Hybrid Nanoparticle/Microfiber‐Filled Injection‐Molded Composites

The structure and some properties of hybrid injection‐molded polypropylene (PP) composites, containing carbon fibers (CF) and nanoparticles, were studied. The effects of nanoparticle type, size, and content were investigated. All studied hybrid composites exhibited unexpected fiber disorientation (oriented even transverse to the melt‐flow direction), disappearance of the typical skin/core structure in fiber‐containing moldings and, surprisingly, very low fiber length. Different types of nanoparticles resulted in different levels of fiber breakage: the shortest fibers were observed in composites containing nano‐TiO₂ and longer ones in composites containing nano‐Al₂O₃ and carbon black (CB). Moreover, smaller nanoparticles resulted in higher levels of fiber length attrition. Electrical conductivity of the composites was found to drop drastically upon incorporation of several volume percents of insulating nanoparticles, although the content of CF when by itself was above the percolation threshold. Tensile modulus values, in the melt‐flow direction, of hybrid composites were also inferior to those of the only–CF‐reinforced composites, while rheological properties were dominated by CF, being practically not affected by the presence of nanoparticles. Based on the present and previous studies, the effect of nanoparticles on fiber orientation and their length attrition in hybrid injection moldings could be generalized to systems containing combinations of brittle fibers and hard nanoparticles.     

Fabrication and Properties of Recycled Cellulose Fibre-Reinforced Epoxy Composites

Epoxy matrix composites reinforced with recycled cellulose fibre (RCF) were fabricated and characterized with respect to their flexural and impact properties. Reinforcement of the epoxy by RCF resulted in a significant increase in the strain at failure, fracture toughness and impact toughness but only a moderate increase in flexural strength and flexural modulus. The effect of accelerated exposure to seawater on the flexural and impact properties was also investigated. The salient toughening mechanisms and crack-tip failure processes were identified and discussed in light of observed microstructures, in particular the orientation of RCF sheets to the applied load.     

Mode I and Mode II interlaminar fracture toughness of CFRP/magnesium alloys hybrid laminates

The fiber metal laminates (FML), consisting of carbon fiber reinforced polymer prepregs and magnesium alloys sheets, were introduced, and the Mode I (peel) and Mode II (shear) interlaminar fracture toughness of the FMLs were investigated. The results show that the Mode I interlaminar toughness (0.23 kJ/m²) of the FMLs is much lower than the Mode II interlaminar toughness (5.81 kJ/m²), due to the fact that the effects of mechanical interlock to hinder crack propagates is smaller under Mode I loading conditions than under Mode II. The FMLs mainly show adhesive failure and interfacial failure under Mode I loading conditions, while for Mode II loading, it exhibits a degree of epoxy cohesive failure except the adhesive failure and interfacial failure.     

Viscoelasticity of a polymer matrix and fracture of heat-resistant fiber composites

This paper reports on the results of investigations into the general regularities of deformation and fracture of fiber composite materials based on new heat-resistant polymer binders. Fiber composites based on these binders can find wide application in various fields of engineering. It is established that an increase in the loss modulus of the polymer matrix decreases the probability of formation of a brittle crack in the matrix at the fiber break and increases the time interval between breakages of adjacent fibers. This leads to retardation of the correlated breakage of the fibers in fiber composite materials under loading, i.e., to an increase in their strength and fracture toughness. The inference is made that the matrix of high-strength heat-resistant fiber composites with a high fracture toughness should possess not only a high elasticity (this has long been known) but also good dissipative properties over the entire temperature range of operation.     

INVESTIGATION OF STATIC AND DYNAMIC FRACTURE TOUGHNESS OF SHORT CERAMIC FIBER REINFORCED POLYPROPYLENE COMPOSITES

Short ceramic fiber reinforced polypropylene composites have been investigated to determine their static and dynamic fracture toughness for different reinforcing fiber contents. The composites were reinforced with fibers produced by a carding technique combined with needle-punching. Static fracture toughness (K _c) was measured on single-edge notched tensile (SEN-T) specimens, while dynamic fracture toughness (K _d) was tested by impact strength Charpy specimens. Specimens in both cases were cut transverse (T) and in longitudinal (L) directions. Test results show that dynamic fracture toughness is larger than the static one. During loading of SEN-T specimens the burst-type acoustic emission (AE) signals were monitored. From AE signals it can be concluded that the main damage form is the pull-out in the T specimens, and debonding in L ones. These results were supported by scanning electron microscopy micrographs taken from fracture surfaces.     

