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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: Matt Wiltshire (mwiltshire@spectraprecisionday.com )
Date posted: Thu May 27 11:51:59 US/Eastern 1999
Subject: MIL-STD-810E Salt Fog Test
Message:
How many hours would the MIL-STD-810E Salt Fog Test (48 hours, 5% salt solution, 95 degrees F) be equivalent to in a worst case land environment such as the coast of Florida in the summer and a bast case in Nebraska (if there is a better one let me know)? If numbers can be directly applied to the 48 hour test, can it be applied linearly to an extended test (much > 48 hours)? Can you recommend any studies that address this? Thanks, Matt Wiltshire


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Reply:

Subject: Salt Fog Acceleration
Reply Posted by: Bruce Dudley (bdudley@alionscience.com )
Organization: Reliability Analysis Center
Date Posted: Mon Jun 7 15:35:37 US/Eastern 1999
Message:
Determining the equivalent use time based on an accelerated test condition is not an easy operation. If you can determine the specific accelerated test paramenter(s) that are under test, one can approximate the equivalent use times using single or multiple variable relationships. Assuming that the item you are considering for the test is an electronic or electromechanical component, this salt fog test could represent a temperature and humidity acceleration test. Temperature/humidity relationship tests are very useful in the electronic accelerated test arena. Mr. D.S. Peck in several reports developed a relationship which is the combination of the humidity acceleration and the arrhenius relationship for temperature acceleration. This model is called the "Peck Model". Since the temperature of this test is only 95 degrees F, it's acceleration factor is close to zero as the normal operating conditions at both Florida and Nebraska have temperatures this high or higher. Therefore, this part of the Peck Model is not considered relevant. For the humidity part, the use humidity is expected to be far less on average than the 85% or greater test condition. The accelerating equation for the humidity is: (RHuse / RHaccel)^n. Where RH is the relative humidity for use and test and n is the accelerating factor usually established by experimental testing. The book by Dr. Wayne Nelson, "Accelerated Testing", John Wiley and Sons, 1990, indicates that a generalized "n" factor for electronic components using 85 degrees C and 85% RH to be 2.7, which was determined by experimental testing. Thus, for the Florida location if the relative use humidity is 75% and the Nebraska location is 50%, the humidity related acceleration factors for each site would be about 1.4 for Florida and 4.2 for Nebraska. This translates to approximately 70 hours and 200 hours of equivalent use time, respectively for the 48 hour stress test. This is an estimation that really needs to be tested in order to determine the actual acceleration factor "n".


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