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     

Dynamic responses of irregular fibers under axial tension

A numerical study on the dynamic responses of irregular fibers under axial tension is presented in this article. The irregularity was represented by a sinusoidal shape profile along the fiber axis. The finite element method was used in the simulations and the maximum first principal stress due to the dynamic pulling has been examined. Our numerical results indicate that the first principal stress mainly varies along the longitudinal direction. Its change in radial direction is negligibly small. The maximum first principal stress in an irregular fiber always appears in the narrowest crosssection, which is the weakest link of the fiber. The maximum first principal stress is very sensitive to the change in the irregular amplitude. The stress value increases dramatically with the increase in the amplitude of irregularity. The frequency of irregularity has a limited effect on the maximum first principal stress. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Modeling the stress‐relaxation behavior of wool fibers

The stress‐relaxation behavior of wool fibers after a pretreatment with a chemical solution is particularly important for evaluating the efficiency of the pretreatment. In this study, three viscoelastic models, including the Maxwell, two Maxwell unit, and modified two Maxwell unit models, were established first. To verify the feasibility of the models, stress‐relaxation experiments for wool fibers were performed. The wool fibers were pretreated with a sodium bisulfite solution (1 and 3%) at various temperatures (293, 298, 303, 308, 313, and 318 K). Then, the experimental values were fitted to the three models to obtain the rate constants of relaxation. The activation energy of the wool fibers was calculated with the Arrhenius equation. The results showed that the modified two Maxwell unit model provided the best fit for the experimental data of the wool fibers. The stress‐relaxation process of the wool fibers could be divided into two stages, a rapid stage followed by a slow stage. The rapid relaxation of stress was attributed to the weak bonds in the wool fibers, and the following slow relaxation stage was attributed to strong bonds. The Arrhenius equation could describe the stress‐relaxation process of the wool fibers very well. Furthermore, the activation energy decreased in the presence of sodium bisulfite. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Modeling the tensile behavior of ultra‐high‐molecular‐weight polyethylene with a novel flow rule

A study of the tensile behavior of ultra‐high‐molecular‐weight polyethylene over a range of strain rates showed that its strain rate sensitivity was a function of the strain. This was related to a flow rule developed for this material in a previous study on compressive behavior. This flow rule is an adaptation of that of Hill, in which the anisotropy coefficients are power‐law functions of the extension ratios. It is used in conjunction with an Eyring process. The observed rate dependence of the tensile behavior conformed with that obtained with the power‐law flow rule and could be used to derive a value of the power‐law coefficient. Independent observations were made of the relationship between the axial and transverse strains in tensile specimens with inhomogeneous strain fields. A constitutive model was developed that incorporates the new flow rule and was implemented in a finite element analysis. When this analysis was used to model the inhomogeneous tensile specimens, it gave predictions of the axial and transverse strain that were consistent with the experiment when the power‐law coefficient was the same value as that derived from the study of the rate dependence. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The breaking strength of imperfect (real) polymer fibers

The thermodynamic fusion theory of strength of perfect polymer fibers of finite molecular weight is extended to include imperfect (i.e. real) fibers of incomplete crystallinity and orientation. Approximate equations for failure strength, strain, and work of failure are derived by extracting from the real visco-elastic fiber an equivalent reversible component suitable for thermodynamic analysis. This is facilitated by an explicit relationship between fiber breaking stress, σ_*, and breaking strain, _*, which is shown to be σ_*=0.632K_* (K=modulus) for constant strain-rate deformations. It is shown that fiber breaking time is equivalent to the fiber visco-elastic mechanical relaxation time. Experimental data shows that the activation energy of rupture of polyethylene fibers is not the activation energy of covalent bond rupture. Instead it agrees with the activation energy expected of crystal melting in accordance with the fusion theory of rupture. The activation volume of the polyethylene fibers also agrees with the value expected from this theory.     