Evaluation on the interfacial fracture toughness of fiber-reinforced titanium matrix composites by push out test

The models for single-fiber push out test are developed to evaluate the fracture toughness G_IIc of the fiber/matrix interface in titanium alloys reinforced by SiC monofilaments. The models are based on fracture mechanics, taking into consideration of the free-end surface and Poisson expansion. Theoretical solutions to G_IIc are obtained, and the effects of several key factors such as the initial crack length, crack length, friction coefficient, and interfacial frictional shear stress are discussed. The predictions by the models are compared with the previous finite element analysis results for the interfacial toughness of the composites including Sigma1240/Ti-6-4, SCS/Ti-6-4, SCS/Timetal 834, and SCS/Timetal 21s. The results show that the models can reliably predict the interfacial toughness of the titanium matrix composites, in which interfacial debonding usually occurs at the bottom of the samples.     

Mechanical and Tribological Properties of Self-Reinforced Polyphenylene Sulfide Composites

25%, 50%, and 75% polyphenylene sulfide (PPS) long fiber reinforced PPS resin were prepared by a hot pressing method. Neat resin PPS and PPS fiber samples were also prepared to compare with the self-reinforced PPS composites. The reinforcing fibers were preheat treated at 240°C for 24 h. The tribological properties of the self-reinforced PPS composites against an AISI 1045 steel ring were determined by a block on ring type friction tester. Differential scanning calorimetry (DSC) results indicated that a higher degree of crystallinity was retained in the self-reinforced PPS composites than in neat PPS resin after hot pressing. Therefore, the addition of PPS fiber improved both the mechanical and tribological properties of PPS resin significantly. Dynamic mechanical analysis (DMA) demonstrated that the PPS fibers increased the glass transition temperature (Tg) of the PPS resin. SEM images of the fracture surfaces indicated that the toughness of the samples increased with increasing PPS fiber content. Additionally, PPS fibers improved the tribological properties of PPS resin by significantly reducing the friction coefficient and wear rate.     

Morphology and Mechanical Performance of Polysulfone Modified by a Glass Fiber Reinforced Liquid‐Crystalline Polymer

Abstract

Hybrid composites based on polysulfone of bisphenol A (PSF) and glass fiber (GF) reinforced copolyester liquid‐crystalline polymer (gLCP) were obtained by injection molding. The viscosity of the 10% and 20% gLCP composites was lower than that of pure PSF. The Young's modulus followed the direct rule of mixtures. This was due to the counteracting effects of the decreasing orientation of the liquid‐crystalline polymer (LCP) in the skin at increasing gLCP contents on the one hand; and either the increasing skin thickness in the PSF‐rich composites or the lower orientation of the core in the PSF‐poor composites on the other. The composites with 10–20% gLCP showed the best mechanical performance, because, besides their enhanced processability, they showed a tensile strength similar to that of PSF and much larger notched impact strength.     

Effects of fibre surface treatment on fracture-mechanical properties of sisal-fibre composites

Sisal fibre reinforced composites, one class of a broad range of eco-composite materials, were studied in connection with the effects of fibre surface treatment on their fracture-mechanical properties. Previous investigations on sisal fibre and its composites have been fully reviewed [1], which provided an impetus for this research. Two fibre surface treatment methods, chemical coupling based on silane and oxidization based on permanganate and dicumyl peroxide, together with untreated sisal fiber composites were used to set up different levels of interface bonding strength. The interface effects on the mechanical properties and fracture toughness of sisal fibre reinforced vinyl-ester composites were completely assessed based on the test results obtained and theoretical analyses. Many aspects of studies reported in this paper are original, such as single fiber pull-out tests and toughness evaluation of sisal composites aided by scanning electron microscopy. The results showed that fibre surface treatment could improve interfacial bonding properties between sisal fibre and vinylester resin. These in turn influenced the fracture-mechanical characteristics of this class of ecocomposites.     

