Robert A. Ainsworth

Assessment of the integrity of structures containing defects

Abstract A number of methods are now available for assessing the integrity of structures containing flaws in all loading regimes. In this paper one of these methods is used to define a route for establishing the integrity of a structure containing defects. It should not be inferred that the approach selected is the only satisfactory one. Other methods of analysis are mentioned in this paper and in some cases these may be preferable to the one described. In developing this route, care has been taken to ensure that, wherever possible, the techniques used have been fully validated. In instances where validation is not available, the suggested methods represent the best judgement of the drafting panel. The status notes in Section 14 of this paper describe the extent of validation of the different steps in the analysis. The user is cautioned against following this paper without first reading that section. The paper contains 14 main sections and a number of appendices. The sections detail the major steps which have to be followed when using this procedure, in some cases giving a number of different options. Since all sections must be followed to perform an R6 analysis, they are obligatory. On the other hand, the appendices are advisory, since they are intended only to give guidance on how to perform the different aspects of the analysis. Techniques other than those contained in the appendices may be used in the analysis provided they are validated. This third revision of R/H/R6, originally published in 1976, supersedes all earlier revisions. It takes cognisance of developments in elastic-plastic fracture mechanics which have taken place up to April 1987. In particular, the failure assessment diagrams given in Addenda 1 and 2 of R/H/R6 Rev. 2 have been incorporated, as have modified methods for dealing with ductile tearing and residual and thermal stresses. This paper has been prepared by the R6 drafting panel in consultation with fracture experts within the CEGB. Their contributions are gratefully acknowledged.

Basic principles of analytical flaw assessment methods

Analytical flaw assessment methods play an important role in the industrial realisation of fracture mechanics application. In the field of general as well as specific standards and guidelines there have been rapid developments in recent years. This paper gives a brief review of some of the more important methods, which have been developed over the last decades. Descriptions are given of various design curves and failure assessment diagrams, net section yielding and EPRI type solutions, the reference stress method and approaches derived from it, the ETM and the recently developed European flaw assessment method SINTAP. The discussion of these approaches is restricted to the basic principles of each method.

Consistency of numerical results for power-law hardening materials and the accuracy of the reference stress approximation for J

Abstract The GE scheme for the integrity assessment of defective structures relies heavily on tabulated finite-element solutions. A consistency check for load-line displacements of simple geometries is proposed, based on a theorem of nesting surfaces introduced by Calladine and Drucker. The check is passed by most of the finite-element results although a few give errors of up to 15% on load. It is argued that the lack of consistency is unlikely to produce non-conservatism in practical assessments using the GE scheme. The finite-element results in the GE scheme are also used to assess the accuracy of references stress J or C ∗ estimates which form the basis of the failure assessment curve in the CEGBs R6 scheme and are used in the CEGBs creep crack growth procedure. The reference stress estimates are shown to reproduce the finite-element results with an average 5% conservatism on load and a scatter of ± 15% . It is argued that non-conservatism relative to the finite-element results is unlikely to produce non-conservatism in practical assessments using the R6 approach, or the creep crack growth procedure.

Background to and validation of CEGB report R/H/R6—Revision 3

Abstract This paper is a companion document to the third revision of the CEGB procedures for the ‘Assessment of the Integrity of Structures Containing Defects’, R6, and provides background information and validation. The background information includes details of the derivation, demonstrating the strong theoretical foundation of the procedures. Validation is addressed by comparison with tests on simple specimen geometries and structural components. This is supported and extended by comparing the procedures with the results of finite-element analyses. The most convincing validation is obtained by the direct application of the procedures to experiments on complex structures. Structural tests are, however, time-consuming and expensive to perform and so this class of validation is necessarily limited. The results on such components demonstrate the conservative nature of the R6 procedures with all failure points falling outside the failure assessment diagram. Contributions to the overall conservatism come from the failure assessment diagram and the procedures themselves, as well as from the calculational inputs and material properties used in individual assessments. The validation on structures is supplemented by comparing R6 assessments with experiments and finite-element analyses on test specimen geometries. As the calculational inputs and material properties are well defined for test specimens, this type of validation primarily addresses conservatisms in the failure assessment diagrams themselves. Results are presented for a range of specimen types, crack depths, specimen sizes and structural steels, and demonstrate the applicability of the Option 1, Option 2 and Appendix 8 failure assessment curves in R6. Finite-element solutions are described which extend the range of the validation by considering geometries with thermal stresses and geometries with shear loadings. Particular emphasis is given to thermal stresses because of the limited structural validation and an extensive series of finite-element solutions has been obtained within the CEGB. The results show that the R6 procedures adequately cover interactions between thermal and mechanical stress systems, although they are overly conservative at high thermal stresses. Similarly, the results for shear loadings show that the empirical approach adopted in Appendix 7 of R6 is reasonable. It is concluded that the procedures presented in the third revision of R6 are valid. The use of moderately pessimistic input data in assessments will ensure that failures are avoided.

