PSI - Issue 14

S. Vishnuvardhan et al. / Procedia Structural Integrity 14 (2019) 482–490 S. Vishnuvardhan / Structural Integrity Procedia 00 (2018) 000–000

487

6

a

b

d

c

Fig. 3. Crack growth images at different cycles of loading for the specimen S5/CFCG8-3DW. (a) at 1,20,000 cycles (crack length: 12.98 mm); (b) at 2,00,000 cycles (crack length: 15.66 mm); (c) at 2,80,000 cycles (crack length: 23.71 mm); (d) at 3,16,000 cycles (crack length: 52.51 mm)

Figure 4(b) shows comparison of crack length vs. number of cycles curves for SA 333 Gr. 6 steel specimens tested at two different levels of corrosion current, viz, 0.2 A and 0.3 A, and at a loading frequency of 0.50 Hz. It is observed that increase in applied current significantly increased the fatigue crack growth rate and reduced the fatigue life. The number of cycles corresponding to maximum crack length satisfying the elastic check criterion for the specimens tested at 0.2 A and 0.3 A corrosion current were 196000 and 166000 respectively. Fatigue life (corresponding to maximum crack length satisfying the elastic check criterion) decreased approximately by 15.0 % when the applied current increased from 0.2 A to 0.3 A. Figure 4(c) shows comparison of crack length vs. number of cycles curves for SA 516 Gr. 70 steel specimens tested at two different levels of corrosion current, viz, 0.2 A and 0.3 A, and at a loading frequency of 0.25 Hz. It is observed that increase in applied current significantly increased the fatigue crack growth rate and reduced the fatigue life. The number of cycles corresponding to maximum crack length satisfying the elastic check criterion for the specimens tested at 0.2 A and 0.3 A corrosion current were 179000 and 142000 respectively. Fatigue life (corresponding to maximum crack length satisfying the elastic check criterion) decreased approximately by 20.5 % when the applied current increased from 0.2 A to 0.3 A.

Table 3. Results of FCG tests. Specimen ID

Crack initiation

No. of cycles satisfying elastic check

End of experiment

Number of cycles

Crack length* mm

Number of cycles

Crack length* mm

S3/CFCG8-1AC S3/CFCG8-2AC S3/CFCG8-3AC S3/CFCG8-4DW S5/CFCG8-1AC S5/CFCG8-2AC S5/CFCG8-3DW

50000 75000 50000 60000 89000 75000 70000

0.80 0.40 0.25 0.60 0.60 1.05 0.40

166000 196000 197000 355000 142000 179000 310000

169302 201433 203239 365292 147000 181326 316426

50.80 55.80 55.80 57.70 47.60 45.35 57.05

* Including initial notch length of 11.25 mm

Plots between crack growth rate ( da/dN ) and stress intensity factor range ( ∆ K ) were obtained for all the specimens. Figure 5 shows da/dN vs. ∆ K curves for the specimens tested under demineralised water environment. Figure 6 shows