Mechanical and tribological enhancement of polyoxymethylene-based composites with long basalt fiber through melt pultrusion

Abstract

The polyoxymethylene (POM)/basalt fiber composites were prepared by use of long fiber-reinforced thermoplastic technology through melt pultrusion. The mechanical and tribological properties, morphology, and thermal stability of the resulting composites were investigated. The composites exhibit significant improvements in tensile, flexural, and notched impact strength. These mechanical strength and toughness are dependent on the fiber content over the full range of the study. The residual fiber length and distribution in the injection-molded specimens were characterized. The prominent reinforcement effect of basalt fiber on POM is derived from the supercritical fiber length, which is much longer than that of the short fiber-reinforced ones and thus makes the composites take full advantage of the strength of the reinforcing fibers. The Kelly–Tyson model was used to predict the ultimate tensile strength of POM composites using the measured values of residual fiber length in the matrix, but the deviations were observed at the high contents of basalt fiber. The morphologic investigation indicates that the fiber pullout and fiber breakage both contribute energy dissipation to the tensile fracture of the composites. The tribological characterization indicates that the friction coefficients and specific wear rates of POM composites also decrease remarkably. Such an improvement of tribological performance is due to the presence of the high wear-resistant basalt fibers on the top of the worn surface bearing the dynamic loadings under sliding. Moreover, the dynamic mechanical analysis reveals that the storage moduli of the composites increase with increasing the fiber content, whereas the loss factors present an opposite trend.     

The influence of interface reaction zone on interfacial fracture toughness of SiC fiber reinforced titanium matrix composites

A fiber-reaction zone-matrix three-phase model is developed to evaluate the interfacial fracture toughness of titanium alloys reinforced by SiC monofilaments. Based on fracture mechanics, theoretical equations of G_IIc are presented, and the effects of several key factors such as crack length and the interface reaction zone thickness on the critical applied stress necessary for crack growth and interfacial fracture toughness are discussed. Finally, the interfacial fracture toughness of typical composites including Sigma1240/Ti-6Al-4V, SCS-6/Ti-6Al-4V, SCS-6/Timetal 834, SCS-6/Timetal 21s, SCS-6/Ti-24Al-11Nb and SCS-6/Ti-15V-3Cr are predicted by the model. The results show that the model can reliably predict the interfacial fracture toughness of the titanium matrix composites.     

Fabrication of continuous fiber-reinforced thermoplastic composites with structural gradient from sheath-core type bicomponent fibers

Sheath-core type bicomponent fibers of polypropylene (PP) as a sheath component and thermotropic liquid crystalline polymer (TLCP) as a core component were prepared by the highspeed melt spinning process. Continuous fiber reinforced thermoplastic composites, in which TLCP acts as a reinforcing fiber and PP as a matrix polymer, were fabricated by the compression molding of these fibers. In the melt spinning, the attainable highest take-up velocity of TLCP was improved by co-processing with PP. Tensile modulus and strength of the TLCP component in the PP/TLCP bicomponent fibers increased with an increase in the take-up velocity. Comparison of wide-angle X-ray diffraction patterns of starting bicomponent fibers and fabricated composites indicated that the orientation relaxation of TLCP did not occur in the compression molding process. Accordingly, the tensile modulus and strength of the PP/TLCP composites were similar to those of the bicomponent fibers. Continuous fiber reinforced thermoplastic composites with various types of fiber content distributions were fabricated from the bicomponent fibers in which sheath-core composition was changed gradually in the spinning process. In the three-point bending test, the composites with two different types of symmetric structural gradients, one with higher TLCP fiber content near the surfaces than in the center and the other with higher TLCP content in the center than near the surfaces, exhibited different flexural moduli even though the overall TLCP contents were comparable. In the three-point bending test of a composite with asymmetric structural gradient, the yielding behavior and maximum flexural load varied depending on the direction of load application although the initial flexural moduli were similar.     

