静电纺丝制备超细纤维复合材料

主要探讨了固化距离、纺丝电压对聚乙烯醇和淀粉、聚乙烯醇和壳聚糖共混液静电纺丝的影响,并尝试了多喷头静电纺丝制备超细长纤维复合材料。运用扫描电镜、红外光谱和差示扫描量热仪等对制得的超细复合材料的纤维形态、结构和力学性能进行研究,制得了纤维形貌与力学性能优异的、结构均匀的超细长纤维复合毡;多喷头电纺时,溶剂挥发影响着复合毡形态与性能。经过乙醇浸泡处理后,纯聚乙烯醇纳米纤维毡的结晶度和力学性能明显提高。     

Highly aligned flax/polypropylene nonwoven preforms for thermoplastic composites

A major challenge for natural fibre composites is to achieve high mechanical performance at a competitive price. Composites constructed from unidirectional yarns and woven fabrics are known to perform significantly better than composites made from random nonwoven mats, but unidirectional yarns and fabrics are much more expensive to manufacture than random nonwoven mats. Here, we report on highly aligned natural fibre nonwoven mats that can be used as a replacement for unidirectional woven fabrics. A drawing operation is added to the conventional nonwoven process to improve fibre alignment in the nonwoven preforms and the final composites. The modified nonwoven manufacturing process is much simpler and cheaper than the unidirectional woven fabric process because of the elimination of expensive spinning and weaving operations. The composites fabricated from the highly aligned nonwoven mats showed similar mechanical strength as the composites made from unidirectional woven fabrics.     

Development of slate fiber reinforced high density polyethylene composites for injection molding

During the last decade the use of fiber reinforced composite materials has consolidated as an attracting alternative to traditional materials due to an excellent balance between mechanical properties and lightweight. One drawback related to the use of inorganic fibers such as those derived from siliceous materials is the relative low compatibility with conventional organic polymer matrices. Surface treatments with coupling agents and the use of copolymers allow increasing fiber–matrix interactions which has a positive effect on overall properties of composites. In this research work we report the use of slate fiber treated with different coupling agents as reinforcement for high density polyethylene from sugarcane. A silane (propyltrimethoxy silane; PTMS) and a graft copolymer (polyethylene-graft-maleic anhydride; PE-g-MA) were used to improve fiber–matrix interactions on HDPE-slate fiber. The effect of the different compatibilizing systems and slate fiber content were evaluated by scanning electron microscopy (SEM), dynamic thermomechanical analysis (DTMA) as well as mechanical properties (tensile, flexural and impact). The results show that the use of silane coupling agents leads to higher fiber–matrix interactions which has a positive effect on overall mechanical properties. Interesting results are obtained for composites containing 30 wt.% slate fiber previously treated with propyltrimethoxy silane (PTMS) with an increase in tensile and flexural strength of about 16% and 18% respectively.     

Studies on lignocellulosic fibers of Brazil. Part II: Morphology and properties of Brazilian coconut fibers

Stress–strain curves for different diameters, tensile properties and thermal behaviour of Brazilian coir fibers are presented. The tensile strength (TS) and Young’s modulus (YM) of these coir fibers were found to decrease, while the percentage (%) strain at break remained constant as fiber diameter increased. With fibers (mean diameter of 0.225 mm), a decrease in TS and % strain at break but an increase in YM with increasing test length of the fiber, and a considerable increase in TS, constant YM and % strain at break with increasing strain rate were observed. The results are discussed in terms of X-ray diffraction and microscopic observations. Thermal behaviour of the fibers revealed degradation of different constituents in an N₂ or O₂ atmosphere. Thermo-mechanical analysis of the fibers revealed increased modulus and decreased tan δ values.     

