid composite laminates were determined, and the effects of hybridization on the mechanical properties were investigated. To examine the effect of hybridization, the mechanical properties of aramid/basalt hybrid composite laminates were compared with those of aramid/epoxy and basalt/epoxy non‐hybrid composite laminates. The mechanical properties, tensile and flexural, of composite laminates were determined by performing uniaxial tensile and three‐point bending tests. The results showed that the employment of basalt fibers for partial substitution of aramid fibers in the composite laminate could provide improvements in the tensile and flexural properties. Furthermore, the results of three‐point bending tests were found that the employment of basalt fibers on compressive side across the thickness of composite laminates were realized significant improvement for flexural properties in comparison to the employment of basalt fibers on tensile side. POLYM. COMPOS., 38:1144–1150, 2017. © 2015 Society of Plastics Engineers     

Mechanical,flammability, and crystallization behavior of polypropylene composites reinforced by aramid fibers

In this article, we report the mechanical and thermal properties, together with the crystallization and flammability behaviors, of pure polypropylene (PP) and PP/aramid fiber (AF) composites with AF loadings of 5, 10, 20, 30, and 40 wt %. The mechanical properties of the samples were evaluated by tensile and izod notched impact tests, and the results show that the tensile strength of the composites could reach up to 67.8 MPa and the izod notched impact strength could rise to 40.1 kJ/m². The structure and morphology were observed by scanning electron microscopy and polarized optical microscopy, respectively. This demonstrated that a solid interface adhesion between the matrix and fibers was formed. The thermal and crystalline behaviors of the PP/AF composites were also investigated by thermogravimetric analysis and differential scanning calorimetry analysis, and the results show that the char residue of the PP/AF composites improved greatly with increasing AF loading, and the highest value could reach up to 23.7% in the presence of 40 wt % AF. The supercooling degree, initial crystallization temperature, and crystallization percentage were used to characterize the crystallization behavior of the PP/AF composites, and the results indicate that the AFs had positive effects on the promotion of PP nucleation, which can usually improve the mechanical properties of composites. Moreover, the flammability analysis of the PP/AF composites demonstrated that the presence of AFs could significantly decrease the peak heat release rate and the total heat release and reduce the melt-dripping of the PP/AF composites. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Mode-II fracture properties of CFRP/steel composite structure reinforced by short aramid fibers

Abstract

To obtain a good bonding strength of steel/CFRP adhesive joint, the steel surface was machined by grooving process. Short aramid fibers were mixed into the adhesive layer to achieve the further adhesion strength. In the pressing process of steel/CFRP specimen preparation, short aramid fibers with the diameter of several micrometers could be embedded in the grooved gap and the rough surface of CFRP. The higher strength aramid fibers had been not only improved interfacial strength of steel/epoxy and CFRP/epoxy, but also reinforced the adhesive layer due to the bridging activities of aramid fibers. In this study, Mode II fracture strength of grooved-steel/CFRP adhesive joints was investigated by end-notch bending test. The ultimate load and fracture energy of specimens have been improved by 15.7 and 6.8%, in comparison to specimens with smooth steel surface, respectively. The reinforcing mechanisms of CFRP/steel bonding joint as a result of short aramid fibers were discussed according to the failure modes of specimens, and scanning electron microscopy observation and experimental results were carried out.     

Creep and strength retention of aramid fibers

Creep tests at ambient conditions have been carried out on Kevlar 49 and Technora yarns covering a wide stress spectrum (10–70% average breaking load) for a long period of time (up to a year). The results confirm that Kevlar 49 and Technora yarns show a nonlinear behavior at stresses below 40% of the breaking load and a linear behavior at stresses above 40%. The strength retention following creep for Kevlar 49 and Technora has also been examined. The results show a significant difference in the behavior of the two materials. Kevlar 49 appears to lose strength almost linearly with time, while Technora seems to lose strength much more rapidly. These results would have significant implications for design. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Numerical modeling of the dynamic tensile behavior of irregular fibers

Most fibers are irregular and are often subjected to rapid straining during mechanical processing and end‐use applications. In this article, the effect of fiber dimensional irregularities on the dynamic tensile behavior of irregular fibers was examined using the finite‐element method (FEM). Fiber dimensional irregularities are simulated with sine waves of different magnitude (10, 30, and 50% level of diameter variation). The tensile behavior of irregular fibers was examined at different strain rates (333, 3333, and 30,000%/s). The breaking load and breaking extension of irregular fibers at different strain rates were then calculated from the finite‐element model. The results indicate that strain rate has a significant effect on the dynamic tensile behavior of an irregular fiber, and that the position of the thinnest segment along the fiber significantly affects the simulation results. Under dynamic conditions, an irregular fiber does not necessarily break at the thinnest segment, which is different from the quasi‐static results. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2855–2861, 2004     

Core structure of vapor grown carbon fibers and morphology dependence of tensile strength

Vapor grown carbon fibers (VGCFs) grown via a liquid pulse injection technique mostly have a turbostratic structure with a smooth surface (ca. 90%). The rest are connected or fused fibers, and fibers with spheroidal grains on their surfaces. It was clarified by TEM observation that the central core of the VGCFs whose diameter is 0.2-0.5 μm has an amorphous structure and is surrounded by graphitic sheets. Graphite layers with polygonal plates were found to be contained among circular graphite layers. The tensile strengths of individual fibers depend on their morphologies and are in the range from 0.6 to 3.7 GPa.     

