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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: Glen Hanington (ghanington@littlefeet-inc.com )
Organization:Littlefeet, Inc.
Date posted: Fri Oct 20 19:58:37 US/Eastern 2000
Subject: Temperature Cycling versus Accelerated Life Testing
Message:
I understand that acceleration of life due to the application of a constant stress from a constant elevated temperature can be calculated using the Arrhenius Equation, assuming that the activation energy is known and the delta T is within a range where the equation does apply. But consider this: in order to maintain the system at a constant temperature, the flow of energy into the system must equal the flow out and be constant. Let's say this value is X joules/minute, and it's giving us an acceleration factor of 5 over ambient. What I want to know is this: Suppose I perform temperature cycling of the system (hot and cold) for a number of cycles and calculate the average flow of energy into the unit as it goes hot, causing its temperature to rise. (This is also equal to the flow of energy out of the unit as it cools down.) If I find this average value to be 10X joules/minute, could this concievably be giving me an acceleration of 5*10=50, since the flow of energy is 10 times greater than that required to yield an AF of 5?


Reply:

Subject: Temperature Cycling vs. Accelerated Life Testing
Reply Posted by: Ken Tonnesen (ktonnesen@alionscience.com )
Organization: Reliability Analysis Center
Date Posted: Wed Nov 1 13:16:22 US/Eastern 2000
Message:
Subject: Temperature Cycling vs. Accelerated Life Testing A cyclic stress, such as a temperature cycle, is a separate stress on the system with its own acceleration factor, based on the upper and lower temperature limits and rate of change of temperature. It would not be additive or linear with the constant elevated temperature acceleration factor, as you suggested. The new acceleration factor for multiple stress tests would have to be estimated from previous empirical data, or determined experimentally on the present system. Actual experimental testing is the best way to find the acceleration factors and any problems associated with multiple levels of stress.


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