A numerical study on the thermal expansion coefficients of fiber
A numerical study on the thermal expansion coefficients of fiber A numerical study on the thermal expansion coefficients of fiber
9 ceramic matrix composites (CMC). The most widely used composites are PMCs. These are mainly used in ambient temperature applications. MMCs are commonly used to increase the strength of low density metals. CMCs are used extensively in high temperature applications which require high strength and toughness characteristics. Metal and ceramic matrix composites are relatively new technologies. This is evident when observing the extent of their application, as it is limited to high performance components and assemblies on advanced equipment (Chawla, 1998). 2.2.1.1 Polymer Matrix Materials Polymers are structurally much more complex than metals or ceramics. They are cheap and can easily be processed. On the other hand, polymers have lower strength and modulus and lower temperature use limits. Prolonged exposure to ultraviolet light and some solvents can cause the degradation of polymer properties. Because of predominantly covalent bonding, polymers are generally poor conductors of heat and electricity. However, they are generally more resistant to chemicals than are metals. Polymers are giant chainlike structures with covalently bonded carbon atoms forming the backbone of the chain. The process of forming large molecules from small ones is called polymerization; that is, polymerization is the process of joining many monomers, the basic building blocks, together to form polymers. Different molecular chain configurations of polymers are given in Figure 2.1. Based on their behavior, there are two major classes of polymers, thermoset and thermoplastic polymers. Polymers that soften or melt on heating are called thermoplastic polymers and are suitable for liquid flow forming. Cooling to room temperature hardens thermoplastics. Their different behavior, however, comes from their molecular structure and shape, molecular size or mass, and the amount and type of bonds (covalent or van der Waals). Examples of thermoplastics include low and high density polyethylene, polystyrene, and polymethyl methacrylate (PMMA). When the molecules in a polymer are crosslinked in the form of a network, they do not soften on heating. Such cross-linked polymers are called thermosetting
10 polymers. Thermosetting polymers decompose on heating. Crosslinking makes sliding of molecules past one another difficult, making the polymer strong and rigid. A typical example is that of rubber crosslinked with sulfur which is called vulcanized rubber. Vulcanized rubber has ten times the strength of natural rubber. Common examples of thermosetting polymers include epoxy, phenolic, polyester, vinyl ester, polyurethane, and silicone. Figure 2.1 Different molecular chain configurations; linear (a), branched (b), crosslinked (c), ladder (d) (Chawla, 1998). Common types of resin used to provide the composite matrix are polyester, vinyl ester and epoxy. Polymers are not recognized as the strongest of materials, but in the thermosetting resin form they provide excellent abilities to be molded into complex shapes and to adhere strongly to the fibers. Accelerator and catalyst elements can be added to the resins in varying amounts to allow the polymerization reaction to be controlled to provide varying material properties. Use of the catalyst element needs to be carefully controlled to ensure the reaction is not too rapid and to avoid insufficient curing. Polyester resins are the most widely used polymer matrix material; this is maybe due to the fact that they can be used to construct composites without the need to introduce pressure. Vinyl ester resins are similar to polyesters but provide better resistance to chemical and water attack and also display improved toughness. Epoxy resins exhibit higher levels of mechanical strength and they can so be more resilient to environmental attack. The reinforcing materials that are widely used for PMCs are glass, carbon, aramid and boron.
- Page 1 and 2: DOKUZ EYLÜL UNIVERSITY GRADUATE SC
- Page 3 and 4: M.Sc THESIS EXAMINATION RESULT FORM
- Page 5 and 6: A NUMERICAL STUDY ON THE THERMAL EX
- Page 7 and 8: CONTENTS Page THESIS EXAMINATION RE
- Page 9 and 10: 4.2.1 Geometry Creation............
