Issue 48

J. Christopher et alii, Frattura ed Integrità Strutturale, 48 (2019) 554-562; DOI: 10.3221/IGF-ESIS.48.53 557 unloading was performed at the same strain rate of 1  10 –4 s –1 . The diagram for loading pattern is shown in Fig. 3a. Load- elongation data were recorded using data acquisition system for all the tests. From the load-displacement data, the stress vs. strain data was obtained. As an example, Fig. 3b shows the stress vs. strain diagram for E911 steel at 873 K for 1.3% strain hold. Using the stress vs. strain and strain vs. time data, the stress-relaxation data was acquired by judiciously selecting small time intervals in terms of relaxation stress (  r ) vs. hold time (t). In Fig. 3b, permanent set defines the strain present in the material when it is unloaded to zero stress. Figure 1: Microstructure of E911 steel in normalised and tempered condition. Figure 2: Specimen geometry used for stress relaxation tests. Figure 3: a) Loading waveform for stress relaxation tests and b) Stress-strain curve for strain hold of 1.3% at 873 K.