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     

Experimental study on the mechanical behavior and failure mechanism of 3d MWK carbon/epoxy composites under quasi‐static loading

Quasi‐static tensile, out‐of compression, in‐plane compression, three‐point‐bending and shear tests were conducted to reveal the mechanical behavior and failure mechanisms of three‐dimensional (3D) multiaxial warp‐knitted (MWK) carbon/epoxy composites. The characterization of the failure process and deformation analysis is supported by high‐speed camera system and Digital Image Correlation. The results show that tensile, bending, out‐of‐plane compression, in‐plane compression stress–strain response exhibit obvious linear elastic feature and brittle fracture characteristics, whereas the shear response exhibits a distinct nonlinear behavior and gradual damage process. Meanwhile, 3D MWK carbon/epoxy composites have good mechanical properties, which can be widely used in the fields of engineering. In addition, the failure for tension behaves as interlayer delaminating, 90/+45/−45° interface debonding and tensile breakage of 0° fibers; the damage for out‐of‐plane compression is mainly interlaminar shear dislocation together with local buckling and shear fracture of fibers; the failure pattern for in‐plane compression is 90° fiber separating along fiber/matrix interface as well as 0/+45/−45° fiber shear fracture in the shear plane. The main failure for bending is fiber/matrix interface debonding and fibers tearing on the compression surface, 0° fibers breakage on the tension surface as well as fiber layers delaminating. Although the shear behavior is characterized by a gradually growing shear matrix damage, 90/+45/−45° interface debonding, +45/−45° fibers shear fracture, and final 0° fiber compression failure. POLYM. COMPOS., 37:3486–3498, 2016. © 2015 Society of Plastics Engineers     

Mechanical properties of short sisal fiber‐reinforced polypropylene composites: Comparison of experimental data with theoretical predictions

Sisal fibers were used for the reinforcement of a polypropylene (pp) matrix. Composites consisting of polypropylene reinforced with short sisal fibers were prepared by melt‐mixing and solution‐mixing methods. A large amount of fiber breakage was observed during melt mixing. The fiber breakage analysis during composite preparation by melt mixing was carried out using optical microscopy. A polynomial equation was used to model the fiber‐length distribution during melt mixing. The experimental mechanical properties of sisal/PP composites were compared with existing theoretical models such as the modified rule of mixtures, parallel and series models, the Hirsch model, and the Bowyer–Baders model. The dependence of the tensile strength on the angle of measurement with respect to fiber orientation also was modeled. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 602–611, 2003     

Modeling the tensile behavior of fiber bundles with irregular constituent fibers

In this article, the effect of fiber dimensional irregularities on the tensile behavior of fiber bundles is modeled with the finite element method. The fiber dimensional irregularities are simulated with sine waves of different magnitudes. The specific‐stress/strain curves of fiber bundles and their constituent single fibers are obtained and compared. The results indicate that fiber diameter irregularity along the fiber length has a significant effect on the tensile behavior of fiber bundles. For a bundle of uniform fibers of different diameters, all the constituent fibers will break simultaneously, regardless of the fiber diameter. Similarly, if fibers within a bundle have the same pattern and level of diameter irregularity along the fiber length, the fibers will break at the same time, also regardless of the difference in the average diameter of each fiber. In these cases, the specific‐stress/strain curve for the bundle overlaps with that of the constituent fibers. When a fiber bundle consists of single fibers with different levels of diameter irregularities, the specific‐stress/strain and load–elongation curves of the fiber bundle have a stepped or ladder shape. The fiber with the highest irregularity breaks first, even when the thinnest section of the fiber is still coarser than the diameter of a very thin but uniform fiber in the bundle. This study suggests that fiber diameter irregularity along the fiber length is a more important factor than the fiber diameter itself in determining the tensile behavior of a fiber bundle consisting of irregular fibers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2664–2668, 2004     

Effects of sunshine UV irradiation on the tensile properties and structure of ultrahigh molecular weight polyethylene fiber

This study investigated sunlight‐simulated ultraviolet (UV) beam irradiation on the tensile properties and structure of ultrahigh molecular weight polyethylene (UHMWPE) fibers. The tensile results showed that after 300 h sunlight UV irradiation, the tensile properties of the UHMWPE fibers were obviously degraded. Investigation of morphology revealed that the crystallinity was slightly increased, whereas the overall orientation and molecular weight of the fibers were decreased. SEM observations indicated that the degradation process was nonuniform throughout the fiber and a change from a ductile to a brittle fracture mechanism was found after UV irradiation. DMA results showed two β‐relaxations and one α‐relaxation in the original single filament, and UV irradiation led to the increased intensity of the high‐temperature β‐relaxation and the lowered position of the low‐temperature β‐relaxation. This indicated that irradiation‐induced molecular scission and branching were located primarily in the amorphous and the interface areas of the fiber. Changes in the thermal behavior were also examined by DSC. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2757–2763, 2003     

