Component (such as light bulbs and electronic elements) is commonly used for electronic product and other mass produced items. Such methods are now being used for equipment-see OREDA database. The component reliability method uses the concept of hazard function, mean time to failure, mean time to repair to determine the reliability. However, reliability of equipment depends on the mode of operation, as not all equipment working at full specification all the time. Moreover, the number of equipment failure is not on par with light bulbs failure, and equipment fails due to various reasons. It seems a better way to determine equipment reliability would be to use FORM, SORM and other similar method.
Where do you stand with regard to such practice?
Dear Sirous,
I am curious as why do you categorize Form and SORM as stress-strain based reliability approaches?
FORM and SORM are general approximate methods for reliability analysis. Both methods are based on taylor series expansions and stand for first and second order reliability methods respectively.
Best regards,
Behrouz
Hi Behrouz,
Perhaps I should have called them load-capacity (or demand-capacity) models. Another name would have been Response-based Design, but this phrase is used for a different purpose.
In FORM & SORM structural response is calculated somehow, albeit approximately- stress-strain curve is used to estimate the capacity. The Taylor series is used both to linearize the capacity-demand model and to estimate the variance. Bu in most cases variance is determined in a different way.
In FORM/SORM methods the time enters indirectly, e.g, loads are defined using the return period. Another assumption is there is no degradation. While, in MTTF approach time is explicit, but the loading history is ignored. I would like to know their correlation, If somebody has investigates it before.
If you know any reference which compares/contrasts structural/mechanical reliability analysis method with the reliability analysis method used for mass products (such as electronic components), I would appreciate highly if you let me know.
Hi Ariandne,
I couldn't find answer to my question in references you have posted.
These references use the structural reliability methods. Structural reliability method attempt to calculate the interference between load distribution and resistant distribution. FORM and SORM are the simplest of them. In fact the probability of overload and probability of under-strength is the basic idea behind the structural reliability methods.
Reliability concept based on Mean Time to Failure (MTTF) and Mean Time to Repair (MTTR) assumes a constant rate of hazard enabling them to average failure rate. This is a valid assumption if failure is random, i.e. not due to an extreme event.
My question is when one approach stops to be true and the other approach becomes valid.
Hi all,
I found the answer. Write back if you are interested to know.
One should consider experimental methods such as (highly accelerated life testing (HALT ) and screening methods to ensuring quality such as (highly accelerated stress screening) HASS.
Dear Dr Kujawski,
Thank you for your contribution. I specialize in subsea engineering which is a very conservative industry. Every equipment is tested at least three times, twice at the factory known as FAT (factory acceptance test). Then Extended FAT. At beach as systems take shape, they are also tested which is known as sub-system integration tests and finally after installation we have the integration testing ( two or three times).
For equipment subject to time degradation, such as valves, an extra product is made and tests are performed on that (opening and closing for a number of times, erosion testing). Accelerated testing is used to determine the creep behaviour. What do you think about the testing on a complete equipment which never would be used?
My job is only reliability and availability calculations. Testing will be done if I ask for it or not.
PS: I have read your paper on refutation of Dr Sauser's system readiness level method. I never found his answer to your criticism. In principle I agree with you, but if this is a mistake that everybody is doing, then it is an accepted norm. I can’t see an alternative for it. If it was a moral issue, then the method would have been the normal behaviour of citizen. Compare Victorian time and the current time.
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