AbstractIn atomistic modeling of fracture, most examinations are based on the Griffith energy, calculated from the surface energy, and the stress intensity factor derived from continuum mechanical formulations. Both approaches, however, reflect continuum mechanics and lack the direct connection to the atomistic crack tip mechanisms, and it is thus appealing to examine if Crack Tip Opening Displacement, CTOD, can be a meaningful physical parameter in atomistic modeling of material failure.In this presentation we first review our previous examinations of the crack tip displacements from fracture of single crystal silicon. We there established a procedure to determine the deformation zone ahead of the crack tip, based on CTOD90degree and CTODblunting, and demonstrated that CTOD could give a realistic estimate of the fracture toughness.The same procedure has then been applied on iron, loaded by a Modified Boundary Layer (MBL) model. In this case the crack flanges were not straight, and we were not able to extrapolate the flanges and define the center of rotation. However, by directly measure the deformed zone ahead of the crack tip, we could obtain realistic estimates of the fracture toughness via the CTOD.