Composites (2)

• Composites


  Combination of two or more chemically distinct and insoluble phases where properties and structural performance are heightened compared to the constituents acting independently
• Addition of straw to clay


  • Oldest example of composites for making mud huts and bricks
• Reinforcing of masonry and concrete with iron rods

  • Steel rods impart necessary tensile strength to the composite, since concrete is brittle and generally has little or no useful tensile strength
  • Concrete itself is a composite material, consisting of cement, sand, and gravel
• Reinforced Plastics (PMC or FRP)


  Consist of fibers in a plastic matrix
• Fibers (the discontinuous or dispersed phase)


  • Strong and stiff and have high specific strength (strength-to-weight ratio) and specific stiffness (stiffness-to-weight ratio)
  • Brittle and abrasive, lack toughness, and can degrade chemically when exposed to the atmosphere
• Matrix (the continuous phase)


  • Less strong and less stiff but tougher than fibers
• Hybrid


  More than one type of fiber is used in a reinforced plastic
• Percentage of fibers (by volume) in reinforced plastics usually ranges between 10% and 60%
• Highest practical fiber content is 65%
• Higher % of fibers generally results in diminished structural properties
• Glass fibers


  • Most widely used and least expensive
  • Composite material is called Glass-fiber reinforced plastic (GFRP): 30-60% glass fibers by volume
  • Made by drawing molten glass through small openings in a platinum die
• Principal Types of Glass Fibers

  • E-type – calcium aluminoborosilicate glass (used the most)
  • S-type - a magnesia-aluminosilicate glass which has higher strength and stiffness and is more expensive
• Graphite fibers


  • More expensive than glass fibers, have a combination of low density, high strength, and high stiffness
  • Composite material is called Carbon-fiber reinforced plastic (CFRP)
  • Made by pyrolysis of organic precursors
• Aramid fibers


  • Among the toughest fibers and have very high specific strength
  • Marketed under the trade name Kevlar
  • Can undergo some plastic deformation before fracture and thus have higher toughness than brittle fibers
  • Absorb moisture (hygroscopic), which reduces their properties and complicates their application
• Boron fibers


  • Consist of boron deposited (by chemical vapor-deposition techniques) on tungsten fibers
  • Favorable properties, such as high strength and stiffness both in tension and compression and resistance to high temperatures
  • Because of the use of tungsten which have high density, they are heavy and also more expensive, thus increasing overall cost and weight of the reinforced plastic component
• Mean diameter of fibers in reinforced plastics is usually less than 0.01mm (0.0004 in.)
• Glass fibers can have tensile strengths as high as 46000 MPa (650 ksi), whereas the strength of glass in bulk form is much lower (glass fibers are stronger than steel)
• Short fibers


  If the mechanical properties improve as a result of increasing the fiber length, then it is denoted as a short fiber
• Long fibers


  When no additional improvement in properties occurs, it is denoted as a long fiber
• Other forms of fibers

  • Chopped fibers
  • Particles
  • Flakes
  • Continuous roving (slightly twisted strand of fiber)
  • Woven fabric (like cloth)
  • Yarn (twisted strand)
  • Mats of various combinations
  • Hybrid yarns
• Matrix functions

  • Support and transfer stresses to the fibers, which carry most of the load
  • Protect the fibers from physical damage and the environment
  • Reduce propagation of cracks in the composite by virtue of the greater ductility and toughness of the plastic matrix
• Common matrix materials

  • Epoxy
  • Polyester
  • Phenolic
  • Fluorocarbon
  • Polyesthersulfone
  • Silicon
• Most commonly used matrix materials are EPOXIES (80% of all reinforced plastics) and POLYESTERS which are less expensive than epoxies
• Properties of composites


  • Depend on the kind, shape, and orientation of the reinforcing material, the length of the fibers, and the volume fraction (percentage) of the reinforcing material
  • Short fibers are less effective than long fiber and their properties are strongly influenced by time and temperature
  • Long fibers transmit the load through the matrix better and thus are commonly used in critical applications, particularly at elevated temperatures
• Fiber-matrix interface

  • Critical factor is the strength of the bond between the fiber and the polymer matrix, since the load is transmitted through the fiber-matrix interface
  • Weak bonding causes fiber pullout and delamination of the structure, particularly under adverse environmental conditions
  • Adhesion at the interface can be improved by special surface treatments
• Optimal configuration of reinforced plastic part


  • Alignment of the fibers with the direction of the tension force for unidirectional reinforced structure
  • Transverse properties are much lower than longitudinal properties
  • Crisscrossed fibers in the matrix if it is to be subjected to forces in different directions; or layers of fibers oriented in different directions
  • Made with various other materials and shapes of the matrix in order to impart permeability and dimensional stability, make processing easier, and reduce costs
• Metal-Matrix Composite (MMC)


  Advantages: higher elastic moduli, resistance to elevated temperatures, and higher toughness and ductility

  Disadvantages: higher density and greater difficulty in processing
• Applications of MMC

  • MMC having silicon-carbide fibers and a titanium matrix are being used for beams, stiffeners, and frames and sporting goods
• Studies are in progress on developing techniques for optimal bonding of fibers to the metal matrix
• Ceramic-Matrix Composite (CMC)

  Ceramics are strong and stiff, and they resist high temperatures, but they generally lack toughness

  Composites are resistant to high temperatures and corrosive environments

  Matrix materials that retain their strength up to 1700 ºC (3000 ºF) are silicon carbide, silicon nitride, aluminum oxide, and mullite (a compound of aluminum, silicon, and oxygen)

  Carbon-carbon matrix composites retain much of their strength up to 2500 ºC (4500 ºF), although they lack oxidation resistance at high temperatures. Fiber materials are usually carbon and aluminum oxide
• Applications of CMC


  • Jet and automotive engines, deep-sea mining equipment, pressure vessels, structural components, cutting tools, and dies for the extrusion and the drawing of metals
• Composites is one of the major developments in materials in recent years and is one of the most important classes of engineered materials because of outstanding properties
• Composites have a wide range of applications in the aircraft, aerospace, and transportation industries, in sporting goods, and in structural components