PSI - Issue 68

A. Barabi et al. / Procedia Structural Integrity 68 (2025) 285–291 A. Barabi / Structural Integrity Procedia 00 (2025) 000–000

287

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2. Experimental procedure The AISI 415 stainless steel has the chemical composition shown in Table 1. The material was austenitized at 1040℃ for 1 hour and then furnace cooled to ambient temperature. The heat treatment process was completed by tempering at 500℃ for 8 hours and cooling at 0.096 ℃/s.

Table 1 The chemical composition of the tested low carbon martensitic stainless steel.

C (wt %) 0.009

Cr (wt %) 12.17

Ni (wt %)

Mn (wt %)

Mo (wt %)

Si (wt %)

Alloy

415 SS 0.37 The tempered microstructure is fully martensitic with a tensile yield strength ( σ ! ) of 872 MPa ± 17 MPa, ultimate tensile strength ( σ "#$ ) of 971 MPa ± 7 MPa, and total elongation of 22% ± 7%, as reported in our previous work (Barabi, 2024). FCG tests were carried out using compact tension (CT) specimens in accordance with ASTM E647 guidelines (ASTM E647, 2016). The dimensions of the CT specimens were length ( W ) 50.8 mm, thickness ( B ) 12.7 mm, and notch length 10.16 mm. Two CT specimens were precracked at a constant ΔK= 15 MPa.m 0.5 resulting in an overall crack length ( a ) of 13 mm. The FCG tests were performed at constant ΔK= 15 MPa.m 0.5 and load ratio ( R ) of 0.1 in two environments, air and synthesized river water. Load frequency ( f ) was adjusted during the tests as the crack propagated. For each combination of R and f , the crack was propagated for at least 750 µm. FCGR measurements were taken every 250 µm of crack growth, resulting in at least three measurements per propagation condition. Detailed testing parameters are listed in Table 2. Crack length was determined using the compliance method and monitored with a travelling microscope. The fatigue crack growth rate was determined by applying linear regression to crack length ( a ) versus cycle count ( N ) data. 4.46 0.77 0.60

Table 2 FCG test conditions of CT samples in air and synthetized river water.

∆K ! MPa.m 0.5 " 15

Sample number

Frequency (Hz)

Crack length (mm)

Environment

R

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

10

13.1 – 15.2 15.2 – 17.3 17.3 – 19.3 19.3 – 21.4 21.4 – 23.0 23.0 – 25.3 20.5 – 25.5 25.5 – 27.5

Synthetic river water Synthetic river water Synthetic river water Synthetic river water Deaerated s ynthetic river water Synthetic river water

15 15 15 15 15 15 15

1

0.1

CT-I

10

1

0.1

10

Air Air

CT-III

1

Synthesized river water (containing 20 ppm NaCl and 130 ppm Na₂SO₄ ) was circulated in the crack using a peristaltic pump. The pH of the solution was 5.5 and its conductivity conductivity 320 µS/cm. Open circuit potential (OCP) variation was monitored during the FCG tests to characterize the electrochemical activity. A three-electrode setup was employed to track the electrochemical properties, with the CT specimen acting as the working electrode (WE), a platinum mesh serving as the counter electrode (CE), and a leak-free 3.5 M Ag/AgCl electrode 1 mm in diameter serving as reference electrode (RE). A portion of the CFCG test was carried out in deaerated solution at f = 1 Hz (conditions in bold in Table 2) to simulate the crack tip environment. The deaeration was performed by blowing argon into the solution.