Frequently Asked Questions
The crack arrest approach can be adopted for determining the fitness for service of a structure. Many structural integrity assessment procedures are aimed at preventing the initiation of fracture. An alternative approach is to assume that unstable crack extension may start in a region of high stress and/or localised embrittlement, but that the surrounding material will have sufficient resistance to crack extension to arrest the running crack.
The crack initiation approach relies on close attention to the workmanship and quality control during welding. However, it can be difficult to guarantee that no areas of local brittle zones exist. It is also difficult to know the exact stress distribution around a crack tip that experiences contributions from residual stresses and local stress concentration. For these reasons the crack arrest approach may be considered a viable alternative. The crack arrest concept is simple in principle, if not always in application: arrest of fast running brittle cracks, that have initiated in a region of low toughness, will occur if the applied crack driving force is smaller than the resistance to crack propagation. By looking at conditions of crack arrest, the focus can be taken away from localised stress concentration or brittle zones and back to the bulk properties of the parent plate, weld metal or heat affected zone and the nominal applied stress.
The conditions for crack arrest to occur can be expressed using one of the following approaches [1] :
- Fracture mechanics comparison, where the applied stress intensity factor (KI) is less than the crack arrest fracture toughness (KIa).
- Ensuring that the temperature at the crack tip is above the crack arrest temperature (CAT)
- An energy balance so that the available energy for crack propagation is less than the energy absorbed by the material during crack propagation.
- Local approach, such that the crack driving force (cdf) is less than the local arrest property. The cdf is determined from mechanical models, and the local arrest is determined from mechanistic models of the crack arrest processes.
The main applications of the crack arrest approach are for ductile fracture propagation control in gas pipelines, [2,3] and fracture control in pressure vessels [4] and LPG storage tanks. [5]
References
- Wiesner, C. S. 'Crack arrest - when brittle fracture stops'. In The life of a Crack: Initiation - Growth - Fracture, 2000. Edited by M Koçak.
- Rothwell, A. B. 'Fracture propagation control for gas pipelines-past, present and future'. Pipeline Technology vol. 1, 2000, pages 387-405.
- Horsley, D. J. 'Background to the use of CTOA for prediction of dynamic ductile fracture arrest in pipelines'. Engineering Fracture Mechanics Vol. 70, 2003, 547-552.
- Wiesner, C. S and Hayes, B. 'A review of crack arrest tests, models and applications'. TWI Members Report, 1995. Available on this website to TWI Industrial Members, PDF file, 10581 kBytes:
- 'Crack arrest checks for new LPG storage tank steels'. TWI's news magazine Connect No. 95, July-August 1998. Available on this website:
Further information
FAQ: What are local brittle zones? FAQ: What is significance of local brittle zones? Characterising crack arrest behaviour in steel (Knowledge summary).
The next item (PDF file, 2198 kBytes) is only available to TWI Industrial Members:
An introduction to K, CTOD and J fracture mechanics analyses and toughness, and the application of these to metal structures.