ch16

• Composite materials


  • Multiphase materials that exhibit a significant proportion of the properties of both constituent phases such that a better combination of properties is realized
  • Composed of just two phases - a matrix phase and a dispersed phase
  • Properties are a function of the properties of the constituent phases, their relative amounts, and the geometry of the dispersed phase
• Main divisions of composite materials

  • Particle-reinforced
  • Fiber-reinforced
  • Structural
  • Nanocomposites
• Large-particle composites


  • Dispersed phase is equiaxed (particle dimensions are approximately the same in all directions)
  • Reinforcement or strengthening mechanism is based on particle-matrix interactions that cannot be treated on the atomic or molecular level
• Dispersion-strengthened composites


  • Dispersed phase particles are much smaller, with diameters between 0.01 and 0.1 μm
  • Strengthening mechanism is similar to precipitation hardening, where small dispersed particles hinder or impede the motion of dislocations
• Rule of mixtures


  Mathematical expressions that predict the dependence of the elastic modulus on the volume fraction of the constituent phases for a two-phase composite
• Cermets are examples of ceramic-metal composites, with extremely hard particles of a refractory carbide ceramic embedded in a ductile metal matrix
• Elastomers and plastics are frequently reinforced with various particulate materials such as carbon black, which enhances tensile strength, toughness, and abrasion resistance
• Concrete


  A large-particle composite in which both matrix and dispersed phases are ceramic materials
• Carbon black


  For the carbon black to provide significant reinforcement, the particle size must be extremely small, with diameters between 20 and 50 nm; also, the particles must be evenly distributed throughout the rubber and must form a strong adhesive bond with the rubber matrix
• Particle reinforcement using other materials (e.g., silica) is much less effective because this special interaction between the rubber molecules and particle surfaces does not exist
• Concrete


  A common large-particle composite in which both matrix and dispersed phases are ceramic materials
• The terms concrete and cement are sometimes incorrectly interchanged, it is appropriate to make a distinction between them
• Concrete


  A composite material consisting of an aggregate of particles that are bound together in a solid body by some type of binding medium, that is, a cement
• Most familiar concretes


  • Those made with Portland cement
  • Those made with asphaltic cement
• Asphaltic concrete is widely used primarily as a paving material, whereas Portland cement concrete is employed extensively as a structural building material
• Portland cement concrete


  The ingredients are Portland cement, a fine aggregate (sand), a coarse aggregate (gravel), and water
• Production of Portland cement and mechanism of setting and hardening

  Discussed in Section 13.7
• Aggregate particles


  Act as a filler material to reduce the overall cost of the concrete product because they are cheap, whereas cement is relatively expensive
• To achieve the optimum strength and workability of a concrete mixture, the ingredients must be added in the correct proportions
• Dense packing of the aggregate and good interfacial contact are achieved by having particles of two different sizes; the fine particles of sand should fill the void spaces between the gravel particles
• Typically, these aggregates constitute between 60% and 80% of the total volume
• The amount of cement–water paste should be sufficient to coat all the sand and gravel particles; otherwise, the cementitious bond will be incomplete
• All of the constituents should be thoroughly mixed
• Complete bonding between cement and the aggregate particles is contingent on the addition of the correct quantity of water
• Too little water leads to incomplete bonding, and too much results in excessive porosity; in either case, the final strength is less than the optimum
• Aggregate particles


  • The size distribution influences the amount of cement–water paste required
  • The surfaces should be clean and free from clay and silt, which prevent the formation of a sound bond at the particle surface
• Portland cement concrete


  • It can be poured in place and hardens at room temperature and even when submerged in water
  • As a structural material, it has some limitations and disadvantages: it is relatively weak and extremely brittle, with tensile strength approximately one-fifteenth to one-tenth its compressive strength
  • Large concrete structures can experience considerable thermal expansion and contraction with temperature fluctuations
  • Water penetrates into external pores, which can cause severe cracking in cold weather as a consequence of freeze–thaw cycles
• Most of the inadequacies of Portland cement concrete may be eliminated or at least reduced by reinforcement and/or the incorporation of additives
• Reinforced concrete


  The strength of Portland cement concrete may be increased by additional reinforcement, usually accomplished by means of steel rods, wires, bars (rebar), or mesh, which are embedded into the fresh and uncured concrete
• Steel as reinforcement material


  • Its coefficient of thermal expansion is nearly the same as that of concrete
  • It is not rapidly corroded in the cement environment
  • A relatively strong adhesive bond is formed between it and the cured concrete, which may be enhanced by the incorporation of contours into the surface of the steel member
• Portland cement concrete may also be reinforced by mixing fibers of a high-modulus material such as glass, steel, nylon, or polyethylene into the fresh concrete
• Care must be exercised in using this type of reinforcement because some fiber materials experience rapid deterioration when exposed to the cement environment
• Prestressed concrete


  Another reinforcement technique for strengthening concrete involves the introduction of residual compressive stresses into the structural member
• Prestressing techniques


  1. High-strength steel wires are positioned inside the empty molds and stretched with a high tensile force, which is maintained constant. After the concrete has been placed and allowed to harden, the tension is released. As the wires contract, they put the structure in a state of compression because the stress is transmitted to the concrete via the concrete–wire bond that is formed.

  2. Sheet metal or rubber tubes are situated inside and pass through the concrete forms, around which the concrete is cast. After the cement has hardened, steel wires are fed through the resulting holes, and tension is applied to the wires by means of jacks attached and abutted to the faces of the structure. Again, a compressive stress is imposed on the concrete piece, this time by the jacks. Finally, the empty spaces inside the tubing are filled with a grout to protect the wire from corrosion.
• Concrete that is prestressed should be of high quality with low shrinkage and low creep rate
• Prestressed concretes, usually prefabricated, are commonly used for highway and railway bridges
• Dispersion-strengthened composites


  Metals and metal alloys may be strengthened and hardened by the uniform dispersion of several volume percent of fine particles of a very hard and inert material
• Strengthening mechanism


  Interactions between the particles and dislocations within the matrix, as with precipitation hardening
• The dispersion strengthening effect is not as pronounced as with precipitation hardening; however, the strengthening is retained at elevated temperatures and for extended time periods because the dispersed particles are chosen to be unreactive with the matrix phase
• Dispersion-strengthened composites


  • Thoria-dispersed (or TD) nickel
  • Sintered aluminum powder (SAP)