Experimental Characterization of Influence of Gaseous Hydrogen on Fatigue Crack Propagation and Crack Tip Plasticity in Commercially Pure Iron

Abstract : The objective of this study is to experimentally characterize Hydrogen-Affected Fatigue Crack Growth (HAFCG) behavior under various conditions and clarify the mechanism by focusing on crack tip plasticity. For this objective, as a first step, the influence of hydrogen on plastic deformation has been investigated by means of tensile tests in a commercially pure iron, Armco iron, under gaseous hydrogen. The results of the tests pointed out that the hydrogen effect on crack propagation is more important than that on uniform plastic deformation. Then, the HAFCG was investigated by means of FCG tests under various conditions of crack tip stress intensity ΔK, hydrogen gas pressure (PH2 = 3.5 and 35 MPa) and loading frequency (f = 0.02 – 20 Hz). It has been revealed that the FCGRs in a high ΔK regime were highly enhanced by hydrogen up to 50 times higher than the one in air. The fracture mode was a brittle intergranular fracture in a low ΔK regime, while it is a brittle transgranular quasi-cleavage one in a high ΔK regime. The value of ΔKtr (value of ΔK triggering the FCGR enhancement) decreases by increasing the pressure PH2. Besides, the FCGR enhancement increases by decreasing the frequency f. Once f becomes lower than a critical value, the HAFCG rate significantly decreases down to the same level as in nitrogen., The crack tip plasticity was analyzed in a multiscale approach by means of optical microscopy, out-of-plane displacement measurement, and scanning transmission electron microscopy of dislocation structure immediately beneath the fracture surface (FIB/STEM). As a result, no clear modification of monotonic crack tip plasticity by hydrogen was observed, while a drastic reduction of cyclic crack tip plasticity associated with the FCGR enhancement was identified. Based on the experimental evidences, models of the hydrogen-induced intergranular FCG mechanism involving microvoid coalescence along grain boundary and the hydrogen-induced transgranular FCG mechanism involving cyclic cleavage due to crack tip plasticity reduction have been proposed. Three characteristic criteria of HAFCG (ΔKtr, hydrogen gradient (PH2 × f)1/2 and upper limit of FCGR) have been established. These criteria are expected to be useful for improving fatigue design and reliability of hydrogen-related equipment.
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Tomoki Shinko. Experimental Characterization of Influence of Gaseous Hydrogen on Fatigue Crack Propagation and Crack Tip Plasticity in Commercially Pure Iron. Other. ISAE-ENSMA Ecole Nationale Supérieure de Mécanique et d'Aérotechique - Poitiers, 2019. English. ⟨NNT : 2019ESMA0003⟩. ⟨tel-02156722⟩

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