13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- 3.5. Influencing factors for degradation Considering the hydrogen segregation and fracture process together with the strain-rate-dependent degradation in the hydrogen-charged specimen, it is possible to postulate the role of graphite in the degradation process as follows. In the deformation process of the non-charged specimen, an interspace generated by interfacial debonding between graphite and matrix maintains a vacuum environment. In this case, the surrounding matrix material can be relatively ductile; thereby, the fracture surface exhibits the dimple fracture accompanied with the interconnection of graphites (cf. Figure 8(a)). On the other hand, in the hydrogen-charged specimen, a great amount of hydrogen exists at or near the graphite/matrix interface zone, as illustrated by Figures 2 and 3. Therefore, in the early stage of the fracture process, the interspace between graphite and matrix is immediately filled with hydrogen gas (cf. Figure 8(b)). It is noted that the hydrogen in the interspace is of molecular in nature. Then, a certain amount of hydrogen as atom can spread into the surrounding matrix near the graphite nodule. Simultaneously, the stress-induced hydrogen diffusion inside the matrix can also contribute to increase the hydrogen concentration near the graphite. It has been shown that solute hydrogen enhances a localization of plasticity [17, 18], which hinders the formation of ductile dimple by facilitating the cracking [5-7, 17]. After the crack initiation from a graphite nodule, hydrogen is incessantly outgassed from the inside of the graphite nodule and supplied to the crack tip via the local hydrogen gas environment. The stress-induced hydrogen diffusion can also attract a certain amount of hydrogen to the crack tip. Such a process results in the localization of plasticity at the crack tip and thereby facilitates the crack growth [17]. As a consequence, smaller CHS enables more hydrogen to be concentrated to the crack tip. This time-dependent process causes the ductility loss associated with a decrease in CHS only for the hydrogen-charged specimen. As was discussed, the DCI possesses the unique characteristics regarding the influence of hydrogen. Therefore, for proper evaluation of the hydrogen-induced degradation in DCI, the pivotal role of graphites as a local hydrogen supplier should be taken into consideration. 4. Conclusions The effect of hydrogen-charging on ductility loss in the ductile cast iron (DCI) was investigated by conducting a series of tensile tests with three different crosshead speeds (CHSs). According to the present study, the following conclusions were obtained: (1) Hydrogen-charging led to a marked decrease in the percentage reduction of area (%RA). In the non-charged specimens, %RA was nearly constant irrespective of CHS, whereas in the hydrogen-charged specimens, %RA was reduced with a decrease in CHS. (2) Thermal desorption spectroscopy (TDS) and the hydrogen microprint technique (HMT) revealed that most of solute hydrogen in the hydrogen-charged specimen was diffusive, and they were mainly segregated at the graphite, graphite/matrix interface zone and the cementite of pearlite. (3) Hydrogen-charging accelerated the coalescence of graphites during the fracture process. In the non-charged specimen, the fracture process involved the ductile dimple fracture associated with the coalescence of neighboring graphites. On the other hand, in the hydrogen-charged specimen, the fracture process involved interconnecting cracks between neighboring graphites, which appeared prominently at lower CHS. (4) The variation in fracture morphology is attributed to a great amount of hydrogen stored in the
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