CRE Failure Analysis & Testing 3 — Questions and Answers
Question 1: A Weibull plot of field failure data shows a shape parameter (β) of 0.5. This indicates the failures are most likely due to:
- Wear-out mechanisms
- Random causes with constant failure rate
- Infant mortality or early-life failures (Correct answer)
- Fatigue with a defined cycle life
Correct answer: Infant mortality or early-life failures
A Weibull shape parameter β < 1 indicates a decreasing failure rate, characteristic of infant mortality failures caused by latent defects.
Question 2: In failure mode and effects analysis (FMEA), the Risk Priority Number (RPN) is calculated as:
- Severity × Occurrence × Detection (Correct answer)
- Severity + Occurrence + Detection
- Severity × Occurrence / Detection
- (Severity + Detection) × Occurrence
Correct answer: Severity × Occurrence × Detection
RPN = Severity × Occurrence × Detection, where each factor is rated on a scale (typically 1–10) to prioritize corrective actions.
Question 3: Which failure analysis method uses dye to reveal cracks and surface discontinuities in non-porous materials?
- Magnetic particle inspection
- Liquid penetrant testing (Correct answer)
- Radiographic testing
- Acoustic emission testing
Correct answer: Liquid penetrant testing
Liquid penetrant testing (PT) uses capillary action to draw dye into surface-breaking discontinuities, which is then revealed with a developer.
Question 4: The Inverse Power Law model in accelerated testing is typically applied when the accelerating stress is:
- Temperature
- Voltage or mechanical load (Correct answer)
- Humidity
- Combined temperature-humidity
Correct answer: Voltage or mechanical load
The Inverse Power Law relates life to non-thermal stresses such as voltage, pressure, or mechanical load, where life is inversely proportional to stress raised to a power.
Question 5: During failure investigation, a 'witness mark' on a bearing component most likely indicates:
- Corrosion fatigue
- Contact or impact with another component during operation (Correct answer)
- Hydrogen embrittlement
- Thermal overload
Correct answer: Contact or impact with another component during operation
Witness marks are impressions or rub marks left on a surface by contact with another component, indicating improper clearance or misalignment.
Question 6: In a step-stress accelerated life test, stresses are:
- Applied at a constant level throughout the test
- Increased incrementally at fixed time intervals (Correct answer)
- Applied randomly within a specified range
- Cycled between high and ambient conditions
Correct answer: Increased incrementally at fixed time intervals
In step-stress ALT, the stress level is increased at predetermined intervals, allowing failures to be induced faster while still providing life-stress data.
Question 7: What is the primary goal of a corrective action verification in the failure analysis process?
- To document the failure for warranty records
- To confirm the corrective action eliminates the root cause without introducing new failures (Correct answer)
- To determine the cost of the failure
- To identify all components that may have been affected
Correct answer: To confirm the corrective action eliminates the root cause without introducing new failures
Corrective action verification confirms that the implemented fix actually prevents recurrence of the failure mode while not creating unintended side effects.
CRE Failure Analysis & Testing 3
A Weibull plot of field failure data shows a shape parameter (β) of 0.5.
This indicates the failures are most likely due to: