PSI - Issue 54

Mihaela Iordachescu et al. / Procedia Structural Integrity 54 (2024) 52–58 Mihaela Iordachescu et. al / Structural Integrity Procedia 00 (2023) 000–000

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successive levels of laths, blocks and packages of distinct orientation, which broadens the range of potential micro mechanisms of damage in aggressive environments. .

Table 1. Chemical composition of the studied steel bars.

C

Mn

Si

P

S

Cu

Ni

Cr

Fe

0.45

0.65

1.77

0.017

0.004

0.11

0.06

0.15

Bal

Fig. 1. (a) Microstructure of the bars in transverse and axial direction, and geometry and instrumentation sketches of the fatigue pre-cracked SENT specimens tested in air (b), and in FIP medium (c); (B=2.5 mm, W=5.3 mm).

Table 2. Mechanical properties the studied steel bars.

Ramberg-Osgood parameters: n, s 0 ( s = s 0 e n ) n=0.12, s 0 =1640 MPa

Elastic modulus, E [GPa]

Yield strength , R p0.2 [MPa]

Tensile strength, R m [MPa]

[%] Maximum uniform elongation

220

988

1138

8

2.2. Fracture tests in air and stress corrosion tests under slow strain rate tensile loading Fig. 1b shows the sketch of the fatigue pre-cracked flat tensile notched specimens used in both types of tests performed in present research, namely the fracture tests in air (FTA) and the stress corrosion cracking tests under slow strain rate tensile loading (SSRT-FIP). These last tests were performed according to the sketch given in Fig. 1c, in a 20% aqueous solution of ammonium thiocyanate (FIP medium) at 50°C, applying a cross-head rate of 0.0005 mm/min. The transparent wall of the vessel containing the FIP solution allowed the VIC-2D digital extensometry technique to be used along the entire SSRT-FIP tests (De Abreu et al., 2019). The flat tensile notched specimens had threaded cylindrical ends prolonged with smooth transitions up to the 25 mm cage length of the rectangular cross section (2.5 x 5.3 mm 2 ). In order to avoid residual stresses (Iordachescu et al., 2008), the specimens were V-notched by electric discharge machining (EDM) to a maximum depth of 0.5 mm. The longitudinal axis of each specimen was parallel to that of the bar from which it was machined. Four fatigue pre-cracked specimens (FTA) with distinct depths were tensile tested in air up to fracture, as a reference to assess the steel sensitivity to stress corrosion. The specimens were loaded at a crosshead speed of 0.05 mm/min and the frontal and lateral crack opening displacements (COD) respectively measured with resistive and VIC 2D digital extensometers, over gauge lengths 12.5 and 7 mm. A total of nine notched specimens were subjected to the slow strain rate stress corrosion cracking test in FIP environment (SSRT-FIP), four of them without fatigue pre-cracking and the other five with fatigue crack lengths that varied from 0.9 to 2.3 mm. The first four tests and one of the tests performed on fatigue pre-cracked specimens were interrupted, heat-treated according to (Iordachescu et al., 2022) to mark the environmentally assisted area of the crack and finally broken in air at 0.05 mm/min. The remaining 4 SSRT-FIP tests ended with the failure of the specimens.