Structure of a High Efficiency Glass Fiber Filter Medium

ABSTRACT

The analysis of the real internal structure of a high efficiency glass fiber filter medium was carried out to provide detailed structure data, e.g., for more realistic calculations of the filter performance. The procedure of the structure analysis is described in detail and the most important results are presented and discussed. The fiber diameter distribution was found to be constant throughout the HEPA medium studied. The fibers tended to be oriented perpendicular to the direction of air flow, but there was no preferred direction of orientation within the plane of the filter medium. The interfiber distance distributions in parallel as well as in perpendicular orientation to air flow direction were found to be of the expected exponential shape for distances > 1 μm. On the other hand, the frequencies of the distances < 1 μm increase and the exponential shape of the curves is no longer observed.     

Influence of out‐of‐plane fiber orientation on reaction‐to‐fire properties of carbon fiber reinforced polymer matrix composites

This work investigates the influence of the out‐of‐plane orientation of carbon fibers on the reaction‐to‐fire characteristics of polymer matrix composites. A deep insight into combustion processes is gained, which is necessary to fully understand and assess advantages of composites with out‐of‐plane fiber angles. Epoxy‐based Hexply 8552/IM7 specimens with primarily low fiber angles between 0° and 15° are characterized by cone calorimetry. Heat release during fire is greatly affected by the out‐of‐plane fiber angle because of the thermal boundaries created by the fibers. The advancement of the pyrolysis front during fire was determined from peak heat release rates and validated by temperature measurements along the back surface of the panels, representing a novel method of determining position‐dependent pyrolysis migration velocity. These measurements show a transverse shift in pyrolysis front velocity for increasing out‐of‐plane fiber angles. Pyrolysis pathways between the fiber boundaries facilitate faster combustion through the composite thickness, especially for increasing angles from 0° to 15°. It was determined that under the chosen conditions, the pyrolysis front advances approximately 4 times faster when propagating parallel to the fibers than perpendicular.     

Effects of Fiber Architecture on Matrix Cracking for Melt-Infiltrated SiC/SiC Composites

The matrix cracking behavior of slurry cast melt-infiltrated SiC matrix composites consisting of Sylramic-iBN fibers with a wide variety of fiber architectures were compared. The fiber architectures included 2D woven, braided, 3D orthogonal, and angle interlock architectures. Acoustic emission was used to monitor in-plane matrix cracking during unload–reload tensile tests. Two key parameters were found to control matrix-cracking behavior: the fiber volume fraction in the loading direction and the area of the weakest portion of the structure, that is, the largest tow in the architecture perpendicular to the loading direction. Empirical models that support these results are presented and discussed.     

Tensile behavior of glass fiber-reinforced acetal polymer

The tensile stress-strain behavior of glass fiber-reinforced polyacetal resin was investigated for various fiber concentrations, fiber length distributions, and finishing agents. The polyacetal fiber blends change considerably in strength and elongation at break when treated with ammonium chloride, but otherwise similar specimens still follow a common stress-strain curve to a point shortly before failure. As the mean fiber length decreases, the modulus and tensile strength fall, but the elongation at break remains almost unchanged. The observed tensile behavior is discussed in terms of a simplified model, which assigns the fibers to two categories: a fraction α parallel to the applied load, and the remainder distributed in a plane perpendicular to the load axis. By fitting this model to the stress-strain curves, two other constants of each system are derived: a length-dependent efficiency factor β for parallel fibers, whose magnitude agrees with the predictions of Rosen and his co-workers, and a factor γ which expresses the constraint of the matrix resin by the “transverse” fibers. The behavior of γ is consistent with Tsai's theory of the transverse modulus of laminates, if a reasonable amount of fiber–fiber contact is assumed. In terms of this model, possible interpretations of the behavior under repeated loading and the mechanism of tensile failure are presented.     

Effect of different fiber orientations on compressive creep behavior of siC fiber-reinforced mullite matrix composites

The compressive creep behavior of monolithic mullite and a composite made of mullite reinforced by 40 vol% SiC fiber were investigated at temperatures from 1100 to 1200°C and under stresses from 5 to 55 MPa in air with a loading direction parallel and perpendicular to the fiber direction. For both situations the composite exhibits better creep resistance than monolithic mullite, although there is a creep anisotropy. The improvement in creep resistance when the fibers are parallel to the loading directions is due to the shedding of the applied stress on the SiC fibers, and the improvement in creep resistance when the fibers are perpendicular to the loading direction occurs because the fibers inhibit the lateral deformation of the mullite matrix along the fibers. The improvement mechanisms of the composites were confirmed further by their creep-recovery study, which indicated that the two types of composite specimens exhibit both an apparent creep-recovery behavior on load removal, due to the relaxation of the residual stress state between the mullite matrix and the SiC fibers after unloading. ©     