A phenomenological study of the mechanical properties of long‐fiber filled injection‐molded thermoplastic composites

Tensile and flexural tests on specimens cut from rectangular injection‐molded plaques show that long‐fiber filled thermoplastic composites are complex, non‐homogeneous, anistropic material systems. Like all fiber‐filled materials, they exhibit through‐thickness nonhomogeneity as indicated by differences between tensile and flexural properties. The in‐plane orientation of fibers in through‐thickness layers causes the material to have in‐plane anisotropic properties. However, these long‐fiber filled materials exhibit an unexpectedly large level of in‐plane nonhomogeneity. Also, the effective mechanical properties of these materials are strongly thickness dependent. The thinnest plaques exhibit the largest differences between the flow and cross‐flow tensile properties. These differences decrease with increasing thickness. A methodology for part design with this class of materials is discussed.     

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.     

Electrospinning of ethyl–cyanoethyl cellulose/tetrahydrofuran solutions

In this study, electrospinning was used to fabricate ethyl–cyanoethyl cellulose [(E‐CE)C] fiber from a solution of (E‐CE)C/tetrahydrofuran. The diameter of the thinnest fiber fabricated during the electrospinning was about 200 nm. It was found that the diameters of the fibers and their distribution depend on the processing parameters and properties of the solution, such as viscosity, temperature, and concentration, for example. The morphology of the fiber was also observed by SEM. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 242–246, 2004     

高性能纤维在预型件加工过程中的损伤分析

采用声发射方法对玻璃纤维、碳纤维和芳纶纤维三种高性能纤维在直拉、弯拉以及摩擦作用下的损伤破坏过程进行了描述、比较和分析。结合纤维断头电镜照片,对各种纤维损伤的机理进行讨论。     

Statistical analysis of the mechanical properties of natural fibers and their composite materials. II. Composite materials

In this study, the tensile behavior of different natural fiber reinforced composite materials were analyzed. The statistical analysis used to study the natural fibers in the first article, has been extended to analyze the behavior of PP‐matrix composites, combining the probability density function estimation of fiber properties with the Halpin‐Tsai equation. In this case, the advanced statistical approach overestimates the mechanical properties of the composites, probably because of the poor matrix‐fiber adhesion between polypropylene and natural fibers in the real system. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Statistics on the fracture strength of bamboo fibers

Tensile strength is a key mechanical property of fibers used as sustainable reinforcements for advanced fiber‐reinforced composites. This study aims to conduct experimental investigation on the fracture strength of bamboo fibers of different dimensions subjected to longitudinal tensile loading. The statistical distributions of the fracture strength in bamboo fibers are correlated with the effects of fiber length and diameter variation. These are described according to Weibull statistics, which exhibit the random nature of fiber strength. The Weibull function parameters used for strength prediction are obtained from the test specimens. A comparison of predicted results and experimental data is presented to assess the accuracy of using weak‐link scaling. Furthermore, the findings of this study also indicate that fiber strength statistics dominate size dependence of tensile strength. POLYM. COMPOS., 221–228, 2016. © 2014 Society of Plastics Engineers     

Effect of the chain‐transfer‐agent content on the emulsion polymerization process and adhesive properties of poly(n‐butyl acrylate‐co‐acrylic acid) latexes

The effect of water on regenerated silkworm silk fibers has been studied and compared with that of water on natural silkworm silk fibers. Regenerated fibers are spun from an N‐methylmorpholine‐N‐oxide (NMMO) fibroin solution through a wet‐spinning process, leading to fibers with two distinct tensile behaviors, labeled as brittle and ductile, respectively. Regenerated fibers show a significant contraction when immersed in water. Contraction increases further after drying. In contrast, natural silkworm silk fibers show a negligible contraction when submerged in water. Regenerated fibers tested in water are considerably more compliant than samples tested in air, though their stiffness and tensile strength are significantly reduced. It has been shown that the tensile properties of brittle regenerated fibers can be modified by a wet‐stretching process, which consists of deforming the fiber while immersed in water. Regenerated wet‐stretched fibers always show a ductile behavior independent from their initial tensile behavior. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

