Plot failure rate against time for a large population of similar assets and the shape that shows up most often looks like a bathtub in cross-section — hence the name:
- Infant mortality — a higher failure rate right after commissioning, usually from installation faults, manufacturing defects or component weaknesses that surface fast under load
- Useful life — a long, roughly flat stretch where failures are largely random rather than age-driven, and MTBF is the meaningful metric
- Wear-out — failure rate climbs again as components age past their design life, and preventive replacement becomes the right call over run-to-failure
Why it matters for scheduling
It's the reason commissioning a new automated line often needs a tighter inspection cadence for the first few months than the steady-state schedule calls for — and why replacing a high-wear part just because it's old, ahead of a scheduled overhaul, is usually the right call once an asset is deep into wear-out.