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!

Related Items

Back to Basics 01 - Functional Safety

Back to Basics 02 - Safety Integrity Level (SIL)

Back to Basics 03 - Safety Instrumented Function (SIF)

Back to Basics 04 - Safety Instrumented System (SIS)

Back to Basics 05 - What is a Safety Function?

Back to Basics 06 – IEC 61508

Back to Basics 07– Safety Lifecycle – IEC 61508

Back to Basics 08 – IEC 61511

Back to Basics 09 – Safety Lifecycle – IEC 61511

Back to Basics 10 – How Does a Product Get a SIL?

Back to Basics 11 – How is SIL Used by an End User?

Back to Basics 12 – What is IEC 61508 Certification?

Back to Basics 13 - How Do I Start IEC 61508 Certification?

Back to Basics 14 - Systematic Capability

Back to Basics 15 - Architectural Constraints

Back to Basics 16 - PFDavg

Back to Basics 17 - PFH (Probability of dangerous Failure per Hour))

Back to Basics 18 – Route 1H

Back to Basics 19 – Route 2H

Back to Basics 20 – Safe Failure Fraction, SFF

Back to Basics 21 – The B10 Method

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