SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and Answers
Topic Posted by: Reliability & Maintainability Forum
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Posted by: C K
Organization:DSO National Laboratories
Date posted: Wed Sep 8 22:57:05 US/Eastern 2004
Subject: ESS vs EWT
Recently, I came across some statements in MIL-STD-810F, Method 514.5 (pg 514.5-1, para 1.2e and Annex B, para 2.1.8) on ESS. In para 1.2e, it was written that "Where units do not necessarily receive the same exposures, such as multiple passes throuth ESS, apply the maximum allowable exposures to the units used for environmental test as pre-conditioning for the environment tests."
In Annex B, para 2.1.8, it says EWT can be used to substitute ESS when the material is to be subjected to worthiness test and then used in the field.
My questions are:
1) Can EWT really be used to substitute ESS for the scenerio mentioned in Annex B?
2) What if my unit is only subject to 1 grms vibration (human transportation) in field, is EWT of 1 grms, 10mins/axis for 3 axes sufficient to replace ESS?
3) If I conduct ESS on the said unit using 6 grms, 10 mins/axis (critical axis only), will it kill the unit?
Kindly advise. Thanks.
Reply Posted by: ERS
Date Posted: Wed Sep 15 14:49:15 US/Eastern 2004
1-ESS thermal profile must first be based on the operating or non-operating limits of the components within the module, and the thermal rate of changes capability of the test chamber. After this is known, then the PE Table 5.15 in MIL-HDBK-344A then can be used to determine the number of thermal cycles required to achieve the desired PE. Table 5.14 provides the vibration PE, but this is not generally used to select the Grms or the vibration time. The vibration requirements are generally pre-established as a design requirement.
2-For electronic components, the typical vibration profile is 6 Grms for 10 minutes per axis. This provides a PE of 0.945. Your plan of 6.3 for 10 minutes per axis is not considered high but not too stringent. The module should only be vibrated once. If for some reason you need to vibrate the module again, reduce the time down to 5 minutes per axis. To understand the impact of vibration at 6.3 Grms for 10 minutes per axis, if you vibrate the module 10 times without a failure, then your screen is not taking any significant life out of the module.
3-ESS is designed to precipitate latent manufacturing defects and should not be used as an accelerated test. Your specification of 1.04 Grms for transportation is from MIL-STD-810E. This level should be applied 60 minutes for each 1,000 miles of common carrier transportation in a vertical axis.