- Page 11 and 12: 2 coefficients of thermal expansion
- Page 13 and 14: 4 coefficients coincide to give res
- Page 15 and 16: 6 concentration from pure metal to
- Page 17: 8 high strength-to-weight and stiff
- Page 21 and 22: 12 Metals are strong and tough. The
- Page 23 and 24: 14 Table 2.1 Properties of reinforc
- Page 25 and 26: 16 2.2.2.2 Carbon Fibers Carbon is
- Page 27 and 28: 18 use is in aircraft industry foll
- Page 29 and 30: 20 strength and a reasonable Young
- Page 31 and 32: 22 1. Processing the conventional f
- Page 33 and 34: 24 (orthorhombic) of polyethylene h
- Page 35 and 36: 26 Whiskers are monocrystalline sho
- Page 37 and 38: 28 3.2 Factors Affecting the Coeffi
- Page 39 and 40: 30 3.2.4 Thermal Cycling The primar
- Page 41 and 42: 32 3.3.1 Mechanical Dilatometry Thi
- Page 43 and 44: 34 absolute accuracy of about ± 0.
- Page 45 and 46: 36 3.3.3 Strain Gauges This relativ
- Page 47 and 48: 38 • The composite is macroscopic
- Page 49 and 50: 40 3.4.1.3 Equation of Van Fo Fy In
- Page 51 and 52: 42 and the thermal expansion coeffi
- Page 53 and 54: 44 P P 11 33 2 A 22 − A = Det A A
- Page 55 and 56: 46 • A perfect bonding exists at
- Page 57 and 58: CHAPTER FOUR FINITE ELEMENT METHOD
- Page 59 and 60: 50 No matter how the geometry is cr
- Page 61 and 62: 52 displacements and/or rotations a
- Page 63 and 64: CHAPTER FIVE MICROMECHANICAL ANALYS
- Page 65 and 66: 56 5.2 Mesh Creation 10-node tetrah
- Page 67 and 68: 58 carbon fibers were assumed to ha
10<br />
polymers. Thermosetting polymers decompose <strong>on</strong> heating. Crosslinking makes<br />
sliding <strong>of</strong> molecules past <strong>on</strong>e ano<strong>the</strong>r difficult, making <strong>the</strong> polymer str<strong>on</strong>g and rigid.<br />
A typical example is that <strong>of</strong> rubber crosslinked with sulfur which is called vulcanized<br />
rubber. Vulcanized rubber has ten times <strong>the</strong> strength <strong>of</strong> natural rubber. Comm<strong>on</strong><br />
examples <strong>of</strong> <strong>the</strong>rmosetting polymers include epoxy, phenolic, polyester, vinyl ester,<br />
polyurethane, and silic<strong>on</strong>e.<br />
Figure 2.1 Different molecular chain c<strong>on</strong>figurati<strong>on</strong>s; linear (a), branched (b), crosslinked (c), ladder<br />
(d) (Chawla, 1998).<br />
Comm<strong>on</strong> types <strong>of</strong> resin used to provide <strong>the</strong> composite matrix are polyester,<br />
vinyl ester and epoxy. Polymers are not recognized as <strong>the</strong> str<strong>on</strong>gest <strong>of</strong> materials, but<br />
in <strong>the</strong> <strong>the</strong>rmosetting resin form <strong>the</strong>y provide excellent abilities to be molded into<br />
complex shapes and to adhere str<strong>on</strong>gly to <strong>the</strong> <strong>fiber</strong>s. Accelerator and catalyst<br />
elements can be added to <strong>the</strong> resins in varying amounts to allow <strong>the</strong> polymerizati<strong>on</strong><br />
reacti<strong>on</strong> to be c<strong>on</strong>trolled to provide varying material properties. Use <strong>of</strong> <strong>the</strong> catalyst<br />
element needs to be carefully c<strong>on</strong>trolled to ensure <strong>the</strong> reacti<strong>on</strong> is not too rapid and to<br />
avoid insufficient curing. Polyester resins are <strong>the</strong> most widely used polymer matrix<br />
material; this is maybe due to <strong>the</strong> fact that <strong>the</strong>y can be used to c<strong>on</strong>struct composites<br />
without <strong>the</strong> need to introduce pressure. Vinyl ester resins are similar to polyesters but<br />
provide better resistance to chemical and water attack and also display improved<br />
toughness. Epoxy resins exhibit higher levels <strong>of</strong> mechanical strength and <strong>the</strong>y can so<br />
be more resilient to envir<strong>on</strong>mental attack. The reinforcing materials that are widely<br />
used for PMCs are glass, carb<strong>on</strong>, aramid and bor<strong>on</strong>.
Change language