Frictional characteristics of stiff,high aspect ratio microfiber arrays based on cyclic olefin polymers

Cyclic olefin polymer (COP) microfiber arrays with diameters in the range of 0.6–5 μm and up to ~20 μm in length are successfully fabricated using polycarbonate membranes as templates, and their macroscale friction properties are evaluated against smooth glass surfaces. Increasing the aspect ratio of the fibers decreases the effective modulus and increases the effective work of adhesion, which favor a better compliance, resulting in higher friction forces. On the other hand, the shape of the fibers as well as inter-fiber adhesion at high aspect ratio complicates the dependence of friction on fiber geometry. Among the arrays investigated, the 2 and 0.6 μm diameter arrays exhibited excellent friction performance (up to ~6 and 9 N/cm², respectively) and high fractional contact area (50–60%). These results suggest that arrays with high friction properties can be easily obtained by molding COPs from polycarbonate membranes.     

Improved strain sensing performance of glass fiber polymer composites with embedded pre-stretched polyvinyl alcohol–carbon nanotube fibers

Polyvinyl alcohol–carbon nanotube (PVA–CNT) fibers differing on their pre-stretching condition were embedded in glass fiber reinforced plastic (GFRP) composites and used as strain sensors for damage monitoring of the composite. Strain sensing of the composite was made by the in situ measurement of the embedded fiber’s electrical resistance change during the mechanical tests. Four glass fiber composite plates were manufactured; each one had embedded a different type of produced PVA–CNT fibers. The multi-functional materials were tested in monotonic tensile tests as well as in progressive damage accumulation tests. The electrical resistance readings of the PVA–CNT fibers were correlated with axial strain values, taking into account the induced damage of the composite. It has been demonstrated that increasing the fiber’s pre-stretching ratio, its electrical resistance response increases due to higher degree of the CNTs alignment in the PVA matrix. Higher fiber pre-stretching degree enables the better strain monitoring of the composite due to higher measured electrical resistance change values noticed for the same applied axial strain values. To this end, it enables for the better monitoring of the progressive damage accumulation inside the composite.     

Macrostructure and mechanical behavior of fibers of poly-p-phenylene benzobisthiazole

Morphology and tensile behavior of wet-spun fibers of poly-p-phenylene benzobisthiazole have been investigated. The as-spun fibers contain a large number of voids, which result in void-localized tensile failure. The stress–strain behavior is elastic–plastic with strain hardening. This behavior is shown to be the result of residual stresses which arise during the wet-spinning process.     

Toward mechanistic understanding the effect of aspect ratio of carbon nanotubes upon different properties of polyurethane/carbon nanotube nanocomposite foam

This study investigated the carbon nanotube's aspect ratio's influence on the nanocomposite foams' cellular structure and mechanical, acoustic absorption characteristics. The free-rising foaming process has been used for producing different flexible polyurethane (PU) foams embedded with other multi-walled carbon nanotubes (MWCNT's). Dynamic mechanical and thermal analysis, flow resistivity, and compressive mechanical measurements were achieved on the prepared samples. The acoustic absorption coefficient in a wide range of frequencies was estimated for the prepared PU/CNT foamed nanocomposite samples. Results indicated that by increasing the aspect ratio of MWCNT, the absorption coefficient's peak shifts toward the lower frequencies and improved sound absorption characteristics of PU foam in the low-frequency region. Moreover, the Young modulus of nanocomposite samples increases by increasing the aspect ratio of MWCNT's, whereas the stored strain energy or area under the stress–strain curve increases. Based on the obtained results, it is observed that the acoustic absorption coefficient of produced nanocomposite foams at the frequency of 800 Hz has been reported to have a 70% improvement in 2 cm samples and a 40% improvement in 3 cm samples compared to obtained results from pure PU foam.     

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     

Numerical simulation of fiber orientation distribution in round turbulent jet of fiber suspension

The Reynolds averaged Navier–Stokes equation was solved numerically with the Reynolds stress model to get the mean fluid velocity and the turbulent kinetic energy in a round turbulent jet of fiber suspension. The fluctuating fluid velocity was described as a Fourier series with random coefficients. Then the slender-body theory was used to calculate the fiber orientation distribution and orientation tensor. Numerical results of mean axial velocity and turbulent shear stress along the lateral direction were validated by comparing with the experimental ones. The results show that most fibers are aligned with the flow direction as they go downstream, and fibers are more aligned with the flow direction within the region near the jet core. The fibers with high aspect ratio tend much easier to align with the flow direction, and the fiber orientation distribution is not sensitive to fiber aspect ratio when fiber aspect ratio is larger than 5. Fiber density has no obvious influence on the fiber orientation distribution and fiber orientation tensor. The randomizing effect of turbulence is insignificant in the regions near outside and jet core, and becomes significant in the region between outside and jet core. The randomizing effect increases with the increasing of the distance from the jet exit. Different components of fiber orientation tensor show a similar distribution pattern.     

