13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- to reach the test stress of 250MPa at 823K, step 2 in Fig. 1 (a). Specimen was then creep deformed to a pre-defined creep duration, which was followed by cooling under the applied load to freeze the creep induced dislocation structure. Finally the specimen was unloaded and dismantled at room temperature. This procedure was adopted for each specimen shown in Table 2. Each specimen was then subject to incremental tensile deformation at room temperature and simultaneously measured using neutron diffraction. This is detailed in the next section. (a) (b) Figure 1. A schematic diagram of the history applied to the specimen strained to the primary creep, followed by the room temperature incremental tensile deformation: (a) strain history and (b) stress cycles used in the incremental tensile deformation at room temperature combined with ND measurement 2.3. Incremental tensile deformation combined with neutron diffraction (ND) The time-of-flight neutron diffractometer, at the Rutherford Appleton Laboratory, UK, is optimised to measure elastic strains at precise locations for the bulk material [13]. A pulsed beam of neutrons with a wide energy range travels to the specimen, see Fig. 2 (a), where a small fraction of the beam is diffracted to both detectors located at an angle of 2 θ=90°. This arrangement provides a measure of axial strain vector from detector 1 and radial strain vector from detector 2, Fig. 2 (a). The wavelength, λ, of the detected neutrons is defined from their time-of-flight, t. 1 2 ( ) h t m L L λ= + (2) where h is the Planck constant, m is the neutron mass and 1L and 2L are the flight paths from the moderator to specimen and from the specimen to detector, respectively. A typical spectrum obtained from stainless steel is shown in Fig. 2 (b). Each diffraction peak, at a specific time-of-flight, according to Bragg’s law, 2 sin hkl hkl d λ θ = , represents a grain family with {hkl} crystallographic orientation under a specific elastic strain. The evaluation of the elastic strain in each grain family of the material requires a measure of the lattice spacing of this grain family under the stress free condition. The elastic strain is determined from the change in the lattice spacing, as compared with the stress free lattice spacing: 0 0 hkl hkl hkl hkl d d d ε − = (3) where hkl ε is the elastic strain in the {hkl} grain family, hkl d is the lattice spacing at a specific strain state and 0 hkl d is the stress free lattice spacing. In this paper, four diffraction peaks are considered: {111}, {200}, {220} and {311} grain families, see Fig. 2 (b). In the time-of-flight instrument, the engineering strain can also be approximated from a Rietveld refinement of the
RkJQdWJsaXNoZXIy MjM0NDE=