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Title: Study of auxetic composites made with multilayered orthogonal structural reinforcement
Authors: Jiang, Lili
Degree: Ph.D.
Issue Date: 2017
Abstract: As good candidates for being used as energy absorption materials, polymeric foams have been widely used in the cushioning, packaging and impact protection areas due to their characteristics of large plastic deformation, inelastic energy conversion and moderate reactive force. But at the same time, the plastic deformation, collapse of cell walls and internal fracture can easily cause the damage and failure of foams. There are ways that are commonly used to improve the mechanical properties of foams like filling the chopped fibers and small particles to the plain foams. However, the continuous long fibers and integral structural reinforcements are seldom used for mechanical enhancement due to the increase of foam weight and the difficulties in manufacturing. Therefore, the light-weight plastic tubes and high-performance yarns are proposed and selected as the reinforcement materials to enhance the plain foams without decreasing the strength to weight ratio. Meanwhile, the negative Poisson's ratio effect will be achieved by creative structure design of reinforcements, which would further enhance the mechanical properties of foams under compression and impact loading. Optimal synchronistic effect could be achieved by structure design and material selection to maximum the peculiarities of each constituent material in composite. The structure for the developed composite in this study is multilayered orthogonal auxetic structure and the component materials are rigid ABS open tubes, high-strength low-extension polyester yarns and flexible open cell polyurethane foam. The newly manufactured composite materials were named "auxetic composites made with multilayered orthogonal structural reinforcements".
Auxetic composites are non-conventional materials with negative Poisson's ratio (NPR). They have been receiving great attentions due to special properties and large application potentials. In this research, novel kinds of auxetic composites were proposed and fabricated via an injecting and foaming techniques by using multilayer auxetic orthogonal structure as reinforcements and flexible open-cell polyurethane foams as matrix. The NPR effect and mechanical behavior of the composites under static and dynamic compression tests were investigated using experimental and numerical methods. The quasi-static compression results showed that auxetic composites exhibited obvious NPR effect and behaved more like damping materials with a big range of deformation strain. Fiber pull-out tests demonstrated that strong interfacial bonding between the reinforcements and matrix could ensure the desired deformation of structural reinforcements and auxetic effect of the composite. The low-velocity drop-weight impact results revealed that the mechanical responses of composites including the impact contact force, impact stress as well as energy absorbed were strain rate sensitive. The cross-sections shape and dimension of ABS tubes could seriously influence the mechanical responses of auxetic composites. A three-dimensional finite element model about low-velocity impact on the auxetic composites was achieved by using in Ansys/Ls Dyna Explicit codes. Good agreement in the overall trends of stress strain curves for the FEM and experimental results were found. The deformation process of composites in the simulation was captured and compared with experimental to explore the damage mechanisms of materials. It was concluded that the auxetic composite had good energy absorption capacities and could be used individually or served as filling materials in sandwich materials under different compression conditions.
Subjects: Hong Kong Polytechnic University -- Dissertations
Plastic foams
Pages: xvi, 168 pages : color illustrations
Appears in Collections:Thesis

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