Moisture sorption/desorption behavior of various manmade cellulosic fibers

The kinetics of dynamic water vapor sorption and desorption on viscose, modal, cotton, wool, down, and polyester fibers and lyocell knit fabrics were investigated according to the parallel exponential kinetics (PEK) model. The total equilibrium moisture regain (M_inf(total)) in all the materials decreased with increasing temperature. However, the partial equilibrium fast sorption, determined by PEK simulation at 60% relative humidity (RH) and 36°C, was larger than that at 20°C, whereas the partial equilibrium slow sorption was smaller. The characteristic times in fast sorption (τ₁) and in slow sorption (τ₂) for lyocell were reduced when the conditions were changed from 60% RH and 20°C to 36°C, whereas those for the other fibers increased. Lyocell exhibited the highest M_inf(total) value and the lowest τ₁ and τ₂ values, and this suggested high equilibrium moisture content and fast moisture uptake/release, that is, high moisture accessibility for lyocell. The relationships between the moisture regain, hysteresis, water retention capacity, and Brunauer–Emmett–Teller surface volume in the materials were also examined. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1621–1625, 2005     

Effect of surface treatments on the thermal behavior and tensile strength of piassava (Attalea funifera) fibers

Piassava (Attalea funifera) fibers subjected to several surface chemical treatments and as‐received raw fibers were compared with respect to their thermal and tensile behaviors. The thermal degradation of the raw fibers was characterized by three main stages that corresponded to water release at low temperatures, decomposition of hemicellulose, and decomposition of α cellulose. Mercerization acted mainly on hemicellulose removal, and there was no change in the hydrophilic behavior of the fibers. The removal of hemicellulose split the fibers into microfibrils and favored the thermal decomposition of α cellulose. The same behavior was observed when the fibers were subjected to mercerization and acetylation. The fibers subjected to only acetylation showed thermal behavior similar to that of the raw fibers. With the acetylation treatment, a minor decrease in the hydrophilic character of the fibers was noted. Despite some differences in the thermal behavior, the tensile strengths of the raw and treated fibers were statistically equal. Complementary Fourier transform infrared and scanning electron microscopy analysis corroborated the thermogravimetric analysis/differential thermogravimetry results. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Internal lipid content and viscoelastic behavior of wool fibers

Wool is a natural keratin fiber made up of cuticle and cortical cells held together by the cell membrane complex (CMC), which contains few internal lipids (IWLs) (1.5% by mass). IWL arouse considerable cosmetic and dermatological interest because of its high proportion of ceramides. In this work, IWLs were extracted with acetone, methanol, and dichloromethane/acetone solvents, and the possible alteration of the extracted fibers with respect to their textile feasibility was analyzed. Parameters of yield, fibril, and matrix viscoelastic behavior, deformation work, and breaking elongation were useful in highlighting the effect of internal wool lipids on the mechanical properties of the fibers. The extraction with acetone and methanol solvents supplied good yields of IWL. Although extraction with methanol achieved the richest extracts, the fibers were chemically modified. By contrast, although acetone‐extracted fibers had similar properties after treatment, alkaline solubility was lower and fiber length and barb were superior. In the mechanical analysis, a prior extraction of IWL increased yield tenacity and decreased the elongation at break of the fibers, maintaining the feasibility of extracted wool for textile purposes. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3252–3259, 2004     

Optothermomechanical device for the interferometric characterization of fibers

An optothermomechanical (OTM) device was designed and constructed for fiber characterization with a double‐beam interference microscope. This device enabled us to correlate both the mechanical properties and the thermal properties with the optical properties of fibers. The OTM device consisted of three parts, which were used for the drawing (stress–strain), cooling, and heating of the fibers. The designed OTM device (cooling and heating) was attached to the Pluta microscope for the determination of the optical properties of high‐density polyethylene (PE) fibers at different temperatures (0–50°C). Also, this OTM device (drawing and heating), connected to the Pluta microscope, was used to study the influence of the temperature (10–50°C) and draw ratio (1–7) simultaneously on the optical properties of polypropylene (PP) fibers. Microinterferograms were provided for illustration. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 95: 647–658, 2005     

