Session: CS-36-01 Combined Session: Master Curve Method and Applications, and Improvement of Flaw Characterization Rules for Fitness for Service

Paper Number: 61934

Start Time: Thursday, July 15, 2021, 09:00 AM

61934 - A Review of Conservatisms in Fracture Mechanics Assessments 

Fracture mechanics assessments are typically carried out in order to be able to evaluate the significance of known or postulated crack-like defects in engineering structures and components.  Such assessments, which are usually deterministic in nature,  commonly entail evaluating  limiting defect sizes and fatigue crack growth based on methodologies such as those contained in R5, R6 and BS7910 in the UK, RSEM in France, KTA in Germany and ASME XI in the USA.  These methodologies generally contain inherent conservatisms such that evaluated limiting defect sizes are underestimated and fatigue crack growth overestimated from the true situation.  Such conservative evaluations are of course generally acceptable to the regulatory bodies in terms of ensuring safe operation.

 

Particularly owing to various material degradation mechanisms, the deployment of assessment methodologies that may have large conservatisms becomes challenging for assuring structural integrity of structures and components in later operating life or for justifying plant life extension. It would therefore be desirable if a better understanding could be gained of the various conservative aspects inherent in assessment methodologies, and in the current practices followed by structural integrity assessment engineers.  This is with a view to modifying aspects of the methodologies and practices in the future that may currently be excessively conservative.  Furthermore, there are indications that probabilistic fracture mechanics assessments may become more prevalent in the future.  For such assessments, it is important to be able to better quantify mean or best estimate values of the various parameters and so better understanding of the conservatism inherent in the deterministic approaches is even more prevalent with respect to this requirement. 

 

The paper provides an overview of the conservatisms inherent in current fracture mechanics assessment methodologies and practices.  This is mainly with respect to the UK R6 and BS7910 methodologies but several of the aspects considered will also be common to other methods.  Where appropriate, recent and current work aimed at understanding and reducing conservatisms are highlighted. 

 

The overview is associated with the fracture evaluations of limiting defect size and with fatigue crack growth.  In terms of fracture evaluations, input data is firstly considered, centred on component geometry, defect geometry, loads/stresses, tensile material properties, fracture toughness properties and assessment location and time through transients.  Actual methodology is then considered, focussing on limit/load/reference stress solutions, stress intensity factor solutions, failure assessment curve and combined primary and secondary stresses.  In terms of fatigue crack growth evaluations, the overview is given with respect to transient grouping, cycle definition, cycle order and life distribution, fatigue crack growth laws, through life defect aspect ratio and forward/reverse crack growth.   

Presenting Author: John Sharples Jacobs Clean Energy

Authors:

John Sharples Jacobs Clean Energy
Peter James Jacobs Clean Energy