13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- complete diffraction spectrum, see Fig. 2 (b) [13]. By knowing two principal strain vectors, the stress state in each grain family can be calculated from the generalized Hooke’s law [14]. In our case, both axial and radial strain vectors have been measured from detectors 1 and 2. (a) (b) Figure 2. Neutron diffraction: (a) arrangement of the in-situ tensile deformation specimen in the ENGIN-X neutron diffractometer; (b) typical time-of-flight diffraction spectrum for the investigated stainless steel In-situ tensile deformation was performed at room temperature on the specimens, which had been subjected to a prior deformation at high temperature, see Figs. 1 (a) and (b). The arrangement of the neutron diffractometer in ENGIN-X with respect to the specimen is shown in Fig. 2 (a). The direction of applied loading for the prior creep deformation and the direction of the incremental tensile deformation were co-axial. As illustrated in Fig. 1 (a), room temperature incremental deformation may be considered to be a continuation of deformation in terms of the total strain accumulated. Fig. 1 (b) shows the applied stress history during the incremental tensile deformation, where the stress level was increased step by step. At the end of each load step, the specimen was held at either a constant stress (elastic region) or at a constant strain (plastic region) for the period of the neutron diffraction (ND) measurement time to provide a measure of hkl d , see Figs. 1 (a) and (b) All the incremental tensile deformation tests were undertaken at a constant strain rate of 5×10-6/s using the ENGIN-X 100kN servo-hydraulic stress rig. A fixed rate was selected because the yield point of stainless steel is very strain rate dependent. An extensometer was attached onto the specimen to measure the bulk axial strain of the material during loading. A 3mm×3mm×4mm gauge volume was used for the neutron diffraction (ND) measurements to ensure the sampled gauge volume was fully contained within the specimen, as illustrated schematically in Fig. 2 (a). A typical measurement time of 540s was selected to ensure good counting statistics for the diffraction peaks. Some stress relaxation was observed when the stress was higher than 300MPa, Fig. 1 (b). To ensure the stress change was less than 3MPa when measurements were undertaken in the plastic region, a pre-defined delay for starting ND measurements, ranging from 180s to 360s, was adopted. Therefore all the neutron diffraction measurements were undertaken at a relatively constant stress. 2.4. Derivation of internal stress and internal resistance due to creep The internal stress, due to the prior deformation at high temperature, was quantified directly by the change in the lattice spacing which was measured from each specimen (without being subjected to the incremental tensile deformation). Specimen 1 was not subjected to any prior deformation at high temperature, and therefore was considered to be a reference state that was free of internal
RkJQdWJsaXNoZXIy MjM0NDE=