220.127.116.11 Step Stress Profile
Using a step stress profile, test specimens are subjected to a
given level of stress for a preset period of time, then they are subjected to
a higher level of stress for a subsequent period of time. The process
continues at ever increasing levels of stress, until either; all specimens
fail, or the time period at the maximum level stress ends, as shown in Figure
8.7-2. This approach provides more rapid failures for analysis, but with this
technique it is very difficult to properly model the acceleration and hence to
quantitatively predict the item life under normal usage.
How much to increase the stress in any single step is a function
of many variables and is beyond the scope of this discussion. However, the
general rule to follow in the design of such a test is to eventually exceed
the expected environments by a comfortable margin so that all members of the
population can be expected to survive both the field environment and the
screen environments, assuming of course that they are defect free.