Slide 1
Repeated loads can accumulate damage
Fatigue is damage caused by repeated loading. A component can fail after many cycles even when each load is smaller than a one-time breaking load.
More to Read
The relevant history includes both the changing stress range and the average stress. Two parts with the same maximum stress can have different fatigue behaviour if their loading cycles differ.
Slide 2
Small features can start cracks
Surface scratches, notches and manufacturing details can concentrate local stress. These sites may become the starting points of fatigue cracks.
More to Read
A nominal stress calculated from load divided by area does not describe every local peak. Surface finish and joint geometry can therefore matter as much as increasing the overall section size.
Slide 3
Cracks can grow gradually
Once a crack exists, repeated loading can extend it cycle by cycle. Eventually the remaining section may no longer support the applied load.
More to Read
Growth rate depends on the load cycle, crack size and environment. The final fracture may be sudden even though a long period of progressive damage preceded it.
Slide 4
Real loading is variable
Machines rarely experience identical cycles forever. Occasional overloads, changing duty and corrosion complicate simple life estimates.
More to Read
Laboratory fatigue curves describe particular test conditions. Applying them to a component requires accounting for geometry, surface treatment and actual loading rather than reading a single universal lifetime from the material name.
Slide 5
Design can include inspection
Damage-tolerant designs allow for detectable cracks and planned inspection. The interval depends on how defects can grow before becoming critical.
More to Read
Inspection methods have detection limits, so an assumed detectable crack size must be realistic. Access, method capability and growth uncertainty belong in the maintenance plan together.
Sources
- Fatigue
MIT / David Roylance
- Introduction to Fracture Mechanics
MIT / David Roylance