Over the life of a complex system, three distinct failure rate phases may become apparent. The first phase or period is referred to as the infant mortality period,which is shown as the decreasing failure rate on the left portion of figure 1.1. The second phase is the random or constant cause failure period, also called “useful life”, which is the period of timeencompassing the flat portion of the curve, where the failure rate remains constant. The last phase is the wearout period, which is shown on the right side of figure 1.1 as an increasing failure rate.The wearout phase is more predominant in mechanical systems than in electronic system.
Figure 1.1 : Reliability bathtub curve showing the infant mortality period as the decreasing failure rate(left), useful life failure period (middle), and wear-out period as an increasing failure rate (right).
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Infant mortality : The initial region that begins at time zero and ischaracterized by a high but rapidly decreasing failure rate. This region is known as Infant Mortality Period. This decreasing failure rate typically lasts several weeks to a few months Infantmortality failures are generally the result of manufacturing imperfections, weak or substandard components, design errors, and installation defects. At the end of the early failure rate region, the failurerate will eventually reach a constant value. Useful life period: In the time period after infant mortality, but before the beginning of wear out, random failures dominate. During the constant failurerate region the failures do not follow a predictable pattern but occur at random Wear-out period: The beginning of the wear-out region is noticed when the failure rate starts to increase significantlymore than the constant failure rate value and the failures are not longer attributed to randomness but are due to the age and wear of the components. To minimize the effect of the wear-out region,...