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Title: A study of vibration behaviour of weft-knitted spacer fabrics
Authors: Chen, Fuxing
Degree: Ph.D.
Issue Date: 2016
Abstract: Weft-knitted spacer fabrics are textile structures commonly consisting of two outer layers that are connected by spacer yarns. As a potential substitute for traditional foams in anti-vibration applications such as gloves and cushion pads, they are superior in terms of air permeability and thermoregulation. In former studies, spacer fabric with top-loaded mass was simply treated as a one-degree-of-freedom mass-spring-damper system. However, due to the nonlinear elastic force of spacer fabric, linear vibration models are only valid for very small excitation levels. To investigate its vibration performance under large excitations, nonlinear force-displacement relationship is considered in building the equation of motion, where models containing the symmetric elastic force and the asymmetric elastic force are compared. Besides, the viscoelasticity of spacer fabric is represented by a fractional derivative term. This study follows a procedure as below. Firstly, weft-knitted spacer fabrics with structural variations are manufactured. Secondly, forced vibration experiments are carried out under different excitation levels and load mass. Thirdly, theoretical models concerned with the nonlinear elastic force-displacement relationship and the viscoelasticity of spacer fabric are established, and then model parameters are identified by fitting models with experimental results. Fourthly, the effects of model parameters on frequency response curves are analysed. Moreover, numerical simulations are also presented for solving the periodic solutions and the aperiodic solutions. Weft-knitted spacer fabrics were firstly knitted on a STOLL CMS 822 computerized flat knitting machine of gauge 14. Polyester monofilaments tucked alternately on two outer layers, forming a convoluted structure. The difference in fabric structure was achieved by varying the tucking distance of monofilaments, and thus two fabric structures were obtained. Two identical fabrics were laminated into one sample in order to balance the transverse shift of fabric under vertical force. In vibration experiments, sine sweep tests were carried out using an electromagnetic vibration exciter to record the acceleration transmissibility curves of the mass-spacer fabric systems. The influences of load mass, fabric structure and excitation level were studied. Results show that increasing the weight of load mass initially shifted the resonance frequency to lower values but then it rose again. Thicker spacer fabric exhibits better force isolation performance. It was also revealed that with a light load mass, increasing the vibration level could give rise to a nonlinear softening type of transmissibility curve, and a broadened isolation region could also be achieved.
To build the equation of motion under forced harmonic vibration, nonlinear stiffness terms were applied. Phenomenological models with the symmetric elastic force and with the asymmetric elastic force were studied separately. The major difference lies in the quadratic stiffness term which breaks symmetry in the elastic force-displacement relationship.Besides, viscoelasticity was expressed as a fractional derivative term in both models. The frequency-domain solutions to the equations of motion were obtained using harmonic balance method (HBM) with the first-order approximation. The effects of model parameters on the transmissibility and amplitude-frequency curves were analysed. Next, model parameters were identified by fitting theoretical models with experimental results. Root mean square error (RMSE) was adopted as the indicator of goodness of fit.Issues including the highest-order stiffness coefficient, the physical significance of the fractional order and the model redundancy were discussed. RMSEs using the symmetric model and the asymmetric model were compared. Results showed that the asymmetric model performed slightly better than the symmetric model. Moreover, the fractional derivative term improved the goodness of fit to a certain extent.Following parameter identification, MATLAB/Simulink block diagram was utilized to obtain periodic solutions of the equation of motion. The difference in the frequency response curves between using numerical simulation and the HBM approximation was explained. Besides, bifurcation and chaotic motions are also observed numerically with varied conditions.In brief, the vibration behavior of weft-knitted spacer fabric has been explored with the use of experimental, analytical and numerical methods. Nonlinear softening phenomenon is correlated with the polynomial force-displacement relationship. And the fractional derivative term is used to account for the viscoelasticity in the system. This study provides a better understanding of the vibration behavior of weft-knitted spacer fabric, and experimental data benefiting applied research in the future.
Subjects: Textile fabrics.
Hong Kong Polytechnic University -- Dissertations
Pages: xix, 228 pages : color illustrations (some color)
Appears in Collections:Thesis

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