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What is the failure rate function of the series system of two independent components when the failure rate functions of the two components are respectively \(r_{1}(t)\) and \(r_{2}(t)\) ?

  • A.\(\min(r_{1}(t), r_{2}(t))\)
  • B.\(r_{1}(t) + r_{2}(t)\)
  • C.\(r_{1}(t) \times r_{2}(t)\)
  • D.\(\max(r_{1}(t), r_{2}(t))\)

Answer: B

The failure rate is \(r_{1}(t) + r_{2}(t)\).

A series system works only while both components work, so its reliability is the product of theirs:

\[ \begin{aligned} R(t) &= R_{1}(t)R_{2}(t) \\ &= e^{-\int_{0}^{t} (r_{1} + r_{2})\,du} \end{aligned} \]

since each \(R_{i}(t) = \exp(-\int_{0}^{t} r_{i})\). Hence the system's failure rate \(-R'/R\) is \(r_{1}(t) + r_{2}(t)\): hazards add in series. The minimum or maximum would ignore one component's risk, and the product has no reliability meaning here.

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