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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The FCG acceleration ratio calculated by the interaction model was low compared to the superposition model but larger than the ratio issued from the process competition model. The results issued from the interaction model are given on Figure 9-b) and show that not only the interaction model didn’t mimic any of the elementary mechanisms, but also successfully reproduced the test frequency dependence of the FCG acceleration ratio of the H1150 steel.

Figure 9 – Prediction of test-frequency dependence on the hydrogen-enhanced FCG acceleration ratio from elementary mechanisms by the superposition and competition models (a) and interaction model (b)

4. Conclusions

This paper investigated the effects of hydrogen on tensile and fatigue crack growth (FCG) properties of the 17 4PH H1150 steel. Smooth and circumferentially-notched axisymmetric specimens were used for slow-strain-rate tensile (SSRT) and fatigue-life tests, respectively. The specimens were precharged by exposure to hydrogen gas at pressures of 35 and 100 MPa at 270°C for 200 h. The notched fatigue test was done to determine the FCG properties. Several testing conditions were considered: the stress amplitude was in the [120 MPa; 400 MPa] range at stress ratio of -1 and the test frequency in the [10 -3 Hz; 10 Hz] range. These investigations have led to the following conclusions: 1. The SSRT tests showed no degradation of an ultimate tensile strength (UTS). The relative reduction in area (RRA) was 0.31 for 100 MPa H-charged specimens and the fractographic analysis showed a mixture of QC and IG surfaces, suggesting a high susceptibility to hydrogen embrittlement (HE). 2. In the fatigue-life test, no degradation of the fatigue limit was observed. Fatigue cracks emanating from notch roots, considered to be non-propagating cracks, were observed in both the non- and H-charged specimens. 3. At low stress amplitudes in the low-cycle regime, the H-assisted FCG acceleration ratio showed an upper bound of ~30 and the H-charged specimens failed accompanied by quasi-cleavage (QC), in accordance with the hydrogen-induced successive crack growth (HISCG) mechanism. In contrast, at high stress amplitudes in the low-cycle regime, the upper bound of the FCG acceleration ratio was nearly equal to 100 and the H charged specimens failed by a mixture of QC and intergranular cracking (IG). Additionally, the proportion of IG facets were quantified and correlated to the test frequency. 4. The H-assisted FCG acceleration ratio was decomposed into cycle-dependent and time-dependent mechanisms. On the hypothesis that the cycle-dependent mechanism follows the HISCG one, the superposition and process competition models available in the literature did not provide satisfactory results. An interaction model, in good agreement with the experimental FCG acceleration ratio, was then proposed.

References

Amaro, Robert L., Neha Rustagi, Kip O. Findley, Elizabeth S. Drexler, and Andrew J. Slifka. 2014. “Modeling the Fatigue Crack Growth of X100 Pipeline Steel in Gaseous Hydrogen Q.” International Journal of Fatigue 59:262 – 71. ANSI. 2014. “ANSI/CSA CHMC 1 - 2014: Test Method for Evaluating Material Compatibility in Compressed Hydrogen Applications - Phase I - Metals.” CSA Group . ASTM. 2004. “G142 -98" Standard Test Method for Determination of Susceptibility of Metals to Embrittlement in Hydrogen Containing Envi ronments at High Pressure, High Temperature, or Both.” ASTM International, West Conshohocken, PA 4.