20-10-2012, 01:03 PM
Composites in Action
Composites in Action.ppt (Size: 3.67 MB / Downloads: 78)
Large particle composites
Large particle composites
Involves large particles that are harder or stiffer than matrix.
The matrix transfers applied stress to the particles, which thus bear a fraction of the load.
Bonding at the interface is necessarily important.
Particles should be:
Equiaxed
Uniformly distributed
Properties generally determined by the rules of mixtures.
Large Particle Composite Examples
Cermets (not cements) are ceramic-metal composites
Cermented Carbide—cutting tools
WC or TiC particles (incredibly hard)
Metal matrix (Co or Ni)
The particles will crack under the high stresses in cutting applications, so the matrix prevents crack propagation between particles by separating them.
Up to 90 volume percent of particles.
Polymer/Carbon composites include
Tires
Elastomer matrix with carbon black particles (15-30 vol%).
Improved tensile strength, tear and abrasion resistance, and toughness.
Small particles are optimal, <50 nm.
Ceramic-ceramic composites include
Concrete is:
~70 vol% sand and gravel particles (different sizes promotes better packing).
Portland cement is the binder once water is added.
Improved tensile, compressive, and shear response by reinforcing with steel rods, bars (rebar), wires, or wire mesh (ceramic-ceramic-metal composite).
Steel is selected for thermal expansion coefficient
Not corroded during cement hardening
Strong composite/matrix bond is possible, especially if the steel surface is contoured
Pre stressing
Fiber composites
Why are we using fibers?
Especially for ceramics, due to Weibull statistics the fracture strength of a small part is usually greater than that of a large component (smaller volume=fewer flaws=fewer big flaws).
Fibers come in three forms
Whiskers (graphite, SiC, Si3N4, Al2O3)
Single crystals
Huge length/diameter
Small, so nearly flaw free
Strongest known materials
expensive
Fibers (aramids, glass, carbon, boron, Si3N4, Al2O3)
Polycrystalline or amorphous
Small diameter
Wires (usually metals)
Large diameter
Matrix phase
Usually a metal or polymer since some ductility is desirable
Serves several functions for fiber composites
Bonds with the fibers (Very important).
Protect fibers from surface damage due to abrasion or corrosion (i.e., avoid cracks on surfaces of fibers).
Separate the fibers.
Prevent propagation of brittle cracks between fibers.
SUMMARY
What is the matrix in a composite and what materials are commonly used as a matrix?
What are the possible strengthening mechanisms for particle reinforced composites (there are 2)?
Be able to calculate upper and lower bounds for the Young’s modulus of a large particle composite.
Know the equation for the critical length (Lc) of a fiber.
Know the stress distribution on fibers of various lengths w/r Lc in a composite.