PSI - Issue 14

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

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using an external current source and the experiments under 3.5% NaCl aqueous environment were conducted at three different levels of corrosion current by applying constant Direct Current (DC) of 0.1 A, 0.2 A and 0.3 A. All the experiments were carried out under constant amplitude cyclic sinusoidal loading; the stress ratio was maintained as 0.1 and the test frequency was either 0.25 Hz or 0.50 Hz. Table 2 gives details of FCG experiments.

Table 2. Details of FCG tests. Sl. No. Material

Specimen ID

Test Environment

Test frequency, Hz

External current, A

1. 2. 3. 4. 5. 6. 7.

SA 333 Gr. 6 steel

0.50

0.3 0.2 0.1

S3/CFCG8-1AC 3.5% NaCl aqueous S3/CFCG8-2AC

S3/CFCG8-3AC

-

S3/CFCG8-4AC Demineralised S5/CFCG8-1AC 3.5% NaCl aqueous S5/CFCG8-2AC S5/CFCG8-3AC Demineralised

SA 516 Gr. 70 steel

0.25

0.3 0.2

-

The maximum and minimum load values were 15 kN and 1.5 kN, which were decided based on ASTM E 1820 (2013). Crack growth in the length direction was continuously monitored and images were recorded at regular intervals of loading cycles. The number of cycles for crack initiation was recorded for both specimens. The experiments were continued till the crack growth in the specimens become unstable. Figure 1 shows the set-up for CFCG experiment on an ESE(T) specimen. Figure 2 shows close-up view of the ESE(T) specimen and corrosion chamber. For the FCG results to be valid, it is required that the specimen be predominantly elastic at all values of applied force.

Fig. 1. Set-up of CFCG experiment on an ESE(T) specimen.

Equation (1) recommended in ASTM E 647 was used to check the elastic condition in the ESE(T) specimen at different cycles of loading. (W - a) ≥ (4/ π ) (K max / σ ys ) 2 (1) Where (W-a) = uncracked ligament mm; σ ys = yield strength in MPa and K max = maximum stress intensity factor in MPa√mm.