Bathtub curve

The bathtub curve is a plot of failure rate over time for a population of products. It shows three periods: an early failure period with a high but rapidly decreasing failure rate, an intrinsic failure period with a roughly constant failure rate, and a wearout failure period with an increasing failure rate. It is used in reliability evaluation to model failure behavior over the product lifecycle.

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Specification

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Study notes

Early Failure Period (also called Infant Mortality Period) begins at time zero and has a high but rapidly decreasing failure rate, typically lasting several weeks to a few months.
Intrinsic Failure Period (also called Stable Failure Period) has a roughly constant failure rate level called the Intrinsic Failure Rate; most systems spend most of their lifetimes in this flat portion.
Wearout Failure Period occurs when materials wear out and degradation failures occur at an increasing rate.
The Bathtub Curve also applies to repairable systems, where the vertical axis is the Repair Rate or the Rate of Occurrence of Failures (ROCOF).