Thermal and ablative properties of low temperature carbon fiber-phenol formaldehyde resin composites

The thermal and ablative properties of phenol formaldehyde resin (PF) composites reinforced with carbon fibers heat-treated at low temperature have been investigated. Low temperature carbon fibers (LTCF) were obtained by a continuous carbonization process from stabilized PAN fibers at 1100 °C. The properties of LTCF reinforced PF (LTCF-PF) composites are compared with those of high temperature carbon fiber (HTCF) reinforced PF (HTCF-PF) composites. The thermal conductivity of the LTCF-PF composite is lower than that of HTCF-PF composite by about 35 and 10% along the directions parallel and perpendicular to the laminar plane, respectively. It was found from the ablation test using an arc plasma touch flame that the erosion rate is higher by about 30% in comparison with HTCF-PF composite. The result suggests that use of LTCFs as reinforcement in a composite may improve the thermal insulation of the composite but decrease the ablative resistance.     

High-Temperature Transverse Fracture Toughness of Nicalon-Fiber-Reinforced CAS-II Glass-Ceramic Matrix Composite

Cracking parallel to the fibers in off-axis plies is usually the initial form of damage in composite laminates. This cracking process has been associated with the (transverse) fracture toughness, defined by the critical strain energy release rate, G _Ic. The measurement of G _Ic provides basic information about the transverse crack resistance. In this study, the utility of the double torsion (DT) test technique to determine _G Ic in a glass-ceramic matrix composite (Nicalon/CAS-II) at temperatures up to 1000°C has been demonstrated. _G Ic did decrease moderately with increasing temperature (as does the bulk matrix); however, no evidence of an interphase oxidizing effect on crack growth (parallel to the fibers) could be found. The inevitable misalignment of fibers in the material was not very efficient at bridging the crack in the DT specimens, in contrast to the significant matrix crack interactions with the fibers reported for other geometries such as double cantilever beam and flexure specimens.     

Cracking and Damage in a Notched Unidirectional Fiber-Reinforced Brittle Matrix Composite

Results of four-point bend tests on notched beams of a laminated unidirectional fiber-reinforced glass matrix composite are presented. The failure sequence has been established through in situ examination. The dominant damage mode is a mixed-mode, split crack that runs parallel to the predominant fiber directions. The crack interacts with and crosses over imperfectly aligned fibers. The resulting bridging tractions are sufficient to cause the critical strain energy release rate to increase substantially as the crack extends. Several other damage modes are also observed. These include mode I (tensile) matrix cracks bridged by fibers, mode II (shear) cracks, and compressive damage at the loading points.     

Interferometric Determination of Optical Properties of Bicomponent Fibers

Abstract

Both double-beam and multiple-beam interferometry were applied for the determination of refractive indices and birefringence of nylon, 6 sheath, nylon 66 core bicomponent fibers. Double-beam technique was applied to determine the mean refractive indices for plane polarized light vibrating parallel and perpendicular to the fiber axis and the mean birefringence. Multiple-beam Fizeau fringes in transmission and in reflection were used to determine these optical parameters for each component of these fibers.     

陶瓷基复合材料的界面相容性研究

有关陶瓷基复合材料（CMC）的界面问题已经得到广泛的重视。为了使材料达到一个很好的刚性，在纤维与基体之间保持尽量小的界面作用力对于陶瓷纤维增强Si-C-O复合材料是非常重要的。在纤维界面上涂层有利于减小它们之间相互作用，涂层处理后的Si-C-O复合材料的弯曲强度比一般无涂层的复合材料高5倍。在介质涂层、基体、以及涂层与纤维间的三相物质中避免化学反应的发生。目前，可利用化学相容性的原理对涂层纤维进行选择。     

Dynamic mechanical properties of cotton/polypropylene commingled composite systems

The dynamic mechanical properties of cotton/polypropylene (PP) commingled composite system was studied with reference to the fiber content, chemical treatments with potassium permanganate and maleic anhydride modified PP, processing conditions and applied frequency. Side by side commingling of matrix and reinforcing fibers was adopted for the fabrication of composite laminates as they provide the shortest melt flow distance during the melting of matrix fibers. This method can also be used for the recycling of textile wastes. The storage modulus was found to increase with the fiber content across a range of temperatures. The loss factor was found to decrease with the increase in fiber content while the glass transition temperature increases. The chemical treatments increase the value of storage modulus. A master curve was constructed and also made a comparison between the experimental results and the theoretically predicted values. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

The mechanical,thermophysical and electromagnetic properties of UD SiCf/SiC composites in different directions

