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MIL-HDBK-189 13 February 1981 Solution for Case 2: The average number of failures for the i-th test phase is determined Dy Kj = N(ti)-N(ti_i) where / tt \l-a N(t^) = Aj tj I J and Aj= 1/Mj- The average MTBF for the i-th test phase is given iSy Mj = -Example of Case 2: In the example in Section 5.2-6.2.1 the first test phase was identified from 0 to 1000 hours. Suppose the program consists of four additional test phases at 1000-2500, 2500-5000, 5000-7000, and 7000-10000 hours. Determine the average MTBF's to be expected over these periods if reliability growth follows the idealized curve M(t) = < 50 01000 from the example in Section 5.2.6.2.1. From the idealized growth curve the parameters are Aj = .02 and a - .23. Therefore, the average number of failures for the i-th test phase is Hi = N(ti)-N(ti_i) where N(t:i) = .02(1000) , for 1 = 1000, t2 = 2500, t3 = 5000, t4 = 7000, t5 = 10000. The average number of failures Hj and the average MTBF M, for each test phase are presented in the table below. The average MTBF's are plotted in Figure 5.28. Phase i "i ti-ti-i Mi 1 20.0 1000 50 2 20.5 1500 73 3 28.6 2500 87 4 20.4 2000 98 5 28.3 3000 106 46

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