Orientation-dependent compression/tension asymmetry of short glass fiber reinforced polypropylene: Deformation,damage and failure

Short glass fiber reinforced polypropylene (sgf-PP) is increasingly employed in structural components which are subjected to a variety of loading conditions including tensile, compressive and bending loading modes. Since typical industrial components exhibit a wide range of fiber orientation distributions, their mechanical response to these loading conditions is also highly anisotropic. In this paper, the compression/tension asymmetry in the stress–strain behavior of sgf-PP is investigated from a macroscopic engineering and a micro-mechanisms of deformation and failure point of view for specimens with varying, precisely defined fiber orientations. Furthermore, we performed volume strain measurements and two-cyclic tests. We used the results to deduce the onset of damage due to cavitational mechanisms under tension and compared this to the onset of deviation of the tensile from the compressive stress–strain behavior. The results showed a good correlation for specimens with high fiber orientation, whereas for specimens with low fiber orientation results deviate due to the high deviatoric matrix volume strain contribution.     

Influence of matrix flowability,fiber mixing procedure,and curing conditions on the mechanical performance of HTPP-ECC

In this study, the influences of matrix flowability, fiber mixing procedure, and curing conditions on the mechanical properties of Engineered Cementitious Composites (ECC) made with High Tenacity Polypropylene (HTPP) fibers are investigated. While the HTPP-ECC examined in this study possesses moderate compressive strengths (30–70 MPa), their tensile ductility (1.91–3.91%) is similar to that of ECC with Polyvinyl Alcohol (PVA) fibers. For the purpose of controlling matrix flowability, different dosages of HRWR admixture were introduced to a matrix with fly ash/cement weight ratio of 2.8 and water/cementitious material weight ratio of 0.23. Dogbone-shaped and 50 mm cube specimens were used to investigate uniaxial tensile and compressive properties of HTPP-ECC, respectively. Test results showed that control of flowability in a certain range is required to achieve robust tensile ductility. A further improvement in tensile ductility and mechanical properties of HTPP-ECC was achieved through water-curing instead of air curing typically used for PVA-ECC. The basic mechanisms that enhance tensile ductility of HTPP-ECC through flowability control, mixing procedure modification, and water-curing are discussed from the view point of micromechanics underlying ECC design, with supporting evidence from fiber bridging stress–crack width (σ–δ) relations.     

Fibre orientation of novel dynamically sheet formed discontinuous natural fibre PLA composites

This paper describes work carried out to fabricate and to assess the fibre orientation in PLA reinforced by aligned discontinuous harakeke and hemp fibre mats produced using a dynamic sheet former (DSF). These mats were combined with PLA sheets to make composites with fibre contents of 5–40 wt% using a hot press. It was found that the fibre orientation factors (K_θ) for both harakeke and hemp fibre composites were higher than those values seen in the literature for composites prepared using injection moulding and hot pressed using randomly oriented fibre mats, but slightly lower than the highest values obtained with aligned fibre nonwoven preform composites utilising more processing stages. The highest K_θ values for harakeke and hemp fibres in this work were found to be 0.58 and 0.44 respectively. K_θ decreased, reflecting increased fibre misalignment as fibre content increased, believed to be due to fibre agglomeration and the higher pressure required during processing.     

Effects of fiber orientation and anisotropy on tensile strength and elastic modulus of short fiber reinforced polymer composites

An experimental study was conducted to investigate anisotropy effects on tensile properties of two short glass fiber reinforced thermoplastics. Tensile tests were performed in various mold flow directions and with two thicknesses. A shell–core morphology resulting from orientation distribution of fibers influenced the degree of anisotropy. Tensile strength and elastic modulus nonlinearly decreased with specimen angle and Tsai–Hill criterion was found to correlate variation of these properties with the fiber orientation. Variation of tensile toughness with fiber orientation and strain rate was evaluated and mechanisms of failure were identified based on fracture surface microscopic analysis and crack propagation paths. Fiber length, diameter, and orientation distribution mathematical models were also used along with analytical approaches to predict tensile strength and elastic modulus form tensile properties of constituent materials. Laminate analogy and modified Tsai–Hill criteria provided satisfactory predictions of elastic modulus and tensile strength, respectively.     