Unidirectional (UD) silicon carbide (SiC) fiber-reinforced SiC matrix (UD SiC_f/SiC) composites with CVI BN interphase were fabricated by polymer infiltration-pyrolysis (PIP) process. The effects of the anisotropic distribution of SiC fibers on the mechanical properties, thermophysical properties and electromagnetic properties of UD SiC_f/SiC composites in different directions were studied. In the direction parallel to the axial direction of SiC fibers, SiC fibers bear the load and BN interphase ensures the interface debonding, so the flexural strength and the fracture toughness of the UD SiC_f/SiC composites are 813.0 ± 32.4 MPa and 26.1 ± 2.9 MPa·m^1/2, respectively. In the direction perpendicular to the axial direction of SiC fibers, SiC fibers cannot bear the load and the low interfacial bonding strengths between SiC fiber/BN interphase (F/I) and BN interphase/SiC matrix (I/M) both decrease the matrix cracking stress, so the corresponding values are 36.6 ± 6.9 MPa and 0.9 ± 0.5 MPa?m^1/2, respectively. The thermal expansion behaviors of UD SiC_f/SiC composites are similar to those of SiC fibers in the direction parallel to the axial direction of SiC fibers, and are similiar to those of SiC matrix in the direction perpendicular to the axial direction of SiC fibers. The total electromagnetic shielding effectiveness (EM SE_T) of UD SiC_f/SiC composites attains 32 dB and 29 dB when the axial direction of SiC fibers is perpendicular and parallel to the electric field direction, respectively. The difference of conductivity in different directions is the main reason causing the different SE_T. And the dominant electromagnetic interference (EMI) shielding mechanism is absorption for both studied directions.     

钢纤维增强水泥基复合材料增韧机理研究

钢纤维可有效阻碍水泥基材料中微裂缝的产生和发展,提高其抗裂能力。基于Eshebly等效夹杂理论和最大周向应力准则,获得了平面应力条件下无限大薄板中裂缝与钢纤维相互作用的应力强度因子(SIF)解,推导了掺入纤维后的Ⅰ-Ⅱ复合型断裂裂缝尖端最大应力表达式。根据掺入钢纤维前后裂缝尖端最大应力比,分析了钢纤维方向、位置和弹性模量对增韧效果的影响。结果表明,钢纤维通过改变裂缝尖端应变场,影响应力强度因子,从而降低尖端最大应力。当钢纤维方向与裂缝方向垂直时,裂缝尖端最大应力比最小,定向后钢纤维的增韧效果最明显。钢纤维的增韧效果主要受纤维方向影响,而受纤维弹性模量的影响小。     

Thermal Expansion of Laminated, Woven, Continuous Ceramic Fiber/Chemical-Vapor-Infiltrated Silicon Carbide Matrix Composites

Thermal expansions of three two-dimensional laminate, continuous fiber/chemical-vapor-infiltrated silicon carbide matrix composites reinforced with either FP-Alumina (alumina), Nextel (mullite), or Nicalon (Si-C-O-N) fibers are reported. Experimental thermal expansion coefficients parallel to a primary fiber orientation were comparable to values calculated by the conventional rule-of-mixtures formula, except for the alumina fiber composite. Hysteriesis effects were also observed during repeated thermal cycling of that composite. Those features were attributed to reoccurring fiber/matrix separation related to the micromechanical stresses generated during temperature changes and caused by the large thermal expansion mismatch between the alumina fibers and the silicon carbide matrix.     

Orientation structure in transverse sections of carbon fibers from dehydrated polyvinyl alcohol

Transverse sections of carbon fibers prepared from two different lots of dehydrated polyvinyl alcohol fibers, with and without graphitizing treatment, were observed by both polarized light and scanning electron microscopy. Several different types of transverse layer plane preferred orientation were found in the graphitized fibers: layers aligned parallel with the section major axis in the central zone of elliptical cross sections; parallel with the fiber surface in concave regions of an irregular cross section shape; and parallel with the fiber surface in a highly crystallized fiber with an irregular cross section shape. These graphitic layer plane configurations are considered in connection with the lamellar structure in crystalline regions of the dehydrated polyvinyl alcohol fibers.     

Perpendicular Alignment of 2D Nanoplatelet Emitters in Electrospun Fibers: A Result of the Barus Effect?

