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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

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Posted by: Hari ( )
Organization:I.I.T Khagapur
Date posted: Sat Feb 22 2:59:45 US/Eastern 2003
Subject: Difference between Reliability and Durability
I am hoping someone could help me out.What is the difference between Durability and Reliability.(Along with some practical example)? How can we specify the both for components?


Subject: Durability /Reliability
Reply Posted by: bdudley ( )
Organization: RAC
Date Posted: Tue Feb 25 14:11:26 US/Eastern 2003
For electronic equipment, the failure rate over the life of the equipment has three distinct periods, characterized by the classic “bathtub curve,”. For mechanical equipment, a similar curve exists, but with a less pronounced constant failure rate region. Infant Mortality. In this first phase of the bathtub curve, the failure rate is relatively high because some of the parts used in the manufacturing process are out of tolerance, or the skills used in manufacturing are inadequate. The shape of the failure rate curve is decreasing, with its rate of decrease dependent on the maturity of the design and manufacturing process, and the applied stresses. Useful Life. The second phase of the bathtub curve is known as the “useful life” and is characterized by a relatively constant failure rate caused by randomly occurring defects. It should be noted that failure rate is only related to the height of the curve, not to the length of the curve, which is a representation of system life. Wearout. The last part of the curve is the wearout portion. This is where components start to deteriorate to such a degree that they have reached the end of their useful life. Durability is defined as the length of failure free or maintenance free operation period. The basic assumption is that all failure are caused by applied mechanical / thermal stresses and there are no failures before the end of the failure free period [useful life] is reached. This process is predicated on being able to quantify the loads or stresses that are applied to the electronic or mechanical components and relating these conditions to cycles to failure for repeating and varying load conditions. Wear-out is the process that results in an increase of failure rate or probability of failure as the number of life units increase. So, the difference is the durability function is the initial part of the life operation up to the point where failure starts. Wear-out is the next step after durability. Reliability descriptions and definitions are contained in the Reliability Toolkit; Commercial Practices Edition and in MIL-HDBK-338 Electronic Reliability Design Handbook.

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