Quality and Reliability Methods - SAS

Chapter 14 Lifetime Distribution II 239
Introduction to Accelerated Test Models
Introduction to Accelerated Test Models
The Fit Life by X platform provides the tools needed for accelerated life-testing analysis. Accelerated tests are
routinely used in industry to provide failure-time information about products or components in a relatively
short-time frame. Common accelerating factors include temperature, voltage, pressure, and usage rate.
Results are extrapolated to obtain time-to-failure estimates at lower, normal operating levels of the
accelerating factors. These results are used to assess reliability, detect and correct failure modes, compare
manufacturers, and certify components.
The Fit Life by X platform includes many commonly used transformations to model physical and chemical
relationships between the event and the factor of interest. Examples include transformation using Arrhenius
relationship time-acceleration factors and Voltage-acceleration mechanisms. Linear, Log, Logit,
Reciprocal, Square Root, Box-Cox, Location, Location and Scale, and Custom acceleration models are
also included in this platform.
You can use the DOE > Accelerated Life Test Design platform to design accelerated life test experiments.
Meeker and Escobar (1998, p. 495) offer a strategy for analyzing accelerated lifetime data:
1. Examine the data graphically. One useful way to visualize the data is by examining a scatterplot of the
time-to-failure variable versus the accelerating factor.
2. Fit distributions individually to the data at different levels of the accelerating factor. Repeat for different
assumed distributions.
3. Fit an overall model with a plausible relationship between the time-to-failure variable and the
accelerating factor.
4. Compare the model in Step 3 with the individual analyses in Step 2, assessing the lack of fit for the
overall model.
5. Perform residual and various diagnostic analyses to verify model assumptions.
6. Assess the plausibility of the data to make inferences.
Launching the Fit Life by X Platform Window
This example uses Devalt.jmp, from Meeker and Escobar (1998), and can be found in the Reliability folder
of the sample data. It contains time-to-failure data for a device at accelerated operating temperatures. No
time-to-failure observation is recorded for the normal operating temperature of 10 degrees Celsius; all other
observations are shown as time-to-failure or censored values at accelerated temperature levels of 40, 60, and
80 degrees Celsius.
1. Open the Devalt.jmp sample data table.
2. Select Analyze > Reliability and Survival > Fit Life by X.
3. Select Hours as Y, Time to Event.
4. Select Temp as X.
5. Select Censor as Censor.