13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- Derby and Ashby [10], Esposito and Bonora [11]. However, there are two challenges remaining before those physically based creep deformation models can be adopted for the extension of life of existing power generation plants as well as the prediction of life for the future designs. First, the distinction between internal stress and internal resistance is often confusing and unclear [5]. Second, a reliable and simple measurement technique to quantify these two terms unambiguously is still required. In this paper, we present a technique, based on neutron diffraction (ND) measurement combined with in-situ tensile deformation, to quantify the internal stress and internal resistance associated with high temperature creep deformation. The evolution of these two terms during creep deformation is measured quantitatively by this method. The crystallographic orientation dependence of the internal stress is discussed with respect to the underlying mechanisms of polycrystalline elasticity and plasticity. A newly developed self-consistent model has been established to interpret the measured results. The model is presented in the companion paper for this conference [12]. The ability of adopting this experimental technique to quantify internal state of the material is critically assessed, followed by concluding remarks. 2. Material and Experimental 2.1. Material Type 316H austenitic stainless steel, provided by EDF Energy plc., with a chemical composition given in Table 1, was examined. The stainless steel had experienced 65,015 hours service at temperatures in the range of 763K to 803K and it was then subjected to a further thermal ageing at 823K for 22,100 hours. The grain size for this stainless steel was measured using the linear intercept method. The averaged grain size was 87±9μm. Table 1. Chemical composition (wt.%) of Type 316H stainless steel C Si Mn P S Cr Mo Ni Co B Fe 0.06 0.4 1.98 0.021 0.014 17.17 2.19 11.83 0.10 0.005 Bal. Table 2. Summary of specimens subject to a prior deformation at high temperature (250MPa and 823K) Specimen ID Creep test duration Plastic loading true strain, % True creep strain, % Specimen 1 No creep 0 0 Specimen 2 As loaded 1.88 0 Specimen 3 Primary, 160h 2.04 0.92 Specimen 4 Secondary, 1000h 1.98 4.86 2.2. Prior deformation at high temperature To study systematically the influence of the prior deformation at high temperature on the internal state of the material, four prior deformation states were considered: (i) no creep, (ii) as loaded, (iii) primary creep, and (iv) secondary creep. Four specimens were prepared from the Type 316H austenitic stainless steel. Prior deformation tests at a temperature of 823K were then carried out. A summary is given in Table 2. Uniaxial round bar specimens with a 28.25mm gauge length and 5.65mm diameter were used. These specimens were deformed at 823K and at a constant stress of 250MPa to different stages of creep deformation. The left hand side in Fig. 1 (a) illustrates the strain history for a specimen, which was strained to reach primary creep. The specimen was heated to a temperature of 823K, step 1 in Fig. 1 (a). This was followed by the application of load
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