Optimization of the arrangement of fiber laser array in beam coherent combining

Based on the expanding of the fundamental mode of a step-index fiber in terms of Laguerre–Gaussian modes, the accurate expression for the beam propagation of a fiber laser in free space is obtained. Thereby, the coherent combining beam of three fiber laser arrays including circular arrangement, square arrangement, and diamond arrangement are numerically analyzed. The study shows that all the beams gradually concentrate centrally on the propagation axis and the highest far-field peak intensity can be obtained by using the circular arrangement. Meanwhile, the far-field intensity of the circular arrangement by using the Laguerre–Gaussian approximation is also compared with that by using the pure Gaussian approximation, which indicates that the pure Gaussian approximation will induce much error in the far-field intensity. Finally, the influence of the radius of a circular fiber laser array on the far-field intensity is studied, of which the result shows that the far-field intensity decreases with increasing radius. Therefore, the fiber laser elements are suggested to be placed as close as possible.     

Direct simulations of fiber network deformation and failure

A finite element model for 3D random fiber networks was constructed to simulate deformation and failure behavior of networks with dynamic bonding/debonding properties. Such fiber networks are ubiquitous among many living systems, soft matters, bio-materials, and engineering materials (papers and non-woven). A key feature of this new network model is the fiber–fiber interaction model that is based on AFM measurements from our earlier study. A series of simulations have been performed to investigate strain localization behavior, strength statistics, in particular, the variations of strength, strain-to-failure and elastic modulus, and their size dependence. Other variables investigated are fiber geometries. The result showed that, in spite of its disordered structure, strength and elastic modulus of a fiber network varied very little statistically, as long as the average number of fibers in the simulated specimen and the degree of fiber orientation are kept constant. However, strain-to-failure showed very significant statistical variations, and thus more sensitivity to the disordered structures.     

X-ray computed tomography used to measure fiber orientation in CFRP laminates

This paper explains a new method to measure the fiber orientation in carbon fiber reinforced plastics (CFRP) laminates from X-ray CT images. In the method, the fiber orientation is analyzed by the application of digital image correlation (DIC) method to the acquired tomographic images. Using DIC, the brightness pattern, which results from the radiodensity difference between fiber and resin, is compared between two different planes in the thickness direction. Then, the three-dimensional displacement of the brightness pattern, which indicates the fiber orientation, can be measured. This study applied the proposed method to a quasi-isotropic CFRP laminate. After X-ray CT imaging, the sample was sectioned and polished. The fiber orientation was then measured experimentally using microscopy. The fiber orientation calculated using the proposed method agrees very well with the experimentally measured one. After demonstrating the validity of the proposed method, we applied it to a plain woven CFRP laminate. Results revealed that an invalid fiber orientation might be calculated for fibers parallel to the plane of the CT image, or for the fiber orientation of the pattern around the outer edge of CT images.     

Bending effect on fiber acousto-optic mode coupling

The acousto-optic effect in a bent fiber is studied experimentally and numerically by using the scalar finite-element method. The resulting transmission spectra show that new mode-coupling peaks appear due to the breaking of the mode spatial symmetry. The strength of new peaks increases as the fiber-bending curvature increases with a redshift or blueshift in wavelength, strongly depending on the orientation of fiber bending with respect to the acoustic-wave vibration direction.     

Mode-field diameter of single-mode optical fiber by far-field scanning

I use the direct far-field method to measure the mode-field diameter of a single-mode fiber with an expanded uncertainty of 30 nm, with a coverage factor of 2. For a step-index fiber with a mode-field diameter of approximately 9 mum, the major sources of uncertainty are nonlinearity in the electronics, angular errors and scattered light in the apparatus, and the polarization and noncircularity of the mode of the fiber. The paper concludes by showing an inconsistency in the derivation of the far-field expression for mode-field diameter.     

