# TECH SPOT: SAMPLE CRE QUESTIONS (Part 26)

1. Use the exponential distribution with MTTF=50,000 hrs.. What is the time to 10% failure?

A, 5000 hrs

B. 5268 hrs

C. 5450 hrs

D.  5333 hrs

2. Three assembly plants produce the same type of parts. Plant A produces 25% of the volume, and has a defect rate of 1%, Plant B produces 30% of the volume and has a defect rate of 1.2%. Plant C makes the remainder and ships 0.6% defectives. If a component is picked at random from the warehouse stocked by the and is defective, what is the probability it was manufactured by Plant A, Plant B, Plant C?

A. 0.25,0.30,.45

B. 0.36, 0.43, 0.21

C. 0.28, 0.41, 0.31

D. 0.333, 0.333, 0.333

 Plant % Prod %Defect Prod x Defect Prob A 25 1 0.0025 0.28 B 30 1.2 0.0036 0.41 C 45 0.6 0.0027 0.31 Sum= 0.0088

3. You are asked to construct a Zero failure test for a redesigned ball bearing

( β=2.5) that the design folks believe should have an  η=1000hrs.  Program Mgmnt wants you to use only 5 tests. How long should you test these five samples to be 90% confident that the ball bearing design is better than 1000hrs?

A. 733hrs,

B. 851hrs

C. 975hrs

D. 1500.hrs

 115 1188 1489 1599 120 1300 1487 1650 205 1380 1505 1670 840 1405 1550 1675 848 1415 1575 1680 890 1437 1585 1700 1160 1449 1590 1710

4. The Reliability analysis of a Submarine fire-control system involved the life-test of 500 fuses put on test at 85% of rated current. After 1800 hr, 28 fuses had blown and the test was stopped. A Weibull plot of data

A. (4, 3721 hrs)

B. (3.8, 3865 hrs)

C. (2.0, 7272 hrs)

D. (5.6, 3130 hrs)

5. Referring to the Weibull plot in 4. above, How many Failure modes are there in  the fuse reliability test data:

A. 2

B. 3

C. 1

D. 4

6. A bearing manufacturer has a bearing with Weibull β=2     and η=600 hrs. The design team made a small change in     the design and a change of material. They now hope they     have doubled the characteristic life at 95% confidence. They tested 10 of the new design bearings for 1800 hrs and Had 5 failures : 700, 900, 1000, 1100, and 1300 hrs. Did the new bearing design have 2X the characteristic life With 95% confidence?

A. YES

B. NO

WHY?

7. For a certain 3-engine airplane, at least two out of its three engines must function. The engine reliability at time t is 0.995. Find the probability that the airplane flies successfully for T hours, assuming identical and independent engines.

A.0.9998

B. 0.985

C. 0.99945

D. 0.999925

8. A fleet of 100 engines is subjected to a known Weibull failure mode. The Weibull has a slope of 3 and a characteristic life of 1000 hours. The current engine times are as follows:

Calculate the expected number of failures now. Calculate the additional engines

that will be expected to fail in 6 months if the utilization rate is 25 hours/month.

What is the total number of engines expected to fail? Assume that failed units

are not fixed.

A. 6.7

B. 1.9

C   4.9

D. 8.6

 No engines T=now T+150 F(Nowtime) F(t+135) (F(t+150)-F(t))/(1-F(t)) Exp Now Exp-6mos Exp(Cum6mos) 20 150 300 0.003369311 0.026638758 0.023348114 0.067386222 0.466962289 0.53277517 20 200 350 0.007968085 0.041968864 0.034273876 0.159361703 0.68547751 0.839377271 20 250 400 0.015503563 0.061995 0.047223571 0.31007126 0.944471422 1.239900009 20 300 450 0.026638758 0.087096409 0.062112244 0.53277517 1.242244875 1.741928184 20 350 500 0.041968864 0.117503097 0.078843193 0.839377271 1.576863862 2.350061948 Sum = 1.908971625 4.916019958 6.704042581

9. A system consisting of 4 subsystems in series is required to have a reliability of 0.99 for a mission time of 50 hours. Allocate the reliability requirements to each of the sub-systems given the following estimates for present sub-system failure rates;

λ1=0.004 Failures/hour,  λ2=0.007 Failures/hour, λ3=0.008 Failures/hour, λ4=0.005 Failures/hour

Hint: Calculate the weighting factors 1st, then solve R=0.99=e-λ* 50   for λ*

λ*1-λ*4:  all x 10-4

A. (0.235,0.486,0.569,0.418),

B. (0.135,0.486,0.569,0.418),

C. (0.135,0.386,0.469,0.418)

D. (0.335,0.586,0.669,0.418)

10. A parallel system contains four identical and independent active units. Each unit may fail due to Hardware or a human error. The unit constant hardware Failure rate =λ1=0.004 failure/hour, The human error rate is λh= 0.0005 error/hr.

Determine the parallel-system MTTF.

Hint:

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