PSI - Issue 60

9

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

173

A tail part can be observed in the initial crack growth region which accounts for at least 40% – 50% of the total crack growth life. In this region, the value of (C t ) avg dips until a minimum (C t ) avg is reached, showing that small-scale creep conditions dominate the initial portions of the test and thereafter, extensive creep conditions dominate (Saxena A. 2002). (da/dt) avg varies linearly with respect to (C t ) avg beyond the tail part as in Fig. 7 and the parametric constants of the corresponding equations derived by fitting the plots, can be found in Table 4. using the Eq. (2) ( ) = ( ) (2) A and B are material constants. The parameters A and B increase with the rise in dwell period. The slopes of the plots corresponding to the samples subjected to 600 sec hold are nearly same irrespective of difference in force ratio, whereas for lesser dwell periods (i.e., 10sec) the slope increases with increase in force ratio. This leads to the inference that the value of slopes are not affected by force ratio for samples subjected to higher hold time. Table 4: Details of fitting of (da/dt) avg vs (C t ) avg plot S. ID R t h (sec) RSOS Pearson’s R R-Square (COD) Adj. R. Square A B

02

0.1

10

0.33323

0.98959

0.97929

0.97873

6.08

1.38

10

0.1

600

1.08567

0.97326

0.94723

0.94584

2.0E3

2.20

05

0.5

10

0.29953

0.99025

0.98059

0.98009

8.14

1.39

09

0.5

600

0.31448

0.97297

0.94666

0.94424

9.4E11

1.53

Fig. 9: Cack length exposure time to air during test at high temperature.

The transition of fracture mode according to the increase of dwell time and force ratio was evaluated in detail as illustrated in Fig. 8. Three different locations are observed to investigate the fracture mechanism at different stages of CFCG i.e., at 2 mm crack length from Pre crack front (initial stage), 2 mm before the CFCG crack terminal (final stage) and in-between these two regions (midterm stage). The oxide layer over the fracture surface is clearly visible and the reason for that is the high temperature exposure of fracture surface to the air due to which the features of fatigue are covered. Fig. 9 shows the exposure time for different stages for all the test condition. The amount of Oxide