M. A. Wilkes

Further Studies of Multiple Co-Planar Surface Breaking Flaws for Cleavage Fracture

In assessing the integrity of structures, complex multiple flaws located in close proximity to each other are generally characterised as one, larger, single flaw. Guidance for the characterisation of multiple flaws is provided in procedures such as R6 and BS 7910, which are routinely used in the UK and elsewhere in the structural integrity assessment of structures and components. For this approach to be valid, the characterisation process must be conservative. That is to say, the probability of failure must be higher for the characterised flaw than for the system of multiple flaws. However, previous studies showed that the current characterisation rules may be non-conservative under some circumstances, in particular under cleavage fracture conditions. A combined experimental and analytical programme of work has been undertaken within the UK in order to further investigate this potential non-conservatism for situations where the possibility of cleavage failure may have to be taken into account when assessing structures or components containing multiple flaws. Details of early stages of the analytical programme were reported at the 2006 and 2007 ASME PVP Conferences and comprised a number of finite element analyses to evaluate cleavage failure probability, via a Master Curve-based approach, for interacting twin flaws and the corresponding characterised single flaw, under applied tensile and bending loads, at low temperatures. These analyses considered surface-breaking semi-elliptical flaws all having the same depth, but with four different aspect ratios. For each aspect ratio the separation of the twin flaws was varied. It was found that non-conservatism of the characterisation rules was indicated for flaws of high depth to length aspect ratio (a/c) in contact. This paper describes further work that has been undertaken to extend the results previously reported. The further work described has been centred on: • for the most onerous aspect ratio (a/c = 1.0) extending the experimental results to higher temperatures in the cleavage transition regime. • performing finite element analyses to complement these experiments. • revisiting the methods for calculation of cleavage failure probability to obtain improved agreement with the experimental results. • examining the rules governing the characterisation process, to determine if modification is necessary.Copyright

An Engineering Approach to Characterisation of Twin Flaws in Cleavage

Multiple adjacent flaws are aligned and combined into simplified flaws for the purpose of flaw assessment in fitness for service procedures. Adjacent co-planar flaws are characterised by a bounding semi-elliptical flaw in accordance with flaw combination rules. Approximately 10 years ago BS7910 and R6 procedures revised flaw combination rules to allow adjacent co-planar flaws to touch prior to flaw characterisation. An extensive experimental and analytical programme has been underway over the past few years in the UK to examine the conservatism of flaw characterisation for fatigue, ductile tearing and cleavage with the new flaw combination rule. It was found that for fatigue and ductile tearing of flaws in contact the flaw characterisation is conservative, while for cleavage the flaw characterisation was found to be non-conservative when failure is accompanied with only small amounts of crack tip plasticity. In this paper an engineering approach is introduced to identify cases when flaw characterisation becomes non-conservative for cleavage. The approach relies on comparing material’s capacity for plastic deformation with the geometry of the complex flaw. In this manner no detailed analyses or extensive material characterisation are required for every examined case. The procedure is developed on the basis of test data and finite element analyses are used to extend its applicability to other flaws.Copyright

“First Estimate” Plastic Collapse Solutions for Flat and Torispherical Ended Pressure Vessels Subjected to Extensive Corrosion Wall Thinning

Plants in safety-critical industries comprise of a number of different components, including various sizes of pressure vessels, and their integrity is often paramount to the safe running of the plant. In many cases these vessels can contain highly corrosive liquids, which, over time can lead to a degree of thinning in the vessel walls. With corrosion wall-thinning come a number of issues, including a reduced plastic collapse load of the vessel and possible fatigue initiation from corrosion pits. Since failure of these vessels could lead to catastrophic failure and the escape of highly corrosive material, the assessment and maintenance of these vessels is of high importance.Current Fitness for Service Codes and Standards provide guidance on the assessment of cylinders and pipes with localized wall thinning and general corrosion, with respect to plastic collapse. This guidance, however, is not currently applicable to components with geometric discontinuities (the pressure vessel base-wall junction).This paper follows a preceding paper presented at the ASME PVP Conference 2010 which derived “First Estimate” plastic collapse solutions for flat-based, corroded, pressure vessels. These solutions have been expanded through further finite element studies to accommodate the estimation of plastic collapse in tori-spherical headed pressure vessels subjected to extensive corrosion wall-thinning. Results from this study are presented in graphical form to enable a quick and efficient first estimate of the effect of wall thinning on the collapse pressure to be attained.© 2012 ASME

