TWI Frequently asked questions
Modelling is generally used to prove the integrity of welded structures. This means that the model results should be used to predict the resistance of the structure to all likely forms of failure mechanism. Fatigue is a problembecause fusion welds generally have low fatigue strength relative to plain material under the same cyclic stresses. Welding defects (e.g. lack of side-wall fusion) or fatigue cracks emanating from welds can be susceptible to fracture,or, if the material is very tough, plastic collapse.
Fatigue of welds has been assessed using fracture mechanics. Alternatively, S-N curves have been developed for a wide range of weld joint types. (An S-N curve gives the relationship between the applied stress range [S] and thenumber of cycles to failure [N]). For instance, in the UK, S-N curves can be found in BS 7608: 1993 for welds in steel structures. Methods of calculating the stress range from model results are described in Niemi et al (see references).
The assessment of fracture and plastic collapse is described in the British Standards Institution BS7910 : 2005 ('Guide to methods for assessing the acceptability of flaws in fusion welded structures'). BS7910 uses a failureassessment diagram (FAD). Points inside the FAD are safe. The user must know the material toughness and have some knowledge of the material stress versus strain curve. The loading must be divided into primary (those which contribute toplastic collapse) and secondary stresses, and these must be further divided into membrane and bending parts. A similar document is used in North America, (API 579).
References
BS 7608: 1993 'Code of practice for fatigue design and assessment of steel structures'.
BS7910:2005 'Guide to methods for assessing the acceptability of flaws in metallic structures'
API579 'Fitness for service', API recommended practice 579, 2 nd ed 2007
E Niemi, W Fricke and S Maddox: 'Fatigue analysis of welded components: Designer's guide to the structural hot-spot stress approach', Woodhead Publishing, 2006.
