In the “real world” fatigue usually – not always, but usually – initiates at a location that acts as a stress concentration, or “stress riser”. A component is most resistant to fracture when the stress is evenly distributed over it. A stress riser disrupts this even distribution and concentrates the stress at a geometric feature or reduction in the component’s area. Typical stress risers include holes, slots, corners and radii, rough surface finish, welds, corrosion pits, cracks and microstructural defects such as inclusions.
The exception to “usually” – the cases where fatigue fractures initiate from component surfaces that are free of stress risers – typically result from one of two causes; under-design of the component, or abusive service conditions.
Just as all materials have an ultimate tensile strength, they also have a fatigue strength, sometimes called the fatigue limit or endurance limit. Once a component is subjected to cyclic stresses that exceed this limit, fatigue fracture occurs, even though no stress riser is present.
Fatigue failures of this type are less common than fatigue failures initiating from stress risers. Usually components are intentionally over-designed to deal with stresses several times greater than those they would be subjected to in service as a safety margin.
Fatigue Crack Initiation – The Critical Event
If the initiation stage can be prevented, fatigue fracture will not occur. It sounds obvious and simple. It’s not. Initiation is the most complex stage of fatigue fracture. A low magnitude load, which would have no effect whatsoever on a component in a single application, can be devastating when repeatedly applied in thousands or millions of cycles. The cumulative effect of these cyclic loads are microscopic “shifts” in the material’s structure which ultimately produce a “dislocation” – at this scale it is too small to be called a crack – and the focal point of stress concentration is born. Vibration harmonics, dampening of the system and the service environment further complicate the issue. Collectively, these affects become difficult to predict in the design stage.
In Failure Analysis of Fatigue – Part 3 we will discuss fatigue prevention at the design stage of a component’s life, with following entries focused on the manufacturing and service environments and their relationship to fatigue failure.