TWI Knowledge Summary

Failure of welded structures

Failures of welded structures can and do occasionally occur, sometimes with serious human, environmental and economic consequences. Table 1 shows approximate failure rates for various types of welded structure. It shows amongst other things how the use of experience-based engineering codes and standards can reduce failure rate (the ASME Boiler Code Committee was established in 1911, when boiler explosions in the USA were occurring at the rate of virtually one per day). Although such occurrences are much less common a century on, the continued prevention of failure requires careful attention to design, materials, construction, inspection and maintenance.

A useful way of categorising failures in welded structures is to distinguish between instant failure modes and time-dependent failure processes. Examples of instant failure modes include brittle fracture, overload failure and buckling. In all cases, the failure occurs when the 'driving force' for failure (e.g. applied stress, applied stress intensity) exceeds the materials resistance (e.g. tensile strength, fracture toughness). Consequently, instant failure modes are quite likely to occur early in the life cycle of the structure, perhaps due to errors in design, construction, materials or inspection. Failure of pressure vessels during their final hydrotest ^[1,2] (see also Catastrophic Failures of Steel Structures in Industry: Case Histories.' TWI report 632/1998, February 1998 ) and of bridges during construction ^[3] , come into this category. On the other hand, 'instant' failures may also occur after many years of operation because of statistically rare events, such as the loading induced by a severe storm or the reduction in fracture toughness caused by unusually low temperatures. It should also be borne in mind that fracture toughness can fall over time as a result of embrittlement (e.g. temper embrittlement, radiation embrittlement) so that a structure which was safe at the start of life may later become unsafe.

Time-dependent failure processes include fatigue and creep crack growth, stress corrosion, other forms of corrosion, and wear. These may eventually lead to final failure by one of the instant failure modes described above, but are more amenable to control by in-service inspection. For example, welded joints are particularly susceptible to fatigue, typically initiating from discontinuities at the weld toe. However, the relationship between applied stress range, joint geometry and fatigue life is well-documented, and failure can be prevented by basing the design on an appropriate design codes. ^[4,5] . The FAQ 'Where a welded structure is subjected to cyclic rather than static loading, how is the allowable stress calculated?' provides further information on this topic.

Although design codes allow a notional design life to be assigned to a welded structure, life extension and/or change of use of a welded structure are often possible through the use of in-service inspection, materials assessment and application of fitness-for-purpose procedures ^[6-10] . additional information is contained in the FAQ 'Are there any codifed procedures to evaluate the significance of weld flaws in metallic structures and components?' .

Table 1. Examples of failure frequencies of welded structures

Type of structure	Failure rate	Reference
Boiler (explosion), USA, c.1900	approx. 400 per year (rate per vessel year not stated)	11
Boiler (explosion), USA, c.1970	approx. 20 per year (rate per vessel year not stated)	11
Onshore gas pipeline, Western Europe	0.6 per 1000km/year	12
Petroleum products pipeline, USA	0.55 per 1000km/year	13
Pressure vessels (catastrophic failure), UK	2x10 ^-6 per vessel year	14,15

References

N°	Author	Title
1	Hayes, B:	'Six case histories of pressure vessel failures', Engineering Failure Analysis, 3/3, 1996, 157-170
2	Hayes, B and Phaal, R:	'Catastrophic Failures of Steel Structures in Industry: Case Histories' TWI report 632/1998, February 1998
3	Hartbower C E:	'Brittle fracture of the tension flange of a steel box-girder bridge'. In Book: Handbook of Case Histories in Failure Analysis, Ed. K A Esaklul, Publ. ASM International, Vol.1, 1992, pp.369-377.
4	BS7608:1993	'Code of practice for fatigue design and assessment of steel structures'
5	AWS D1.1:2000:	'Structural welding code - steel', 17th edition
6	BS7910: 1999	'Guidance on methods for assessing the acceptability of flaws in metallic structures (incorporating Amendment 1)'
7	SINTAP:	'Structural integrity assessment procedures for European Industry', Final Procedure, November 1999
8		IIW guidance on assessment of the fitness for purpose of welded structures, IIW/IIS-SST-1157-90, 1990
9	API 579:	Recommended practice for fitness-for-service, API, January 2000
10	WES 2805:1997:	'Method of assessment for flaws in fusion welded joints with respect to brittle fracture and fatigue growth'. Japanese Welding Engineering Society, 1997
11	Walters, S:	'The beginnings', Mechanical Engineering, April 1984, 38-62
12	Jones, D:	'Inspection - the key to a reliable future', Pipes and Pipelines International, March-April 1997, 32-43
13	Hovey D J and Framer E J:	'Pipeline accident, failure probability determined from historical data', Oil and Gas Journal, July 12 1993, 104-107
14	Smith, T A. and Warwick, R G:	'A survey of defects in pressure vessels in the UK for the period 1962-1978 and its relevance to nuclear primary circuits', International Journal of Pressure Vessels and Piping, 1983, 11, 127-166.
15	Davenport, T J:	'A further study of pressure vessel failures in the UK', International Conference on Reliability Techniques and their application, Reliability '91, London, UK, 10-12 June 1991

Further information

You can use the Weldasearch literature database to supplement what you find in JoinIT.