Multi-scale modeling of refractory woven fabric composites

Thermomechanical analysis of a refractory, woven fabric composite was conducted using a multi-scale analysis technique. The composite was made of carbons and ceramic materials. The fibers were made of carbons and the outer coating was made of a ceramic material. In order to reduce the thermal stress in the carbon fibers and the ceramic material caused by mismatch of coefficients of thermal expansion between the two materials, a graphitized carbon layer was introduced between the fiber and the ceramic coating. For the multi-scale analysis, a new analysis model was developed and used to bridge the micro-scale characteristics, i.e. the constituent material level such as carbon and ceramic materials, to the macro-scale behavior, i.e. the woven fabric composite level. Furthermore, finite element analyses were undertaken with discrete modeling of the representative fibers, coating, and the graphitized middle layers. Then, both multi-scale analytical and numerical results were compared. In this study, thermal stresses at the micro-level, i.e. in the fibers and coating materials, as well as effective thermomechanical properties of the refractory composites were computed using the multi-scale technique.     

Mitigation of alkali–silica expansion in pulp fiber–mortar composites

The physical and microstructural effects of kraft pulp, thermomechanical pulp, and polypropylene fibers for restraining expansion due to alkali–silica reaction are investigated through expansion measurements, environmental scanning electron microscopy, and energy dispersive spectroscopy. This preliminary research revealed that thermomechanical pulp fibers are more effective at reducing expansion due to alkali–silica reaction than kraft pulp and polypropylene fibers. It is shown that the open lumen of the TMP fibers, and to some degree, the kraft pulp fibers, serves as a reservoir for alkali–silica gel, potentially minimizing internal expansive pressures. Furthermore, increases in the fiber volume fraction and fiber–cement bond strength (accomplished through extended initial moist curing) decreased expansion for all composites.     

Microstructure and mechanical properties of Al–2Mg alloy base short steel fiber reinforced composites prepared by vortex method

Fabrication and characterization of cast Al–2Mg alloy matrix composites reinforced with short steel fibers are dealt with in the present study. Three types of steel fiber were used: uncoated, copper coated and nickel coated. All the composites were prepared by the liquid metal route using vortex methods. When tested in tension, all composites exhibited improvement in strength due to high relative strength of steel fibers. The ductility was lowered except for the composite with copper coated fibers. Copper coated fiber reinforced composites gave the highest strength. Higher strength accompanied with appreciable ductility demonstrated by composites with copper coated fibers is attributed to the solid solution and fiber strengthening as well as good bonding at the interface. Composites reinforced with uncoated and Ni coated steel fibers did not exhibit strengthening to the level exhibited with copper coated fibers because brittle intermetallic phases are formed at the interface. These phases promote initiation and facilitate propagation of cracks. The observed fracture mechanism of composites was dimple formation, fiber breakage and pullout of fibers. Fracture surface of uncoated and Ni coated composites showed extensive pull out of fibers as well as fiber breakage confirming the above inference. In case of the copper coated composites dimple formation and coalescence was more extensive. EDX analysis showed a build up Cu, Ni, and Fe at the interface.     

碳密封涂覆光纤在编织复合材料中的应用

碳密封涂覆光纤由于具有良好的强度性能和抗疲劳能力,已应用于海底光缆、军用制导光纤以及苛刻环境下的光纤传感系统中。本文作者研究了碳密封涂覆光纤在三维碳/环氧编织复合材料中的应用。将该光纤成功地编入编织复合材料试件并成型。对该光纤在编织前、后以及成型后的光学性能进行了测试。设计了使用碳密封涂覆光纤的偏振式光纤传感器,并用其对试件处于弯曲受力状态的情况进行了测试。     

Thermomechanical response of TiNi fiber-impregnated CFRP composites

The focus of this work is the thermomechanical characterization of pre-strained TiNi fiber impregnated CFRP composites. TiNi/CFRP composites were fabricated by hot-pressing at a temperature of 180 °C. The compressive recovery force induced by thermomechanical actuation of TiNi fibers depends on the pre-strained level. The shrinkage strain due to the thermomechanical response of the composites increased with increasing pre-strain level and number of impregnated TiNi fibers. Transverse cracks which formed at 90 ° to the CFRP layers under tensile loading could be healed by the compressive recovery force introduced by pre-strained TiNi fibers when electrically heated.     

