COMPOSITE_MATERIALS_module_2_FIBER_a

COMPOSITE_MATERIALS_module_2_FIBER_a - 1 ME 624: MECHANICS...

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Unformatted text preview: 1 ME 624: MECHANICS OF COMPOSITE MATERIALS Module 2: Fiber Reinforcement (Material Based on: Composite Materials by Matthews & Rawlings and other References) 2 I. Introduction In fiber-reinforced composites, fibers used to reinforce the matrix typically possess these desirable properties: 1. high strength 2. high stiffness 3. low density (weight) 4. etc ….. 3 II. Natural Fibers 4 II. Natural Fibers: re mostly plant fibers (such as otton, jute, emp , ..) s well as naturally occurring mineral (rock) fibers as asbestos 5 II. Synthetic Fibers 6 III. Synthetic Fibers: he “ Fiber” Reinforcement Constituent: Fibers made from their respective bulk materials possess UCH higher strengths . This is due to the fact that materials fail due to the presence of inherent surface flaws. Due to the presence of inherent surface flaws: The larger the critical flaw, the lower the fracture stress. The lower the sample volume, the smaller on average the size of the maximum flaw Æ the greater the strength. 7 he “ Fiber” Reinforcement Constituent: So, what is the secret behind Fibers that are many times Stronger than their monolithic Siblings?? 8 In PMC’s , the common fibers are: Carbon Glass Aramid (Kevlar) Polyethylene (PE) he “ Fiber” Reinforcement Constituent: In MMC’s and CMC’s , the use of Ceramic reinforcements in fiber, whisker, and particle forms are common. 9 The use of fibers as high-performance engineering materials is based on that 1. A small diameter with respect to its grain size or other micro- structural unit . This allows a higher fraction of the theoretical strength to be attained than that possible in a bulk form . This is a direct result of the so-called size effect; that is, the smaller the size, the lower the probability of having imperfections in the material. The following Figure shows that the strength of a carbon fiber decreases as its diameter increases . 10 2. A high aspect ratio (length/diameter L/d) allows a large fraction of the applied load to be transferred via the matrix to the stiff and strong fiber. The use of fibers as high-performance engineering materials is based on that 3. A very high degree of flexibility that is a characteristic of a material having a high modulus and a small diameter. This flexibility permits a variety of techniques to be employed for making composites ith these fibers. A measure of the flexibility is taken as 1/MR here M is the bending moment and R is the radius of curvature . plot of the various materials in fibrous form which have a flexibility (1/MR) of a 25 icrometer diameter flexible nylon fiber (d in figure is the diameter). 11 he “ Fiber” Reinforcement Constituent: herefore the fracture stress of a fiber is much greater than hat of a bulk (monolithic) sample of the same material. 12 13 Again, the larger the diameter of the fiber, the lower the fiber’s strength: he “ Fiber” Reinforcement Constituent: 14 (Lack of) Toughness: Most synthetic fibers show a brittle behavior w/purely elastic deformation up to failure he “ Fiber” Reinforcement Constituent: 15 A Quick Materials Properties Guide to the selection of synthetic fiber reinforcement: 16...
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COMPOSITE_MATERIALS_module_2_FIBER_a - 1 ME 624: MECHANICS...

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