TABLE OF CONTENTS If we know the shape of the pdf curve, we can estimate the item s survival probability. If the item has not failed till time t, the reliability function R(t) gives us the probability of survival up to that point. As discussed above, this is the same as the instantaneous availability.
In the case of hidden failures, we will never know the exact time of failure. We need to collect data on failures by testing the item under consideration periodically. It is unlikely that a single item will fail often enough in a test situation to be able to evaluate its failure distribution. So we collect data from several similar items operating in a similar way and failing in a similar manner, to obtain a larger set (strictly speaking, all the failures must be independent and identical, so using similar failures is an approximation). We make a further assumption, that the hazard rate is constant. When the hazard rate is constant, we call it the failure rate. The inverse of the failure rate is the Mean Time To Failures or MTTF. MTTF is a measure of average operating performance for non-repairable items, obtained by dividing the cumulative time in service (hours, cycles, miles or other equivalent units) by the cumulative number of failures. By non-repairable, we mean items that are replaced as a whole, such as light bulbs, ball bearings or printed circuit boards. In the case or repairable items,...
