Rheological characterization of long fiber thermoplastics – Effect of temperature,fiber length and weight fraction

Isothermal squeeze flow tests were conducted on E-Glass/polypropylene long fiber thermoplastics (LFT) to obtain the rheological characteristics of the material over a range of squeeze rates (0.5–60 mm/min). A transversely isotropic power-law model has been incorporated to capture the combined effect of shear and extensional flow behavior. Scott’s approach [Bird RB. Useful non-Newtonian models; 1976. p. 13–34] was used to obtain the shear power-law parameters, which were then used to calculate the radial velocity in the r-direction. The continuity equation was used to calculate transverse velocity in the z-direction. Radial and through the thickness velocity profiles were determined to obtain the extensional and the shear strain rates. Finally the extensional and shear viscosities were determined at strain rates calculated. Good agreement between the experimental applied stress and the predicted curves from the model was achieved. Effects of mold separation, mold temperature, and fiber length on viscosity at constant fiber weight fraction were examined. Effect of fiber weight fraction on viscosity at constant fiber length, mold separation and temperature was examined. Results indicate that viscosities decrease with either increase in mold temperature or decrease in fiber length at constant mold separation and fiber weight fraction. Viscosities also decreased with decrease in fiber weight fraction.     

Tunable single longitudinal mode S-band fiber laser using a 3 m length of erbium-doped fiber

In this paper a tunable single-longitudinal mode (SLM), short-wavelength band (S-band) fiber laser using a conventional erbium-doped fiber (EDF) with a length of 3?m and a step index erbium dopant profile as opposed to the commonly used depressed cladding erbium-doped fiber (DC-EDF) is proposed and demonstrated. The proposed SLM fiber laser has a tuning range of 1496 to 1507?nm in a ring configuration using two 0.15?m of EDF which acts as saturable absorbers (SAs). The highest peak power measured is about ?0.6?dBm at a wavelength range of 1502 to 1507?nm. The measured signal-to-noise ratio (SNR) is approximately 74?dB for the same wavelength range. The line-width of the SLM output is measured to be 140?kHz.     

Hierarchical wood cellulose fiber/epoxy biocomposites – Materials design of fiber porosity and nanostructure

Delignified chemical wood pulp fibers can be designed to have a controlled structure of cellulose fibril aggregates to serve as porous templates in biocomposites with unique properties. The potential of these fibers as reinforcement for an epoxy matrix (EP) was investigated in this work. Networks of porous wood fibers were impregnated with monomeric epoxy and cured. Microscopy images from ultramicrotomed cross sections and tensile fractured surfaces were used to study the distribution of matrix inside and around the fibers – at two different length scales. Mechanical characterization at different relative humidity showed much improved mechanical properties of biocomposites based on epoxy-impregnated fibers and they were rather insensitive to surrounding humidity. Furthermore, the mechanical properties of cellulose-fiber biocomposites were compared with those of cellulose-nanofibril (CNF) composites; strong similarities were found between the two materials. The reasons for this, some limitations and the role of specific surface area of the fiber are discussed.     

Experimental study of a bow-tie-shaped optical fiber ring resonator with two 2 × 2 fiber couplers

An experimental study of a bow-tie-shaped double-coupler fiber ring resonator is presented. Multiple resonances of the transmitted output intensity and the splitting of the main resonance dip or peak have been observed. The experimental results are discussed and compared with theoretical results. The observed output property suggests the possible applications of the resonator as periodic Butterworth-like, narrow-band passing, and blocking filters.     

Effective mechanical properties of “fuzzy fiber” composites

In this paper we investigate the mechanical behavior of carbon fiber composites, where the carbon fibers are coated with radially aligned carbon nanotubes. For this purpose we develop a general micromechanics method for fiber composites, where fibers are coated with radially aligned microfibers (“fuzzy fiber” composites). The mechanical effective properties are computed with a special extension of the composite cylinders method. The in-plane shear modulus is determined using an extended version of the Christensen’s generalized self consistent composite cylinders method. The proposed methodology provides stress and strain concentration tensors. The results of the method are compared with numerical approaches based on the asymptotic expansion homogenization method. The combination of composite cylinders method and Mori–Tanaka method allows us to compute effective properties of composites with multiple types of “fuzzy fibers”. Numerical examples of composites made of epoxy resin, carbon fibers and carbon nanotubes are presented and the impact of the carbon nanotubes length and volume fraction in the overall composite properties is studied.     

