ME Polymers and Composite Materials 1 — Questions and Answers
Question 1: The glass transition temperature (Tg) of a polymer is the temperature at which:
- The polymer melts and becomes a liquid
- The amorphous regions transition from a glassy, rigid state to a rubbery state (Correct answer)
- Crystallization of the polymer begins
- Crosslinks between polymer chains begin to break
Correct answer: The amorphous regions transition from a glassy, rigid state to a rubbery state
Below Tg, amorphous polymer chains are frozen in place (glassy); above Tg, they gain sufficient mobility to produce rubbery, viscoelastic behavior.
Question 2: In fiber-reinforced polymer composites, the primary function of the matrix is to:
- Carry the majority of applied tensile loads
- Transfer load to fibers and protect them from the environment (Correct answer)
- Increase the stiffness of the fibers
- Reduce the density of the composite
Correct answer: Transfer load to fibers and protect them from the environment
The matrix in a composite holds the fibers in position, transfers applied loads to the fibers, and protects them from environmental damage.
Question 3: Which type of polymer has a three-dimensional crosslinked network that makes it infusible and insoluble once cured?
- Thermoplastic
- Elastomer
- Thermoset (Correct answer)
- Semi-crystalline polymer
Correct answer: Thermoset
Thermosets form irreversible covalent crosslinks during curing, creating a rigid 3-D network that cannot be re-melted or reshaped.
Question 4: The rule of mixtures for a unidirectional fiber composite loaded parallel to the fibers gives the composite modulus as:
- E_c = E_f / V_f + E_m / V_m
- E_c = E_f × V_f + E_m × V_m (Correct answer)
- E_c = (E_f × E_m) / (V_f × V_m)
- E_c = V_f / E_f + V_m / E_m
Correct answer: E_c = E_f × V_f + E_m × V_m
For loading parallel to fibers (isostrain condition), the composite modulus is the volume-fraction-weighted sum: E_c = E_f·V_f + E_m·V_m.
Question 5: Vulcanization of rubber introduces which type of bonds to improve strength and reduce creep?
- Hydrogen bonds between polymer chains
- Covalent sulfur crosslinks between polymer chains (Correct answer)
- Van der Waals forces between polymer segments
- Ionic bonds within individual polymer chains
Correct answer: Covalent sulfur crosslinks between polymer chains
Vulcanization uses sulfur to form covalent crosslinks between rubber polymer chains, greatly improving strength, elasticity, and resistance to creep and permanent set.
Question 6: Carbon fiber reinforced polymer (CFRP) composites are preferred in aerospace applications primarily because they offer:
- Low cost and easy repairability
- High specific stiffness and specific strength (high stiffness/strength per unit weight) (Correct answer)
- Superior electrical insulation
- Better fire resistance than aluminum
Correct answer: High specific stiffness and specific strength (high stiffness/strength per unit weight)
CFRP composites offer exceptional specific strength and specific stiffness (properties per unit density), enabling significant weight savings in aerospace structures.
ME Polymers and Composite Materials 1
The glass transition temperature (Tg) of a polymer is the temperature at which: