Comparison Between Two Assessment Methods for Defects in the Creep Range

Abstract

In recent years considerable effort has been made to understand the behaviour of creep defects at elevated temperature. A large number of experimental studies have been devoted to creep crack growth behaviour. In Europe, both in U.K. (CEGB) and in France (EMP), attention is focussed not only on crack growth but also on creep crack initiation behaviour in the assessment of defects at high temperature. This paper describes and applies both methods, first to laboratory test results and then to a cracked component. The comparison with test data is made with three materials, a ferritic 1 Cr-lMo-0.25 V steel and two austenitic stainless steels, while the application to a cracked pressure vessel deals with a f Cr-MeV steel.For these materials which are creep ductile, the relevant load-geometer parameter for stationary creep cracks is the C* parameter, which relies on the concept of reference stress and reference length. The expressions used for these parameters in both procedures are compared. Then the methods used for calculating the time for incubation prior to crack extension, t,, the time for subsequent growth, t,, and the time to failure, t, = ti + t, are compared. This comparison is made not only for laboratory test data but also for a cracked pressure vessel. It is shown that, in spite of different approaches, especially in the assessment of t,, both methods provide comparable t, -C* and t, = C* diagrams. The reasons for this situation are briefly discussed. NOMENCLATURE a, a,,, aF = crack length, initial and final defect size ri, da/df =crack propagation rate B, Be, B, = gross, effective and net specimen thickness C*, C& = creep load parameters K =elastic stress intensity factor L,,, R = reference length m ( a / w ) = limit load ratio n =constitutive law stress exponent t = time ti = time for creep crack incubation tF = predicted failure time (ti + 2,) t, = time for crack propagation P, P, = load and limit load tCD = time for structural failure due to continuum damage ?,(a) = creep rupture time from uniaxial data obtained at a given stress, cr t, = creep crack growth stage I-stage I1 transition time W = specimen width A ' , = critical distance c((n2) = constant in crack blunting analysis y = constant in reference length expression A = load point displacement rate A, Aexp = load point displacements 6, 6, = crack tip opening displacement, value at incubation c,(a, 1 ) =creep strain in uniaxial test at stress n and time t E,, imin = creep strain rate and minimum creep strain rate 6 , =creep ductility u , , = reference stress oy = yield stress 87 1 872 R. PIQUES et al.

Specimen geometry effect on creep crack growth in 316L(N)

“…The analytical expressions for respectively CT 1,4,11 , DENT 4,7,11 and CCRB 4,7,12 specimens are given below: and where n 2 is the creep‐stress exponent, ( W − a) the length of the remaining ligament and R min the minimum radius of CCRB specimens.…”

The Use of a Failure Assessment Diagram for Initiation and Propagation of Defects at High Temperatures

“…Following Green [7], failure assessment curves are produced in Fig. 2 for austenitic steel at 600°C using equation (3) and the average isochronous curves in [12]. Also shown in Fig.…”

“…the time for growth to initiate from a pre-existing defect, t i , is related to C* by Under widespread creep conditions, the R5 approach [2] reduces to that of Piques et al [3] and where B and CD are material constants. Using the reference stress estimates of C * in [l, 2,9] for a constant secondary creep rate, i k f .…”

“…Using the isochronous data, the failure assessment diagram has been constructed using equation (3) and the result is shown in Fig. 5.…”

Langzeitiges Kriechrißverhalten kennzeichnender Kraftwerksstähle

“…Fur die nachfolgende Bewertung der Parametereinflusse und fur die Streubandbestimmung wurde im wesentlichen der Hauptbereich herangezogen. Auf eine teilweise andere Deutung der RiBbereiche in[16] sei hier nur hingewiesen. Zunachst wird auf die Ergebnisse an dem bei 550°C untersuchten Stahl 30 CrMoNiV4 11 eingegangen.…”