Elimination of Kerr bistability in an optical fiber ring resonator with a 3 × 3 planar fiber coupler

A special property of a type 3 optical fiber ring resonator with a 3 × 3 planar coupler in which the circulating intensity is independent of the phase change of the ring resonator can be used to eliminate unwanted Kerr bistability. At the same time the device can be used as a two-channel frequency division multiplexer or demultiplexer or a switch. Another method for the elimination of Kerr bistability is the use of two fibers whose nonlinear refractive-index coefficients have opposite signs to build the ring resonator.     

Shot peen forming of fiber metal laminates on the example of GLARE®

GLARE (GLAss-fiber REinforced aluminum) is a sandwich material that combines thin aluminum sheets with intermediate layers of glass fiber that are bonded using epoxy. Due to the resulting low specific weight and high strength as well as superior deterioration resistance the material has found its application in aircraft structures. GLARE parts are typically manufactured using the so-called self-forming technique, which is a very expensive and labor-intensive manufacturing process. If it was feasible to form GLARE from flat stock material using conventional forming processes, substantial savings could be achieved. Several attempts to form GLARE from flat stock reported in the literature are restricted by the limited formability of the glass fibers and/or delamination of the layers. This work analyses the possibilities to form GLARE using shot peen forming (SPF), which is an established forming process, e.g. for the production of fuselage parts. It is shown that GLARE shows a similar deformation behavior as monolithic sheets under quasi-static indentation with single steel balls. The process limits are analyzed using SPF tests and lock-in thermography, which is a non-destructive testing procedure for the detection of delamination. A process window for shot peen forming of GLARE is established, and it is shown that curvature radii of less than 2500 mm can be accomplished with no evidence of failure, which is a typical curvature radius of fuselage components for the Airbus A380.     

Predictions of Young’s modulus of short inorganic fiber reinforced polymer composites

An expression of Young’s modulus of short inorganic fiber reinforced polymer composites was derived based on the tensile strength equation proposed in the previous paper, and the factor affecting the Young’s modulus was analyzed. This equation was applied to estimate the Young’s modulus of short inorganic fiber reinforced polymer composites. The results showed that the relative Young’s modulus increased nonlinearly with increasing fiber volume fraction, while increased linearly with an increase of fiber length-diameter ratio. Finally, the equation was verified preliminarily by using the measured Young’s modulus of the short glass fiber (SGF) reinforced polycarbonate/acrylnitrile–butadiene–styrene copolymer composites and the polypropylene reinforced respectively with SGF and short carbon fiber reported from literature, good agreement was found between the predictions and the experimental data.     

A study of the surface of carbon fiber by means of X-ray photoelectron spectroscopy—III

This paper reports a study of the surface composition of carbon fibers treated by various methods by means of X-ray photoelectron spectroscopy (SPS). C_1S X-ray photoelectron spectra showed that after the surface treatment of carbon fibers, the carbon atoms in the hydrocarbon were changed into

etc. oxygen-containing groups, that is, the results of surface oxidation and concentration increased with time but finally reached a constant level. Comparing experimental results for the treatments used, we found that all of these methods resulted in concentrations of oxygen groups on the surface in the order:

.Evidence was found for the formation of lactone groups

during treatment in an oxygen or nitrogen plasma, but not during treatment by nitric acid or anodic oxidation.     

Investigation of freeze–thaw effects on mechanical properties of fiber reinforced cement mortars

Fibers are used for improving some properties of conventional concrete (which is a brittle material) such as tensile strength, abrasion resistance, absorption and crack control. This study investigates the usability of fibers against the harmful effects of freeze–thaw cycles on cement mortars. For this objective, five different types of fibers, i.e., Polypropylene (PP), Carbon (CF), Aramid (AR), Glass (GF) and Poly vinyl alcohol (PVA) in four different ratios (0.0%, 0.4%, 0.8% and 1.2%) were added to cement mortars along with an amount of air agent. These samples were then subjected to five different freeze–thaw cycles (0, 25, 50, 75 and 100). Thus, mechanical behaviors were investigated under freeze–thaw effects.The most important results of the study are summarized; the fibers increase flexural strength and deflection ability of the samples while decreasing compressive strength, dynamic modulus of elasticity and specific mass. The highest flexural strength was obtained with a 1.2% addition of CF fiber for the samples in normal conditions. The mechanical properties of the samples subjected to repetitive freeze–thaw cycles were also investigated; the best flexural strength was provided with 1.2% CF addition, while the highest dynamic modulus of elasticity was obtained with a 1.2% PP addition.     

