BME Materials Science 3 — Questions and Answers
Question 1: Which ceramic failure mode is associated with slow crack growth under sustained static loading below the fracture toughness?
- Fatigue fracture
- Creep rupture
- Static fatigue (stress corrosion cracking) (Correct answer)
- Ductile fracture
Correct answer: Static fatigue (stress corrosion cracking)
Static fatigue in ceramics involves subcritical crack growth driven by chemical attack at crack tips under sustained stress.
Question 2: The Avrami equation is used to describe:
- Creep strain rate in metals
- Kinetics of phase transformations (Correct answer)
- Diffusion coefficient temperature dependence
- Fatigue crack growth rate
Correct answer: Kinetics of phase transformations
The Avrami (JMAK) equation models the fraction of material transformed as a function of time during nucleation and growth processes.
Question 3: In fiber-reinforced composites, the rule of mixtures predicts longitudinal stiffness when fibers and matrix are loaded:
- In series (parallel loading)
- In parallel (isostrain condition) (Correct answer)
- At 45° to fiber axis
- Under hydrostatic pressure
Correct answer: In parallel (isostrain condition)
Under isostrain (parallel) loading along the fiber axis, both constituents experience the same strain, giving the upper-bound rule of mixtures for stiffness.
Question 4: Which diffusion mechanism dominates in metals at temperatures near the melting point?
- Interstitial diffusion
- Vacancy diffusion (Correct answer)
- Grain boundary diffusion
- Surface diffusion
Correct answer: Vacancy diffusion
At high homologous temperatures, vacancy concentration is large and vacancy diffusion (substitutional diffusion) dominates bulk transport.
Question 5: A steel component undergoes cyclic loading. The S-N curve shows an endurance limit at 10^7 cycles. This means:
- The part will fail after exactly 10^7 cycles
- Below this stress amplitude, fatigue failure will not occur (Correct answer)
- The material loses all strength at 10^7 cycles
- The part must be replaced every 10^7 cycles
Correct answer: Below this stress amplitude, fatigue failure will not occur
The endurance limit is the stress amplitude below which steel can theoretically withstand infinite fatigue cycles without failure.
Question 6: Which type of polarization in dielectrics has the slowest response to an alternating electric field?
- Electronic polarization
- Ionic polarization
- Orientational (dipolar) polarization
- Space charge polarization (Correct answer)
Correct answer: Space charge polarization
Space charge polarization involves migration of free charges to interfaces and has the lowest frequency response, occurring only at very low frequencies.
Question 7: The Pilling-Bedworth ratio (PBR) predicts oxidation resistance. A PBR value between 1 and 2 indicates:
- The oxide layer is porous and non-protective
- The oxide layer is protective and adherent (Correct answer)
- The metal undergoes catastrophic oxidation
- No oxidation occurs
Correct answer: The oxide layer is protective and adherent
A PBR between 1 and 2 means the oxide volume slightly exceeds the metal volume consumed, creating a continuous, protective barrier.
BME Materials Science 3
Which ceramic failure mode is associated with slow crack growth under sustained static loading below the fracture toughness?