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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: Grib khider (kgrib@zodiac.com )
Organization:Aérazur - Zodiac group
Date posted: Mon Nov 8 7:10:55 US/Eastern 2004
Subject: Accelerated testing
Message:
We have performed an accelerated test using Arrhenius acceleration factor in order to obtain a service life of fuel tank (acceleration factor between life t at temperature T and life t’ at reference temperature T’). For the test we have filled a tank with air instead of fuel. I am looking for a factor between air and fuel to apply in order to be more representative of tank utilization. I will really appreciate if someone have a method to obtain this factor or how we can do to be more realistic?


Reply:

Subject: Fuel Tank Accelerated Testing
Reply Posted by: bwd (bdudley@alionscience.com )
Organization: RAC
Date Posted: Tue Nov 9 14:57:20 US/Eastern 2004
Message:
Accelerated Testing Fuel Tanks Accelerated testing is a difficult process to determine the life of a product unless one knows the failure mechanisms and failure modes of the materials under test. Each life analytical model describes physical change mechanisms associated with specific material characteristics. The first step in constructing accelerated tests is to define the failure mechanisms in terms of the materials used in the product to be tested. The next step is to determine the environmental parameters the product will be exposed when operating and when not operating or stored. Based on the failure mechanisms most likely to limit the life of the product, one can choose a test or tests that will accelerate that failure mechanism. Some of the models that can be considered are: Miner's Rule for accumulated fatigue, Arrhenius Temperature test for accelerated temperature and chemical aging effects, Peck's Model for temperature and humidity combined effects, Coffin-Manson non-linear mechanical fatigue damage model and Eyring/Black/Kenney models for temperature and voltage acceleration. If the fuel tank is a metal product, then corrosion is a possible failure mechanism. Corrosion is a very slow acting mechanism in the presents of air and high temperature. The activation energy associated with this mechanism using a constant high temperature test could be as low as 0.3 electron volts. The corrosion factor is much higher when in the presents of corrosive containments which is in the fuel product itself. This condition would result in a much higher activation energy which could be as high as 0.5 electron volts. If the fuel tank is a plastic or a fiber glass product, the constant high temperature test is useless as there is no failure mechanism to drive. The only way to test this product is to perform temperature cycling testing and vibration testing. Accelerated test for these types of testing require a study of fatigue failure mechanisms. Fuel would not be a consideration in cycle testing as fast transition from high to low temperature is the driving concern. The vibration testing is the use of higher stresses in a shorter time period. The RAC has developed a series of accelerated tests for fuel tanks with fuel pumps for automotive use.


Reply:

Subject: Fuel Tank Accelerated Testing
Reply Posted by: Ken
Date Posted: Thu Nov 11 16:29:00 US/Eastern 2004
Message:
In addition to RACs reply, consider the following: Air is compressible and will not apply the same stresses as non-compressible fluids; therefore, I doubt any reliable conversion exists. Testing of a tank that is designed to carry liquid fuel by filling it with air is not a comprehensive test. The weight of the fuel is needed to stress the seams and any integral hanger brackets/attach points. Also, some of the testing should be performed using a full tank and something other than full to account for sloshing forces on the tank, baffles, internal plumbing, and hanger sway stresses. Operational profile needs to be applied to determine the amount to fill or pick a worse case. A full tank becomes something less than full as soon as it is placed in use. Fill lines and discharge lines need to be attached at both ends, filled with fluids as they would be in operation, and pressurized, as they would be in operation, to stress fittings to the tank.


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