Asset Structural Integrity Seminar Highlights Constraint Effects in Fracture
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The Structural Integrity Technical Group of The Welding Institute (TWI) and the UK Forum for Engineering Structural Integrity (FESI) recently addressed a topic that is currently of substantial research and industrial interest by jointly holding a full day’s Constraint Effects in Fracture seminar. The event focused on how to take advantage of constraint issues when carrying out fracture mechanics-based safety cases for pressure containing equipment and included the roots of constraint-based assessment, providing studies, validation and an overview of current research.

Geoff Booth, Chairman of the TWI Structural Integrity Technical Group and John Sharples, Wood Plc and Chief Technical Advisor to FESI co-chaired the event with Geoff opening proceedings and welcoming delegates.

First to speak was Professor Steve Garwood of Imperial College London, describing how to optimise constraint treatments in pressure vessel safety case assessment. Professor Garwood explained the background to British and European pressure vessel design codes approaches to avoid brittle fracture, as well as discussing constraint effects under biaxial loading and in fracture toughness specimens.

Next, Dr Gery M. Wilkowsi, Engineering Mechanics Corporation of Columbus, USA talked about the full scale validation of constraint effects on fracture transition temperature and upper shelf behaviour, and presented work on how the R-curve (CTOD or J) can be treated for any other flaw depth by having two SENT tests with different flaw depths.

He was followed by Professor Bob Ainsworth, University of Manchester whose presentation comprised: the current constraint based procedures in R6 and BS7910 standards and the inputs needed to apply them; a planned revision to the constraint-based methods in R6 2018, illustrated by considering application to a pipe under pressure and bending; and ongoing activities on fracture modelling.

The programme continued with a talk by Dr Isabel Hadley of TWI on how to assess constraint effects using BS 7910 Annex N and R6 section III.7 and covered the approaches of constraint-based methods described in both standards, together with TWI’s work on validation of BS and R6 fracture assessment procedures. Dr Hadley also included a case study on a tested pressure vessel steel and completed her talk with two additional constraint-based methods.

Dr Catrin Mair Davies, Imperial College London, spoke about: the challenges in experimentally determining constraint effects in nuclear steel; the range of specimen geometries, high and low constraint, which may be used to simulate different constraint conditions in components; and numerical and experimental results from research on RVP steel.

Next on the programme was a case study presented by Anthony Horn, Wood Plc on how constraint-based fracture assessments are applied in practice, and detailing the background to the assessment and testing which was performed on modified compact tension, and conventional low constraint, specimens such as shallow cracked bend specimens. A series of R6 constraint modified assessments on a semi-elliptical crack at the root of a screw thread was also shown.

The last session of the day was an overview of the potential benefits in claiming constraint corrections and modifications, when building up the materials toughness estimates used in a safety assessment in low alloy pressure vessel steels – given by Dr Dan Cogswell from Rolls-Royce Plc – who also explained some of the pitfalls and metallurgical reasons why it may not be possible to use a correction as widely as desired.

In summing up the event, Geoff Booth, Chairman of the TWI Structural Integrity Technical Group said ‘It is now clearly evident that there are substantial financial and technical benefits to be gained when constraint effects are included in safety cases for pressure containing equipment such as pipelines and nuclear reactor vessels.’ Adding ‘The technology is ready for industrial exploitation and ongoing research continues to obtain further improvements.’

John Sharples, Wood and Chief Technical Advisor to FESI explained ‘The better understanding of crack-tip constraint effects has clearly been a significant advancement in fracture mechanics in recent years. This event has been timely therefore, in enabling state-of-the-art developments and understanding to be outlined and discussed. In addition, the event has been successful in demonstrating the benefits of employing constraint-based methodology to plant components when the use of conventional methods is not sufficient to demonstrate adequate structural integrity and safety margins.’