A cycle test is done on a set of products (>20) until 10% of the units under test fail.
The number of cycles is converted to a time period by knowing the cycles per hour in any particular application.
A failure rate is calculated by dividing the 10% failure count by the time period.
The cycle test method assumes that the constant failure rate during the useful life is due entirely to premature wear-out AND all other failure modes are insignificant. Research shows other failure modes become significant when these products do not move frequently – some failure modes become significant if a product is static (motionless) for 100 hours.
Low demand applications can generate failures that high demand applications cannot develop. When O-rings and other seals are part of a product, many failure modes become significant when the product remains static for a week or more. There include stiction, cold-welding, corrosion binding, etc. Most of these failures are dangerous. Therefore, cycle test data is NOT applicable to most applications including the process industry applications.
Cycle Testing is useful for estimating failure rates when the dominant mechanical failure rates are due to (premature) wear-out of components. This occurs in applications with frequent dynamic movement, lubrication and mechanical loading. Testing must be done until at least 10% of the population has failed.
This method is NOT APPLICABLE to static applications such as low demand mode safety systems as it does not account for failure modes like stiction, cold welding, corrosion, etc. This is for high or continuous demand applications only. While cycle testing is an acceptable failure rate prediction technique for high or continuous demand applications, using it for low demand is DANGEROUS!