A load of insufficient magnitude to cause failure in a single application may lead to failure if it is removed and reapplied repeatedly. This mechanism of failure is known as fatigue. In unwelded metals and alloys the failure process consists of initiation of microscopic cracking, frequently at a surface feature such as a change of section, followed by propagation, with each load cycle causing minute crack extension. Fatigue cracks are in many cases very fine, remaining tightly closed at minimum load and hence difficult to find by visual examination alone. As the crack extends, the remaining intact area of the cross section reduces, possibly leading to complete fracture, or failure by another mode such as jamming or seizure of a mechanism.
In welded components, pre-existing flaws provide sites for early fatigue crack formation. The fatigue process then consists almost entirely of propagation, the initiation phase being much shorter or entirely absent. Planar fabrication flaws such as lack of penetration or lack of fusion provide ideal sites for fatigue cracking. Even joints proved free of fabrication flaws by NDT will contain microscopic planar features at the weld toe, for example entrapped slag intrusions, which allow early fatigue cracking. As a result, the fatigue performance of welded joints is generally poor by comparison with unwelded material. For example, in mild steel plate, the allowable fatigue stress range for a typical fillet welded detail is roughly one third of that for the unwelded material.
Premature fatigue failure is prevented by careful attention to detail at the design stage to ensure that cyclic stresses are sufficiently low to achieve the required endurance. Stress concentrations should be avoided where possible; a design with smooth 'flowing' lines is usually the optimum. Textbooks and design handbooks are available giving fatigue data for a wide range of materials, see References 1 & 2 below for example. National and international codes provide guidance on allowable cyclic stresses in structural joints, including welds and mechanical fasteners, see References 3 and 4. Reference 5 provides detailed background on fatigue of welded joints.
- Fatigue Design Handbook, Society of Automotive Engineers, Warrendale PA, 1988.
- Metal Fatigue, Frost N E, Marsh K J, & Pook L P, Clarendon Press, Oxford, 1974.
- Fatigue Design of Welded Joints and Components, Recommendations of IIW Joint Working Group XIII-XV, Abington Publishing, 1996.
- BS 7608:1993 Fatigue Design and Assessment of Steel Structures. BSI London.
- Fatigue Strength of Welded Structures, Maddox S J, Abington Publishing, Second Edition 1991.