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.

The warm pre-stress (WPS) of a flawed structure occurs when it is pre-loaded at high temperature in the ductile domain then cooled and loaded up to fracture in the brittle to ductile transition temperature domain. This load history is a feature of RPV accidental transients of LOCA type. Numerous tests on non irradiated specimens and structures have shown the favourable effect of WPS on fracture behaviour. Theorical knowledge let expect that the WPS effect occurs by the same way on irradiated material, but experimental approach had to be completed in such conditions. The experimental program presented in the present article consists in fracture toughness tests under WPS loading conditions performed on two RPV steels irradiated up to a fluence of 6,5.1019 n/cm2. The CT12.5 specimens used for these tests had been irradiated in the capsules of the pressure vessel surveillance program of two french reactors. Different types of WPS load history have been applied to cover typical accidental transients. All the results obtained confirmed for an irradiated steel the two assumptions generally made about the WPS effect: no fracture occurred during the cooling step of the loading even at high load level and the mean fracture toughness value is higher than that measured with conventional mono-temperature tests.

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