This paper presents an experimental study on the vibration isolation performance of weft-knitted spacer fabrics under forced harmonic excitation. The weft-knitted spacer fabrics with two different thicknesses were first designed by varying the linking distance of the spacer monofilament and fabricated using an electronic flat knitting machine. Then, their vibration isolation performance was tested under forced vibration condition via sinusoidal sweeps from low to high frequencies. The typical acceleration transmissibility curve and effects of fabric thickness, load mass and excitation level were discussed in detail. The results obtained show that the thicker spacer fabric has a lower resonance frequency than the thinner fabric due to lower stiffness, and thus can isolate the vibration at a lower frequency level. The results also show that changing the load mass and excitation level changes the loading conditions of the fabric structure, and thus also changes fabric stiffness and vibration isolation performance due to nonlinear behavior of spacer fabrics. It is expected that this study could provide some useful information to promote the application of weft-knitted spacer fabrics for vibration isolation.
Negative stiffness refers to a negative ratio of the force to the displacement in a deformed material system. It can be very useful in the design and fabrication of vibration isolation systems. In this study, a new kind of weft-knitted spacer structure that can achieve negative stiffness under compression was specially developed by using elastic yarn to knit the outer layers of spacer structure. Twelve fabric samples were knitted on an electronic flat knitting machine with three different linking distances and four diameters of spacer monofilaments. The compression tests were conducted to verify the negative stiffness effect of the fabrics after a steaming treatment. The results obtained have shown that the negative stiffness effect can be obtained for weft-knitted spacer fabrics in a special range of compression displacement if suitable fabric structure and fiber materials are used, and the decrease of the linking distance of spacer monofilaments can enhance the negative stiffness effect. It is expected that this study could provide useful information in the design and fabrication of spacer fabrics for vibration isolation.
Herein, a novel form of bicomponent nanofiber membrane containing stereo-complex polylactic acid (SC-PLA) was successfully produced by the side-by-side electrospinning of Poly (L-lactic acid) (PLLA) and Poly (D-lactic acid) (PDLA). We demonstrate that through these environmentally sustainable materials, highly efficient nanofiber assemblies for filtration can be constructed at very low basis weight. The physical and morphological structure, crystalline structure, hydrophobicity, porous structure, and filtration performance of the fibrous membranes were thoroughly characterized. It was shown that the fabricated polylactic acid (PLA) side-by-side fiber membrane had the advantages of excellent hydrophobicity, small average pore size, high porosity, high filtration efficiency, low pressure drop as well as superior air permeability. At the very low basis weight of 1.1 g/m2, the filtration efficiency and pressure drop of the prepared side-by-side membrane reached 96.2% and 30 Pa, respectively. Overall, this biomass-based, biodegradable filtration material has the potential to replace the fossil fuel-based polypropylene commercial meltblown materials for the design and development in filtration, separation, biomedical, personal protection and other fields.
Knitted spacer fabrics can be an alternative material to typical rubber sponges and polyurethane foams for the protection of the human body from vibration exposure, such as automotive seat cushions and anti-vibration gloves. To provide a theoretical basis for the understanding of the nonlinear vibration behavior of the mass-spacer fabric system under harmonic excitation, experimental, analytical and numerical methods are used. Different from a linear mass-spring-damper vibration model, this study builds a phenomenological model with the asymmetric elastic force and the fractional derivative damping force to describe the periodic solution of the mass-spacer fabric system under harmonic excitation. Mathematical expression of the harmonic amplitude versus frequency response curve (FRC) is obtained using the harmonic balance method (HBM) to solve the equation of motion of the system. Parameter values in the model are estimated by performing curve fit between the modeled FRC and the experimental data of acceleration transmissibility. Theoretical analysis concerning the influence of varying excitation level on the FRCs is carried out, showing that nonlinear softening resonance turns into nonlinear hardening resonance with the increase of excitation level, due to the quadratic stiffness term and the cubic stiffness term in the model, respectively. The quadratic stiffness term also results in biased vibration response and causes an even order harmonic distortion. Besides, the increase of excitation level also results in elevated peak transmissibility at resonance.
Weft-knitted spacer fabrics, a kind of three-dimensional fabric structure, are potential substitutes for typical rubber sponges and polyurethane foams for the protection of human body from exposure to vibrations. To explore the capability and the designability of weft-knitted spacer fabrics as the functional material of personal protective equipment against vibration, such as anti-vibration gloves and car cushions, the vibration behavior and physical properties of weft-knitted spacer fabrics manufactured using flat knitting technology were studied. In the first part of the article, the vibration behavior of top-loaded weft-knitted spacer fabric under harmonic base excitation was analyzed. The effects of monofilament diameter, linking distance and excitation acceleration level on the transmissibility curve of the mass-spacer fabric were evaluated. It was shown that to broaden the frequency range for vibration isolation, spacer fabrics with smaller monofilament diameter and longer linking distance were preferred. In the second part of the article, the effects of monofilament type, monofilament diameter, and spacer structure on the physical properties of spacer fabric including fabric shrinkage coefficient, fabric thickness, fabric stitch densities, and fabric areal mass were analyzed. It was found that fabric thickness was increased by employing spacer structures with longer linking distance and lower filling density of spacer monofilament. In addition, in order to obtain an optimized high fabric thickness, nylon monofilament was preferred to polyester monofilament. On the other hand, monofilament diameter has a significant influence on stitch densities and fabric areal mass. Larger monofilament diameter resulted in lower stitch densities and heavier areal mass for spacer fabric.
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