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December 4, 2015

Thesis defence of Mouhcine KAHZIZ

of center of Materials of MINES ParisTech

Thesis defence of Mouhcine KAHZIZ

Mouhcine Kahziz will defend his thesis which called

"Experimental and numerical investigation of ductile damage mechanisms and edge fracture in advanced automotive steels"

on december, 4 at 1:00 pm

at Ecole des Mines de Paris, 60 Bld Saint-Michel, Paris 6ème, in room L224

abstract (in pdf file) : The mechanical properties of automotive structures made of AHS Steels is often seen reduced by the presence of cut edges. Here this phenomenon is investigated for ferrite-bainite steel (FB600) and martensite ferrite steel (DP600), the latter having higher work hardening and phase hardness contrast than FB600.
Damage micromechanisms for these two base materials were assessed using in situ synchrotron tomography, in situ SEM and SEM on cross sections. It was revealed for the DP600 steel that damage nucleated from particles and ferrite-martensite interfaces. In addition, needle shaped voids, that are consistent with the
presence of segregation lines, were seen. For the FB steel, the same observations hold true except that the decohesion on interfaces sets in at higher strains. Quantitative image analysis also showed that the initial number of voids and the number of nucleating voids was higher for DP steel than for FB steel which was also seen to be more damage tolerant. Punched and machined edges made of DP600 and FB600 steel were mechanically loaded during in situ laminography testing. It was found that the fracture zone of the punched edge was rough and that needle-shape voids at the surface and in the bulk followed material ow lines. During mechanical in situ testing the needle voids grew from the fracture zone surface and coalesced with the sheared zone. In contrast, for the machined edge the damage started away from the edge ( 800
m) where substantial necking has occurred. Three-dimensional image analysis was performed to quantify the initial damage and its evolution. The FB600 was more resistant to cut edges than the DP600 steel. 3D elasto-plastic FE calculations were carried out to investigate mechanical elds, potentially aected by the edge prole and pre-hardening prole. These parameters were not found to substantially modify the mechanical elds. Finally, axisymmetric 2D simulations for hole expansion were carried out for dierent sheet thicknesses using a post-treated damage evaluation calibrated on in-situ tomography data.

Thesis defence - MINES ParisTech

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