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Example 4


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Maintenance Optimization

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A new product is required to demonstrate 99% reliability at 1.5 million cycles with 80% confidence.  Given a Weibull shape parameter of 2.8, how long must 8 units be tested to demonstrate the required reliability assuming no failures?

Solution

  1. Select "Testing" from the menu, and the following screen appears.

    Testing Screen
  2. Enter the shape parameter of 2.8, a sample size of 8, a confidence level of 0.8, reliability of 0.99 and a time of 1.5.  Click the "Test Duration" radio button, and the test duration will be computed.  To demonstrate 99% reliability with 80% confidence at 1.5 million cycles requires testing 8 units for 4 million cycles with no failures.  This is shown in the figure below.

    Example Screen
  3. Change the Test Duration to 1.5, and click the "Sample Size" radio button.   To demonstrate 99% reliability with 80% confidence at 1.5 million cycles requires testing 161 units for 1.5 million cycles with no failures.  By testing a sample size of 8 for 4 million cycles ( a total of 32 million test cycles) instead of 161 samples for 1.5 million cycles ( a total of 241.5 million test cycles), the total number of test cycles is reduces by 80%.  If the shape parameter is greater than 1, the total number of test cycles decreases as the test duration increases.

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