Two-Stress Example

For life-stress data only.

A team of reliability engineers is instructed to perform an accelerated life test and use the accelerated test data to extrapolate a product's use level failure behavior. They are asked to estimate, with 90% confidence, the time at which the product will have an unreliability of 10% (i.e., the B10 life).

The normal operating temperature for this product is 328 K, and the normal operating voltage is 2 V. It was decided to accelerate both stresses in the test. The following table shows the data from the test.

Time Failed  (Hr)

Temperature (K)

Voltage (V)

620

348

3

632

348

3

658

348

3

822

348

3

216

378

3

246

378

3

332

378

3

400

378

3

380

378

5

416

378

5

460

378

5

596

378

5

Create a Life-Stress Data Folio

  1. Choose Home > Insert > Life-Stress Data. Since the data set contains only exact failure times, do not select any of the check boxes in the Options for the Times-to-Failure Data Type area. Select Hour (hr) from the Units drop-down list and click Next.
  2. On the second page of the setup window, select Temperature and Voltage as the stress types. Set the use level temperature to 328 with K as the unit, and set the use level voltage to 2 with V as the unit. Click OK to create the folio.
  3. In the data sheet of your new folio, select the check boxes inside the Temperature and Voltage column headings. Both stresses will now be used in the calculation for the selected model.
  4. Enter the given data into the data sheet.

Analyze the Data Set

  1. On the Main page of the folio’s control panel, select the TNT-Weibull model (i.e., the temperature-nonthermal life-stress relationship combined with the Weibull life distribution) and click the Calculate icon. The parameters of the model will be automatically computed and displayed in the Analysis Summary area of the control panel.

  1. To obtain the two-sided 90% confidence bounds on the product's B10 life, click the Quick Calculation Pad icon on the control panel.

  1. In the QCP, choose to calculate the BX% Life with Two-Sided Bounds. Select Hour for the time units and then make the following inputs:
    • Stress = 328; 2
    • BX% Life At = 10
    • Confidence Level = 0.9
  2. Click Calculate to display the results, as shown next. The results show, with 90% confidence, that the time at which the product will have an unreliability of 10% is between 459.7681 and 853.1253 hours.

  1. To view a plot of the result, click the Plot icon on the Main page of the control panel.

  1. On the Plot page, select Unreliability vs. Time from the Plot Type drop-down list.


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