Constitutive relations for the thermomechanical behavior of fiber-reinforced inelastic laminates

The thermomechanical behavior of laminated composites in which every lamina is unidirectional fiber-reinforced thermoinelastic material is determined by a micromechanical analysis followed by a macromechanical one. In the micromechanical analysis, effective constitutive relations are derived for unidirectional fibrous materials in which the matrix and fiber phases are thermoelastic in the linear region and thermoinelastic in the nonlinear region. The derivation is based solely on the material properties of fibers and matrix and amount of reinforcement. By a macromechanics analysis the gross behavior of the laminated composite in stretching and bending deformation is determined. Applications are given for the deformation field developed in cooling and reheating of graphite/aluminium laminated plates.     

Use of biochar-coated polypropylene fibers for carbon sequestration and physical improvement of mortar

Biochar is widely recognized as an effective material for sequestration of carbon dioxide. The possibility of using it as a coating material on polypropylene fibers to improve mechanical properties and permeability mortar is explored in this study. Effectiveness of two types of biochar – fresh biochar and biochar saturated with carbon dioxide prior to application as coating – on compressive and flexural strength, post-cracking behavior and permeability of mortar is studied. The biochar used was derived from mixed wood saw dust by pyrolysis at 300 °C. Experimental results show that application of fresh biochar coating offer significant improvement in compressive strength and flexural strength of mortar. Residual strength and post-cracking ductility of mortar with biochar coated fibers is found to be higher than control samples, although fresh biochar coating offers the best performance. Mortar with polypropylene fibers coated with fresh biochar shows higher impermeability, compared to reference samples and mortar with saturated biochar coated fibers. The findings suggest that biochar coating could be a potential solution to improve properties of fiber reinforced cementitious composites that also promotes waste recycling and carbon sequestration.     

C6位选择性氧化竹浆纤维的壳聚糖改性处理

采用H_3PO_4/HNO_3-NaNO_2氧化体系对竹浆纤维C6位进行选择性氧化,然后与壳聚糖溶液交联反应制备生态的抗菌壳聚糖改性竹浆纤维。固相CP/MAS 13 C核磁共振分析显示,H_3PO_4/HNO_3-NaNO_2已将竹浆纤维C6位上的伯羟基选择性氧化成羧基,且红外光谱和扫描电镜测试表明,壳聚糖的氨基与氧化竹浆纤维分子上的羧基发生酰胺化反应,壳聚糖分子通过C-N化学键共价交联在竹浆纤维上,并在竹浆纤维表面形成壳聚糖薄膜。通过分析H_3PO_4/HNO_3-NaNO_2氧化和壳聚糖处理过程中竹浆纤维羧基含量、机械强力、壳聚糖含量等指标,得出最佳工艺参数。Kjeldahl定氮分析得出壳聚糖在氧化竹浆纤维上的最大含量为3.92%。壳聚糖接枝竹浆纤维的断裂强度变化不明显,但其断裂伸长率降低。抗菌试验结果显示,与壳聚糖处理的原竹浆纤维抗菌性相比,壳聚糖改性竹浆纤维对大肠杆菌和金黄色葡萄球菌的抗菌效果明显提高,抑菌率均在96%以上,且壳聚糖改性竹浆纤维具有良好的抗菌耐洗涤性能。     

Optical fibers coated with non-linear Langmuir-Blodgett films

We report second harmonic generation (SHG) in structures composed of optical fiber cores coated with Langmuir-Blodgett non-linear films. Such structures may serve as attractive alternatives to currently available coated slab non-linear waveguides. The fabrication of frequency doublers, couplers and modulators on optical fibers would eliminate the difficult problems of coupling from planar signal conditioners, sensors and detectors onto fiber transmission systems. In addition, the small size of fibers allows for the high energy densities required for non-linear interactions to exist over large distances.

Our results confirm that fibers can be coated and the deposition ratio monitored, provided that a sufficient number are dipped simultaneously. The dependence of SHG on distance as well as the variation with number of layers for a hemicyanine dye deposited on multimode fiber cores are discussed.     

Uniaxially aligned electrospun fibers for advanced nanocomposites based on a model PVOH-epoxy system

This work demonstrates the potential of aligned electrospun fibers as the sole reinforcement in nanocomposite materials. Poly(vinyl alcohol) and epoxy resin were selected as a model system and the effect of electrospun fiber loading on polymer properties was examined in conjunction with two manufacturing methods. A proprietary electrospinning technology for production of uniaxially aligned electrospun fiber arrays was used. A conventional wet lay-up fabrication method is compared against a novel, hybrid electrospinning–electrospraying approach. The structure and thermomechanical properties of resulting composite materials were examined using scanning electron microscopy, dynamic mechanical analysis, thermogravimetric analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy, and tensile testing. The results demonstrate that using aligned electrospun fibers significantly enhances material properties compared to unreinforced resin, especially when manufactured using the hybrid electrospinning–electrospraying method. For example, tensile strength of such a material containing only 0.13 vol% of fiber was increased by ∼700%, and Young’s modulus by ∼250%, with concomitant increase in ductility.     

