Cegb Assessment Procedure for Defects in Plant Operating in the Creep Range

Abstract

In recent years considerable progress has been made in research work addressing the behaviour of defects at elevated temperature. Developments have been made both in structural assessment techniques and in methods for analysing representative materials data. On the basis of these developments a procedure has been produced within the CEGB for the assessment of defects in plant operating in the creep range under loadings for which creep rather than creep-fatigue is the dominant failure mechanism. This paper describes the CEGB procedure. Calculations are required for three events: the time for overall structural failure by continuum damage mechanisms; the time for incubation prior to crack extension; and the time for subsequent growth to a maximum tolerable defect size. Methods are presented for calculating these times and for obtaining the materials data required to perform the calculations.

A unified European approach to high temperature defect assessment code and its incorporation in a knowledge base system

“…Number of cycles to failure at each strain range is determined from fatigue endurance data. In this method non-linear fracture mechanics based modelling is employed to analyse the initiation period of a small crack extension [1][2][3]. This method uses the reference stress method for evaluating C* and assumes that the crack tip stresses can be described by steady state C*.…”

“…It is usual for the codes to discount initiation when fatigue is present. The methods proposed have been successfully employed in different European assessment codes [1][2][3][4][5][6][7][8][9]. Since little universal validation exist to justify the choice of one method over another it is proposed to implement four models for crack initiation at this stage.…”

“…For example the German flat bottom hole approach (FBH) considers crack detection and characterisation and the German two criteria method [7,8] regards initiation as the important factor in life assessment and does not deal with the crack growth regime. The more advanced codes dealing with defects over the range of creep and creep/fatigue interaction in initiation and growth of defects are the British Energy R5 [3], the French A16 [4,5] and BS-7910 [6] which have clear similarities in terms of methodology.…”

“…It is clear that Europe is a leader in this developing field. Comparison of the existing European high temperature life assessment codes [1][2][3][4][5][6][7][8][9] lead to the conclusion that crack initiation is dealt with in different ways by the codes whereas creep and creep/fatigue crack growth are considered using the same models for analysis. At the same time whilst increasing sophistication is implemented in the analysis at the deterministic level insufficient emphasis is placed on the effects of the sensitivity and applicable bounds of the various parameters and material properties.…”

“…A number of defect assessment procedures (e.g. the British Energy R5 and the French A16, BS-7910 [1][2][3][4][5][6] and the German FBH and two-criteria method) [7][8] based on experimental and analytical models are available to assess crack initiation and growth and to determine the remaining useful life of such components.…”