13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- 0,00 0,20 0,40 0,60 0,80 1,00 1,20 1,40 1,60 -150 -100 -50 0 50 Temperature (°C) CTOD (mm) T50_L=-68ºC modeling 0 50 100 150 200 250 300 350 -200 -150 -100 -50 0 Temperature (°C) Impact toughness (J) T50_L=-74ºC modeling Figure 2 DBT behavior of UNS S32750 SDSS, (a). CTOD (b). Impact toughness. Table 2 Influences of the different factors on DBTT Reference Ferrite content (%) Cold deformation (%) Austenite spacing (μm) DBTT (°C) 45 61 68 0 5 10 15 18 23 32 T50 -86 -74 -58 -41 -74 -76 -72 T90J -100 -92 -74 -54 -85 -76 -58 -52 -92 -94 -90 As known, the tendency for cleavage in the ferritic phase can increase when temperature decrease [22]. At -75°C, isolated cleavage fracture can be observed (Fig. 3a). It is in the transition regime as shown in Fig. 2. At -130°C, cleavage fracture is now dominant (Fig. 3b). It seems that brittle fracture could have also occurred in the austenitic phase. As discussed in [15, 16], the ferrite that has a BCC structure obeys the weakest link theory and shows a brittle fracture. For the austenitic phase, the weakest link theory can not be applied, but it follows the coupling effect. The effect of crack front length on fracture toughness is now practically absent when the hardness of austenitic phase is high enough. A sharp cleavage crack in the ferrite can lead to a stress concentration that can be higher than the critical shear stress for a cleavage fracture in the austenitic phase. In this investigation, quasi-cleavage or cleavage fracture can be observed in the austenitic phase. The cleavage in the ferrite shows multiple lines, not converge but are vertical to single big line like a “river” (Fig. 3c). The cleavage in the “river” may nucleate at some nuclei (Fig. 3c). A small cracking inclusion can be one of them (Fig. 3d). (a) (b)
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