TWI Technology Briefing 632 - February 1998
B Hayes and R Phaal
FULL REPORT
A series of fourteen case studies is described, where failure has occurred by fracture, and the lessons which can be learnt from these failures are discussed.
Background
Fortunately, catastrophic structural failures are rare. However, when significant structures such as pressure vessels, storage tanks, bridges or ships fail, the ramifications can be extensive, in terms of human injury, together with loss of capital revenue. In order to reduce the incidence of costly failures in the future, it is important to learn from past events. These lessons are incorporated into industrial and national codes of practice, and are valuable to practising engineers responsible for the management of critical structural plant.
Objectives
- To describe events relating to significant structural failures, together with the associated failure investigations.
- To summarize the lessons which can be drawn from these failures.
Approach
This report describes a series of fourteen industrial failures, where the mechanism of failure was by catastrophic fast fracture. Failure case studies are presented for six pressure vessels, three ships, two bridges, two storage tanks and one offshore rig, in chronological order. For each case study, the failure events are described, together with an account of the main contributing factors and failure mechanisms. (Fracture mechanics analysis of the failures are not included in this report, see TWI Members' Report 512.) The lessons which can be learnt from these failures are discussed.
Discussion
A range of issues has been identified which have contributed significantly to the structural failures described in this report:
- Fracture toughness is of particular importance for welded fabrications, where fracture toughness is dependent on chemical composition, microstructure, joint configuration, loading rate and temperature. Low fracture toughness was afactor which contributed to most of the failures which have been discussed in this report.
- All of the failures which have been described in this report were associated with welds. Weldments are associated with a higher risk of fracture due to the combination of complex metallurgy, welding residual stresses, stressconcentrations and higher constraint associated with the joint configurations, together with the inherent flaws which are present in all welds.
- Many of the failures associated occurred at relatively low temperatures (-20 to 13°C). Ferritic steels undergo a transition from ductile behaviour at lower temperature.
- Many of the failures occurred during hydrotests.
- The environmental/service conditions to which critical structures are exposed are important. These include any factors which could lead to embrittlement of the component materials during its anticipated lifetime.
- The effective management of fracture control in critical structures implies an ongoing commitment to effective maintenance, inspection and quality assurance, with regard to issues relating to management of fracture risk.
Main conclusions
The lessons which are learnt from structural failures influence the industrial and national codes of practice for design, fabrication and operation of critical plant.
Structural failures have also resulted in the development of 'fitness-for-purpose' assessment methods, such as BSI PD 6493: 1991 'Guidance on methods for assessing the acceptability of flaws in fusion welded structures'. These methods are based on fracture mechanics principles, and allow the significance of weld flaws to be assessed in terms of structural integrity assessment. PD 6493-type methods are used extensively, on an international basis, for many applications, including pressure vessels, pipelines, storage tanks, ships, bridges, buildings and other structural components.
Recommendations
In order to reduce the incidence of costly failures in the future, it is important to learn from past events. These lessons need to be incorporated into industrial and national codes of practice, and are valuable to practising engineers responsible for the management of critical structural plant.
Member Report No. 632-1998
Catastrophic failures of steel structures in industry: case histories