Melt rheological behavior of starch‐based matrix composites reinforced with short sisal fibers

A systematic analysis of the melt rheological behavior of a commercial starch‐based (MaterBi®) matrix composite reinforced with short sisal fibers is presented. The effects of shear rate, temperature, fiber content and treatment were analyzed by parallel‐plate rheometry, and classical non‐Newtonian models were applied to analyze the pseudoplasticity behavior of the molten composite systems. It is reported that shear rate is the most influential processing condition, while, from the point of view of the material structure, the intercalation effectiveness of the matrix in the fibers is directly linked to the rheological behavior. In fact, processing techniques with high stresses and more efficient mechanical mixing promote the opening of fiber bundles, increasing the aspect ratio of the fibers and the average viscosity of the molten composite. A similar effect on the increase of the aspect ratio and composite viscosity is observed when treated fibers are used. Polym. Eng. Sci. 44:1907–1914, 2004. © 2004 Society of Plastics Engineers.     

Mechanical characterization of lightweight engineered geopolymer composites exposed to elevated temperatures

In this study, the effect of different factors, such as PVA fibers (2% by total volume) and precursor type (slag, fly ash, or a combination of both), on the behavior of green lightweight engineered geopolymer composites (LEGC) and lightweight engineered cementitious composites (LECC) after exposure to temperatures up to 800 °C for 1 h is investigated. Expanded glass granules were used as lightweight aggregate instead of silica sand to reduce the spalling tendency and density of the composite. The flowability, density, color change, mass loss, spalling resistance, residual mechanical properties (compressive strength, stress-strain diagram, tensile stress-strain diagram, load-deflection response, failure mode), and microstructural analysis (by scanning electron microscopy) were investigated before and after exposure to thermal deterioration. The findings pointed out that the dry density, compressive strength, fiber bridging stress, strain capacity, maximum load, and maximum deflection of the developed mixtures before exposure to fire deterioration were in the range of 1703–1883 kg/m³, 16.66–64.11 MPa, 2.66–4.97 MPa, 2.40–3.33%, 1573–4824 N, and 2.92–5.53 mm respectively. It's worth mentioning that the substitution of 50% slag in the lightweight EGC mixture demonstrated the optimal tensile strain capacity and deformation capacity and further enhanced both ultimate tensile strength and flexural strength of fly ash-based EGC (FA-EGC) mixtures. After heat exposure, both LEGC and LECC composites demonstrated strain hardening behavior and deflection hardening behavior up to 300 °C of heat treatment, while after exposure to a temperature of 300 °C and above, both deflection hardening behavior and strain hardening behavior are dramatically damaged. This is attributable to the melting of the PVA fibers. Also, the microstructural analysis showed that incorporating fly ash into lightweight EGC mixtures can effectively reduce the melting point of PVA fibers and further improve the fire resistance of EGC mixtures.     

Shear flow rheological properties,fiber damage,and mastication characteristics of aramid-, glass-, and cellulose-fiber-reinforced polystyrene melts

An experimental study of the viscosity and principal normal stress difference of a polystyrene melt filled with aramid (Kevlar), glass, and cellulose fibers is reported. The influence of loading level and mastication on the rheological properties is discussed. The effects of mixing and mastication on fiber damage are considered. Glass fibers break down rapidly to very small aspect ratios, while aramid shows a “kinked” structure, with kinks occurring every 100 μm. A mechanism is proposed for fiber breakage based on buckling during rotation in shear flow. It is found that addition of fibers increases the viscosity in the same manner as a reduction in temperature, and data may be superposed by reduced plotting. This indicates that the viscosity increase is due solely to enhanced viscous dissipation in the matrix and not to interparticle forces as is the case with smaller particles. The principal normal stress difference increases at fixed shear stress with fiber loading. The extent of increase depends upon fiber loading, aspect ratio, and modulus.     