Effects of surface treatment and weave structure on interlaminar fracture behaviour of plain glass woven fabric composites: Part II. Report of the 2nd round robin test results

This paper reports the second part of the results from the round robin test program proposed by the Society of Interfacial Materials Science (SIMS) to characterise the interlaminar fracture behaviour of E-glass woven fabric reinforced vinylester composites. Special emphasis was placed on the study of loading direction (i.e. weft and warp directions) effect on interlaminar shear strength and fracture toughness. Ten laboratories worldwide participated in this test (Table 1). Each laboratory was supplied with composite laminates and conducted the tests according to its own procedure. The results showed that although there were large variations in absolute magnitude between laboratories, a general trend was established with higher interlaminar fracture resistance in the weft direction than in the warp direction for a given silane agent. The larger number of strands running in the warp direction with rougher, more undulating areas perpendicular to the direction of crack propagation was mainly responsible for this result. The results also confirmed the previous finding that the mode I interlaminar fracture toughness increased with increasing silane agent concentration.     

Predictions of Young's Modulus of Polymer Composites Reinforced with Short Natural Fibers

Polymers reinforced with natural fibers are beneficial to prepare biodegradable composite materials. A new expression for the Young's modulus of short, natural fiber (SNF) reinforced polymer composites was derived based on a micro-mechanical model. The Young's moduli of poly(lactic acid) reinforced with reed fibers and low-density polyethylene (LDPE) reinforced with sisal fibers, from literature data, were estimated in the fiber weight fraction range from 0 to 50% using the equation and both the compounding rule and the Halpin–Tsai equation, and the estimations were compared with the reported measured data. The results showed that the predictions of the Young's moduli by means of the new Young's modulus equation were close to the measured data from the low density polyethylene/sisal fiber composites, as well as the poly(lactic acid)/reed composites at high fiber concentration. Comparing with other Young's modulus equations, the new Young's modulus equation would be more convenient to use owing to the parameters in the equation being easily determined.     

光纤环形镜线形腔掺Er3+光纤激光器输出特性的数值分析

在分析光纤环形镜工作原理的基础上,给出了基于光纤环形镜的线形腔掺Er3+光纤激光器相位和幅度的振荡条件.通过求解速率方程,理论分析了其输出特性,获得了稳态条件下激光器输出功率、阈值泵浦功率和斜率效率的解析表达式.推导出激光器工作所需掺Er3+光纤最短长度,并在给定泵浦光功率时,在特定输出波长上获得最大输出功率所需最佳掺Er3+光纤长度的表达式,且通过实验进行了验证.     

Ultrasonic polar scans: numerical simulation on generally anisotropic media

Ultrasonic polar scans are based on the recording of the reflected or transmitted amplitude of sound, impinging a fiber reinforced composite from every possible angle of incidence. The mechanical anisotropy of such materials makes the reflection coefficient direction dependent, whence an ultrasonic polar scan forms a fingerprint of the investigated material. Such scans have already proved to be very valuable in the characterization of composites. Simulations have been performed for single layered and multi-layered systems, for pulsed and harmonic waves. Fiber reinforced composites are mostly orthotropic. The current report presents simulations not only on orthotropic materials but on materials of any kind of anisotropy. These extended numerical simulations are not only valuable in the characterization of highly sophisticated composites, but may also be used to characterize thin slices of crystals and even layered crystals.     

Lignocellulosic composites with grafted polystyrene interfaces

The effects of surface grafting of a polymer onto lignocellulosic fiber surface and processing methods on both the interfacial interactions and the resulting composite properties of the fiber-reinforced thermoplastic composites were investigated. Chemithermomechanical pulp (CTMP) wood fiber was used as a reinforcement, which has been chemically modified by radical polymer grafting of styrene onto the fiber surfaces. The chemically modified CTMP fiber was then compounded with polystyrene (PS). Two different processing methods, both compression and injection moldings, were performed to prepare the wood-fiber-reinforced composites. Experimental results showed that surface modification of wood fiber leads to an obvious increase in mechanical properties of the fiber-reinforced composites as compared to the untreated fiber composites. The enhancement of mechanical properties is much greater through injection molding compared with compression molding owing to occurrence of orientation, and better mixing and interaction between the fiber and the matrix by injection molding. An improvement in fiber wetting properties and adhesion by the matrix was observed through scanning electron microscopy for the surface grafted fiber reinforced composites. Untreated wood fiber exhibited a smooth surface without adhered polymer, indicating poor adhesion, while polymer attached to the surface was seen on treated cellulose fiber due to the higher fiber-matrix interactions.     