基于宏观各向异性碳纤维增强树脂基复合材料的切削仿真

为了对碳纤维增强树脂基复合材料切削加工过程中的基体破坏及亚表层损伤机制进行研究,借助数值仿真方法建立了基于宏观各向异性的复合材料正交切削有限元模型。采用Hashin-Damage失效准则,通过定义纤维拉伸断裂、压缩屈曲极限应力及基体横向拉伸断裂、剪切断裂极限应力等数值,建立了复合材料切削加工动态物理仿真模型。通过切削力仿真值与实验值的比较,验证了仿真模型的有效性。通过对0°和90°纤维方向复合材料基体开裂和压溃的分析发现,当进入稳定切削后,基体开裂方向与纤维方向平行,而基体的压溃主要发生在刀尖周围。分析了纤维方向对复合材料亚表面损伤深度的影响,随着纤维方向角度的增加,工件亚表面裂纹损伤深度呈增长趋势。     

Bearing strength of commingled boron/glass fiber reinforced aluminum laminates

The bearing properties of recently developed hybrid fiber/metal laminates, or COmmingled Boron/glass fiber Reinforced Aluminum laminates (COBRA), are investigated in this study. The bolt-type bearing tests on GLass REinforced aluminum laminates (GLARE), non-commingled hybrid boron/glass/aluminum fiber/metal laminates (HFML) and COBRA were carried out as a function of e/D ratio, metal volume fraction, fiber volume fraction, and fiber orientation. Experimental results show that with the same joint geometry and metal volume fraction, the commingling of boron fibers improves the bearing strength of fiber/metal laminates. Observations show the boron/glass fiber prepreg, transverse to the loading direction, results in a bearing mechanism that effectively increases the bearing strength. The bearing strength of COBRA with longitudinal fibers is lower than that with transverse fibers due to the fact that shearout failure takes place before maximum bearing strength is reached. The experimental results show that, with only either transverse fiber orientation or longitudinal fiber orientation, COBRA with 18% boron fiber volume fraction possesses a higher bearing strength when compared to HFML with 6% boron fiber volume fraction. In addition to the properties in COBRA with parallel-plies commingled prepreg, the bearing properties of various COBRA with [0°/90°] and [0°/90°/90°/0°] cross-ply commingled prepregs are also discussed.     

Creating three-dimensional (3D) fiber networks with out-of-plane auxetic behavior over large deformations

Fiber networks with out-of-plane auxetic behavior have been sporadically investigated. One of the major challenges is to design such materials with giant negative Poisson’s ratio over large deformations. Here in, we report a systematic investigation to create three-dimensional (3D) fiber networks in the form of needlepunched nonwoven materials with out-of-plane auxetic behavior over large deformations via theoretical modeling and extensive set of experiments. The experimental matrix has encapsulated the key parameters of the needlepunching nonwoven process. Under uniaxial tensile loading, the anisotropy coupled with local fiber densification in networks has yielded large negative Poisson’s ratio (up to ?5.7) specifically in the preferential direction. The in-plane and out-of-plane Poisson’s ratios of fiber networks have been predicted and, subsequently, compared with the experimental results. Fiber orientation was found to be a core parameter that modulated the in-plane Poisson’s ratio of fiber networks. A parametric analysis has revealed the interplay between the anisotropy of the fiber network and the out-of-plane Poisson’s ratio based upon constant volume consideration.     

Characterization of fiber orientation in short fiber reinforced composites with an image processing technique

An experimental method is developed to measure the three-dimensional fiber orientation in short fiber reinforced composites by utilizing an image processing technique. The second order orientation tensor can be calculated with geometrical data that were obtained from two parallel planar cross-sections. The orientation state of individual fibers is determined from the geometry of the elliptical cross-sectional shape on the polished surface. The basic concept in determining the three-dimensional fiber orientation tensor is to slice the composite sample twice in the same direction within a small distance. The tensor is determined by using a digital image processing technique and a computational code which calculates the tensor from the geometrical characteristics obtained for the elliptical fiber cross-sections. Experiments are performed to measure the three-dimensional orientation tensor of composite specimens and good results are obtained by using the method proposed in this study Electronic Publication     

Analysis of effect of fiber orientation on Young’s modulus for unidirectional fiber reinforced composites

Young’s modulus of unidirectional glass fiber reinforced polymer (GFRP) composites for wind energy applications were studied using analytical, numerical and experimental methods. In order to explore the effect of fiber orientation angle on the Young’s modulus of composites, from the basic theory of elastic mechanics, a procedure which can be applied to evaluate the elastic stiffness matrix of GFRP composite as an analytical function of fiber orientation angle (from 0° to 90°), was developed. At the same time, different finite element models with inclined glass fiber were developed via the ABAQUS Scripting Interface. Results indicate that Young’s modulus of the composites strongly depends on the fiber orientation angles. A U-shaped dependency of the Young’s modulus of composites on the inclined angle of fiber is found, which agree well with the experimental results. The shear modulus is found to have significant effect on the composites’ Young’s modulus, too. The effect of volume content of glass fiber on the Young’s modulus of composites was investigated. Results indicate the relation between them is nearly linear. The results of the investigation are expected to provide some design guideline for the microstructural optimization of the glass fiber reinforced composites.     

