8.7.3 Equipment Level Accelerated
Accelerated testing of equipment is usually quite limited.
Creating a valid model relating the rate of equipment failures at a high
stress - to that at normal operating conditions - is extremely difficult.
Likewise it is very difficult to formulate stress conditions that do not
change the failure mechanisms occurring within the equipment.
One example of an accelerated test that can be used effectively
on equipment is that of increasing the duty cycle. Take for example an
equipment normally operated at some given duty cycle, e.g., running only
during one shift, or avionics equipment operating only a few hours before and
during a flight. In such cases a higher duty cycle could easily be used during
the test. The system undergoing test could be operated continuously for three
shifts a day or the avionics equipment might be cycled continuously, with only
enough time between simulated flights to permit the temperature within the
equipment to stabilize during non-operating conditions. Although the failure
rate per operating hour does not change, the number of failures accrued per
day is increased.
This type of accelerated testing is commonly done in reliability
qualification test, and although it is not usually recognized as such, this is
actually a form of accelerated testing.
Another example of equipment level accelerated testing is ESS.
In this case equipment is often subjected to higher stresses, particularly
thermal cycling and vibration, as part of the ESS program. Here the purpose of
the stresses are to detect defects induced into the equipment during the
manufacturing process, e.g., weak solder joints, etc. Assuming that each
defect is removed when it is discovered, with ESS there is no need of a model
to correlate the rate of failure under stress to the rate of failure under
Given these specific exceptions, accelerated testing is seldom
applied at the equipment level. However, accelerated testing is an extremely
important concept for component testing.