Wearout period: bathtub curve

As described in Problem HZA.7, the failure rate of equipment frequently exhibits three stages a break-in stage with a declining failure rate, a useful life stage characterized by a fairly constant failure rate, and a wearout period characterized by an increasing failure rate. Many industrial parts and components follow this path. A failure rate curve exhibiting these three phases is called a bathtub curve. 

The lifetime of a population of units at the component, board, box, or system level can be divided into three distinct periods. This is most often defined by the so-called reliability bathtub curve (Fig. 6.16). The bathtub curve describes the cradle-to-grave failure rates or frequency of failures as a function of time. The curve is divided into three distinct areas early failure rate (also known as infant mortality), the useful life period, and the wearout failure period. The infant-mortality portion of the curve, also known as the early life period, is the initial steep slope from the start to... 

Statistical values can also be used to determine expected periods of optimum performance in the life cycle of products, systems, hardware, or equipment. For example, if the life cycle of humans were plotted on a curve, the period of their lives that may be considered most useful, in terms of productivity and success, could be represented as shown in Figure 5.4. This plotted curve is often referred to as the bathtub curve because of its obvious shape. A similar curve can be used to determine the most productive period of a product s life cycle according to the five known phases of that life cycle, as discussed in Chapter 3. The resultant curve, known as a product s reliability curvey would resemble the curve that appears in Figure 5.5. During the breakin period, failures in the system may occur more frequently, but decreasingly less frequently as the curve begins to level toward the useful life period. Then, as the system reaches the end of its useful life and approaches wearout, more frequent failure experience is likely until disposal. 

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. 

For nonrepairable systems (a system composed of many subassemblies and components), the instantaneous failure rate, termed the hazard rate h t), follows a pattern that changes with time. It is usually represented by a bathtub-shaped failure curve over time, as shown in Fig. 1 [1,1a]. It begins with an initial decreasing hazard rate attributed to premature failure due to defects. This is followed by a useful life period with an almost constant hazard rate due to intrinsic failures and, finally, a wearout period where the hazard rate increases rapidly with time. 

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