Cylindrical vector beam generation in multiple elliptical core fiber with gold wire

A new method to generate cylindrical vector beam carrying helical phase is proposed based on a multiple elliptical core fiber integrated with gold wire. Both the light field distributions in near- and far-field are considered. The simulation results show that the multiple core fiber filled with gold wire in the center can only support low-loss transmission for the analogous azimuthally polarized (AP) light field with discrete rotational symmetric profile in a broadband range, because the resonant coupling between the surface plasmon modes and the fiber core-guided supermodes. The far-field patterns for different number core are demonstrated to represent an effective AP beam. It also acts as an effective broadband transmission filter for azimuthal modes.     

Characterization of the orientation profile of steel fiber reinforced concrete

The orientation of fibers has a pronounced influence on the tensile behavior of steel fiber reinforced concrete (SFRC) and, consequently, this aspect should be considered in modeling the material constitutive law. Previous works have shown that the tensile strength of SFRC is directly related to the average orientation of the fibers. However, few studies have investigated the correlation between the variation of distribution of fiber orientation and material strength. This paper introduces a new concept of orientation profile in order to characterize fiber orientation through an unambiguous method. From the investigation on experimental data it could be observed that fiber orientation follows a Gaussian law and that the distribution and average values of single fiber orientations are correlated with each other. Conclusions from this paper are particularly relevant for the development of micromechanical models for SFRC.     

考虑纤维方向分布的玻纤增强PP复合材料拉伸性能

纤维方向及其分布对玻纤增强PP复合材料的力学特性具有至为关键的影响。提出了一种快速获取纤维数量及每根纤维方向的方法。通过引入方向张量, 利用Moldflow软件进行玻纤增强PP树脂注塑成型模拟获得纤维方向的平均分布, 结合显微方法观察判断特定点的纤维沿厚度方向的分层情况及定量判断纤维方向的分布。对轿车玻璃纤维增强注塑仪表板的纤维方向相对一致处取与纤维方向呈0°、45°、90°的样条, 通过拉伸实验测得拉伸模量, 利用所提出的方法研究了仪表板内玻纤方向的分布及其对拉伸模量的影响。研究结果表明: 玻纤增强注塑仪表板的力学性能是各向异性的, 其沿厚度方向纤维按方向大致可分为三层。     

Numerical simulation of the variation of fiber orientation distribution during flow molding of Ultra High Performance Cementitious Composites (UHPCC)

In this paper, the variation of the fiber orientation distribution along the flow of fresh UHPCC was studied. In order to describe the rotational motion of a single fiber, Jeffery’s equation was adopted, in which the interaction among fibers is neglected. Two cases of flow patterns were considered: shear flow and radial flow. Starting with a three-dimensional random distribution of fibers, the fiber orientation distribution along the flow distance was simulated. These results reveal that fibers gradually become more parallel (in the case of shear flow) and perpendicular (in the case of radial flow) to the flow direction as the flow distance increases. This approach will be useful to predict flow-dependent tensile behavior considering the change of fiber orientation distribution.     

Influence of processing methods and fiber length on physical properties of kenaf fiber reinforced soy based biocomposites

Biocomposites from kenaf fiber and soy based bioplastic were fabricated by extrusion, followed by injection or compression molding. The impact of fiber length and the processing method on the thermal and mechanical properties of the composites were characterized with dynamic mechanical analysis (DMA) and mechanical properties measurements. The morphology was studied with optical and electron microscopy. Compression molded specimens have a similar modulus to injection molded specimens at room temperature, but exhibit a higher heat deflection temperature (HDT) and notched Izod impact strength. The improved HDT and impact strength are derived from an increase in modulus at high temperature and fiber bridging effects, respectively. The modulus, impact strength and HDT of kenaf fiber reinforced soy based biocomposites increase with increases in fiber length, fiber content and fiber orientation. Through microscopy observations, it was found that the fractured fiber length on the impact fracture surface increased with increasing fiber length and fiber content. This indicates that the role of fiber bridging effects is predominant on impact strength of the biocomposites.     

