Introduction to fatigue (Topic: 12033)

Structures under repeated stress applications may get fatigue damages which will reduce their working life or limit their use.

Fatigue fractures may occur in areas with stress concentrations or at defects as fractures and lack of fusion. Critical areas will be where the cross section changes shape, i.e. holes, ends of welds, tapped holes and defects in the material.

Fatigue fractures will grow 90° on the main stress direction. Defects laying around 90° on the main stress direction will for this reason be especially critical.

The growth of fatigue fractures is exponential. When the fractures are small, they will increase slowly, and they can be hard to detect. At the end of the working life, however, they move very rapidly.

The fatigue resistance of a structure is dependent on:

- The different dynamic loads during its working life

- The variation of stress at the actual detail or component

- Material properties (S-N-curves or fracture mechanics properties)

The national codes deal with fatigue in different ways. It is, however, a lot of research work going on in Europe on the fatigue behaviour of aluminium alloy structures. A recommendation is issued (march 1992). It is called the "European Recommendation for Aluminium Alloy Structures. Fatigue design." from ECCS TC2.

References: [17, 18]