composites

• Composites


  Mixtures of the other three types of materials (metals, ceramics, polymers)
• Composite material


  A material system composed of two or more physically distinct phases whose combination produces aggregate properties that are different from those of its constituents
• Composites


  • Can be designed to be very strong and stiff yet very light, having high strength-to-weight and stiffness-to-weight ratios compared to steel and aluminum
  • Have better fatigue properties and toughness than common engineering metals
  • Can be designed to not corrode like steel
  • Can achieve combinations of properties not attainable with metals, ceramics or polymers alone
  • Allow better appearance and control of surface smoothness
• Disadvantages and limitations of composites

  • Properties of many important composites are anisotropic (properties differ depending on the direction they are measured)
  • Many of the polymer-based composites are subject to attack by chemicals or solvents
  • They are generally expensive
  • Some of the manufacturing processes are slow and costly
• Examples of composites


  • Cemented carbides (tungsten carbide with cobalt binder)
  • Plastic molding compounds that contain fillers (e.g., cellulose fibers, wood flour)
  • Rubber mixed with carbon black
• Traditional composites


  Those that occur in nature or have been produced by civilizations for many years (e.g. wood, concrete, asphalt)
• Synthetic composites


  Modern material systems normally associated with the manufacturing industries, in which the components are first produced separately and then combined in a controlled way
• Composite material components


  Primary phase (matrix) and secondary phase (reinforcing agent)
• Possible matrix materials


  • Polymers
  • Metals
  • Ceramics
• Possible reinforcing materials


  • Fibers
  • Particles
  • Flakes
  • Infiltrated phase in a skeletal or porous matrix
• A composite, in the present context, is a multiphase material that is artificially made, as opposed to one that occurs or forms naturally, and the constituent phases must be chemically dissimilar and separated by a distinct interface
• Main types of composite materials

  • Particle-reinforced
  • Fiber-reinforced
  • Structural composites
• Properties of composite materials

  • Determined by the materials used as component phases, the geometric shapes of the constituents and resulting structure, and the manner in which the phases interact with one another
• Rule of Mixtures


  1. Density of composite = (mass of matrix * density of matrix + mass of reinforcing phase * density of reinforcing phase) / (volume of matrix + volume of reinforcing phase + volume of voids)

  2. Modulus of elasticity of fiber-reinforced composite can be estimated using the rule of mixtures
• Eq. (3)


  Equation 3
• can substitute these terms into Eq. (3) and conclude that (4)
• Rule of Mixtures


  The rule of mixtures can sometimes be used to estimate the modulus of elasticity of a fiber-reinforced composite made of continuous fibers
• Stress calculations


  Phase cross-sectional areas are necessary
• Hybrid composites


  Obtained by using two or more different kinds of fibers in a single matrix, have a better all-around combination of properties than composites containing only a single fiber type
• Fiber combinations


  • Carbon and glass fibers incorporated into a polymeric resin
• Carbon fibers


  • Strong and relatively stiff, provide a low-density reinforcement, but expensive
• Glass fibers


  • Inexpensive, lack the stiffness of carbon
• Glass-carbon hybrid


  • Stronger and tougher, has a higher impact resistance, and may be produced at a lower cost than either all-carbon or all glass reinforced plastics
• Ways the two different fibers may be combined

  Aligned and intimately mixed, or laminations with alternating layers of single fiber types
• Hybrid composites


  • Properties are anisotropic, failure is usually noncatastrophic (does not occur suddenly)
• Principal applications for hybrid composites

  • Lightweight land, water, and air transport structural components, sporting goods, and lightweight orthopedic components
• Advantages of composites over traditional engineering materials

  • Part integration, in-service monitoring/online process monitoring with embedded sensors, high specific stiffness, high specific strength, high fatigue strength, high corrosion resistance, design flexibility, net-shape or near-net-shape parts, complex parts and special contours, greater manufacturing feasibility, good impact properties, better noise/vibration/harshness characteristics, cost-effective manufacturing, design freedom by tailoring material properties, low smoke and toxicity, lower tooling cost
• Drawbacks of composites


  • High materials cost, lack of high-volume production methods, lack of design databases, limited temperature resistance, limited solvent/chemical resistance and environmental stress cracking, moisture absorption affecting properties and dimensional stability