CRE Reliability Fundamentals 3 — Questions and Answers
Question 1: A non-repairable component has a Weibull distribution with β = 2.5 and η = 1000 hours. What does η represent?
- The characteristic life at which 63.2% of units have failed (Correct answer)
- The median life of the component
- The B10 life of the component
- The mean life of the component
Correct answer: The characteristic life at which 63.2% of units have failed
The Weibull scale parameter η (characteristic life) is the time at which 63.2% of units will have failed, regardless of β.
Question 2: Which of the following is the correct formula for the reliability of a two-component active parallel system?
- R_sys = 1 - (1 - R₁)(1 - R₂) (Correct answer)
- R_sys = R₁ × R₂
- R_sys = R₁ + R₂
- R_sys = min(R₁, R₂)
Correct answer: R_sys = 1 - (1 - R₁)(1 - R₂)
For an active parallel system, the system fails only if all components fail: R_sys = 1 - (1-R₁)(1-R₂).
Question 3: The B10 life of a bearing is the time by which what percentage of bearings will have failed?
- 10% (Correct answer)
- 50%
- 90%
- 63.2%
Correct answer: 10%
BX life is the time by which X% of a population will have failed, so B10 life corresponds to 10% failure.
Question 4: A system's hazard function h(t) is constant. What does this imply about its failure behavior?
- Failures are memoryless and occur randomly (Correct answer)
- The system is in the wear-out phase
- Failures are due to infant mortality
- MTTF equals MTBF
Correct answer: Failures are memoryless and occur randomly
A constant hazard rate implies the exponential distribution and memoryless property — past operation does not affect future failure probability.
Question 5: What is the primary purpose of accelerated life testing (ALT) in reliability engineering?
- To precipitate failures quickly by applying elevated stress to estimate normal-use life (Correct answer)
- To improve product reliability by burn-in screening
- To identify design flaws through destructive testing
- To measure MTBF under field conditions
Correct answer: To precipitate failures quickly by applying elevated stress to estimate normal-use life
ALT applies stresses above normal use levels to induce failures faster, then uses models to extrapolate life at normal conditions.
Question 6: Which of the following correctly distinguishes MTTF from MTBF?
- MTTF applies to non-repairable items; MTBF applies to repairable systems (Correct answer)
- MTTF includes repair time; MTBF does not
- MTTF is always larger than MTBF
- MTTF is the reciprocal of MTBF
Correct answer: MTTF applies to non-repairable items; MTBF applies to repairable systems
MTTF (Mean Time To Failure) is used for non-repairable items; MTBF (Mean Time Between Failures) applies to repairable systems.
Question 7: In a reliability context, what does 'inherent availability' exclude that 'operational availability' includes?
- Inherent availability excludes logistics and administrative downtime; operational availability includes them (Correct answer)
- Inherent availability excludes preventive maintenance; operational availability includes it
- Inherent availability uses MTBF; operational availability uses MTTF
- Inherent availability includes standby time; operational availability does not
Correct answer: Inherent availability excludes logistics and administrative downtime; operational availability includes them
Inherent availability (Ai = MTBF/(MTBF+MTTR)) only considers active repair time, while operational availability adds logistics and administrative delay.
CRE Reliability Fundamentals 3
A non-repairable component has a Weibull distribution with β = 2.5 and η = 1000 hours.
What does η represent?