Effect of glass fiber sizing on the cure kinetics of vinyl–ester resins

The cure characteristics of thermosetting resins are affected by the presence of reinforcements as a result of surface–resin interactions. Surface treatments and sizing can significantly affect such interactions; hence, sizing or surface treatment selection may significantly affect resin cure characteristics. This is of particular concern in the processing of composite materials, since neat resin cure characteristics often will not provide the appropriate basis for predicting the cure behavior of the composite. In this work, the effect of several commercially sized S-2 glass systems on the cure of vinyl–ester resin was investigated. Generally, a significant increase in the cure rate of the glass-modified systems is observed. Furthermore, a relationship between the surface energy characteristics of the fibers and the degree of cure acceleration is established, and possible mechanisms for the effect are discussed. It is apparent that sizing selection can significantly affect cure processes for vinyl–ester systems.     

Structural changes of polyacrylonitrile precursor fiber induced by γ-ray irradiation

The structural changes of polyacrylonitrile precursor fibers under γ-ray irradiation were studied. The Fourier transform infrared spectroscopy (FTIR) results indicated that chemical reactions occurred in the irradiated fibers. The thermal and thermal mechanical behaviors of the fibers, which were characterized by differential scanning calorimetry (DSC), thermal gravimetry (TG) and thermal mechanical analysis (TMA), changed under irradiation, since ladder structure had formed in the fibers under irradiation prior to the heating process. The crystallinity and crystallite size were found to decrease with the increase of irradiation time, as the chemical reactions induced by γ-ray irradiation affected the crystal structure of the fibers. γ-ray irradiation may be useful in accelerating the thermal stabilization of PAN precursor fibers.     

Enhancement of fiber–matrix adhesion by laser ablation-induced surface microcorrugation

Micrometer-sized surface corrugations produced on Kevlar fiber surfaces by laser ablation were found to dramatically enhance the mechanical adhesion between the fibers and the epoxy matrix in a fiber-reinforced composite. Symmetric and asymmetric corrugation structures were produced by irradiating the fibers with high-fluence UV laser pulses at various incidence angles. The interfacial shear strength (IFSS) between the fibers and the matrix was measured using the microbond fiber-pullout method. Upon laser ablation treatment, the IFSS increased by 120% with symmetric corrugation profiles obtained with laser irradiation normal to the fiber axis, and 5-fold with asymmetric corrugation profiles obtained with the laser incidence angle at 45° to the fiber axis. A similar enhancement was observed in pullout tests under wet conditions. A simple model based on an elementary analysis of the expected strain field in the presence of interface corrugation is found to provide a quantitative explanation of the observed strength enhancement factors.

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Effects of Υ-ray irradiation on carbon fiber surface graft modification

为提高碳纤维／环氧树脂复合材料的界面粘结性能,采用7射线共辐照接枝方法对碳纤维表面改性,利用X光电子能谱仪（XPS）、扫描电子显微镜（SEM）、电子万能材料试验机,研究了在缩乙二醇丙酮溶液和环氧氯丙烷丙酮溶液中经200kGy剂量的Υ射线辐照接枝后,碳纤维的表面化学元素及官能团组成、表面形貌、复合材料剪切断面形貌及其层间剪切强度（ILSS）的变化。研究表明,缩乙二醇类接枝液的接枝效果较理想,碳纤维接枝率达7％;辐照处理碳纤维表面O／C比值和含氧官能团含量增加,以此制备的碳纤维／环氧复合材料的ILSS提高,最大提高率达31．2％;同时还发现辐照接枝后的碳纤维表面粗糙度增大。     

Compressive stress–strain behavior of steel fiber reinforced-recycled aggregate concrete

The use of recycled aggregate from construction and demolition waste (CDW) as replacement of fine and coarse natural aggregate has increased in recent years in order to reduce the high consumption of natural resources by the civil construction sector. In this work, an experimental investigation was carried out to investigate the influence of steel fiber reinforcement on the stress–strain behavior of concrete made with CDW aggregates. In addition, the flexural strength and splitting tensile strength of the mixtures were also determined. Natural coarse and fine aggregates were replaced by recycled coarse aggregate (RCA) and recycled fine aggregate (RFA) at two levels, 0% and 25%, by volume. Hooked end steel fibers with 35 mm of length and aspect ratio of 65 were used as reinforcement in a volume fraction of 0.75%. The research results show that the addition of steel fiber and recycled aggregate increased the mechanical strength and modified the fracture process relative to that of the reference concrete. The stress–strain behavior of recycled aggregate concrete was affected by the recycled aggregate and presented a more brittle behavior than the reference one. With the addition of steel fiber the toughness, measured by the slope of the descending branch of the stress–strain curve, of the recycled concretes was increased and their behavior under compression becomes similar to that of the fiber-reinforced natural aggregate concrete.     

