PSI - Issue 60

5

Chitresh Chandra et al. / Procedia Structural Integrity 60 (2024) 165–176 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

169

the crack initiation cycle increases from 534 to 428. This can be attributed to the weakening of creep-fatigue interaction with decrease in hold period and increase of force ratio. (Xu L. et. al. 2017)

Fig. 3: The stereo microscope image of the fracture surface after the CFCG test

Fig. 4: Crack length and crack growth rate variation with number of cycles

The crack growth phenomenon can be divided into three stages. The first is slow crack growth domain in the initial portion of the test. During second stage the crack growth stabilizes and progresses steadily, which eventually transitions into a third phase with accelerated crack growth. The third phase of crack growth starts at around 70% of the total crack growth life. CFCG testing can be used to classify material responses into two categories: Creep-ductile and creep-brittle (Narasimha Chary et. al. 2013). Creep-ductile behaviour is represented by the significant increment in crack propagation resulting due to strains caused by creep. On the contrary, Creep-brittle behaviour is characterised by minimal creep zones (Saxena A. 2015). The creep response of the P91 samples tested under various conditions was evaluated by plotting ∆ V e /∆V vs normalized cycles as in Fig. 5. ∆ V e /∆V values for all the samples were lower than 0.5 through the entire duration of the test. Hence, P91 is termed creep-ductile and thus, (C t ) avg is the more appropriate parameter to quantify crack propagation (ASTM E2760 2019). Fig. 6 shows the trend of ∆K versus cyclic crack growth rate. During the initial period of the test, the crack growth rate is high, it drops to a minimum and then again increases, resulting in a hook shaped portion. Similar trends have