TECH SPOT: SAMPLE CRE QUESTIONS (Part 17)

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1. The failure rate on a jet engine is λ = 10−3/hr. What is the probability that more than two engines on a four-engine aircraft will fail during a 2-hr flight? Assume that the failures are independent.
A. 1.0 x 10-8
B. 4.7 x 10-7 .
C. 3.2 x 10-8
D. None of these

 

2. Suppose we have a generator with a failure rate λ1=0.0002 and a standby battery with failure rate λ2=0.001 when in operation. The switching circuit reliability RSS=0.99 for one switching operation.
The reliability block diagram:

What is the reliability of this system for a 10 hour mission?

A. 0.995

B. 0.999

C. 0.9998

D.0.99998

 

3. A pressure vessel is equipped with six relief valves. Pressure transients can be controlled successfully by any three of these valves. If the probability that any one of these valves will fail to operate on demand is 0.04, what is the probability that the relief valve system will fail to control a pressure transient (on demand)? Assume the failures are independent.

A. .3.56×10-5

B. 2.13×10-5

C. .00025

D. 0.000077

 

4. Testing is underway on a new product. Ten events have occurred so far at the hours listed below. The reliability requirement is to have a failure rate that is 1.0 per 1000 hours. The test plan calls for at total of 10,000 hours of testing. What is the predicted instantaneous failure rate at the end of planned testing.
If needed, how much additional testing will be required to achieve the reliability requirement.
A. A. 0.98/1000, NONE

B. 1.14/1000,1360 hrs

C. 0.95/1000,NONE

D. 1.25/1000, 1705hrs

Beta = 0.571, Lambda = 0.1037. At 10000 cumulative hours,
instantaneous failure rate= (20/10000)*.571=1.14/1000 hours.
Testing to a cumulative hours of 13650 achieves 1.0/1000 hours instantaneous failure rate.

 

5. When considering seals to maintain a lubricant within a vessel, which failure mechanism is likely caused by extreme temperature?
A. Wear

B. Elastic deformation

C. Seal distortion

D. Surface damage

 

6. The benefits of HALT testing include all of the following except:
A. Rapid discovery of low‐incidence failure modes
B. The ability to model time‐to‐failure reliability metrics under high stress conditions.
C. The ability to combine stress factors such as temperature and vibration to reduce discovery times.
D. The capability to identify operating limits of the item on test.

 

7. A small, low‐power electronics device in a plastic enclosure connects by way of a flexible cable to home stereo equipment and provides reception for a remote control. Which of the following sets of failure modes should the design team consider in the design phase and testing of the new product?
A. Corrosion, plastic creep and fading due to heat generated by the stereo equipment
B. Solder fatigue, mechanical wear, and electro‐migration due to heat generated by the stereo equipment
C. Oxidation, plastic creep, and electro‐migration due to heat generated by the stereo equipment
D. Fading, plastic creep, and mechanical fracture due to mishandling and drops

 

8. Given a flange under load, we know that strength of the flange is a random variable with a mean and standard deviation of 25,000 and 3000 psi, respectively. The stress is also a random variable with a mean and standard deviation of 17,000 and 1400 psi, respectively. Determine the probability of failure.
A. 0.0078

B. 0.078

C. 99.22

D. 92.2

 

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