Characterization and modeling of the moisture diffusion behavior of natural fibers

Natural fibers have good properties to be used as reinforcement in composite materials. The main issue is their hydrophilic behavior. So we propose here to investigate the diffusion phenomenon in such fibres. First, a brief characterization of four vegetal fibers has been achieved. We show that all fibers have a similar composition and structure despite their different origin. Then, their moisture diffusive behavior was investigated. The samples were submitted to hygro‐thermal aging either in total water immersion at room temperature or in an environmental chamber at 80% relative humidity and 23°C. Various predictive models were used to simulate experimental curves. Results show that all fibers exhibit a similar diffusive behavior in a same environment. In immersion, specimens show anomalous absorption kinetics and Langmuir theory actually describes very well the diffusion kinetics in such conditions, whereas the same fibers follow a Fickian diffusion when they are exposed to vapor during relative humidity aging. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Effect of polypropylene fiber processing conditions on fiber mechanical behavior

An investigation into the mechanical behavior of melt‐spun isotactic polypropylene (iPP) fibers is reported. Two different iPP formulations, PH835 and Exxon3854, synthesized using Ziegler–Natta and metallocene catalysts, respectively, and spun at take‐up velocities ranging from 1000 to 3000 m min^?1 were subjected to uniaxial tensile loading, cyclic loading and creep tests. The strain rate sensitivity was determined by performing strain rate jumps. Injection molded specimens from the same iPP formulations were tested under the same conditions. The fiber birefringence increases slightly with increasing take‐up velocity, while the crystallinity is approximately insensitive to this process parameter in this range of velocities. Fibers from the two iPP samples behave differently at large plastic strains despite having the same birefringence and crystallinity. Differences are also seen in creep. The behavior of fibers is significantly different from that of the injection molded samples of the same iPP and same crystallinity. These have lower strain hardening rate, smaller failure strains, close to zero strain rate sensitivity and exhibit a yield point phenomenon. The difference is associated with the different nature and spatial organization of the crystals and inter‐crystalline amorphous and mesomorphic phases. © 2014 Society of Chemical Industry     

Damage analysis of composites reinforced with Alfa fibers: Viscoelastic behavior and debonding at the fiber/matrix interface

In this article, we first review state‐of‐the‐art experimental techniques and measurements to characterize the mechanical properties of anisotropic vegetal alfa fibers, epoxy‐resin, and the behavior of the interphase between the matrix and alfa fibers. Second, we conduct experimental tests to determine the mechanical properties of fibers, resin, and the interphase. Third, we carry out a series of finite element simulations to predict damage initiation and to estimate crack propagation in alfa‐fiber/epoxy‐resin (AFER) composites. Different tests to determine the longitudinal Young's modulus of alfa fibers and epoxy resin as well as nanoindentation tests to obtain the transverse stiffness of the fibers are presented. Experimental results from the characterization are introduced in a micromechanical model to estimate, using the concept of the energy release rate (ERR), the matrix crack, and its interaction with interfacial debonding. The wettability problems in the preparation of vegetable composites and their effect on fiber‐matrix interfacial debonding are also addressed. The analysis of the damage behavior of AFER composites demonstrates that under load transverse to the fiber axis, a crack initiated in the matrix is propagated perpendicular to the direction of the load. Near the interface, the ERR decreases and this energy is higher in the presence of interfacial debonding areas generated by problems of fiber wettability. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43760.     

