TECH SPOT: SAMPLE CRE QUESTIONS (Part 15)

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1. The Arrhenius model is used to predict part reliability. Which of the following are valid Arrhenius model statements?
I. The rate of chemical reaction doubles for every 10°F rise.
II. It is useful for all accelerated testing plans.
III. It is useful for significant thermal stresses.
IV. It provides a relationship of failure to temperature.
A. I and II only
B. II and III only
C. III and IV only
D. I, III and IV only

 

2. A fault tree analysis (FTA) is a design analysis technique constructed from a “top event”. A basic problem with
this technique in comparison to a FMECA would be which of the following?
A. Assigning probabilities to the various events.
B. Needing a different FTA for each defined top event.
C. Understanding logical symbols.
D. Only electronic systems can be analyzed.

 

3. Subcontractor ABC has provided a new canister design. The canisters are rated at 1000 psi, with a standard deviation of 150 psi.
The expected stress will be 750 psi with a standard deviation of 200 psi.
What is the probability of failure of the canister ?
A. 0.3085
B. 0.1587
C. 0.1056
D. 0.0228

 

4. Which of the following is required to establish a reliability specification ?
A. The usage environment.
B. A robust design.
C. The reliability policy.
D. A mathematical model..

 

5. Using the failure data below, determine what distribution fits best from an engineering standpoint:

A. Lognormal,
B Smallest Extreme value,
C. Normal,
D. 3-parameter Weibull

 

6. Who should be chairperson of a design review committee ?
A. A senior member of management.
B. The chief designer.
C. The person best able to make decisions on a design.
D. The reliability engineer

 

7. The design system predicted B.1 life for a part is 1000 hours. Five parts have accumulated 1500 hours and five have 2000 hours without any failures.
If most parts of this type have a failure mode with a Weibull β = 3.0, is this success data sufficient to increase the predicted design life?


A. YES,
B. NO
(Hint: see USAF Weibull Handbook, 1983, Chapter 4 – Weibayes)

 

8. Ten failures occurred in the field out of a population of 2000. The ten failure times are: 51, 79, 116, 164, 197, 230, 232, 327, 414, and 451 hours.
Generate a Weibull distribution that best represents the total population
(Hint: see USAF Weibull Handbook, 1983, pp 127ff)
Answer: β=1.84, η= 4027hrs

 

9. Maintainability considerations are important in the design of :
A. Unmanned satellites.
B. Integrated circuits.
C. Modern automobiles.
D. Consumer televisions.

 

10. A stress test was conducted for a mechanical part by subjecting a sample of 25 parts to increasing load stress.
The results of the test are identified in the table.
The sample was subjected to a 50% added stress each week for 5 weeks.
The test terminated after 5 weeks.
To determine whether failures observed in the test reflect actual use failures or are the result of high test stresses, which of the following must be known ?


A. The geometry of the test setup.
B. The precise history of cumulative stress.
C. The exact times of the failures.
D. The mechanisms of the failures.

 


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