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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: D. Fazioli (fazioli_dan@si.com )
Organization:Smiths Aerospace
Date posted: Tue Feb 5 10:25:04 US/Eastern 2002
Subject: Traditional ESS IAW NAVMAP
Message:
Is 2 recorded failures which occurred after cycle 5 justification to reduce the ESS tempature cycles from 10 to 5? This failure data is based upon a review of ESS data with respect to 200 WRAs tested over the last 6 months. These WRAs are ultimately used in a military fighter inhabited environment. For the same 200 WRAs, the number of failures recorded was 120 before completing the 5th ESS tempature cycle. We want to reduce cycle time but not at the risk of jeopardizing reliability of the WRAs. Is this adequate data to convince the customer to reduce the number of cycles to 5 in lieu of 10 temperature cycles?


Reply:

Subject: ESS Testing
Reply Posted by: B.Dudley (bdudley@alionscience.com )
Organization: RAC
Date Posted: Thu Feb 14 15:41:48 US/Eastern 2002
Message:
Reducing the number of test cycle in a Environmental Stress Screening (ESS) program is not a simple process. In addition to the reduction of failures shown, that is, only two out of 122 in the last five cycles, you must demonstrate that this testing is not an anomaly. This means that other data must be established or this six month trend could be considered an extreme situation. MIL-HDBK-344 (Environmental Stress Screening of Electronic Equipment) has some equations and terms based on ramp rates , cycle times and other parameters that could be used to demonstrate if the ESS program could be reduced. The last thing that needs to be done is to determine the "root" cause of the failures to see if the "time' to failure is stress dependent. If it is not, then reducing the cycle time could be a risk.


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