Cantor set fiber Bragg grating

The theory of fractals has already been applied to many fields in science, such as physics, biology, and chemistry. One of the most commonly used fractals in these applications is the Cantor set. Novel fiber Bragg gratings are proposed that combine the present technology of fiber Bragg gratings with the theory of Cantor sets. The principal goal of this work is to analyze how Cantor sets, applied to gratings, can alter their reflectivity spectra. Specifically, it is observed that, as the order of the Cantor set increases, the bandpass reflectivity spectra of these gratings broaden and evolve into more-complex patterns. Also, self-similarity properties can be observed in the spectra of these gratings. Numerical examples demonstrate variations in the spectra of these structures as the fractal order increases.     

界面对硅酸铝短纤维增强AZ91D镁基复合材料蠕变性能的影响

界面对复合材料蠕变性能的影响很大。在试验分析的基础上建立了硅酸铝短纤维增强AZ91D镁基复合材料理论分析模型,利用三维有限元分析方法,系统研究了界面特性、界面上应力应变分布和短纤维位向变化对硅酸铝短纤维增强AZ91D镁基复合材料蠕变性能的影响。研究表明：界面特性,如厚度、模量,均对纤维最大轴应力和稳态蠕变速率有影响,当界面厚度增加,纤维最大轴应力减小而稳态蠕变速率增大;当界面模量增大,纤维最大轴应力增大而稳态蠕变速率减小,但当界面模量高于基体模量时,纤维最大轴应力和稳态蠕变速率均保持不变;纤维位向也影响轴应力分布和稳态蠕变速率,纤维在其末端界面上存在较大的应力和应变,此处容易产生微裂纹而使材料抗蠕变能力下降;界面对硅酸铝短纤维增强AZ91D镁基复合材料的蠕变曲线和蠕变断裂机制也有影响,其影响程度还与纤维位向有关。     

Effects of fiber orientation angles of fiber-reinforced plastic on sand solid particle erosion behaviors

Solid particle erosion in industrial applications has been a serious problem in many engineering fields. Earlier studies on fiber-reinforced plastic (FRP) composites were mainly focusing on the erosive wear behavior at several different impact angles. However, the effect of fiber orientation on FRP composites has not been thoroughly investigated. Since fiber orientation is one of the important factors in which causing erosive wear damages to FRP composites, in order to understand the virtue of this problem, it is important to investigate the effect of fiber orientation at different impact angles. In this research, the effect of fiber orientation of unidirectional fiber-reinforced plastic composites on erosive wear behavior was studied. Sandblasting-type erosion tests were conducted on the FRP composites with fiber orientation ranging at three impact angles to clarify the relation between fiber orientation and erosive wear behavior. The Dyneema fiber (ductile material) and the carbon fiber (brittle material) were used for the reinforcement fiber in FRP. From the result, it is confirmed that CFRP composites with higher fiber orientation angle erode faster than the composites with lower fiber orientation angle. But the erosion characteristic of DFRP was almost the same regardless of the fiber orientation angle. The damaged surfaces of the FRP composites were then analyzed using scanning electron microscopy and the possible erosion wear mechanisms were investigated.     

纤维悬浮剪切湍流中纤维脉动取向张量变化率分析

根据张量运算原理,推导了纤维悬浮剪切湍流中纤维脉动取向张量变化率的表达式,该式由流场脉动速度梯度的二阶关联和由方位角与纤维长径比表示的关联值系数组成.基于不同的流场脉动速度梯度二阶关联值量级相当的考虑,计算了二维流场中不同长径比和方位角情况下关联值系数的值,发现一般情况下纤维脉动取向变化率取决于流场的剪切与拉伸,当纤维长径比为1时,则只取决于流场的剪切.对长径比大于1的纤维而言,不同长径比下关联值系数的变化趋势一致,大长径比情况下,长径比对纤维脉动取向变化率没有影响.纯拉伸项对于纤维脉动取向变化率的作用在θ=45°处最显著.由拉伸项与剪切项乘积构成的速度梯度项,其对应的关联系数的最大值出现在θ=30°或60°的位置.