PSI - Issue 19

Jean-Gabriel SEZGIN et al. / Procedia Structural Integrity 19 (2019) 249–258 Jean-Gabriel Sezgin et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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2.2. Samples and experimental protocol

Slow-strain-rate tensile (SSRT) and fatigue-life tests were performed on non-charged and H-charged specimens to quantify the effects of hydrogen on tensile and FCG properties of this alloy. The SSST tests were carried out on smooth axisymmetric specimens with 7.5 mm in diameter at ambient temperature and a displacement rate of 1 mm.min -1 . Fatigue-life tests were carried out on circumferentially-notched specimens. The gross-section diameter of the specimen was 12.8 mm. The net-section one was 6.4 mm and the notch radius was 0.083 mm. The corresponding stress concentration factor was then 6.6. This geometry respects with the prescriptions of the ASTM G142-98 standard (ASTM 2004). The tests of the non-charged and H-charged specimens were performed in air at ambient temperature under various stress amplitudes and test frequencies. The stress ratio, R , was equal to -1 and the test frequencies were included between 0.001 Hz and 10 Hz. The H-charged specimens were charged by exposure to gaseous hydrogen at pressures of 35 MPa and 100 MPa at 270°C for 200 h prior testing. According to calculations using the solution of a diffusion equation, these charging conditions result in a uniform distribution of dissolved hydrogen through the specimens. In order to quantify a potential outgassing of hydrogen in solution during testing, the hydrogen content was measured on 12.8-mm-diameter and 5 mm-thick cylindrical specimens withdrawn from the fatigue-life specimens after testing (at 5 mm from the failure surface). The hydrogen content was determined by thermal desorption analysis (TDA) at a heating rate of 100°C.h -1 .

3. Results and discussion

3.1. Tensile properties

The results of three conditions were gathered in Figure 1-a). The nominal stress-strain curves suggested that no significant effect on neither the elastic domain nor the UTS was observable. The UTS was indeed equal to 1016 MPa (35 MPa H-charged), 1026 MPa (100 MPa H-charged), and 1013 MPa (non-charged). In contrast, the tensile ductility was significantly degraded by the presence of hydrogen. Namely, the relative reduction of area (RRA), which translates the ductility loss of the material in presence of hydrogen (ANSI 2014), was 0.43 (35 MPa H-charged) and 0.31 (100 MPa H-charged). Figure 1-b) shows the fracture surface morphology of the non-charged specimen, whereas Figure 1-c) presents the one of the H-charged specimens. The failure mode characterized by a cup-and-cone failure in the non-charged specimen was affected by hydrogen, resulting in brittle failure consisting in a mixture of quasi cleavage (QC) and intergranular cracking (IG).

Figure 1 – Effect of hydrogen on the SSRT property: a) nominal stress-strain curves related to non- and H-charged specimens, b) fractographic observation of the non-charged specimen, c) fracture surface observed on the H-charged specimen