Fracture energy of UHP-FRC under direct tensile loading applied at low strain rates

This research investigates the fracture energy of ultra-high performance fiber reinforced concretes (UHP-FRC) under direct tensile loading applied at relatively low strain rates. Nine UHP-FRC series incorporating three types of steel fibers (straight, end-hooked, and twisted fibers), each in three different fiber volume fractions, are tested under uniaxial tensile loading at four different strain rates, ranging from 0.0001 s⁻¹ to 0.1 s⁻¹. Particular attention is given to clearly distinguish between the dissipated energy during the strain hardening and softening portions of the loading regime. The test results show that: 1) the fracture energy is mainly influenced by a parameter, termed fiber factor, which is a function of the fiber volume fraction and slenderness, and 2) all three types of UHP-FRCs exhibit increases in fracture energy with increasing strain rates. The observed strain rate sensitivity of the fracture energy suggests it is likely associated with the strain sensitive micro-cracking that occurs during fiber pull-out.     

Mechanical properties of basalt fibers and their adhesion to polypropylene matrices

This work is aimed to study the mechanical properties of basalt fibers, and their adhesion to polypropylene (PP) matrices. Single filament tensile tests were used to calculate the strength of different types of fibers, characterized by different providers and surface treatment. Single fiber fragmentation tests (SFFT) were used to calculate the critical length of the fibers, in a homopolymer PP matrix and in a maleic anhydride modified PP matrix. It was shown that the tensile strength of the fibers is not significantly influenced by the origin or the surface treatment. Only fibers without any sizing show very reduced mechanical properties. On the other hand, the tensile strength was shown to be severely dependent on the filament length. Weibull theory was used in order to calculate the fitting parameters σ₀ and β, which were necessary in order to extrapolate the tensile strength to the critical length determined by SFFT. This allowed calculating the adhesion properties of the basalt fibers. It was shown that fiber–matrix adhesion is dependent on both the presence of sizing on the fiber surface, as well as on the modification of the matrix.     

载药电纺丝素蛋白纤维毡的结晶含量对其药物释放的影响

以水溶液体系的再生丝素蛋白为原料,利用静电纺丝技术制备了载罗丹明B的丝素蛋白纤维毡。利用甲醇水溶液对其进行后处理,考察了不同处理时间下丝素蛋白的结晶含量对药物释放的影响。通过水溶性质量损失率、接触角、扫描电镜、傅里叶红外光谱、X射线衍射光谱及紫外光谱对处理前后的丝素蛋白纤维毡的表面形貌和结构以及药物释放行为进行了表征。结果表明,随着甲醇水溶液处理时间的延长,电纺丝纤维的质量损失率减小,亲水性减弱,丝纤维中的Silk II结晶(由β-折叠结构组成)含量增加,药物释放速率也增加。即通过控制处理时间,可以控制丝纤维中Silk II结晶的含量,从而控制药物的释放速率。     

Tyranno ZMI fiber/TiSi2–Si matrix composites for high-temperature structural applications

A Tyranno ZMI fiber/TiSi₂–Si matrix composite was fabricated via melt infiltration (MI) of a Si–16at%Ti alloy at 1375 °C under vacuum. The Si–Ti alloy was used as an infiltrant to conduct MI processing below 1400 °C and inhibit the strength degradation of the amorphous SiC fibers. The alloy matrix formed was dense and comprised primarily of TiSi₂–Si eutectic structures. The TiSi₂–Si matrix composite melt-infiltrated at 1375 °C showed a pseudo-plastic tensile stress–strain behavior followed by final fracture at ∼290 MPa and ∼0.9% strain. When the MI temperature was increased to 1450 °C, however, substantial reduction in the stiffness and ultimate strength occurred under tensile loading. Microstructural observations revealed that these degradations were attributed to the damages that occurred on the reinforcing fibers and pyrolytic carbon interfaces during the MI process. The present experimental results clearly demonstrated the effectiveness of the low-temperature MI process in strengthening Tyranno ZMI fiber composites and reducing the processing cost.     