The tensile behavior of carbon fibers at high temperatures up to 2400 °C

The tensile behavior of four different brands of carbon fibers (a rayon-based, a PAN-based, and 2 pitch-based fibers) has been investigated at various temperatures up to 2400 °C. The tests were carried out using an original fiber testing apparatus. Various mechanical properties including strength and Young's modulus, as well as Weibull statistical parameters were extracted from test data. Typical tensile behaviors were evidenced such as an essentially linear elastic behavior at room temperature and intermediate temperatures up to 1400-1800 °C, then a nonlinear elastic delayed response at higher temperatures and ultimately an inelastic response with permanent deformations at very high temperatures. Such unusual nonlinear responses for homogeneous materials were related to structure and texture features at the nanometer scale, that were described through an X-ray diffraction technique.     

Crystallization behavior of polyhydroxybutyrate in model composites with kenaf fibers

The kinetics of the nonisothermal crystallization process of polyhydroxybutyrate in polyhydroxybutyrate/kenaf fiber model composites (with 80/20 and 70/30 w/w matrix/kenaf fibers) were investigated with differential scanning calorimetry. An analysis of the data was carried out with the Avrami, Ozawa, and modified Avrami and Ozawa models, as well as the Kissinger approach, for the determination of the crystallization activation energy. The Ozawa model was unsuitable for analyzing the nonisothermal data, whereas the other models described these systems very well. By the analysis of all the relevant parameters, the nucleation activity of the kenaf fibers was confirmed. The activation energies from the Kissinger method were evaluated to be 41.2, 32.6, and 26.3 kJ/mol for the pure polymer resin and 80/20 and 70/30 (w/w) polyhydroxybutyrate/kenaf fiber composites, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 804–809, 2006     

Rheological behavior of polymer melts with natural fibers

Flow behavior of polymer liquids filled with short fibers (particulate fillers) was theoretically analyzed from the point of view of the free volume theory. Assuming that the filler addition changes the occupied volume, while the temperature variations cause mainly the free volume changes, a general expression describing the viscosity of the system as a function of the filter content, temperature variations, and rheological properties of the pure polymer liquid was derived. If the viscosity curve of the unfilled polymer is described by the Carreau equation, the corresponding viscosity curve of the filled polymer is also represented by an equation of Carreau type. However, this equation has other values of Newtonian viscosity and the power exponent in comparison with the initial equation. Both parameters depend on the filler content and temperature. The derived equation predicts a viscosity rise and a stronger non‐Newtonian behavior of the system with increasing filler content. The temperature rise exerts an opposite effect on the rheological behavior. The theoretical predictions are in good accordance with viscosity measurements for low‐density polyethylene and polystyrene melts filled with short cotton, flax, and hemp fibers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1401–1409, 2005     

Fatigue behavior and relation between fatigue resistance and tensile properties of poly(para-phenylene-co-3,4'-oxydiphenylene terephthalamide) fiber

The poly(para-phenylene-co-3,4′-oxydiphenylene terephthalamide) (PPODTA) fiber is one of the high strength organic fibers, and it has been reported that the PPODTA fiber has superior fatigue resistance. The high strength fibers are used in the applications to utilize their high mechanical properties in general. Therefore, the long-term durability of these fibers is also required. In this study, the fatigue tests were conducted for the PPODTA fibers. As a result, it was found that the PPODTA fibers were able to be fractured by the cyclic tensile stress, and the fatigue behavior was influenced by the stress conditions. In addition, the single fiber tensile tests were also conducted for the PPODTA fibers, and the relation between the tensile properties and the fatigue resistance of the PPODTA fiber was investigated. The fatigue resistance of the PPODTA fiber was increased with the decrease of the fiber diameter and the increase of the tensile modulus.     

