CRE Design for Reliability 1 — Questions and Answers
Question 1: What is the primary purpose of derating in Design for Reliability?
- To reduce component cost by using lower-grade parts
- To operate components below their maximum rated stress levels to extend life (Correct answer)
- To increase performance by overclocking components
- To eliminate the need for redundancy in a design
Correct answer: To operate components below their maximum rated stress levels to extend life
Derating operates components below their maximum rated stress levels, reducing failure rates and extending operational life.
Question 2: Which type of redundancy involves switching to a backup component only when the primary fails?
- Active (hot) redundancy
- Passive (cold) standby redundancy (Correct answer)
- Majority voting redundancy
- Load-sharing redundancy
Correct answer: Passive (cold) standby redundancy
Passive (cold) standby redundancy keeps the backup component unpowered or inactive until the primary fails, reducing wear on the standby unit.
Question 3: In a Reliability Block Diagram (RBD), components arranged in series will result in a system reliability that is:
- Greater than any individual component reliability
- Equal to the average of all component reliabilities
- Less than or equal to the lowest individual component reliability (Correct answer)
- Independent of any individual component reliability
Correct answer: Less than or equal to the lowest individual component reliability
In a series configuration, system reliability is the product of all component reliabilities, making it always less than or equal to the least reliable component.
Question 4: Which Design FMEA output metric prioritizes failure modes by combining severity, occurrence, and detection ratings?
- Mean Time Between Failures (MTBF)
- Risk Priority Number (RPN) (Correct answer)
- Failure Rate (λ)
- Reliability Growth Index (RGI)
Correct answer: Risk Priority Number (RPN)
The Risk Priority Number (RPN) = Severity × Occurrence × Detection, and is used in FMEA to rank failure modes and prioritize corrective actions.
Question 5: What is the main goal of a Design Review for Reliability (DRR)?
- To approve the final production drawings for manufacturing release
- To verify that reliability requirements are addressed and risks identified before design is frozen (Correct answer)
- To conduct accelerated life testing on prototype hardware
- To finalize the bill of materials for procurement
Correct answer: To verify that reliability requirements are addressed and risks identified before design is frozen
A Design Review for Reliability ensures that reliability requirements are incorporated and potential risks are identified and mitigated before the design is committed to production.
Question 6: In the context of Design for Reliability, what does 'physics of failure' (PoF) approach emphasize?
- Using historical field failure data exclusively to predict future failures
- Understanding the physical mechanisms and root causes that drive component degradation and failure (Correct answer)
- Applying standard MIL-HDBK-217 failure rate models to all components
- Performing only environmental stress screening on finished assemblies
Correct answer: Understanding the physical mechanisms and root causes that drive component degradation and failure
The physics of failure approach focuses on understanding the underlying physical, chemical, and mechanical mechanisms that cause failures, enabling proactive design improvements.
Question 7: What is a 'single point of failure' (SPOF) in a system design?
- A component that has the highest individual failure rate in the system
- A component or subsystem whose failure alone causes the entire system to fail (Correct answer)
- The first component that fails during accelerated life testing
- A design deficiency found during a formal design review
Correct answer: A component or subsystem whose failure alone causes the entire system to fail
A single point of failure is any element whose failure results in complete system failure, making it a critical reliability risk that typically requires redundancy or redesign.
CRE Design for Reliability 1
What is the primary purpose of derating in Design for Reliability?