Strain birefringence of Kevlar aramid fibers

We have measured the strain birefringence of Kevlar aramid fibers under static loading via a spectrophotometric method. At very low strain rates, the birefringence of Kevlar fibers increases markedly with increasing strain. On initial straining, the birefringence increases even at relatively constant modulus. This is attributed to the orientation of macroscopic species in the fiber which were observed by visible light microscopy. Such behavior is supported by the experimental observation of laser diffraction patterns and optical transmission images of Kevlar fibers under load. In the final stage of straining where the fiber modulus increases rapidly, the birefringence increase is attributable to crystalline orientation. The spectrophotometric method is useful for the simultaneous measurement of stress, strain, and birefringence of highly oriented, highly crystalline fibers such as Kevlar aramid. It is particularly useful to study the morphological inhomogeneity of a fiber which is undetectable by the conventional tensile test.     

Tensile fracture and failure behavior of technical flax fibers

The apparent tensile strength of technical flax fibers was determined in single‐fiber tests at various clamping lengths (20, 40, and 80 mm) and the outcome was compared with literature data. It was demonstrated that the strength of flax at each clamping length obeyed the two‐parameter Weibull model. The failure mode and sequence were studied in situ (i.e., during loading) by SEM and acoustic emission (AE). The failure sequence (axial splitting of the technical fiber along its elementary constituents, radial cracking of the elementary fibers, multiple fracture of the elementary fibers) concluded reflected the hierarchical build‐up of the flax bast fibers. To the above failure events AE amplitude ranges were assigned. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 3638–3645, 2003     

Unsaturated polyester-based hybrid nanocomposite: fracture behavior and tensile properties

In this study, unsaturated polyester (UP) resin was reinforced by using an organically-modified montmorillonite (OMMT) and toughened with poly(n-butyl acrylate)/poly(vinyl acetate-co-methyl methacrylate) core-shell rubber (CSR) particles. The effects of OMMT and CSR levels on the fracture behavior and tensile properties of UP were investigated. The results showed that the incorporation of OMMT of up to 3?wt.% increased the UP fracture toughness (K_IC) to some extent, while further addition caused the fracture toughness to reach a constant level. Furthermore, the dispersion state of OMMT platelets and CSR particles inside the UP matrix was studied by means of transmission electron microscopy (TEM). TEM micrographs showed a good dispersion of organoclay tactoids with an intercalated structure or partial exfoliation for the UP reinforced by 1 and 3?wt.% OMMT. On the other hand, addition of 5 and 10?wt.% CSR particles to the UP increased the fracture toughness much more than the OMMT. Locally clustered but globally good CSR particle dispersion inside the UP matrix was observed in toughened UP specimens. Interestingly, a synergistic effect in fracture toughness was only observed for the UP hybrid composite containing 1?wt.% OMMT and 10?wt.% CSR particles, when compared with other reinforced, toughened, and hybrid specimens. In this case, the OMMT platelets were found to act as bridges between the small rubber-particle agglomerates, which may cause alternating clay debonding/CSR particle cavitation and improve plastic deformation inside the UP matrix. The incorporation of OMMT increased Young??s modulus and also decreased the tensile strength of the neat and CSR-toughened UP specimens with increasing the amount of OMMT.     

Development of electroless silver plating on Para‐aramid fibers and growth morphology of silver deposits

The development of a conductive fiber with flame resistance is an urgent concern particularly in national defense and other specialized fields. Aramid fibers (para‐ or meta‐) exihibit high strength and excellent fire resistance. Electroless silver plating on para‐aramid fibers and growth morphology of silver deposits was investigated in the present work. The surface of para‐aramid fibers was roughened using sodium hydride/dimethyl sulfoxide to guarantee successful electroless plating. Two complexing agents (ethylene diamine/ammonia) and two reducing agents (glucose/seignette salt) were used for the electroless silver plating bath design. Structure and properties of the resulting silver‐deposited para‐aramid fibers were evaluated based on scanning electron microscopy, silver weight gain percentage calculation, electrical resistance measurement, crystal structure analysis, and mechanical properties test. The results showed that a higher silver weight gain was advantageous to the improvement of conductivity for the silver‐deposited para‐aramid fibers. The obtained silver deposit was homogenous and compact. Electroless silver‐plating deposits were considered to be three‐dimensional nucleation and growth model (Volmer–Weber). Black, silver gray, and white deposits appeared sequentially with progressive plating. The breaking strength of silver‐deposited para‐aramid fibers remained at value up to 44 N. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

A preliminary study of the fracture morphology of acrylic fibers

A preliminary study has been made of the tensile and fatigue fracture morphology of acrylic fibers. Tensile breaks show a characteristic granular fracture surface which suggests the separate failure of fibrillar units in the fiber structure. In some instances, there are separate crack and final failure regions; in others, the fracture is in transverse steps linked by axial splits. The main characteristic in tensile fatigue is axial splitting of the fibers. The loading conditions for fatigue failure are less severe than in steady loading there is no zero minimum load criterion for fatigue failure as found in some other fibers.