Chapter 16 Flashcards
What is a composite?
- A combination of two or more individual materials
- Multiphase material that is artificially made
What are the two different phase types?
- Matrix
- Dispersed
What is the matrix phase type?
The continuous phase that surrounds the dispersed phase
What is the dispersed phase type?
Discontinuous phase and is surrounded by the matrix phase
What is the purpose of the matrix phase?
- Transfer stress to the dispersed phase
- Protect the dispersed phase from the environment
What are the main types of composites?
- MMC (metal matrix composite)
- CMC (ceramic matrix composite)
- PMC (polymer matrix composite)
What is the goal of composites?
To obtain a more desirable combination of properties
What are the types of dispersed phases?
- Particle
- Fiber
- Structural
What is the purpose of a dispersed phase in an MMC?
To increase yield strength, tensile strength, and creep resistance
What is the purpose of a dispersed phase in a CMC?
To increase fracture toughness
What is the purpose of a dispersed phase in a PMC?
Increase Young’s Modulus (stiffness), yield strength, tensile strength, and creep resistance
What are the four main classes of composites?
- Particle-reinforced
- Fiber-reinforced
- Structural
- Nano
What are the two types of particle-reinforced composites?
- Large-particle
- Dispersion-strengthened
What are the two types of fiber-reinforced composites?
- Continuous (aligned)
- Discontinuous (short)
> Aligned or randomly oriented
What are the two types of structural composites?
- Laminates
- Sandwich panels
What are some examples of particle-reinforced composites?
- Spheroidte steel
> Ferrite matrix (ductile) & cementite (brittle) - WC/Co cemented carbide
> Cobalt matrix (ductile, tough) & WC (brittle, hard) - Automobile tire rubber
> Rubber matrix (compliant) & carbon black (stiff) - Concrete
What is the composition of concrete?
- Gravel
- Sand
> Sand fills the voids between gravel - Cement
- Water
What are the types of concrete treatments?
- Reinforcing
- Prestressing
- Posttensioning
What is reinforced concrete?
- Concrete reinforced with steel rebar or remesh
- Increases strength, even if concrete matrix cracks
What is prestressed concrete?
- Rebar/remesh is placed under tension during setting
- Tension is released after setting which places concrete in compression
- To fracture, tensile stress must exceed the compressive stress
What is posttensioned concrete?
- Nuts are placed on ends of rebar
- Tightened to place concrete under compression
What is the rule of mixtures?
- The properties of a composite are based on an average of the properties of the constituents
- Often based on volume
What is dispersion strengthening?
- Dispersing very small particles of a hard, inert phase within a load-bearing matrix phase
- Particles hinder displacement motion
What is large-particle strengthening?
- Larger particles are placed within a matrix
- Matrix transfers stresses to particles
What are the properties of particle-reinforced composites?
Isotropic
Why are fibers used to reinforce composites?
Fibers are very strong in tension
What is fiber-glass?
- Glass fibers in a polymer matrix
- Fibers provide strength and stiffness
- Polymer matrix holds, protects, and transfers loads to fibers
What are the three fiber types?
- Whiskers
- Fibers
- Wires
What are whiskers?
- Thin single crystals with large length-to-diameter ratios
- High crystal perfection, making extremely strong
- Expensive and hard to disperse
What are examples of whiskers?
- Graphite
- Silicon nitride
- Silicon carbide
What are fibers?
- Polycrystalline or amorphous
- Generally polymers or ceramics
What are some examples of fibers?
- Alumina
- Aramid
- E-glass
- Boron
- UHMWPE
What are wires?
- Metal wires with relatively large diameters
Ex: steel, molybdenum, tungesten
What is the difference between longitudinal and transverse directions?
Longitudinal: along fiber alignment
Transverse: across fiber alignment
What are the three types of fiber alignments?
- Aligned continuous (longer fibers)
- Aligned (shorter fibers)
- Random
What are the properties of fiber-reinforced composites?
Isotropic or anisotropic
What are examples of applications of discontinuous fibers?
- Disk brakes
- Gas turbine exhaust flaps
- Missile nose cones
What is the critical fiber length?
- Length for effective stiffening and strengthening
> Depends on tensile strength, diameter, and matrix shear strength
How does the efficiency of short and long fibers compare?
- Short, thick fibers are less efficient than long, thin fibers
- Short fibers cannot reach maximum strength
- Long fibers can maintain maximum stress for longer
What are the efficiency factors of fibers?
K = 1 when aligned parallel
K = 0 when aligned perpendicular
What is pultrusion?
- Continuous fibers are pulled through a tank of resin and covered in a thermoset
- Fibers pass through a die and are preformed to a shape
- The preformed shape passes through a die that imparts the final shape and cures the thermosetting resin
What are the two types of composite production methods?
- Pultrusion
- Filament winding
What is pultrusion used for?
- The manufacture of components having continuous lengths and a constant cross-sectional shape (rods, tubes, beams, etc.)
What is filament winding?
- Continous fibers are positioned in a pattern to form a hollow shape
- Fibers are covered in a thermosetting resin
- Fibers are wound onto a mandrel
- After the desired amount of layers is added, thermoset is cured either in an oven or at room temperature
- Mandrel is removed and final product is left
What are the three types of winding?
- Helical (helix)
- Circumferential (circular)
- Polar (pole to pole)
What are laminates?
- Stacked and bonded fiber-reinforced sheets
- Have stacking sequences
- Balanced in-plane stiffness
What are sandwich panels?
- Two strong/stiff outer faces with a lightweight core
- Results in low density and high bending stiffness
Ex: honeycomb core between two facing sheets
