ME Polymers and Composites 1 — Questions and Answers
Question 1: The glass transition temperature (Tg) of a polymer is the temperature at which:
- The polymer melts and flows freely
- Amorphous regions transition from glassy to rubbery behavior (Correct answer)
- Crystalline regions begin to melt
- Chain scission occurs
Correct answer: Amorphous regions transition from glassy to rubbery behavior
Below Tg, amorphous polymer chains are frozen in a glassy state; above Tg, segments gain sufficient mobility to exhibit rubbery, viscoelastic behavior.
Question 2: Thermoplastics differ from thermosets in that thermoplastics:
- Are permanently cross-linked after curing
- Can be repeatedly melted and re-formed (Correct answer)
- Cannot be recycled
- Have higher service temperatures
Correct answer: Can be repeatedly melted and re-formed
Thermoplastics consist of linear or branched polymer chains that soften when heated and solidify when cooled, allowing repeated processing and recycling.
Question 3: In a fiber-reinforced polymer composite, the primary function of the matrix material is to:
- Carry the main tensile load
- Transfer loads to fibers and protect them (Correct answer)
- Provide electrical conductivity
- Increase density
Correct answer: Transfer loads to fibers and protect them
The matrix binds and protects the fibers, transfers applied loads to the fibers via shear, and maintains fiber alignment.
Question 4: The rule of mixtures predicts the longitudinal elastic modulus of a continuous fiber composite as:
- E_c = V_f x E_f x V_m x E_m
- E_c = V_f x E_f + V_m x E_m (Correct answer)
- E_c = E_f / V_f + E_m / V_m
- E_c = (V_f + V_m) / (E_f + E_m)
Correct answer: E_c = V_f x E_f + V_m x E_m
The longitudinal (isostrain) rule of mixtures gives composite modulus as the volume-fraction-weighted sum of fiber and matrix moduli: E_c = V_f times E_f + V_m times E_m.
Question 5: Epoxy resins used as matrix materials in aerospace composites are classified as:
- Thermoplastics
- Elastomers
- Thermosets (Correct answer)
- Semicrystalline polymers
Correct answer: Thermosets
Epoxy resins are thermosets that form a three-dimensional cross-linked network upon curing with a hardener, providing high stiffness, strength, and temperature resistance.
Question 6: Carbon fiber reinforced polymer (CFRP) composites are favored in aerospace because they have:
- High density and high modulus
- Low strength but excellent thermal resistance
- High specific strength and specific stiffness (Correct answer)
- Low cost and high toughness
Correct answer: High specific strength and specific stiffness
CFRP provides extremely high strength-to-weight (specific strength) and stiffness-to-weight (specific stiffness) ratios, enabling significant weight savings in structural applications.
ME Polymers and Composites 1
The glass transition temperature (Tg) of a polymer is the temperature at which: