O failure reliability

[sabrina]

Hello ,

Can I have a explanation,
Tarik has already done me a few explanations , but I would like more informations .

Here is the problem :

The same parts are getting a sequence of different tests, (stress1 then stress2.....There are 3 independent failure modes a, b, c.
There is no failure.


I used weibull++ software then Blocksim to determine the total reliability of the system


With weibull++,I found each reliability :
Ra = reliability of mode a
Rb = reliability of mode b
Rc= reliability of mode c
With Blocksim I found the total reliability (Ra*Rb*Rc)which is smaller than Ra or Rb or Rc ( Rsystem < Ra, Rb, Rc)??? It's surprising ??

Please help
Thank you
Best regards

[tarik]

You shouldn’t bee surprised. When you have competing failure modes, the overall reliability of the product will be smaller than any of the Ri (1<i<n), where n is the number of competing failure modes you have which is also the normal result of multiplying all the reliabilities together.
This idea is that product has a higher probability of failure as the number of failure modes increases, also, this probability is higher then any of the probabilities of failure due any particular failure mode. Does this clarify your question?

[sabrina]

Yes, it's clarify my question and thank you for your answer.

A colleague told me that I can use Blocksim if failure modes are dependent in a test sequence .

Could you confirm if it's valid in my case (eg : the same parts are getting different stresses , stress 1, stress 2...and the failure modes a, b,c are dependent) .
And in this case how to calculate the total reliability ?

I'll appreciate once more your comments
Thank you once more for your answer.
Best regards

[tarik]

What do you mean by DEPENDENT? Do you mean that the life accumulated in a test AFFECTS the life of the part when it’s undergoing the subsequent test? Meaning that if a part is tested under stress type 'B' (which could only produce failures of type Mode 'B'), then the fact that it was put under a test of type 'A' before makes the part’s life shorter.
Can you please confirm if this is the case?

[sabrina]

Hello Tarik ,

Yes, I confirm this is the case .

Thank you for your answer.

Best regards