How Do I Use the Expected Failure Times Plot in Weibull++ 8?
Weibull++ 8 includes an Expected Failure Time plot that estimates the sequential failure times you will observe when you test a specified number of units in a planned reliability life test. You can use the plot to:
- Estimate the duration of a planned test. For example, if you're planning to test all units to failure, you can estimate the test duration by looking at the last estimated failure time.
- Monitor whether an in-progress test is proceeding as planned by comparing the actual and predicted failure times. This can provide an early warning that adjustments may be needed.
To create a plot, choose Insert > Tools > Test Design, then choose Expected Failure Times Plot in the Test Design Assistant. In the folio that appears, enter the number of units that will be tested in the Sample Size field. You will also enter the confidence level to be used for the estimated failure times and the acceleration factor for the stress level that will be used in the test (e.g., if the units will be tested under normal operating conditions, then the acceleration factor is 1).
Next, provide estimates for the parameters of the product’s failure model. By clicking inside the Expected Failure Model area, you can either enter the parameters manually, or you can use the Quick Parameter Estimator to calculate them using information you have about the product's reliability. If the parameters you wish to use were calculated in a folio within the current project, you can copy them from the folio by clicking the Get Failure Model from Folio icon on the control panel.
The plot will show the predicted failures times for each test unit with a margin of error corresponding to the confidence level you specified. The three vertical tick marks on each line display the lower confidence bound, median value and upper confidence bound for the expected failure time for one of the test units. Click the Results (…) button to view all the calculated values. If you want to visually compare actual failure times to the predicted times, click inside the Actual Failures field and enter the times in the table, as shown next.
The actual failure times will appear as blue circles on top of the predicted intervals, as shown next.