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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum (src_forum@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Original Message:

Posted by: Berni (berni.itNO_SPAM_REMOVE@tiscali.it )
Organization:Student
Date posted: Thu Jun 3 2:46:33 US/Eastern 2004
Subject: Part Stress Calculation
Message:
When I perform a part stress analysis (MIL 217F2)which electrical condition should I use? For example, if I have a resistor with a sinusoidal tension applied, how can I compute the actual power dissipation? Should I use the max or the mean tension?


Reply:

Subject: Stress Analysis
Reply Posted by: bwd (bdudley@alionscience.com )
Organization: RAC
Date Posted: Wed Jun 9 15:37:04 US/Eastern 2004
Message:
The part stress analysis for a resistor is computed by dividing actual power dissipated by rated power. Rated power is the manufacturers specified values and the dissipated is calculated from the circuit design. if you do not have such information, then one should use the part count section of MIL-HDBK-217 as it contains default values for the variables. With regard to the sinusoidal tension, I do not believe that this is significant in the determination of the device failure rate.


Reply:

Subject: Part Stress Calculation
Reply Posted by: Berni Student
Date Posted: Fri Jun 11 7:53:39 US/Eastern 2004
Message:
> the dissipated is calculated from the circuit > design. > [...] > I do not believe that this is significant in the > determination of the device failure rate. Usually a circuit doesn't work in a stationary state and so currents and tensions aren't constant. If in a resistor R the tension applied is V = A + B*sin(w*t) How can I compute the dissipated power? P = V^2/R = A^2/R Or P = V^2/R = (A + B)^2/R And what about if the tension is switched on and off?


Reply:

Subject: extreme value analysis
Reply Posted by: bwd (bdudley@alionsience.com )
Organization: RAc
Date Posted: Fri Jun 11 9:30:15 US/Eastern 2004
Message:
If the stresses are variable as you are indicating, you need to perform a worst case extreme value analysis of the individual components. There are a number of books that describe this process which is time consuming but very exacting when this type of detail is necessary. The RAC web site for products describes our worst case handbook. Generally, when one is predicting reliability of components, you select a default parameter that represents the average stress conditions. But in those circumstances where safety is a concern, you should apply the worst case analysis.


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