Characterisation of Twin Co-Planar Flaws: Guidance for the Lower Shelf Regime

Structural integrity assessments use characterised flaws derived from simplifying the real flaw indications into a simpler flaw for the purpose of analysis. The guidance is historically derived from fatigue data and applied to other failure modes. Under cleavage conditions the guidance may not always be conservative as shown by the authors in earlier publications. This paper follows on to the investigation reported in PVP2010-25134 by examining the guidance for the characterised flaw on the lower shelf. Two options are examined: increasing the characterised flaw size or increasing the stress intensity factor for the original flaw. Both approaches result in a more pessimistic characterised flaw which is required to recover the conservatism of the characterisation procedure on the lower shelf.Copyright

Further Analysis of Multiple Co-Planar Flaws Under Cleavage Failure Conditions

In assessing the integrity of structures, complex multiple flaws located in close proximity to each other are generally characterised as one, larger, single flaw. Guidance for the characterisation of multiple flaws is provided in codes such as R6 and BS 7910, which are routinely used in the UK and elsewhere in the structural integrity assessment of structures and components. Recent studies have shown that the current characterisation rules may be non-conservative under some circumstances. A combined experimental and analytical programme of work is underway within the UK in order to further investigate this potential non-conservatism for situations where the possibility of cleavage failure may have to be taken into account when assessing structures or components containing multiple flaws. Details of early stages of the analytical programme were reported at the 2006 ASME PVP Conference and comprised a number of finite element analyses to evaluate cleavage failure probability, via a master-curve based approach, for interacting twin flaws and the corresponding characterised single flaw under an applied tensile load. These analyses considered surface-breaking semi-elliptical flaws of the same depth but three values of aspect ratio and a range of separations. It was found that non-conservatism was indicated for flaws of high aspect ratio in contact. This paper describes further analytical work that has been undertaken to extend the results previously reported. The further work described has been centred on: • extending the three aspect ratios (a/c = 0.2, 0.44 and 0.8) for the twin surface flaws previously considered to a/c = 1. • extending the study to include an applied bending load. • extending the study on surface breaking flaws to include embedded flaws (for a/c = 0.8 and tensile loading only). The paper also includes results of the experimental work completed to-date.Copyright

Analysis of Multiple Co-Planar Surface Breaking Flaws Loaded in Tension Under Cleavage Failure Conditions

In procedures such as R6 and BS 7910 for assessing the structural integrity of structures, complex multiple flaws located in close proximity to each other are generally characterised as one, larger, single flaw. Recent studies have shown that the current characterisation rules may be non-conservative under some circumstances. Concurrent experimental and analytical programmes are being undertaken in order to further investigate this potential non-conservatism for situations where the possibility of cleavage failure has to be taken into account when assessing structures or components containing multiple flaws. The analytical work has involved inelastic finite element modelling in conjunction with numerical analysis based on the weakest link methodology for cleavage initiation to determine the probability of cleavage failure. This has allowed the probability of failure for the situation of multiple adjacent flaws to be compared with that for the single characterised flaw to determine whether the characterisation rules are conservative. Initial results from the cases studied so far indicate that: • For flaws that do not touch, the probability of cleavage failure for the single characterised flaw is higher than the original flaws, confirming that the characterisation process is conservative in this case (perhaps significantly so). • For low aspect ratio (depth/length ≪ 1.0) flaws in contact, the probability of cleavage failure for the single characterised is higher than the original flaws, confirming that the characterisation process is conservative in this case. • However, for high aspect ratio (depth/length ∼ 1.0) flaws in contact, the probability of cleavage failure for the single characterised flaw is lower than the original flaws, indicating that the characterisation process may be non-conservative in this case. This paper covers the initial stages of a much larger programme. The analyses presented in this paper are in the process of being extended to allow for crack-tip constraint effects. Early results from these further analyses indicate that the loss of constraint between interacting flaws may be a significant factor and could eliminate at least some of the perceived non-conservatism for the high aspect ratio flaw. Further work is also underway to examine the effect of any localised coalescence of defects (a re-entrant region); the current analyses consider defects that are only just in contact. Further papers are therefore expected.Copyright