TABLE OF CONTENTS The quantification of the reliability of parallel units, as determined previously, is based on the assumption that, when a redundant unit fails, the failure rate or the reliability of the surviving units does not change during the mission. There will be situations, however, when this will not prevail and the failure rate, or the reliability, of the surviving units will change. Usually their failure rate will increase and their reliability will decrease, because the surviving units will be sharing the load during the mission; consequently, their share of the load will increase, as indicated in Fig. 8.1. To correctly determine the reliability of such units, the change of the failure rate, or of the reliability, of the surviving units has to be properly taken into account.
The reliability of the two load-sharing exponential units shown in Fig. 8.1 is given by
| R( t) | = | Probability that Units 1 and 2 complete their mission successfully with pdf's f 1( T) and f 2( T), respectively, or the probability that Unit 1 fails at t 1 < t with pdf f 1( T), and Unit 2 functions till t l with pdf f 2( T) and then functions for the rest of the mission, or in ( t - t 1), with pdf f 2'( T) ,... |
