Validation of constraint-based assessment methodology in structural integrity

Constraint modified specimens, which are the features test specimens as well as regular specimens with shallow cracks and center cracked tension specimens, have been investigated by means of numerical methods to predict the constraint initiated shift in fracture toughness values of the two RPV materials under investigation. Different methods have been applied: the Weibull stress method, the energy method, T-stress and Q-parameter. It turned out that in principle all methods lead to a correct description of the constraint effect due to crack or specimen geometry for these uniaxially loaded specimens. In some cases the experimental scatter was very large because of a strong variation of crack depths, which has an essential influence on the crack tip constraint at all for the shallow cracks. It makes it difficult to validate the numerical prediction with the experimental results The analyses and their outcome demonstrate that the tools for prediction of crack tip constraint effects due to different specimen and crack geometries are available and are ready for the application to large scale tests and components.

This result can be divided in two parts: - Energy-based approach to account for the effects of crack-tip constraint on cleavage: developed at EDF. - Energy-based approach to account for the effects of crack-tip constraint on ductile fracture: developed at CEA. The aim of the result is to improve the methodologies used in Fracture Mechanics for structural assessment. It needs further development to improve the efficiency of the approaches. Nevertheless it can be already used in order to: - Contribute for discussions with Regulatory bodies on the problem of safety margin factors, - Be applied in internal projects of EDF and CEA related to lifetime extension of RPV.

VOCALIST has provided validation of the procedure whereby the effects of crack-tip constraint loss on cleavage fracture toughness may assessed by comparing To values determined from deep-notch specimens with non-standard To values determined from shallow-notch fracture toughness specimens. Data obtained within VOCALIST lend support to the shape of the Master Curve being constraint independent, at least for temperatures in the range -50° < (T - To) < 50°C. The classical two-parameter constraint-based fracture mechanics methodology was integrated with the Master Curve and local approach descriptions of cleavage toughness to describe cleavage fracture behaviour in Features and Benchmark tests. The usefulness of correlating the Master Curve reference temperature, To, with the crack-tip constraint parameter, Q, has been demonstrated; however, the trend curves derived require further development and validation before they can be used in fracture analyses.

Important outputs from VOCALIST are the D5 and the D12 reports. The first of these, D5, "Issue 1 of the Handbook of Best Practice for Application of Constraint-Based Structural Integrity Procedures" presented the main issues and current state-of-the-art techniques as they existed at the commencement of the VOCALIST project. D12, "Issue 2 of Handbook of Best Practice for Application of Constraint-Based Structural Integrity Procedures", is essentially an updated version of D5, which incorporates the experimental and analytical results obtained from the VOCALIST Work Packages described in detail in other Results sections, and gives more recent advice. These documents, especially the latter, are of benefit to engineers who have responsibility for using defect assessment procedures or for managing and planning R&D programmes designed to improve or extend defect assessment technology. A wide range of techniques are discussed, including two-parameter constraint based fracture models, energy approaches, failure assessment diagram approaches, the Master Curve statistical method for analysis in the ductile-to-cleavage transition region, and the application of Local Approach methods. Advice and the experience developed by or known to the participants is documented. References are given to articles in the open literature relevant to a wide range of influential developments are also given. The Reports are therefore effectively compendia of information of considerable practical utility.

The applied micro-mechanistic models of cleavage and ductile fracture require a calibration of the micro-structural parameters using the results of the small-scale specimens. The Beremin model represents the basis for the local approach models in the brittle to ductile regime together with a modified Weibull stress model, whereas in the ductile regime the Rousselier model is applied to describe the ductile crack initiation and growth. In the Weibull stress model the parameters are determined by a calibration procedure using the fracture toughness values of high and low constraint specimens, in the case of VOCALIST the results of SE(B) pre-cracked Charpy V size specimens. By an iterative process where all results are mapped back to the SSY space the necessary parameters are obtained. In the ductile regime the Rousselier model is calibrated by using notched round bars and CT specimens. Several methods have been applied to achieve the necessary parameters. Although the parameters of each obtained set varied significantly, the final results in the predictive analyses were comparable. It turned out that the stability of the numerical analysis is a good indicator for the appropriate calibration parameters. Convergence problems occurred during the crack growth analysis, depending on the parameter set.

In Vocalist a substantial database of test results has been compiled describing the effects of crack-tip constraint on the cleavage and ductile fracture behaviour of as-received and degraded materials relevant to ferritic RPV components and piping. The characterisation test results have provided added confidence in the use of sub-size specimens to determine the Master Curve reference temperature, T0, for as-received and degraded ferritic RPV materials.

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