Shape memory and thermo-mechanical properties of shape memory polymer/carbon fiber composites

In this study, chopped carbon fiber reinforced trans-1, 4-polyisoprene (TPI) was developed via a proposed new manufacturing process with the aim of improving weak mechanical properties of bulk TPI bulk. Specimens of the developed shape memory polymer (SMP) composites were fabricated with carbon fiber weight fraction of 5%, 7%, 9%, 11% and 13%, respectively. Measured are the effects of chopped carbon fiber and temperature on: (a) shape recovery ratio and rate; (b) stress–strain relationship; (c) maximum tensile stress, strain and Young’s modulus; and (d) maximum stress and residual strain under a constant strain cyclic loading. In addition, SEM micrographs were also presented to illustrate the fracture surface. The present experimental results show that the SMP with 7% carbon fiber weight fraction appears to perform best in all the tests. This indicates that the 7% carbon fiber weight fraction could be the optimum value for the SMP developed using the proposed manufacturing process.     

Active ingredient-containing chitosan/polycaprolactone nonwoven mats: Characterizations and their functional assays

This study demonstrates a facile method developed to generate a chitosan/polycaprolactone (CS/PCL) nonwoven mat. All nonwoven mats are composed of microfibers with an average diameter of 2.51 ± 0.69 μm. The X-ray photoelectron spectroscopy data indicate that positively charged nitrogen was generated on the surface of the mats after undergoing CS coating. By using a non-contacting electrostatic voltmeter, we determined that the nonwoven mats exhibited a positive potential and the charge density of the CS/PCL nonwoven mat was in proportion to the thickness of the CS overlayer. Moreover, platelet aggregation and anti-bacterial ability were enhanced by the CS/PCL nonwoven mat as compared to that of PCL nonwoven mat alone. The enhancements of the CS/PCL nonwoven mat on platelet aggregation are further promoted by incorporating a 1 mM calcium ion in its CS overlayer. We also find that the addition of tea tree oil in the CS overlayer significantly inhibited LPS-induced nitrite formation in Raw 264.7 macrophages. In conclusion, our CS/PCL nonwoven mat possesses pharmacological effects including an increase of platelet aggregation, anti-bacterial, anti-adhesive, and anti-inflammatory activities. The performance of this CS/PCL nonwoven mat can be further promoted by incorporating active compounds to exert therapeutic effects in wound healing.     

Tensile behavior of glass/epoxy laminates at varying strain rates and temperatures

In the present work tensile tests at different strain rates and temperatures were performed in glass fiber reinforced polymer (GFRP). It is observed that such kind of composite presents an elasto–viscoplastic behavior – the rate dependency only occurs for loading levels above a given elasticity limit. Strain rate strongly affects the ultimate tensile strength (σ_u) and the modulus of elasticity is almost insensitive to it while temperature only influences the modulus. Analytical expressions to predict the modulus of elasticity and (σ_u) as a function of the temperature and strain rate are proposed and compared with experimental data showing a reasonable agreement.     

浸渍过程中树脂流体在纤维集合体内的流动行为

建立了浸渍过程中树脂基体在纤维集合体内流动的统计力学模型。将树脂流体的流动过程视为纤维/树脂系统降低能量达到平衡的过程,从微观角度研究树脂流体在纤维集合体内的流动行为。在建模中,不仅考虑了代表系统内能的Hamilton函数和界面张力及驱动流体的压力对系统所作的功,还考虑了流动过程中纤维集合体对流体的摩擦阻力所作的功,完整地反映了在流体的流动过程中系统能量的变化。模拟了水在聚酯纤维非织造织物内及不饱和聚酯树脂在玻纤机织布内的平面径向流动,并设计了相应的实验,以检验模型对不同流动性能流体和不同结构纤维集合体的适应情况。实验与模拟结果良好的一致性表明,所建立的模拟能正确地反映流体在纤维集合体内的流动特征。     

Surface structures of PAN-based carbon fibers and their influences on the interface formation and mechanical properties of carbon-carbon composites

Defects and microvoids in the surface region not only influenced the tensile strength and strain of carbon fibers but also affected the interface formation with pyrocarbon. The interface formation in carbon-carbon composites was closely correlated to rearrangement of carbon atoms and the evolution of surface structure of carbon fiber. Half-open elliptic microvoids or edge planes at the fiber surface were beneficial to the mechanical interlocking as well as chemical bonding with pyrocarbon, contributing to a compatible interface with high interlaminar shear strength of the composites. The closed microvoids in the surface region of carbon fiber would hardly open up to bond with pyrocarbon, which brought negative effects to the mechanical properties of composites. Carbon fiber without obvious microvoids or surface defects tended to have better tensile strain but form weak interface with pyrocarbon, leading to a better pseudo-ductility and ability to absorb more fracture energy under load.     

