The bathtub curve is a concept used in reliability engineering to describe the failure rate of a system or component over time. It is named for its resemblance to the shape of a bathtub. Three regions are distinct in the bath Tube Cruve. 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 life.” The final portion of the curve is called the “end of life” or “wearout region.”
1. Infant Mortality
In the early portion of the bathtub, the failure rate is higher because weaker units in the population will fail first. As these weaker units are removed from the population, the failure rate goes down. This often occurs over a period of time involving a few weeks. A considerable effort has gone into the creation of procedures to identify and weed out weak units in manufacturing processes. Using accelerated screening techniques, most weak units can be identified within a few days. This effort has been quite successful for manufacturers having good quality programs, and since the 1990s, the end user rarely sees most of these failures.
2. Useful Life
The middle portion of the bathtub has a failure rate that remains relatively flat or declining as a function of operating time interval. Failures are primarily due to random stresses in the environment. During this period of time it is reasonable to assume that the failure rate is constant. While many consider this to be too conservative due to the fact that the failure rate is probably declining, this assumption simplifies the math and is very appropriate for probabilistic SIF verification.
3. Wearout Region
The end of the bathtub curve occurs when the strength of the product declines (susceptibility increases). This is commonly known as “wearout.” Wearout occurs after several years but the mechanisms vary considerably depending on the type of component. When the susceptibility of a component goes up, the failure rate increases rapidly. Manufacturers of instrumentation for safety instrumented applications will typically publish the useful life numbers for their products. A mechanical integrity program needs to address all instruments in safety applications before the end of their useful life.
