Issue 48

J. Christopher et alii, Frattura ed Integrità Strutturale, 48 (2019) 554-562; DOI: 10.3221/IGF-ESIS.48.53 560 reported in literature for 9% Cr steels [13,19]. It was demonstrated that the stress exponent values obtained from stress- relaxation data for 9% Cr steel compared favourably with monotonic creep tests and falls in dislocation creep regime [19]. Since dislocation creep is the main dominating mechanism for 9% Cr steels, the power law dependency between variations of internal stress and relaxation stress proposed by Argon and Takeuchi [10] has been implemented in the present investigation. The reported value of 0.79 for P9 steel is close to the observed exponent value of 0.81 for E911 steel (Tab. 2). It indicates that the power law relation between internal stress and applied stress observed for monotonic creep can be successfully implemented for stress-relaxation studies in 9% Cr steels. Moreover, present investigations clearly suggested that Model-II provides better description of stress-relaxation behaviour of 9% Cr steels than Feltham model i.e. Model-I. Figure 5 : Deviation in predicated relaxation stress with reference to the experimental value i.e.  r as a function of hold time. Figure 6: Variations in relaxation stress (  r ), internal stress (  i ) and effective stress (  e ) with hold time (t) for the strain hold 1.3% for E911 steel at 873 K. Figure 7: Variations in inter-barrier spacing (  ) and activation volume (  V) with hold time (t) for the strain hold 1.3% for E911 steel at 873 K. C ONCLUSIONS omparative evaluation of two physically based models has been performed for the description of stress-relaxation behaviour of E911 steel for the strain holds of 1.3 and 2.5 % at 873 K. As compared to Model-I (Feltham model), Model-II proposed by Christopher and Choudhary provides better prediction towards relaxation stress vs. hold C