Effect of fiber volume fraction on the thermal and mechanical behavior of polylactide‐based composites incorporating bamboo fibers

Biodegradable composites reinforced with natural fibers are emerging as advanced materials in structural applications. In this work, green biocomposites are fabricated using hot pressing molding technique, polylactic acid selected as a matrix. The samples are prepared with different fiber volume fractions (30%, 40%, and 50%). Tensile tests are conducted on the specimens to investigate the composite mechanical behavior, and the influences of fiber content on the morphological and thermomechanical properties are evaluated using scanning electron microscopy, differential scanning calorimetry, and thermogravimetric analysis. There are higher tensile modulus and lower elongation at break for composites with increasing fiber content, respectively. Much variation in the tensile strength is observed when the fiber content is varied, which could be attributed to fiber agglomerations that affect the dispersion of fibers in the matrix, as evidenced by fracture surfaces. Thermal tests demonstrate that the increment of fiber content enhances the glass transition temperature and crystallization temperature of composites. Besides, a comparative analysis of the composites is performed, and the properties of the treated fiber composites are found to be improved compared to those from untreated fibers. Detailed analysis confirms the possibility of the addition of bamboo fibers to a biodegradable matrix for a specific application. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46148.     

Dependence of the liquid absorption behavior of nonwovens on their material and structural characteristics: Modeling and experiments

Nonwovens are widely used as liquid absorbent products. Baby diapers, sanitary napkins, adult incontinence pads, oil sorbents, wet wipes, and wound dressings, to name a few, are excellent examples of the use of nonwovens as absorbent media. The performance of nonwoven absorbent media is determined by its liquid absorption behavior, which is characterized by the capacity of absorption and the rate of absorption. In this article, we report on the effects of the physical characteristics of the constituent fibers and the internal structure of the nonwovens on their liquid absorption behavior. A theoretical model of liquid absorption behavior of nonwovens was developed, and this model was verified with a set of experimental results obtained on real nonwoven materials. The nonwoven materials were prepared with polyester fibers with different cross‐sectional sizes and their liquid absorption properties were measured with the gravimetric absorbency testing system. We observed that the size of fiber cross sections and the porosity of the nonwovens played very important roles in determining their absorbent capacity and rate of absorption. The results of the experiments were discussed in light of the theoretical model. The theoretical results were found to be in good agreement with the experimental results. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Capacitance behavior of activated carbon fibers with oxygen-plasma treatment

Modified activated carbon fibers (ACFs) were used as the electrodes of an electric double-layer capacitor and showed an enhanced capacitance effect after a RF-plasma treatment. The capacitance and the surface functional groups of the ACFs were studied. For the plasma-treated ACFs having a specific surface area of 1500 m² g⁻¹, the capacitance increased by 28% compared to the untreated sample and the highest electric capacitance value of 142 F g⁻¹ was achieved with an oxygen feed concentration of 10 vol.%. The Brunauer-Emmett-Teller (BET) surface area was 2103 m² g⁻¹, which was 34% higher than that of the untreated sample. The pore volume was similarly increased to 483.1 cm³ g⁻¹ STP, and from the pore distribution plot, quantities of mesopores of 10 nm or less and micropores also increased. However, in order to enhance the capacitance, the quinone functional group had a significant influence in addition to the BET surface area. The correlation between the capacitance and the number of quinone functional groups was confirmed because quinone is an electron acceptor.     

Effects of heat treatment on tensile properties of diacetate fibers

Diacetate filaments were heat‐treated (without tension or with tension) under dry‐heat or wet‐heat environment, respectively. The effects of temperature, time, and tension on tensile properties of diacetate fibers after heat treatments were discussed. The results show that diacetate fibers present no obvious improvement on its tensile properties after dry‐heat treatments without tension. It was also found that during dry heat treatments with tension, the increase in tensile properties of fibers mainly depends on temperature and tension. Moreover, being dry‐heat treated with tension instant after wet‐heat treatment without tension, diacetate fibers exhibit a higher improvement on its tensile properties comparing with dry heat method with tension. The shrinking measurement for the fibers indicates different supermolecular structures were developed in the fiber before treatment and after treatment, which leads to the different extent in the improvement of tensile properties for the fibers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101:787–791, 2006