13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- (a) Spheroidal graphite (b) Concave apart from graphite (c) Less ductile surface Figure 3. HMT images on the fracture surface of the hydrogen-charged specimen (Crosshead speed: 1.0 mm/min) 3.3. Results of tensile testing under different CHSs Figure 4 displays the engineering stress-strain curves. As demonstrated, hydrogen-charging markedly reduced the percentage elongation after fracture. In addition, Figure 5 illustrates the relationship between the percentage reduction of area (%RA) and CHS. For the sake of more general illustration of the experimental data, the approximated scale of strain rate, ε , is additionally indicated on the top of the figure. It is noted that the strain rate was calculated by the CHS divided by the gage length of the specimen. In the non-charged specimen, %RA was nearly constant irrespective of CHS. On the other hand, in the hydrogen-charged specimen, %RA was gradually reduced with a decrease in CHS, i.e. a strain-rate-dependent ductility loss was manifested. 3.4. Fracture process Figure 6 shows the SEM micrographs at the central part of fracture surfaces, and the non-charged (left-hand side) and the hydrogen-charged (right-hand side) specimens are compared. In the non-charged specimens, the ductile fracture with dimples was dominantly observed regardless of CHS. In the hydrogen-charged specimen tested at a CHS of 50 mm/min, the ductile dimple fracture was still dominant, but a small fraction of less ductile fracture surface appeared. In contrast, in the hydrogen-charged specimen tested at a CHS of 0.02 mm/min, the fracture morphology apparently became much less ductile. Engineering strain (%) Engineering stress (MPa) Non-charged CHS: 50 mm/min CHS: 1 mm/min CHS: 0.02 mm/min CHS: 0.02 mm/min CHS: 1 mm/min Hydrogen-charged CHS: 50 mm/min 0 5 10 15 20 25 100 200 300 400 500 600 Percentage reduction of area, %RA (%) Crosshead speed, CHS (mm/min) Strain rate, (s1) Non-charged Hydrogen-charged . 0.01 0.1 1 10 100 0 5 10 15 20 10-5 10-4 10-3 10-2 Figure 4. Engineering stress-strain curves Figure 5. Relationship between the percentage reduction of area and the crosshead speed
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