Fig. 3 3D views of the damage at the center of the specimen strained at a) εloc=0.5 (just before fracture) b) DP steel just before fracture. 0 2000 4000 6000 8000 1 104 1,2 104 0 0,1 0,2 0,3 0,4 0,5 0,6 density of voids in XiP(N/mm3) density of voids in IF(N/mm3) Density of voids(N/mm3) local strain Fig. 4 Density of voids as a function of the local strain for the TWIP steel and a ferritic one. Fig. 5 SEM fractography of the in situ tensile test Anyway, as it could be deduced from the absence of necking even at strain close to fracture, the triaxiality in the TWIP steel remains almost constant as it can be seen in Figure 6. Thus considering the Rice and Tacey law, the cavity growth should be limited in the case of TWIP steel, nucleation should be thus the major phenomenon governing the damage behavior. To check that point the evolution of the cavity size is calculated from the 3D images and plotted as a function of the local strain. It can be seen that if the entire population of cavities is considered, the average diameter of the cavity remains constant. This could be due to the fact that in that case, nucleation is also considered. Thus it is worth considering only the 20 largest voids. It leads to the conclusion that growth is experienced during the deformation. This growth is almost equivalent to the one experienced by other steel grade. Thus either the mechanisms governing the growth of cavities is different from the others steels and then growth is possible for low triaxiality or locally the triaxiality could be higher than the macroscopic one. One more interesting feature about the damage behavior of these TWIP austenitic steel is shown in figure 7. This figure shows a 3D picture of the TWIP steel just before fracture. It is clearly seen some shear bands in the material. This is in accordance with previous study on the same alloy but in a different stress state [1]. Primary 30
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