A Simple Demonstration of Accelerated Life Testing Analysis
Accelerated life testing consists of tests designed to quantify the life characteristics of a product, component or system under normal use conditions by testing the units at higher stress levels in order to accelerate the occurrence of failures. Performed correctly, these tests can provide valuable information about a product’s performance under use conditions that can empower a manufacturer to bring its products to market more quickly and economically than would be possible using standard life testing methods.
Test a sample of six paper clips using the procedure for the 90° bend shown below. Use a similar procedure to test samples of six paper clips each at both the 180° and 45° angles. Different paper clips yield different results. Use clips of similar size. Jumbo clips, capable of enduring repeated bending, are used in this example.
The accelerated test data were analyzed with ReliaSoft’s ALTA accelerated life testing analysis software, assuming a lognormal life distribution (fatigue) and an inverse power law relationship (non-thermal) for the stress-life model. By using the IPL relationship to analyze the data, we are actually using a constant stress model to analyze the cycling process. Caution must be taken when performing the test. The rate of change in the angle must be constant and equal for both the 90° and 180° bends and constant and equal to the rate of change in the angle for the use life of 45° bend. Rate effects influence the life of the paper clip. By keeping the rate constant and equal at all stress levels, we can then eliminate these rate effects from our analysis. Otherwise, the analysis will not be valid.
The parameters of the IPL-lognormal model are:
The parameters of the lognormal distribution for the base test data set are:
The analysis and some of the results are shown in Figures 1, 2, 3 and 4. Figure 2 shows the analysis of the base data in Weibull++ (with the “Expert” default settings) and the base MTTF estimate. In this case, our accelerated test correctly predicted the MTTF as verified by our base test.
The objective of the accelerated test was to quickly estimate the mean life of the paper clip at the 45° bend. Has this objective been achieved? From the data, it can be seen that the accelerated test yielded significantly reduced cycles-to-failure than the base test. Even though it is obvious from the data, a look at the acceleration factor will quantify this observation. The acceleration factor for the 45° bend is plotted in Figure 3. Using the QCP, a more exact value for the acceleration factor can be obtained, as shown in Figure 5.
Therefore, by testing at 90° we accelerated the occurrence of failure by a factor of 3.7 (or reduced the cycles-to-failure by 3.7 times), and by testing at 180° we accelerated the occurrence of failure by a factor of 13.4. Note that the accuracy of the measured cycle-to-failure becomes extremely important as the accelerating stress increases, since the failures occur very closely in time to each other.
[Editor's Note: In the printed edition of Volume 2, Issue 1, the data for the 180 degree bend was reported incorrectly. This has been corrected in this online version. We apologize for any inconvenience.]