Composites

• Composites


  Any material made from two or more distinct constituent materials (e.g. wood)
• Dispersed Phase


  It provides desirable material properties like high strength or improved ductility, usually ceramic or metal
• Composites categorized by dispersed material form

  • Particle-reinforced composite
  • Short fiber-reinforced composite
  • Continuous fiber-reinforced composite
• Matrix Phase


  It is used to form a mechanical and chemical bond with the elements of the dispersed phase and allows loads to be transferred between them— protecting dispersed phase from the environment
• Composites categorized by matrix material

  • Polymer matrix composite
  • Ceramic matrix composite
  • Metal matrix composite
• Fiber-reinforced, Polymer Matrix Composites


  Most widely used composite materials in engineering applications
• Fiber-reinforced polymer matrix composites

  • Glass Reinforced Polymers (GRP or Fiberglass)
  • Carbon Fiber Reinforced Polymers (CFRP)
• Fiber-reinforced polymer matrix composites

  • Have an epoxy matrix which is a thermosetting polymer and the dispersed material is glass or carbon fibers which make up around 60% of the material by volume
  • Have many useful properties including internal damping, good corrosion resistance, and interesting thermal properties— relatively poor conductors of heat and low thermal expansion coefficients
  • Have drawbacks including high cost, difficult to design and model behavior due to anisotropic nature and varied failure modes, difficult to integrate into assembly, relatively brittle, and not suitable for high temperatures (not much higher than 100 or 200 degrees Celsius)
• Unidirectional Tape


  The most basic form of fiber reinforcement; fiber reinforcement with all fibers running in the same direction
• Unidirectional Tape


  1. The individual fibers are grouped together into bundles which are held together with stitching or using a chemical binder

  2. In the case of carbon fibers, these bundles are called tows

  3. Each tow usually contains anywhere from 3000 to 24000 individual fibers

  4. A typical fiber is around 10 microns in diameter which is 10 times thinner than human hair
• Fiber-reinforced materials with fibers in the same direction

  • Highly anisotropic, with different material properties in different directions
  • If you apply a load along the axis of the fibers, the material will be much stronger and stiffer than if you apply it perpendicular to the axis because the load is taken by the stronger and stiffer fibers instead of by the matrix
• Pressure Vessels


  1. Fibers can be aligned mostly in the hoop direction to handle hoop stress (the largest stress) when pressurized

  2. Axial or helical fibers provide reinforcement in the axial direction
• Components made from fiber-reinforced materials

  1. Built up by stacking multiple layers with different fiber orientations, each layer called a lamina or ply, and the stack called the laminate

  2. The 0 degree layer in laminate provides strength and stiffness in the axial direction while the 90 degree layer provides it in the transverse direction

  3. The 45 degree layers provide it in the shear direction
• Quasi-isotropic Laminate


  If enough layers are stacked with the correct orientations, the laminate can have very similar properties in all of the in-plane directions
• Weave patterns

  • Have good stiffness and strength along the two fiber axes but are weak at 45 degrees and should be layered in different directions if quasi-isotropic properties are needed
  • A twill weave is more flexible and will conform more easily to a curved surface than a plain weave
• Assembling the laminate structure

  The different fiber layers need to be assembled and combined with the polymer matrix to create the final composite part
• Wet Layup Method

  Fiber layers are built up in a mold and resin is applied to each layer using a roller or brush
• Prepreg Method


  Tapes or sheets of fibers pre-impregnated in partially cured epoxy resin are applied to the mold without needing additional resin
• Polymer matrix curing


  Usually a thermoset that irreversibly hardens when heated, curing is done at elevated temperatures in an oven
• Filament Winding


  Machine winds unidirectional tape impregnated with resin around a mandrel, then the structure is cured
• Injection Molding


  Used for composites reinforced with short fibers, where fiber orientation can be arbitrary
• Young's Modulus


  Represents the stiffness of the material by the ratio of tensile stress to tensile strain
• Impressive strength of fiber-reinforced composites

  • Due to the small diameter of the reinforcing fibers
  • The larger the fiber, the more likely it will contain defects
  • Bundles with smaller fibers will be stronger
  • The smaller the fibers, the larger the surface area between the fibers and the matrix which means better load transfer between the two
  • Strength of a fiber increases significantly as the fiber diameter reduces
  • CFRP will fail at very low strains
  • The main thing limiting the use of ever thinner fibers is manufacturing constraints
• Kevlar


  • A type of Aramid fiber
• Kevlar-reinforced Polymers


  • Stiffer and stronger than GRP, more ductile than CFRP, and lighter than both, making them ideal for applications requiring excellent impact resistance like body armor
• Composite materials

  • Significantly increase toughness when fibers are added to the matrix
• Composite materials

  • Adding silicon carbide fibers to a silicon carbide matrix is used in high temperature jet engine turbine blades
  • Carbon-carbon composites have applications in spacecraft heat shields to protect from the extremely high temperatures during atmospheric re-entry
• Metal matrix composites

  Often used to improve strength or stiffness of a metal
• Metal matrix composites

  • Magnesium in biomedical engineering is used for implants designed to heal bone fractures and it biodegrades in the body, eliminating the need for a second surgery to remove the implant
  • Replacing pure magnesium with a composite of magnesium matrix and dispersed ceramic particles controls degradation rate and improves material strength and properties
• Particle-reinforced materials

  • Heat spreaders with copper matrix and diamond particles have higher thermal conductivity allowing the dissipation of heat more effectively
  • Concrete is an example of a particle-reinforced material with cement as the matrix phase and aggregate (mixture of sand and crushed stone) as the dispersed phase
• Sandwich Composites


  • Lightweight core material (typically foam or honeycomb structure) sandwiched between thin skin layers made of stronger and stiffer materials such as metals like aluminum or composites like CFRP
  • Has high bending stiffness
  • Under loading, it behaves in a similar way to an I-beam, with outer layers carrying bending loads and the core carrying shear loads and increasing the second moment of area of the cross-section
  • Inserts are incorporated into the panel to allow the use of threaded fasteners
• Sandwich Composites

  • Honeycomb panels are used extensively in satellites as structural panels to which instruments and communication equipment can be attached
• E-glass- the most commonly used type and was developed for electrical insulation applications.
• S-glass- developed for structural applications and has improved strength.
• Glass-fiber reinforced polymers- have lower stiffness but very good tensile strength
• Carbon fiber reinforced polymers- have unbelievable strength-to-weight and stiffness-to-weight ratios and is why they are commonly used in industries where weight reduction is critical like aerospace, automotive industries, and sports.
• Glass-fiber reinforced composites- have lower stiffness than CFRP but excellent strength properties on a per-weight basis and are more cost-effective, often used in wind turbine blades and in the construction of boats where light weight, high strength, and low cost are critical parameters.