Physical acceleration means that operating a unit at some higher stress level(s) (i.e, higher temperature, voltage, humidity or duty cycle, etc…) should produce similar failures as would occur at typical-use stresses, except that they are expected to occur much sooner. Failures may be due to mechanical fatigue, corrosion, chemical reaction, diffusion, migration, etc. These are the same cause of failures under normal stress condition; the only difference is the time scale (the time to failure). When there is true acceleration, changing stress is equivalent to transforming the time scale used to record when failures occur. The transformations commonly used are linear, which means that time-to-fail at high stress just has to be multiplied by a constant (the Acceleration Factor or AF) to …
Modeling Methods
Functional Safety: Reliability Prediction vs. Reliability Demonstration
An adaption of the Functional Safety standards IEC 61508 and IEC 26262 by the European Union brought a new life into slowly fading activity of reliability prediction. Both reliability prediction and reliability demonstration are now key parts of many product development programs, however despite phonetic similarity those two have little in common as well as the result they generate. While reliability prediction is an analytical activity often based on mathematical combination of reliabilities of parts or components comprising the system; reliability demonstration is based on product testing and is statistically driven by the test sample size. Therefore the obtained results could drastically differ. For example, a predicted system failure rate of 30 FIT (30 failures per billion (109) …
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Robust Design and Reliability Activities
Robust Design & Reliability I delivered a webinar recently to describe the differences and similarities between robust design (RD) activities and reliability engineering (RE) activities in hardware product development . A survey we took from several hundred attendees indicated a diversity of opinions. About half the participants indicated they did not differentiate at all between the two methodologies. Approximately 20 % indicated they did differentiate between the two methodologies, and about 30% indicated that they did not know. I was quite surprised at the result, especially since participants came from working quality engineers, reliability engineers, engineering directors, system engineers, etc. Somewhere along the way, the differences and similarities …
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Reliability of Repairable Systems vs. Non-Repairable Systems
When performing various reliability tasks, non-repairable systems or products are treated differently from repairable systems or products. Some of the tools that are used for one type are not applicable to the other. Obviously, at some level, repairable systems are composed of non-repairable parts. Examples of non-repairable systems would be “one-shot” devices like light bulbs or more complex devices like pacemakers. Examples of repairable systems are computers, automobiles, and airplanes. What is unique about repairable systems? Availability becomes a key measure of importance. In simple terms, availability is the percentage of time that the product or system is able to perform its required functions. When the required functions cannot be performed because a failure has …
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