Thermo-electro-mechanical response of 1–3–2 piezoelectric composites: effect of fiber orientations

A micromechanics-based analytical model is developed to evaluate the performance of 1–3–2 piezoelectric composite where both matrix and fiber materials are piezoelectrically active. A parametric study is conducted to investigate the effects of variations in the poling characteristics of the fiber phase on the overall thermo-electro-mechanical behavior of a 1–3–2 piezocomposite. The performance of the 1–3–2 composite as a transducer for underwater and biomedical imaging applications is analyzed. The proposed model is capable of predicting the effective properties of the composite subjected to thermo-electro-mechanical loading conditions. The predicted variations in the effective elastic, piezoelectric and dielectric material constants with fiber volume fraction are nonlinear in nature. It is observed that the influence of thermal effects on effective properties of the composite also induces polarization in the composite. The analytical results show that an appropriate selection of the poling characteristics of the individual fiber and matrix phases could lead to the development of a piezocomposite with significant effective properties.     

Influence of the fiber volume fraction and the fiber Weibull modul on the behavior of 2D woven SiC/SiC — a finite element simulation

Summary The behavior of two-dimensional woven SiC/SiC ceramic matrix composite (CMC) is studied by numerical simulations based on the finite element method (FEM). Starting point of the investigations is a micromechanical model regarding a three-dimensional unit cell, which takes damage and fracture of the single components—fiber bundles and inter yarn matrix—into account. The scattering of the strength values which is characteristic for ceramic material is involved using Weibull distribution. In a first step the unit cell regarded within the simulations is cooled down to consider the residual thermal stresses resulting from the fabrication process. In a second step the unit cell is subjected to tensile loading and its behavior—especially the influence of the scattering of the strength values—is studied. To be able to estimate the influence of important parameters on the behavior of the composite a macrostructure is built up using the results obtained for a large number of unit cell. Thus an averaging effect is reached and the behavior obtained for the macrostructure should be characteristic for the composite. Doing so, the influence of the fiber volume fractionv _f and the fiber Weibull modulM _f on the composite behavior can be studied.Dedicated to Prof. Dr.-Ing. Dr.-Ing. E. h. mult. Oskar Mahrenholtz on the occasion of his 70th birthday     

Seismic behavior of hybrid fiber reinforced cementitious composite beam–column joints

This paper presents the experimental results of six exterior beam–column joints with different concrete composites under cyclic loading. Engineered cementitious composite with polypropylene fiber and hybrid cementitious composites (HCC) using three different types of fiber namely hooked end steel fiber; brass coated steel fiber and polypropylene fiber are explored in this study. The hysteresis behavior, ductility response, energy dissipation with damping characteristics, crack patterns and damage index of all tested specimens are analyzed and compared with the cyclic response of conventional specimens. The test results indicate that HCC increases load carrying capacity and enhances energy dissipation with increased stiffness retention over conventional specimens. At higher rotation, joint specimens with HCC manifest better damage tolerance capacity over conventional specimens. This investigation implies that the use of HCC in the joint region may be an alternative solution to significantly increase the shear capacity, damage tolerance capacity and member ductility.     

Thermal postbuckling analysis of fiber–metal laminated plates including interfacial damage

Considering the effects of interfacial damage, geometric nonlinearity and transverse shear deformation, thermal postbuckling of fiber–metal laminated plates including interfacial damage is analyzed in detail. Firstly, the Heaviside step function and higher order shear deformation functions are introduced into displacement field so that the damage degree can be characterized. Then, the shape functions can be determined by using the stress continuity conditions between interfaces and the stress boundaries on surfaces. By using the generalized variational principle, the thermal postbuckling equilibrium equations of fiber–metal laminated plates including interfacial damage are established. Finally, the thermal postbuckling problem is solved by adopting finite difference method and iteration method. In numerical examples, the effects of interfacial damage, width-to-thickness ratio and thermal load on the thermal postbuckling of fiber–metal laminated plates including interfacial damage are investigated.     

Pickett effect and creep in flexure of steel‐fiber reinforced concrete

Abstract

Creep and shrinkage are of great concern in the design of steel fiber reinforced concrete structures. This is especially true for a prestressed flex‐ural member with thin section. The test results of creep of steel‐fiber rein‐foced concrete in flexure are presented. The concrete beams made with various fiber volume contents were tested in flexure under drying or standard moist conditions. The Pickett effect in steel‐fiber reinforced concrete was investigated. This research shows that fibers can effectively restrain the bending creep of concrete. The Pickett effect can be reduced with the addition of fibers to plain concrete beam subjected to fiexural loading.