The effect of fiber orientation on the toughening of short fiber‐reinforced polymers

The effect of fiber orientation on the toughening of polymers by short glass fibers generally below their critical length was investigated using specimens with either well‐aligned or randomly oriented fibers. The fibers were aligned by an electric field in a photopolymerizable monomer, which was polymerized while the field was still being applied. These materials were fractured with the aligned fibers in three orientations with respect to the crack plane and propagation direction. Specimens with fibers aligned normal to the fracture plane were the most tough, those with randomly oriented fibers were less tough, and those with fibers aligned within the fracture plane were the least tough. The fracture behaviors compared favorably with predictions based on observed processes accounting for fiber orientation. The processes considered were fiber pull‐out (including snubbing), fiber breakage, fiber–matrix debonding, and localized matrix‐yielding adjacent to fibers bridging the fracture plane. Fibers not quite perpendicular to the fracture plane provided the greatest toughening; these fibers pulled out completely and gave a significant contribution from snubbing. Fibers at higher angles provided less toughening, involving nearly equal contributions from pull‐out, breakage, and debonding. Fibers within the fracture plane provided the least toughening, involving debonding alone. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2740–2751, 2003     

Tensile strength and its variation of PAN‐based carbon fibers. III. Weak‐link analysis

Both the strength and its variance of carbon fibers depend on the worst flaw that exists in the fiber, or more exactly speaking, on the structure of the “fiber weak link” (FWL). To better understand the strength–structure relationship, the fracture‐ends morphologies were examined by the scanning electron microscope (SEM). The weak links of carbon fibers were divided into three groups according to its tensile strength, and the effect of the carbon FWLs on the strength variance was also discussed. The observation by SEM, the analysis on fiber tensile properties, and the corresponding discussion of the two sorts of results indicate that both surface flaw and the incompact structure decrease the strength of carbon fiber and enlarge the strength variance of carbon fiber. The modulus seems to influence the strength of carbon fibers too. It is also confirmed that not only the size of the fracture mirror but also the ratio of the size of the mirror to the fracture surface area (not cross section area) is important for judging the strength of brittle fibers. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Characterization of the draping behavior of jute woven fabrics for applications of natural‐fiber/epoxy composites

In this study, we investigated the draping behavior of jute woven fabric to study the feasibility of using natural fabrics in place of synthetic glass‐fiber fabrics. Draping behavior describes the in‐mold deformation of fabrics, which is vital for the end appearance and performance of polymer composites. The draping coefficient was determined with a common drapemeter for fabrics with densities of 228–765 g/m² and thread counts under different humidity and static dynamic conditions. The results were compared to glass‐fiber fabrics with close areal densities. Characterization of the jute fabrics was carried out to fill the knowledge gap about natural‐fiber fabrics and to ease their modeling. The tensile and bending stiffnesses and the shear coupling were also characterized for a plain woven jute fabric with a tensile machine, Shirley bending tester, and picture frame, respectively. As a case study, the draping and resin‐transfer molding of the jute fabric over a complex asymmetric form was performed to measure the geometrical conformance. The adoption of natural fibers as a substitute for synthetic fibers, where the strength requirements are satisfied, would thus require no special considerations for tool design or common practices. However, the use of natural fibers would lead to weight and cost reductions. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1453–1465, 2013     

Factors influencing the tensile properties of ramie

Many investigators have claimed that tensile breakage of native cellulose fibers occurs primarily by repture of covalent bonds in the cellulose molecules rather than by chain slippage resulting from repture of such interchain bonds as hydrogen bonds. This claim has been made partly on the basis of a comparison of tensile properties of ramie fiber and of the fully esterified counterpart. This comparison indicated that the breaking load of ramie was similar to that of the fully esterified fiber. In studying the tensile properties of ramie fiber and of fully acetylated ramie fiber, we found that the degree of polymerization of the fiber was lowered during the acetylation process. Also, it was evident that both degree of polymerization and degree of crystallinity are important factors to be considered when comparing the tensile strength of native cellulose fibers and their acetylated counterparts. Although the primary cause of tensile breakage of native cellulose fibers may be due to chain scission rather than to chain slippage, it is difficult to claim supporting evidence for this theory from studies made so for on the tensile properties of esterified ramie fibers.     