Methods for including constraint effects within the SINTAP procedures

Abstract To enable constraint-based approaches to be applied to the practical assessment of defective components, development work within SINTAP has addressed a number of issues: constraint parameters for surface defects, constraint parameters for secondary stresses, simplified weight function methods for calculation of T-stress and relevant fracture toughness data. This paper describes these developments and indicates how the methods have been included in the overall SINTAP procedures.

SINTAP defect assessment procedure for strength mis-matched structures

Abstract This paper describes the defect assessment method for strength mismatched welded structures, resulting from the Brite Euram structural integrity assessment procedures for European industry (SINTAP) project. Finite element and experimental validation of the proposed method is provided. This shows that the proposed method is conservative, and that the degree of conservatism is similar to that embedded in the methods for homogeneous structures. It provides confidence in the use of the proposed SINTAP method for assessing defective weld strength mismatched structures.

Simplified J-estimations based on the Engineering Treatment Model for homogeneous and mismatched structures

Abstract This paper presents simplified J-estimation equations for defective homogeneous and strength mismatched structures, based on the Engineering Treatment Model (ETM). For homogeneous structures, two modifications are made to the existing ETM J-estimation equations; firstly a modification of the contained yielding solutions, and secondly modification of J-estimation equations for materials exhibiting Luders strain. Based on the proposed J-estimation equations for homogeneous structures, J-estimation equations for welded structures with strength mismatch are proposed. The proposed J-estimation equations for both homogeneous and mismatched structures are then compared with selected elastic–plastic finite element results. The comparisons show excellent agreement and so provide confidence in the use of the proposed J-estimation equations for defect assessment. Finally, comparison with another simplified method, the R6 method, is discussed.

Analysis levels within the SINTAP defect assessment procedures

Abstract Within the SINTAP defect assessment procedures, there are a number of levels of analysis. An increase in level corresponds to increased complexity and requires more detailed data but also leads to improved accuracy. Some levels of analysis are based on existing procedures such as the R6 approach used in the UK nuclear industry and the engineering treatment model developed in Germany. Other levels have been derived during the course of the SINTAP project. In this paper, the levels of analysis for homogeneous components are described and the background to their derivation is discussed. The resulting approaches are illustrated by examples for a range of engineering metals.

Chapter 5 – The Model Parameters

Table 3 shows the hyperparameters used to train the models. We have trained each model up to 100 iterations and kept the one with the highest LAS score on the development set. The models were optimized with Stochastic Gradient Descent (SGD) with a batch size of 8. During multitask learning, dependency parsing as the main task was weighted 1.0 while the gaze data treated as auxiliary task had a weight of 0.1.

Workshop 1 Worked Examples Using The CEGB Procedure For The Assessment Of The Integrity Of Structures Containing Defects

This series of worked examples is designed to illustrate as simply as possible the procedures in the second revision of CEGB Report R/H/R6 “Assessment of the Integrity of Structures Containing Defects” (1980). It is also intended to demonstrate how extra information may be gained by performing sensitivity analyses. The examples are artificial and the first 5 are based upon one type of problem, that of a crack in a pressurised cylinder. The level of complication is increased from one example to the next so that a working knowledge of the procedures can be built up to the level of sophistication needed. In the final example, the geometry of a crack in a flat plate is used to demonstrate how inconsistencies arise from the conventional application of rigid safety factors in fracture problems. Each example is structured so that it can be read without reference to the others.