PSI - Issue 13

Y. Charles et al. / Procedia Structural Integrity 13 (2018) 896–901 Yann Charles / Structural Integrity Procedia 00 (2018) 000–000

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of the ratio between the values calculated with the crystalline plasticity and those calculated with the isotropic behavior.

(a) (c) Fig. 5. Distribution of the ratios between the values from crystal plasticity computations and the ones from isotropic behavior for (a) the maximal principal stress, (b) the diffusive and (c) trapped hydrogen concentration after various exposure time. Curves with circles correspond to instantaneous trapping. The spreading of these distributions illustrates the influence of crystal anisotropy, impacting directly both the stress values and the trapped hydrogen. The principal stress in the polycrystal might be twice greater than the one computed in the homogeneous sample, and the trapped hydrogen might be 3 times greater, leading to potential hydrogen-related failure earlier than expected without considering microstructural heterogeneities. It can be observed from Fig. 5b that C L ratio in the polycrystal are much more important when considering instantaneous trapping, whatever the exposition time, due to the higher importance of the plastic strain heterogeneities on diffusion. Fig. 5c shows that the C T ratios appear to be almost time independent for instantaneous trapping, consistently with the faster trap filling leading to quasi saturation after t =5s (see Fig. 4d and Fig. 3b). 4. Conclusion The U-bend test has been modelled by finite element at both macroscopic and polycristal scale, thanks to the implementation of coupled hydrogen transport and trapping equation at both scales, and considering either a transient or an instantaneous trapping. The importance of the stress heterogeneities in polycrystals is pointed out by comparing the results obtained at the two scales, on the diffusion process and on the stress repartition. Considering the crystalline nature of metals and transient trapping for various initial boundary values problems are expected to contribute to more realistic approach of hydrogen embrittlement. Such a multiscale analysis might help to define relationships between failure stress and hydrogen concentration, provided experimental data for comparison with simulations. References [1] ISO 11114-4, Transportable gas cylinders – compatibility of cylinder and valve materials with gas contents – part 4: test methods for selecting metallic materials resistant to hydrogen embrittlement., 2005. [2] Y. Charles, M. Gaspérini, J. Disashi, P. Jouinot, Numerical modeling of the Disk Pressure Test up to failure under gaseous hydrogen, J Mater Process Technol. 212 (2012) 1761–1770. doi:http://dx.doi.org/10.1016/j.jmatprotec.2012.03.022. [3] ASTM, Standard Practice for Making and Using U-Bend Stress-Corrosion Test Specimens, 2003. [4] P. Sofronis, R.M. McMeeking, Numerical analysis of hydrogen transport near a blunting crack tip, J Mech Phys Solids. 37 (1989) 317–350. doi:http://dx.doi.org/10.1016/0022-5096(89)90002-1. [5] A.H.M. Krom, R.W.J. Koers, A.D. Bakker, Hydrogen transport near a blunting crack tip, J Mech Phys Solids. 47 (1999) 971–992. doi:http://dx.doi.org/10.1016/S0022-5096(98)00064-7. [6] R.A. Oriani, The diffusion and trapping of hydrogen in steel, Acta Metall. 18 (1970) 147–157. doi:10.1016/0001-6160(70)90078-7. [7] A. McNabb, P.K. Foster, A new analysis of the diffusion of hydrogen in iron and ferritic steels, Trans Metall Soc AIME. 227 (1963) 618– 627. [8] S. Benannoune, Y. Charles, J. Mougenot, M. Gaspérini, Numerical simulation of the transient hydrogen trapping process using an analytical approximation of the McNabb and Foster equation, Int J Hydrog Energy. 43 (2018) 9083–9093. doi:10.1016/j.ijhydene.2018.03.179. [9] Y. Charles, T.H. Nguyen, M. Gaspérini, Comparison of hydrogen transport through pre-deformed synthetic polycrystals and homogeneous samples by finite element analysis, Int J Hydrog Energy. 42 (2017) 20336–20350. doi:10.1016/j.ijhydene.2017.06.016. [10] Y. Charles, T.H. Nguyen, M. Gaspérini, FE simulation of the influence of plastic strain on hydrogen distribution during an U-bend test, Int J (b)