MTTF Versus MTBF

陌影

Theoretical Definitions and Alternative Uses for MTBF
Question
According to some sources, MTBF is applicable only when failure times follow exponential distributions. According to other sources, MTBF is applicable only for repairable systems. I've also heard that MTBF is nothing more than MTTF. What do MTTF and MTBF really mean? Are there differences between these terms?
The Short Answer
MTBF (mean time between failures) is the expected time between two successive failures of a system. Therefore, MTBF is a key reliability metric for systems that can be repaired or restored. MTTF (mean time to failure) is the expected time to failure of a system. Non-repairable systems can fail only once. Therefore, for a non-repairable system, MTTF is equivalent to the mean of its failure time distribution. Repairable systems can fail several times. In general, it takes more time for the first failure to occur than it does for subsequent failures to occur. Therefore, MTTF for a repairable system can represent one of two things: (1) the mean time to first failure (MTTFF) or (2) the mean uptime (MUT) within a failure-repair cycle in a long run.