PSI - Issue 60

4

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

168

different force ratios (i.e., 0.1 and 0.5) and two different hold periods (i.e., 10 and 600 sec). For this investigation, the trapezoidal loading cycle is used. The specimen was heated using a 3-zoned hinged tube furnace (MPFU-10) with a tungsten heating source. Three S-type thermocouples, were used to measure and keep the sample's temperature constant with an accuracy of ±1.5 °C. An LLD (Heidenham: ST 1288) with gauge 5.5 mm and a travel of about 10 mm measures the Crack Mouth Opening Displacement (CMOD) within the sample. Through the use of direct current potential drop (DCPD), the real fracture length is calculated.

Table 1: Chemical composition of P91 Steel Element C Mn Si

P

S

Cr

Ni

Mo

V

Nb

Al

N

Fe

wt.%

0.097

0.037

0.31

0.0018

0.0047

9.29

0.38

0.92

0.26

0.08

0.006

0.057

bal.

The test was terminated before complete failure; the specimens were unmounted and fatigue-loaded until the fracture reached a specified size, at which point the two pieces were separated at room temperature using a servo hydraulic UTM. A crack length measurement is applied to the separated specimen. The summary of the CFCG test is presented in Table 2. The list of symbols is given in Appendix – A and the calculation done for CFCG analysis is given in Appendix – B. The optical and SEM (Scanning Electron Microscope) micrographs are depicted in Fig. 2 (a) and (b). Prior austenitic grain boundaries (PAGBs) are seen in the lath martensite optical microstructure. On the other hand, the martensite matrix's carbide is well apparent in the SEM microstructure. According to the literature, these carbides are carbonitrides MX (VN, VC, and NbC) and M23C6 (Cr23C6) carbides (Pandey C. et. al. 2018)

Table 2: Overview of CFCG Test S. Id. R

t h (sec)

(∆ K) initial (MPa √ m)

P max (kN) 10.95 10.91 19.78 19.77

a i (mm) 22.65 23.58 22.19 23.36

(∆ K) final (MPa √ m)

a f (mm)

Time (Hr)

Cycle (N) 21860 14920 16540

02 10 05 09

0.1 0.1 0.5 0.5

10

20 20 20 20

54.33 50.26 34.54 29.46

35.4

87

600

34.49 3033 28.27

2503

10

67

600

309

1820

3. Result and Discussion The fracture surfaces of the CT specimens subjected to multiple dwell periods and load ratios, were observed under a stereo microscope and the resultant images were depicted in Fig. 3. The fatigue pre-crack originating from the base of v-notch terminates at the pre-crack front. The crack propagation during the loading cycle extends from the pre crack front and ends at the terminal crack front which has been denoted with red colour. It is evident from the image that the pre-crack and terminal-crack fronts are not perfectly linear throughout the transverse section of the C(T) specimen. Measurements of the initial and final crack growth lengths ( a i & a f ) were taken along five equi-spaced lines across the fracture surface. The trend in crack growth and crack growth rate with respect to number of cycles has been depicted in Fig. 4. For higher hold times crack growth life decreases with increase in force ratio. The tests with the dwell times of 10 s and 600 s with force ratio of 0.1 required about 21860 and 14920, respectively, whereas the tests with force ratio of 0.5 with a dwell of 10 sec and 600 sec required 16540 and 1820, respectively. It is clear from Fig4 that the initiation cycle only occupies a minute fraction of the total duration of the CFCG tests. The cycle corresponding to a crack growth of 0.2 mm is designated as the crack initiation cycle (N 0.2 ) (Cu L. et. al. 2017, Shi K. et. al. 2014). From the figure, it is evident that the crack initiation cycle decreases with increase in dwell period and force ratio. For instance, upon increasing dwell time from 10 sec to 600 sec for 0.1 force ratio, the crack initiation cycle changes from 534 to 365, similarly, with change in force ratio from 0.1 to 0.5 for 10 sec dwell time,