Characterization For The Compressing, Air Permeating And Acoustic Insulating Behaviors Of Fiber Assemblies With Density Variation In Situ

With fiber assembly bulk density variation, the permeability of fiber assemblyhas not been studied because there still has been no such method and apparatus formeasuring the fiber assembly permeability with bulk density variation in situ. Thisstudy just presented a new apparatus, a FACBES-01 (Fiber Assembly ComplexBehavior Evaluation System) developed by authors, for measuring the permeability offiber assemblies with different bulk density. This apparatus provides a kind ofmulti-functional measurement so that the compression property, the air permeabilityand acoustic insulation of a fiber-plug could be characterized simultaneously with thedensity variation for fiber assemblies. Its excellences are precision, multifunction andautomatization. It fit to the measurement for various fiber assemblies.Firstly, the compressional behavior of five fiber has been investigated in thispresent thesis, i.e. wool, cashmere, acrylic, goose down and kapok. Except acrylicassemblies, the top force and bottom force put on the assemblies for the other fourfibers were almost same. The difference between these two forces was little, and theloss work was also small. That is to say the top force may get balance with the bottomforce instantly. The difference between the two forces acted on a acrylic assembly wasenhanced in compressing due to the slabby and rough surface of acrylic fiber.The compression process could be divided into three areas: linear area, transitionarea(or slipping area) and linear strengthen area for modulus. And the transition areawas more complicated because of the fibers slipping. For one kind of fiber assembly,these three phases maybe couldn't appear simultaneously. For various fiberassemblies, the form of their compressive curves and recovery curves weredistinguishing. And the compression moldings were also discrepant.The compressional behavior of fiber assemblies depends on fiber arrangementintensively. A random-arranged fiber plug could be considered as a kind of isotropy and uniformity structure, while fiber balls random arrangement was a multi-boundaryconfiguration. The structure of the fiber random arrangement was soft comparing withthe fiber balls random arrangement, which was a relative rigid structure. With fibercompressing going on, the force of the latter increased more rapidly than that of theformer. The compressive recovery capability of fiber assembly wasn't affected by thefiber arrangement which was decided mainly by the fiber elastic recovery capability.Comparing the forces compressing on the fiber assemblies in cylinder andwithout cylinder, it was found that the force of the former was much larger than thatof the latter. Because the fiber assembly swelled during compressive process, as thecylinder binding up the fibers, so there appeared acute extrusion and friction betweenthe fibers and cylinder. Without the cylinder, the fibers could swell freely and theforce and compression molding increased placidly.Through taking photos during the compression process, we could get differentconfiguration characters for different fibers. Compression just made the relativeposition of fibers change and fiber bending was not dominant. The fiber locationheight in the assembly was linear correlation with the compressing displacement. Thatwas to say the location of a fiber relative to the whole assembly didn't change duringthe compressive process. Compressing a fiber plug, the increasing of the bulk densityin its top layer was faster than that in the mid and bottom layer, and that in the bottomone was the slowest, thus the whole tiber assembly was not compressed equably.Secondly, the air permeability of fiber assemblies for various fibers and differentarrangements have been studied. The Kozeny's equation could be written as thepolynomial of order 2 for the relationship between the air pressure difference for thetop and bottom of assembly and the bulk density of assembly. So such experimentmethod could reflect the air permeability of various fiber assemblies expediently.At a low bulk density of random-arranged fiber assemblies, the air permeabilityfor the five sorts of fiber assemblies was little deference. With the increasing of bulkdensity, the discrepancy of air permeability occurred. Under the condition of samebulk density, the sequence on air permeability of the five assemblies from best toworst could be derived from the pressure difference: wool, cashmere, acrylic, goose down and kapok because of the fiber morphology. While under the condition of samefiber volume fraction, we got the sequence for air permeability from best to worst was:wool, acrylic, cashmere, kapok and goose down. These two sequences were differentand it shows that the air permeability relies not only on porosity but also on the voidform, sinuosity, arrangement and distribution.The values of K for three kinds of fiber arrangements were different. With thefiber volume fraction increasing, the values of K all reduced. There was also differentair permeability for different fineness fibers.Thirdly, the acoustic insulation of fiber assembly was analyzed. The cylinderplayed an important role during the sound transmitting. The cylinder wall could makethe sound echoing, refracting, and interfering. That was why the sound could transmitto the bottom of cylinder and the receiving distance affected the acoustic attenuation.For different frequencies, the degree and trend of acoustic attenuation inassembly were distinct. But for the five fiber assemblies, this rule was similar. Thetrend of acoustic intensity ratio decreasing becomes balanced with the frequenciesincreasing. That was to say for high frequency sound, its acoustic attenuation almostwasn't affected by assembly structure. The acoustic attenuation for fiber assembly todifferent frequencies was also contrast. This paper advanced the bulk density as astructure parameter to design and optimize the acoustic insulating material. There wasan optimum bulk density to best acoustic attenuation for fiber assembly.For the acoustic attenuation of three kinds of fiber arrangements, the bulkdensities affected the acoustic attenuation faintly for the fiber ball arrangement andparallel arrangement. It showed that acoustic attenuation had something to do with theporosity, void distribution and fiber arrangement. Therefore, porosity and structurefactor were two main characters for acoustic attenuation in fiber assembly.