On the use of energy methods for interpretation of results of single-fiber fragmentation experiments

_We consider fragmentation experiments as a set of experimental results for fiber break density as a function of applied strain. This paper explores the potential for using fracture mechanics or energy methods in interpreting fragmentation experiments. We found that energy does not control fiber fracture; instead, fiber fracture releases much more energy than required to fracture the fiber. The excess released energy can lead to other damage mechanisms such as interfacial debonding. By assuming that all the excess released energy causes interfacial debonding and balancing energy using the energy release rate for debonding, we were able to determine interfacial toughness from fragmentation experiments. A reliable determination of interfacial toughness requires prior knowledge of interphase stress-transfer properties, fiber failure properties, actual damage mechanisms, and the coefficient of friction at the interface.     

Interfacial adhesion between semi-crystalline polymers (polypropylene and nylon-6): in situ compatibilized interface and fracture mechanism

The interfacial fracture toughness between semi-crystalline polymers (polyamide/polypropylene) were studied to understand the failure mechanisms at the interface, especially when the interface was reinforced by an in situ compatibilizer. Based on the observation of the interface using scanning electron microscopy and wide angle X-ray spectroscopy, it was revealed that crystalline structure of polypropylene was not affected by the in situ compatibilizer at the interface. The reinforcing mechanism could be qualitatively identified by investigating the evolution of fracture toughness as a function of annealing time and temperature. The adhesion strength increased with the annealing time. Depending on the annealing temperature, the fracture toughness passed a peak value and then reached a plateau after some bonding time. As long as the chain length of the compatibilizer is long enough to form entanglements with the molecules at both bulk sides, the fracture at the interface is decided by the balance between adhesion strength at the interface and cohesive strength in the weak modulus side; the failure locus follows the lower one. Thus, adhesive failure occurred first when the reaction at the interface did not occur long enough to provide high adhesive strength at the interface, but the cohesive failure occurred in the crack propagation side after the adhesive strength value became higher than the cohesive strength value.     

Influence of Carbon Nanotubes on the Crystallization and Directional Tensile Behavior of High-Density Polyethylene Prepared by Dynamic-Packing Injection Molding

The influence of multi-walled carbon nanotubes (MWCNTs) on the crystallization and directional tensile properties of high-density polyethylene (HDPE) was studied for samples prepared by dynamic-packing injection molding (DPIM). Oscillatory shear was imposed on the gradually cooled melt during the packing solidification stage of DPIM. For the oriented composites containing 1.8 wt% MWCNTs, the tensile fracture behavior showed typical brittle features along the flow direction (FD) and perpendicular direction (PD), which were almost the same as those that occurred in oriented pure HDPE. The elongation at break along both directions decreased due to the incorporation of MWNCTs in the oriented composites compared with the oriented pure HDPE. However, the tensile strength of the oriented HDPE/MWCNT composites was greatly improved along the FD due to the presence of carbon nanotubes; meanwhile, it was not weakened along the PD. In scanning electron microscopy observations, it was found that there were some oriented hybrid shish-kebab structures in a nanometre scale in the oriented HDPE/MWCNT composites, but not in its isotropic composites. This suggests that MWCNTs were involved in the shear-induced crystallization of HDPE. Differential scanning calorimetry measurements confirmed that the crystallinity of oriented HDPE composites with 1.8 wt% MWCNTs was higher than those of isotropic HDPE and isotropic composites, but was not obviously higher than that of oriented pure HDPE. These findings demonstrate that MWCNTs indeed affected the formation of crystalline structures, but did not greatly influence the crystallinity of HDPE under shear flow. The transition of crystalline morphology might be the reason for change in tensile behavior for the oriented HDPE/MWCNT composites compared with the oriented pure HDPE.