连续变化界面层对复合材料弹性性能影响

界面层对复合材料的变形和破坏有着重要的影响,实际界面层的性能是随位置而连续变化的,但目前大多数考虑界面层影响的工作都假设界面层材料性能均匀或分层均匀。假设界面层的性能是空间位置的幂函数形式,给出了具有上述界面层的纤维和球形夹杂在球压和剪切载荷下的解,然后利用平均场方法建立了上述复合材料的有效弹性性质与微结构的联系。还将上述方法与均匀界面层模型进行了比较,计算结果表明,界面层的性质对复合材料有效性质和局部应力的分布有着重要影响。      

包覆型短碳纤维等效介电常数的模拟计算

基于等效电磁理论,推导了频率小于2GHz的包覆型短碳纤维的等效电磁参数方程.采用Matlab6.5软件对包覆型短碳纤维的等效电磁参数进行了模拟和计算,并讨论了包覆层体积分数和短碳纤维长径比对等效电磁参数的影响.结果表明,等效介电常数随着包覆层体积分数的增大而增大,当短碳纤维的长径比小于30时影响较大.包覆型短碳纤维的等效电磁参数方程的模拟结果与实验结果相对误差小于10%.     

Multiresponsive Graphene‐Aerogel‐Directed Phase‐Change Smart Fibers

Wearable devices and systems demand multifunctional units with intelligent and integrative functions. Smart fibers with response to external stimuli, such as electrical, thermal, and photonic signals, etc., as well as offering energy storage/conversion are essential units for wearable electronics, but still remain great challenges. Herein, flexible, strong, and self‐cleaning graphene‐aerogel composite fibers, with tunable functions of thermal conversion and storage under multistimuli, are fabricated. The fibers made from porous graphene aerogel/organic phase‐change materials coated with hydrophobic fluorocarbon resin render a wide range of phase transition temperature and enthalpy (0–186 J g^?1). The strong and compliant fibers are twisted into yarn and woven into fabrics, showing a self‐clean superhydrophobic surface and excellent multiple responsive properties to external stimuli (electron/photon/thermal) together with reversible energy storage and conversion. Such aerogel‐directed smart fibers promise for broad applications in the next‐generation of wearable systems.     

A shear lag model of Piezoelectric composite reinforced with carbon nanotubes-coated Piezoelectric fibers

A novel hybrid piezoelectric composite in which the microscopic piezoelectric fiber reinforcements are coated with radially aligned carbon nanotubes (CNTs) is analyzed in this study. A shear-lag model is developed to analyze the load transferred to such coated fibers from the aligned-CNT reinforced matrix in a hybrid composite application in the absence and the presence of the electric field along the length of the fiber. It is found that if the aligned CNTs are radially grown on the surface of the piezoelectric fiber then the axial load transferred to the fiber is reduced in the absence of the electric field while the axial stress in the fiber increases in the presence of the electric filed only. The radial stress in the active piezoelectric fiber significantly increases due to the radial growth of aligned CNTs on the surface of the fibers. This indicates a probable critical window for engineering the surface of the piezoelectric fiber for improving the effective piezoelectric properties. Effects of the variation of the aspect ratio of the piezoelectric fiber and the CNT volume fraction on the load transferred to such CNT-coated piezoelectric fibers are also investigated.     

PAN纤维热松弛行为控制与聚集态结构调控EI北大核心CSCD

聚丙烯腈(PAN)纤维在纺丝过程中会形成受迫高弹形变,在受热条件下会出现物理收缩为代表的热松弛行为,发生解取向。通过热力学分析仪(TMA),动态力学分析仪(DMA),广角X射线衍射法(WAXD)等手段研究PAN纤维的热松弛行为,并且通过张力和温度对其进行控制。结果表明:PAN纤维纺丝过程形成的高弹形变约占10%以上,在高温下会迅速回复发生解取向。通过施加适当的张力和提高温度可以在不损失取向结构的同时将高弹形变转化为塑性形变。在张力和温度的作用下,纤维内部聚集态结构重排,分子链取向进一步提高,晶态结构进一步完善,纤维尺寸稳定性提高50%以上。研究表明经此方法处理的碳纤维微晶沿分子链排列更加规整,性能得到了有效提升。     

