Wearout region

Figure 2.5-2 depicts the force of mortality as a bathtub curve for the life-death history of a component without repair. The reasons for the near universal use of the constant X exponential distribution (which only applies to the mid-life region) are mathematical convenience, inherent truth (equation 2.5-19), the use of repair to keep components out of the wearout region, startup testing to eliminate infant mortality, and detailed data to support a time-dependent X. 

The shape of the plot in Figure 3-2 is characteristic of many components and well known to reliability engineers. The shape is called the "bathtub curve." Three regions are distinct. In the early portion of the plot, failure rates are higher. This area is called "infant mortality." The middle portion of the curve is known as "useful Ufe." The final portion of the curve is called "end of tife" or "wearout region."... 

The reliability bathtub curve, shown in Rgure 2.5, represents the change in probability of failure over time of a component. The bathtub can be divided into three regions bum-in period, useful life period, and wearout period. 

The IFR region is the wearout period, which is characterized by an increasing rate of failure as a result of equipment deterioration due to age or use. For example, mechanical components such as transmission bearings will eventually wear out and fail, regardless of how well they are made. Wearout failures can be postponed, and the useful life of equipment can be extended by good maintenance practices. The only way to prevent failure due to wearout is to replace or repair the deteriorating component before it fails. 

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