Composition,tensile properties,and dispersion of polypropylene composites reinforced with viscose fibers

The reinforcement mechanics of viscose‐fiber‐reinforced polypropylene (PP) composites were studied. The effect of the coupling agent, maleated polypropylene (MAPP), was of special interest. The fibers, coupling agent, and PP were extruded and injection‐molded. The composition, mechanical properties, fracture morphology, and dispersion of the composites were examined. Thermogravimetric analysis showed that the fiber content in the tensile specimens varied slightly with the sample location; however, the differences in the values were within 1.0%. Scanning electron microscopy images of the fracture surfaces of the composites showed that the surfaces of the composites without MAPP were covered with fibers pulled out from the matrix. A lack of adhesion further appeared as a cracked matrix–fiber interface. A new scanning thermal microscopy method, microthermal analysis, was used to study the dispersion of the fibers in the composites. Local thermal analyses gave further information about the location of the fibers. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2676–2684, 2004     

Mechanical properties and morphology of flax fiber reinforced melamine‐formaldehyde composites

The mechanical performance of natural fiber reinforced polymers is often limited owing to a weak fiber‐matrix interface. In contrast, melamine‐formaldehyde (MF) resins are well known to have a strong adhesion to most cellulose containing materials. In this Paper, nonwoven flax fiber mat reinforced and particulate filled MF composites processed by compression molding are studied and compared to a similar MF composite reinforced with glass fibers. Using flax instead of glass fibers has a somewhat negative effect on tensile performance. However, the difference is relatively small, and if density and material cost are taken into account, flax fibers become competitive. Tensile damage is quantified from the stiffness reduction during cyclic straining. Compared to glass fibers, flax fibers generate a material with a considerably lower damage rate. From scanning electron microscopy (SEM), it is found that microcracking takes place mainly in the fiber cell walls and not at the fiber‐matrix interface. This suggests that the fiber‐matrix adhesion is high. The materials are also compared using dynamic mechanical thermal analysis (DMTA) and water absorption measurements.     

Gauge length effect on the tensile properties of leather

Tensile properties are important basic characteristics of materials and influence their end‐use and performance. More importantly, in the case of leather due to end‐use applications such as shoe uppers, automotive and furniture upholstery, mechanical properties such as tenacity are of extreme importance. Therefore, fundamental studies on the tensile properties of leather are needed. In this study, an attempt has been made to examine the effect of gauge length (GL) on the tensile properties of shoe upper leather. Two different specimens in the form of rectangular and dumbbell shapes have been cut from parallel and perpendicular directions to the body axis of the leather and have been tested. Results showed that the maximum breaking load and the percentage extension at break decreased with the increase in GL. Rectangular specimens showed a 30% decrease in maximum breaking load and a 13% decrease in percentage extension at break, while dumbbell specimens showed reductions in the order of 28 and 6%, respectively, as the GL increased from 9.53 cm to 23.5 cm. Highly varying supramolecular architecture of the collagen matrix and the frictional slippage caused by the free ends present in the collagen fibrils, which induce a weak‐link effect similar to the one found in cotton fibers and yarns, are considered to be the probable reasons for this behavior. A limited scanning electron microscopic study has been undertaken to pictorially represent the breakage of leather at different GLs. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1202–1209, 2006     

Preparation,morphology and pH sensitivity of hybrid hydrolyzed polyacrylonitirile‐blend‐gelatin hydrogel fibers

BACKGROUND: Polyacrylonitrile (PAN) artificial muscles have attracted considerable attention for their fast responses. This research work is based on the preparation of novel pH‐sensitive hydrogel fibers derived from hydrolyzed PAN and gelatin by wet spinning and chemical modification. RESULTS: Through characterization of the fiber dynamic and static pH‐sensitive behavior, pH response times were found to improve greatly with increasing gelatin content. At a weight ratio of 3 to 7 (PAN:gelatin), the best response times were obtained at 0.59 s for elongation and 1.14 s for contraction. Study of the chemical structures of hydrolyzed PAN and gelatin, as well as the surface morphology of the hydrogel fibers, indicated that the mechanism of formation of hydrogel fibers is closely interconnected with their pH‐sensitive behavior. From the standpoint of the mechanism we also found that the addition of urea gave rise to hydrogel fibers with a controllable morphology, influenced by the pH‐sensitive behavior. CONCLUSION: The hydrogel system reported here is simple in preparation, but quite complex in chemical structure. The strong response of the fibers to pH provides some idea on the development of new artificial muscle systems. Copyright © 2008 Society of Chemical Industry