防热复合材料高温力学性能

通过对高温环境下防热材料内部热化学烧蚀机理的分析,利用Eshelby等效夹杂方法研究了组元材料烧蚀-相变特性和高温力学性能的变化规律。假设材料热化学反应后的热解(热氧化)生成相介质统计均匀分布,考虑了烧蚀反应产生的气孔与固体相介质之间的相互作用,预报了单向纤维增强复合材料微结构与宏观性能之间的变化关系,并进行了数值计算。研究结果表明:单向复合材料纵向杨氏模量随温度升高而衰减,并与加温速率有关,典型热防护材料的高温力学性能的理论预报与实验数据进行比较,结果吻合较好,说明理论模型正确,为防热复合材料热结构分析奠定了基础。      

Elastic–plastic thermomechanical response of composite co-axial cylinders

A new method is developed in this paper to deal with the thermomechanical response of continuous fiber-reinforced composites. Treating the matrix as an elastic-perfectly plastic solid, the analytical formulae of the deformations and stresses of the matrix are obtained from the plasticity theory, axisymmetric equilibrium equation, and stress–strain and strain–displacement relations. The fiber is taken to be an anisotropic, elastic material, and the formulae calculating its deformations and stresses are also presented. The boundary conditions and the consistence of deformations and stresses between the fiber and matrix, and between elastic and plastic regions of the matrix are employed to determine the unknown constants in the analytical formulae. With the developed method, the thermomechanical stress distributions in composites reinforced with circumferentially orthotropic, radially orthotropic and transversely isotropic fibers are investigated, and how the elastic-perfectly plastic property and different materials of the matrix affect the thermomechanical response of the composites is discussed. For the thermomechanical loads and composite systems given in this paper, the elastic-perfectly plastic property of the matrix can reduce the compressive stresses in the fiber, and the tensile circumferential and axial stresses in the matrix. A strong matrix can raise the compressive stresses in the fiber, and the tensile circumferential and axial stresses in the matrix.     

SiC/PyC复合涂层碳纤维微观结构及氧化行为研究

采用两步法在碳纤维表面制备了碳化硅/热解碳(SiC/PyC)复合涂层,PyC内涂层的制备采用等温化学气相渗透法,SiC外涂层的制备采用碳热还原法.借助X射线衍射、场发射扫描电镜、透射电镜分析了SiC/PyC复合涂层碳纤维的物相组成以及微观结构,利用热重分析研究了SiC/PyC复合涂层、PyC涂层以及无涂层碳纤维的氧化行为.结果表明,在碳纤维表面制备的SiC/PyC复合涂层连续致密、厚度均匀,PyC内涂层厚度约为200nm,SiC外涂层厚度约为160nm,SiC层中存在大量孪晶面高度有序的SiC孪晶.SiC/PyC复合涂层能够有效地改善碳纤维的抗氧化性能,较无涂层碳纤维起始氧化温度提高了近250℃.     

Enhanced thermomechanical properties of long and short glass fiber-reinforced polyamide 6,6/polypropylene mixtures by tuning the processing procedures

Long and short glass fibers (GF) were incorporated into the polyamide 6,6 (PA66)/polypropylene (PP) mixtures in order to enhance the thermomechanical properties. The effect of fiber length and processing procedures on tensile strength, flexural modulus, impact strength, and heat deflection temperature has been investigated. Miscibility behavior of the PA66/PP mixtures has been examined by performing differential scanning calorimetry analysis and theoretical calculation. The mixtures exhibiting broad coexistent regions such as crystal + crystal (Cr₁ + Cr₂), crystal + liquid (Cr₁ + L₂), and liquid + crystal (L₁ + Cr₂) revealed a significant improvement in thermal and mechanical properties by the addition of GF. Especially, long fiber-reinforced thermoplastics showed better performances compared to short fiber-reinforced thermoplastics at the same filler loading. From the morphological observation of the fractured surface, it was realized that the incorporation of long GF after the melt blending of PA66 and PP was very effective to attain high thermomechanical properties due to the better homogeneity and compatibility.     

Thermomechanische Eigenschaften von Feuerbetonen mit eingelagerten,zunderbeständigen Stahlfasern

Thermomechanical Properties of Castable Refractories with Embedded Scaleresistant Steel Fibers In the last years steel fibers were embedded in castable refractories for certain application to increase the thermomechanical properties of refractory concrete. When cheap carbon steel fibers were used, the less oxidation resistance in the high temperature range was a serious disadvantage. Stainless steel fibers were oxidation resistant only up to about 1000 °C when practical used at long time. Now, stainless steel fibers with high chromium (24–26%) and high nickel content (34–36%) were put on the market for embedding in castable refractories. The thermomechanical properties of these composites will be compared with those of earlier investigations. Their will be used steel fibers not only fabricated by cutting of drawn wires but also by melt extraction processing. The interfacial area steel fiber/refractory matrix will be examined and the reaction or oxidation products analyzed by microprobe.