Pore size characteristics of nonwoven structures under uniaxial tensile loading

Nonwovens are highly porous structures consisting pores of complex shapes and sizes which are responsible for desired functional characteristics. In general, a nonwoven is often subjected to uniaxial tensile loading in various applications and it is of paramount importance to account for changes in structural characteristics including pore sizes during the loading conditions. In this research work, the pore size of thermally bonded nonwoven structures under uniaxial tensile loading at various levels of strains has been investigated. A theoretical model has been proposed that accounted for fibre reorientation and changes in the fibre volume fraction during the application of tensile strain. A comparison has been made between theoretical and experimental pore size distributions of thermally bonded nonwoven structures at defined levels of strains. Moreover, an attempt has been made to rationalise some of the contradictory literature results of pore size distributions of nonwoven structures under uniaxial tensile loading.     

Modeling of mechanical damage detection in CFRPs via electrical resistance

Carbon fiber reinforced polymer composites (CFRPs) are inherently multifunctional materials that, in addition to their primary function as a structural material, allow for the sensing and monitoring of in situ damage nucleation and evolution by the measurement of the material electrical resistance. Here an analytic model is developed for the transverse (perpendicular to the fibers) electrical resistance of pristine and damaged unidirectional composites, complementing earlier work on the longitudinal resistance. The ratio of transverse to longitudinal resistance for undamaged materials provides a direct measure of the internal density of fiber–fiber electrical contacts, a key material parameter in linking to the response of damaged materials. Under uniaxial loading with evolving fiber breakage, the normalized transverse resistance versus strain is predicted to have exactly the same form as that for the longitudinal resistance. Numerical studies show this agreement for uniform fiber–fiber contact distributions but, for random contact distributions, the longitudinal resistance is larger than predicted while the transverse resistance is smaller; these differences are shown to arise as a result of the statistically-preferential breaking of longer fiber segments. Analysis of multiple numerical simulations shows that variations in the electrical resistance are not directly correlated with variations in the stress–strain response. Thus, statistical methods are required to relate resistance to strain or damage. The Weibull modulus of the resistance change increases with increasing applied strain, with values exceeding 10 and 20 for the transverse and longitudinal resistance, respectively, demonstrating increasing reliability at higher damage levels and good correlation of average resistance change to applied strain. The present study shows that both longitudinal and transverse resistance changes are sensitive to damage in a predictable manner and can be used together to improve the reliability of damage assessment during loading of CFRPs.     

Characterization of the in-situ non-linear shear response of laminated fiber-reinforced composites

A simple procedure to determine the non-linear in-plane lamina shear response of laminated composites is presented. Using the ±45° symmetric laminate tensile test results, in conjunction with computational micromechanics, a method was developed and validated to characterize the lamina shear response and the in-situ matrix shear response. Load, and axial and transverse strains measured in the tests were used to calculate the non-linear shear stress–shear strain response of the composite. From this result, the in-situ matrix equivalent stress–strain response was obtained, with some simplifying assumptions, and subsequently used in a micromechanics-based representative finite element (FE) model of the ±45° symmetric laminate tensile test to determine the accuracy of the non-linear response of the in-situ matrix. Results from the FE model of a representative cell (RC) that depicts fiber diameter, fiber volume fraction (V_f) and angled fiber packing of the ±45° symmetric laminate were found to match the tests result well. Thus, the procedure to extract the non-linear lamina shear response and the non-linear in-situ matrix response from the ±45° symmetric laminate tensile test was validated.