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An example of avoiding possible over-conservatism in engineering critical assessment

The results of an engineering critical assessment (ECA) could be unduly conservative. This case study demonstrates how this can be avoided and the subsequent outcome.

Background

ECA is based on fracture mechanics and widely used in many industries to determine the maximum tolerable, initial flaw size in metallic structures. However, it is well recognised that the results of an ECA could be unduly conservative and the accuracy of an ECA strongly depends on how precise the input data are. Material fracture toughness is one of the key input parameters.

Although Annex J in BS 7910 (2013) provides methods to estimate fracture toughness – Kmat of material from charpy V-notch impact energy, Cv – it recommends that direct determination of fracture toughness by testing is always preferable where possible.

Objective

The generation of an example to demonstrate how to reduce over-conservatism of ECA by conducting fracture toughness testing.

Figure 1. A defect found at the fillet radius of a flange
Figure 1. A defect found at the fillet radius of a flange

Work programme

During an inspection of a refrigeration system operated by a TWI Industrial Member company, some crack-like defects were found at the fillet radius of the flanges (Figure 1) which were made of steels in compliance with ASTM standard. TWI was awarded a project to carry out ECA in order to determine the maximum tolerable defect size for each flange for the design life.

As no fracture toughness or even Cv values were available, the initial assessments were based on the fracture toughness values estimated from the specified minimum Cv value for the steel, as specified in the relevant standard, i.e. 27J at 20oC. The maximum tolerable flaw size determined was less than some of the defects found.

Subsequent Charpy testing of the steel did not produce a significant increase in the estimated fracture toughness. Finally, fracture toughness testing at the appropriate temperature was performed to directly obtain the fracture toughness values.

Project outcomes

The fracture toughness values of the steel in several different size of flanges were obtained from crack tip opening displacement (CTOD) testing at the anticipated service temperature of -20oC.

The exercise showed that they were all significantly greater than those estimated from the Cv values, by factors more than 2.6. As a result, the maximum tolerable flaw sizes, assessed using the experimentally determined fracture toughness values, significantly increased. Therefore, all the defects found were acceptable and there was no need to replace these flanges. This has avoided downtime of the system and also reduced cost.

References and credits

BS 7910:2013+A1:2015 (Incorporating Corrigendum No.1): 'Guide to methods for assessing the acceptability of flaws in metallic structures', British Standard Institution, London, UK, 2013.

Please email contactus@twi.co.uk to discuss your requirements.

Avatar Yanhui Zhang Consultant – Fatigue Integrity Management

Yanhui has a background in metallurgy, and graduated from the University of Science and Technology Beijing in 1982 with a Bachelor’s degree, before obtaining a PhD from the Open University, UK in 1992. Before joining TWI in 2001, he worked on Ni-based super-alloys as a Postdoctoral Researcher at the University of Cambridge. Yanhui’s expertise includes fatigue design, fatigue and creep life evaluation, engineering critical assessment (ECA), fatigue and creep testing, and failure investigation. He is also highly experienced in establishing relationships between mechanical properties and the microstructure of materials. Yanhui has published over 60 academic papers in journals and at international conferences.

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