13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Thus, to estimate the serviceability of alloy 04Cr16Ni56 it is suffices to test the specimens in gaseous hydrogen, while for alloy 05Cr19Ni55 there is a need for preliminarily hydrogenation. 0 5 10152025 60 80 100 120 4 3 Kc,MPa√m Р, MPа 1 2 Figure 2. Fracture toughness Kc of 05Cr19Ni55 (1, 2) and 04Cr16Ni56 (3, 4) alloys versus hydrogen pressure P at 293 K: 1, 3 – non-hydrogenated specimens; 2, 4 – hydrogenated specimens. Hydrogen decreases the stress intensity factor Kc and affects the fracture morphology. Under the conditions of maximum hydrogen embrittlement, the load-displacement diagrams become linear with sharp maxima (as functions of the load) (Fig.3a) and correspond to type I [10]. The fracture surfaces of the specimens are intergranular with cleavage facets typical of brittle fracture (Fig.4b), when in helium the honeycomb relief with ductile intergranular fracture portions is prevailed (Fig.4a). The values of Kc can be regarded as equal to K1c, i.e., they satisfy the condition l,b > 2.5(Kc H /σ0,2 H)2, where l is the crack length and b is the thickness of the specimen [10]. The indicated condition is satisfied at room temperature for alloy 04Cr16Ni56 under pressures of hydrogen higher than 10 MPa for which K1c is equal to 52 MPa·m1/2. The plane-strain state conditions were not fulfilled for the specimens made of alloy 05Cr19Ni55. 0.0 0.5 1.0 1.5 0 5 10 15 20 25 30 2 P, kH V, mm 1 a 0 10 20 30 0 400 800 1200 1600 S, MPa ε, % 2 1 b Figure 3. Diagrams “loading – crack opening displacement” (a) and true “stress-strain” diagram (b) of 04Cr16Ni56 alloys in helium (1) and hydrogen (2) under pressure 30 MPa at 293 K. Hydrogen produced changes in true stress diagrams of both alloys in wide pressure and temperature range: essentially decrease failure stress and deformation of specimens (Fig.3b). Figure 4. Fracture surface of 04Cr16Ni56 alloys under static loading of compact specimens in helium (a) and hydrogen (b) under pressure 30 MPa at 293 K.
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