Fig.5 Evolution of N, the number of cavities per cubic mm in the four studied samples measured during the in-situ tensile tests [21] Fig.6 Micrograph of fractured specimen. Voids appear in black, ferrite in light gray and martensite in dark gray [21] 3-2 Void nucleation modeling As demonstrated by [6], the energy criterion necessary for the creation of new surfaces at the inclusion/matrix interface is satisfied at the onset of plastic deformation in materials containing inclusions bigger than about 25 nm in diameter. Only a stress criterion will therefore be used to model the interface decohesion in DP steels as the observed inclusions are about 100 times larger than this. The Argon's criterion [1] is a critical stress criterion stating that the void nucleation occurs when a critical stress state, necessary for the interface decohesion, is reached in the material. This stress state involves a contribution of the hydrostatic stress σm and the equivalent stress σeq. σeq σm=σC (3) where σC is the interface strength, e.g. the maximum shear stress that the interface can support without breaking. The interest in using the Argon's criterion lies in the fact that it accounts for the triaxiality T (T being the ratio between σeq and σm ). T = σeq σm (4) Combining Eq. (3) and Eq. (4), the criterion can be expressed as: σeq1 T =σC (5) In the original Argon's criterion, the triaxiality used is the macroscopic triaxiality. However, decohesion is a local 14
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