Issue 35

X.C. Arnoult et alii, Frattura ed Integrità Strutturale, 35 (2016) 509-522; DOI: 10.3221/IGF-ESIS.35.57 512 Charpy impact toughness properties The Charpy impact test may be used to evaluate the fracture energy at different temperatures, to characterize qualitatively the fracture mode (ductile or brittle facture) and to estimate the ductile-brittle-transition temperature (DBTT). This test is interesting to understand in which fracture mode the delamination cracks are displayed. All studies, cited in this section, used specimens with a V shape notch. Bramfitt and Marder [22] obtained four different Charpy curves at different heat treatment finishing temperature before being cooled down and the range of test temperature was from 24°C to -150°C. As heat treatment finishing temperature decreases, the upper shelf energy decreases from 160 J to 50 J and the DBTT was shifted to lower temperature and disappeared for specimen having a finishing temperature at 316°C. Charpy specimens manufactured from the plate finished at 960°C and tested at 24°C showed a fully ductile fracture surface. The same material tested at -18°C and -73°C showed a completely developed cleavage fracture. For the Charpy specimens manufactured form the plate finished at 707°C that were tested at 24°C and -18°C, the fracture surface showed delamination cracks which were parallel to the rolling plane. At -18°C the number of delamination cracks was higher than at room temperature. When specimens were tested at -73°C, the fracture appearance was fully cleavage. The same phenomenon was observed for specimens manufactured from the plate finished at 538°C. However, the number of delamination cracks at equivalent test temperature was observed higher compared with the previous plate. For the specimens made from the plate having finishing temperature at 316°C, delamination occurred at all three temperatures, and the delamination has still occurred at -129°C where only 5% of the area was a cleavage fracture. This study shows the influence of heat treatment and test temperature on the number of delamination cracks developed and their length present on the fracture plane, however, these qualitative results did not provide a clue on the role that the delamination cracks play in the fracture behavior of metal alloys. Baldi 1978 [23] and Song et al [18] observed a similar evolution of the number and length of delamination cracks related to the decrease of test temperature. Figure 4 : Effect of banding concentration on DBTT and upper shelf energy [24]. Shanmugan and Pathak [24] demonstrated the influence of the number of delamination cracks present on the fracture surface and how the Charpy toughness properties are influenced by these special cracks. They used a micro-alloyed steel and subjected it to a suitable heat treatment in order to implement a variation of ferrite bands in terms of density per mm. Figure 4 shows the effect of ferritic banding density present inside the microstructure on the Charpy toughness properties. During the Charpy test, the delamination crack density on the fracture surface corresponded approximately with the density of ferrite bands per mm. For temperatures higher than 10°C, a high number of ferrite bands led to a decrease of upper shelf energy and a shift of the DBTT to lower temperatures has been observed, as compared to the case of no ferrite bands present in the specimen. Contrarily, a low density of ferrite bands led to a higher upper shelf energy than in the case without ferrite bands. Similarly to Bramfitt and Marder [22], Shanmugan and Pathak [24] observed that delamination cracks seem to disappear with decreasing test temperature. According to them, this is due to the fact that at very low temperature, not enough time is given for delamination cracking process to take place. In this study, it was