Rheological behavior of coir‐fiber‐filled polypropylene composites at constant shear stress

The rheological behavior of coir‐fiber‐filled polypropylene (PP) composite has been studied at constant shear stress. The shear stress versus shear rate relationship for the composite follows power law model of viscous flow. Unlike similar studies in the literature, the viscosity is treated as a stress‐independent parameter, which increases with the increase of fiber loading; but decreases with the rise of temperature. The SEM reveals that the fibers are loosely bound to the polymer matrix and the outer surface of the composite is rough and irregular, making it susceptible to high friction with the wall of the flow channel. With analogy to nth order chemical reaction, new formula has been derived for the activation energy of viscous flow, which is found to increase with the increase in the fiber content. The die‐swell ratio decreases with the increase of fiber loading, but increases with the rise of temperature. The elastics parameters of the composite such as the recoverable shear strain, the first normal stress difference, and the elastic strain induced by the stored energy in the capillary reservoir have been estimated based on the die‐swell data. POLYM. COMPOS., 36:51–61, 2015. © 2014 Society of Plastics Engineers     

The influences of fiber feature and polymer melt index on mechanical properties of sugarcane fiber/polymer composites

The fiber characteristics (i.e., the fiber type, morphology, and dimension) and polymer melt flow index (MFI) significantly affected mechanical properties of sugarcane fiber/HDPE composites. The length and diameter of sugarcane fibers followed a lognormal distribution before and after compounding. The long fibers had a significant reduction in the dimension and aspect ratio during compounding. However, the short fibers had close values in these two properties before and after compounding. For the resultant sugarcane fiber/polymer composites, the HDPE resins with a low MFI value presented high tensile and impact strengths. Because of high sugar content, the pure rind fiber had a poor performance as filler in the HDPE resins with respect to the raw bagasse fiber and alkali‐extracted bagasse fiber. On the other hand, the aspect ratio was proportional to the mechanical performance of the fibers in the HDPE resins. As a result, the fibers with a large aspect ratio and low sucrose content improved the strength properties of the resultant composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5607–5619, 2006     

Micromechanical modeling of large deformation in sepiolite reinforced rubber sealing composites under transverse tension

This article presents an experimental and numerical study on mechanical behavior of sepiolite reinforced rubber sealing composites (SRRC), which are subjected to the transverse tensile loads. A finite element model of composites with fibers in square and random distribution is adopted for the numerical study. The representative volume elements with different fiber volume fraction are established and analyzed. A successive remeshing strategy is employed to achieve the large deformation of SRRC. The results show that the tensile strength and breaking elongation of SRRC can be improved by addition of sepiolite fiber, and they reach a maximum at 42% fiber volume fraction. The stress‐strain curve of SRRC with fibers in the random distribution agrees well with that measured in the experiments. However, there exists a significant difference between experimental and numerical results as fiber volume fraction increases. POLYM. COMPOS., 38:381–388, 2017. © 2015 Society of Plastics Engineers     

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.     

Mechanical and tribological properties of electrospun PA 6(3)T fiber mats

The mechanical and tribological properties of electrospun fiber mats are of paramount importance to their utility as components in a large number of applications. Although some mechanical properties of these mats have been reported previously, reports of their tribological properties are essentially nonexistent. In this work, electrospun nanofiber mats of poly(trimethyl hexamethylene terephthalamide) (PA 6(3)T) with average fiber diameter of 463 ± 64 nm are characterized mechanically and tribologically. Post-spin thermal annealing was used to modify the properties of the fiber mats. Morphological changes, in-plane tensile response, friction coefficient and wear rate were characterized as functions of the annealing temperature. The Young's moduli, yield stresses and toughnesses of the nonwoven mats improved by two- to ten-fold when annealed slightly above the glass transition temperature, but at the expense of mat porosity. The coefficient of friction and the wear rate decrease by factors of two and ten, respectively, under the same conditions. The wear rate correlates with the yield properties of the mat, in accord with a modified Ratner–Lancaster model. The variation in mechanical and tribological properties of the mats with increasing annealing temperature is consistent with the formation of fiber-to-fiber junctions and a mechanism of abrasive wear that involves the breakage of fibers between junctions.     

Deformation kinetics of polypropylene hollow fibers in a continuous drawing process

Effects of isothermal drawing conditions on the deformation kinetics and dimensional change of polypropylene (PP) hollow fibers in a continuous drawing process were investigated. The deformation behavior of solid PP polymers during stretching between two rolls in the isothermal bath was analyzed by a simple model describing the continuous drawing process with a constitutive relation that can express a true (stress–strain–strain rate) surface of solid semicrystalline polymers. Necking profiles during drawing can be calculated from this model without any special assumption for neck criterion, and the calculated results predict that the localization of deformation is promoted with the increase of applied draw ratios. It is also found that at 20°C, the neck is observed apparently both from the calculated and experimental results, and the strain‐rate sensitivity parameter is considered to be a critical factor that determines the intensity of the neck geometry. The calculated drawing forces are shown to increase with increasing the applied draw ratio and decreasing the drawing temperature, and these trends were verified by experimental results. The hollowness, defined as the ratio of inner to total cross‐sectional area, increases as it is drawn at 30°C, but decreases as drawn above this temperature compared with that of the undrawn fiber. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 1836–1845, 1999     

