TECH SPOT: SAMPLE CRE QUESTIONS (Part 15)

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)

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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1. A computer chip manufacturer averages 7 defects per chip. What is the probability of a chip containing less than 3 defects?A. 0.00638 B. 0.02234 C. 0.02963 D. 0.02872     2. Given the following data: (where (-) indicates censored) -309,229,386,-104,180,-217,-167,168,122,138Determine the best fit Weibull parameters β,η, t0? A. β=2.76,η=261.1, t0= 0.0 B. β=0.88,η=147.86, t0=113.76

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