TWI Knowledge Summary

Fatigue

by Peter Tubby

Fatigue is a mechanism of failure experienced by materials under the action of a cyclic stress. It involves initiation and growth of a crack under an applied stress amplitude which may be well within the static capacity of the material. Discontinuities such as changes in section or material flaws are favoured sites for fatigue initiation. During subsequent propagation the crack grows a microscopic amount with each load cycle. The crack so-formed often remains tightly closed, and thus difficult to find by visual inspection during the majority of the life. If cracking remains undiscovered, there is a risk that it may spread across a significant portion of the load-bearing cross section, leading to final separation by fracture of the remaining ligament, or another failure mode may intervene such as jamming of a mechanism. Fatigue occurs in metals, plastics, composites and ceramics. It is the most common mode of failure in structural and mechanical engineering components.

The phenomenon has been investigated extensively over many decades, particularly in metals and alloys. As a result, design guidance is readily available in many texts and is widely codified. Joints in materials are particularly susceptible to fatigue and therefore need to be designed with care for cyclic loading. Fatigue design rules for welded and bolted connections in steel and aluminium alloys have been produced by the International Institute of Welding (IIW) see Ref. 1 ; similar rules can also be found in many national standards, e.g. BS 7608 (Ref. 2 ) is widely used in the UK. Data for other alloys and joining methods have not been codified to the same extent, but a good deal of published information exists which may be accessed via Weldasearch .

In welded joints, fabrication flaws may give rise to premature fatigue failure, particularly planar flaws such as lack of fusion. Using fracture mechanics, the rate at which fatigue cracking will grow from such features can be estimated, and in this way tolerable flaw sizes can be derived. British Standard 7910 (Ref. 3 ) provides detailed guidance on this method of assessment.

Fatigue continues as a subject of current research worldwide, to the extent that several technical journals are devoted solely to the subject. With respect to the behaviour of joints, fatigue data are lacking for all but the most commonly used alloys and joining methods. Other areas of current research include improved methods of predicting fatigue crack growth, refining theoretical stress analysis methods for fatigue sensitive structures and development of fatigue sensors.

References

1. Fatigue design of welded joints and components
   Recommendations of IIW joint working group XIII and XV
   Edited by A Hobbacher
   Abington Publishing, 1996
   ISBN 1 85573 315 3
   
   
2. BS 7608 , Fatigue design and assessment of steel structures, British Standards Institution, 1993
   
   
3. BS 7910 , Guide to methods for assessing the acceptability of flaws in metallic structures, British Standards Institution, 2005