Stable jet electrospinning (SJES) is a special form of optical fiber generation that prevents chaotic fiber whipping typical for conventional electrospinning procedures. Incorporation of highly emissive semiconductor nanoplatelets (NPLs) in such fibers has very high potential in optical data transmission, optological circuits, fiber lasers, solar light concentrators and many other fields because NPLs exhibit strongly directed emission from their surface plane due to various in-plane transition dipole moments. However, potential orientation control of 2D-NPLs in SJES is entirely unknown as electric fields and various mechanical forces contribute in a complex manner simultaneously. Here, the observation of counter-intuitive yet very beneficial orientation of rectangular CdSe/CdS 2D-NLP in SJES perpendicular to the fiber drawing axis is reported. Scanning electron microscopy, 3D-single particle excitation polarization microscopy, 3D-photogoniometry, polarized emission spectroscopy and small angle X-ray scattering (SAXS) demonstrate aggregation free perpendicular alignment of the NPLs in poly(methyl methacrylate) (PMMA) fibers, resulting in dominant emission in directions parallel to the fiber. It is suggested that the observed vertical alignment is due to normal forces resulting from viscoelastic expansion when the polymer solution leaves the cannula (Barus effect) and that using such perpendicular nano-emitter alignment forces allows for the generation of novel materials also beyond fibers.     

Deviation of Fiber Tracts in the Vicinity of Brain Lesions: Evaluation by Diffusion Tensor Imaging

Diffusion Tensor Imaging (DTI) is used to characterize the diffusion properties of deviated white matter caused by brain tumors. DTI was recently shown to be very helpful in delineating white matter both within brain lesions and surrounding them. Displacement of white matter fibers may be one of the consequences of tumor growth adjacent to white matter. The combination of white matter mapping with DTI and gray matter mapping using functional MRI, in some cases, facilitated assessment of the relation between the shifted cortical areas and the corresponding white matter tracts. We found that the fractional anisotropy extracted from DTI is increased by 38% in areas of non-edematous shifted white matter fibers. By contrast, trace apparent diffusion coefficient (ADC) values in those areas were found to be similar to contralateral side and normal control values. Analysis of the three diffusion tensor eigenvalues revealed that the increase in the fractional anisotropy is a result of two processes. The first is the increase in the diffusion parallel to the fibers—λ₁ (by 18%), and the second is the decrease in the diffusion perpendicular to fibers—λ₃ (by 34%) as compared with the contralateral side. These opposing changes cause an increase in the diffusion anisotropy but no change in the trace ADC. It is suggested that the pressure caused by the tumor may lead to an increase in white matter fiber tension, thus causing an increase in λ₁. On the other hand, the same pressure causes increased fiber density per unit area, leading to a higher degree of restricted diffusion in the extracellular space and, hence, a reduction in λ₃.     

The Aerodynamic Behavior of Fibers in a Linear Shear Flow

Lung deposition behavior for straight versus curved aerosol fibers is known to be different based on existing experimental data. However, our understanding of actual fiber dynamics in the respiratory system remains far from being complete. In particular, it is not clear how fiber shape influences particle motion in the lungs. This article presents the results of direct numerical simulations in a linear shear flow of the rotational dynamics of high aspect ratio fibers with complex shapes such as elliptic rods, torus segments, and helices. Our findings show that as expected the rotational behavior of complex fibers is different from that observed for straight symmetrical particles. In particular, we observe secondary rotation for our particles, which is perpendicular to the shear plane (a plane with the unit normal parallel to the local vorticity) as well as more frequent flipping (for helical particles) than observed for straight fibers with the same lengths and diameters. In view of our results, it can be suggested that respiratory tract deposition for particles with complex shapes will exhibit enhanced interception compared with the deposition of particles with simpler shapes.     

超细二氧化硅纤维/橡胶复合材料的结构与性能研究

通过静电纺丝技术制备了直径为300~500 nm的超细二氧化硅纤维,制备的纤维进一步研磨和超声处理得到二氧化硅短纤维,然后将其填充到胎面胶中,分析了不同取向的二氧化硅纤维对胎面胶物理机械性能与动态力学性能的影响。结果表明：二氧化硅纤维在胎面胶基体中有着良好的分散,可以明显提高复合材料的100%定伸模量。在动态性能上,当纤维取向方向与分子链方向一致时,其60~80oC损耗因子最小。当纤维取向方向与分子链方向垂直时,0~-20oC损耗因子最大。因此该二氧化硅纤维作为一种新型增强填料在胎面胶上有着良好的应用前景。     

Fiber-Reinforced Diamond Matrix Composites

The feasibility of fiber-reinforced diamond matrix composites was examined. For this purpose, monofilament composite specimens were fabricated using SiC fibers. The diamond was deposited onto the fibers using plasmae-enhanced chemical vapor deposition. The mechanical properties of the composite were characterized using uniaxial tensile tests. The specimens exhibited multiple matrix cracking, followed by fiber fracture and fiber pullout. The measurements were used to obtain an estimate of the interface sliding stress and the in-situ fiber strength.