Fatigue Life Prediction Methods in Welded Joints: Case Studies

The increasing need for greater reliability from structures and components coupled with the need to remain competitive has focused attention on weld quality and rational methods for assessing weld quality requirements. As a result, the fitness-for-purpose concept has evolved, whereby features which could be regarded as imperfections are acceptable provided they do not reduce the fatigue life of the component below that required. An important part of this evolution has been a development of methods for calculating the fatigue lives of welds containing flaws. Fracture mechanics is now established as the most suitable method for considering crack-like flaws. As well as its use for judging the acceptance of flaws by comparing calculated and required lives, fracture mechanics can also be used for calculating the remaining lives of service-cracked structures. Welding Institute experience of the use of fitness for purpose-based assessments has led to enormous cost savings in terms of elimination of unnecessary repairs and allowing structures to remain in service until it is convenient to repair fatigue cracks (1). The methods used follow closely those outlined in British Standard PD6493 (2), which are closely based on recommendations developed by the International Institute of Welding, which is currently being updated and improved.