SS - Fatigue

Structural design takes into consideration two different modes for failure - fatigue and fracture. Whereas it is commonplace (and correct) to believe fracture is the more dangerous form for failure, the fatigue of a member is more critical to design for. If a material is about to fail due to cyclic loading, this means the maximum stress values are less than the ultimate tensile stress limit, and thus the material will eventually fail to a smaller subjected load.

Continual loads will form gradual cracks that accumulate at a microscopic level in a material. These cracks eventually find their ways to the face of a structural member, and the compilation of the cracks will eventually result in fracture. If a structural member is formed into a certain shape, such as rectangular holes or sharp edges, like the reentrant corners in a building undergoing seismic surface waves, the material will be subjected to elevated local stresses, which will result in the increased rate of fatigue in the member.

Fatigue Life

The fatigue life, N(f), is the number of cyclic loadings a material sustains before it fails. It is NOT measured in seconds or time. Only in number of loadings.

Fatigue Observations from Cracks

The process of fatigue is a gradual one, taking an extend period of time, so the slow build-up of stresses typically results in a darker, more layered cross section when a structural member finally breaks. The brighter more silvery portion of the cross is where a sudden fracture occurs in a member.

Also, fatigue is a stochastic process, meaning that it is random. The path for fatigue cannot be determined, although it might look similar from a quick observation.

Some obvious factors influencing fatigue would be temperature, surface finish, grain size, material type, texture, direction of loading, geometry, atomic structure, internal stresses and oxidation (if present). Fatigue strength can be measured to a maximum of 10^3 to 10^8 cycles (this is typically for steel members, which provide the greatest overall fatigue strength for an extended period of cycles).

Designing for Fatigue

1. Design to keep a structural member below it's fatigue limit

2. Design for a fixed life after which a user should replace the member with a new one (called a "lifed part" and this process is called "safe-life" design practice)

3. Inspect the member periodically for cracks. Replace when cracks exceed a critical length. Employ nondestructive techniques to determine crack length.

Where Fatigue Occurs:

In the graph above, fatigue occurs between the ultimate tensile strength and fracture stress. Although cracks form before the ultimate tensile strength, it is more important to design for this later region at an early stage. That way, one will prevent a critical build-up of stresses at any portion in the cross-section of structural member.