Rheology of LDPE‐based semiflexible fiber suspensions

Molten LDPE suspensions containing fibers of different flexibilities have been investigated in simple shear and small and large amplitude oscillatory shear (LAOS) flow. The suspensions exhibited viscosity and normal stress overshoots in stress growth experiments, and the magnitude and width of the overshoots became larger as the fiber flexibility increased. LAOS was used to help understanding the relationship between stress growth and fiber orientation. For all composites, the stress signal decreased with time in LAOS, and this behavior was more pronounced in the case of the more rigid fibers. The energy dissipated per LAOS cycle was evaluated for each composite, and it showed that less energy was dissipated as fiber flexibility decreased. In addition, the dissipated energy decreased with time and this has been interpreted in terms of a reduction of fiber contacts. The first normal stress difference showed a nonsinusoidal periodic response, and fast Fourier transform analysis indicated the presence of a first harmonic corresponding to the applied frequency for the fiber‐filled systems, in addition to the second harmonic observed for the neat LDPE. It resulted in asymmetrical strain‐normal force Lissajou curves for the suspensions, with this asymmetry being more pronounced in the case of the more rigid fibers. This has been attributed to a more extensive fiber orientation for the latter. POLYM. COMPOS., 31:1474–1486, 2010. © 2009 Society of Plastics Engineers     

Fatigue behavior of polyamide 66/glass fiber under various kinds of applied load

In this study, the fatigue behavior of polyamide 66 reinforced with short glass fibers and especially the role of glass fibers has been investigated under two kinds of cyclic loading. tension–tension fatigue tests with stress controlled and alternative flexural fatigue test with strain controlled were carried out. The main topics include microscope damage observation, described by fiber/matrix debonding and interfacial failure, endurance limit with Wohler curves, effect of self‐heating temperature. For both tests, the surface temperature increases with an increasing applied load. The results show that the self‐heating has an important effect in the failure point where the Wohler curves join each other. The fracture surface was analyzed by scanning electron microscope for both applied loads. The stress ratio is −1 for alternative flexural fatigue test and 0.1 and 0.3 for tension–tension fatigue test ones at frequencies ranging 2–60 Hz. POLYM. COMPOS.,, 2012. © 2012 Society of Plastics Engineers     

Strengths of ceramic fiber bundles: Theory and practice

The Weibull modulus and reference strength of ceramic fibers can be inferred from measurements of the tensile stress‐strain response of a bundle of such fibers. The goal of the present article is to address issues in the fidelity of results stemming from fiber bundle tests and strategies to optimize outcomes. The issues are addressed through established theorems in uncertainty propagation, Monte Carlo simulations of fiber bundle fracture, and experimental measurements on bundles of SiC fibers of various length and surface condition. The study shows that optimal results are obtained when: (i) tests are performed on fiber bundles with a gauge length that exceeds a critical value (specifically, that needed to prevent mechanical instabilities in the post‐load‐maximum domain); (ii) bundles are lubricated with a low‐viscosity oil, to mitigate both inter‐fiber friction and dynamic coupling associated with release waves following fiber fracture; and (iii) the Weibull parameters are obtained by directly fitting the measured stress‐strain curves with the function predicted by fiber bundle theory, rather than using methods based on either linear regression analysis of Weibull probability plots or fitting of the peak stress and strain alone.     

Failure of elastic‐plastic core–shell microcapsules under compression

Characterization of the failure behavior of microcapsules is extremely important to control the release of their core actives by mechanical forces. The strain and stress of elastic‐plastic uninflated core–shell microcapsules at failure (rupture or bursting) has been determined using finite element modeling (FEM) and micromanipulation compression experiments. The ductile failure of polymeric microcapsules at high deformations is considered to occur when the maximum strain in the shell exceeds a critical strain, resulting in their rupture. FEM has been used to determine the maximum strains present in the capsule wall at different deformations for three types of shell material: elastic, elastic—perfectly plastic and elastic—perfectly plastic with strain hardening at large strains. The results obtained were used to determine the failure strain and stress of melamine‐formaldehyde microcapsules, with average population values of ～0.48 and ～350 MPa, respectively. Thus, the elastic‐plastic stress–strain relationship has been determined for the core–shell microcapsules tested. © 2011 American Institute of Chemical Engineers AIChE J, 2012