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The energy associated with shearing of planes of atoms in a crystal is the generalized stacking 10 fault energy (GSFE). It is a crucial material property for describing nanoscale plasticity phenomena in crystalline materials, such as dislocation dissociation, nucleation, and twinning. The dependence of the GSFE on applied stress normal to the stacking fault has been suggested to influence such phenomena. Here, the stacking fault stress dependence is analyzed through (i) the generalized stacking fault potential energy (GSFE) and (ii) the generalized stacking fault enthalpy (GSFH). At an 15 imposed shear displacement, there is also an associated inelastic inter-planar normal displacement around the fault. Extensive molecular statics simulations with interatomic potentials and/or first principle calculations in Ni, Cu, Al and Mg reveal that GSFE and inelastic normal displacement both increase with tensile stress. An increasing GSFE contradicts long-standing wisdom and previous studies. Positive inelastic normal displacement coupled to the applied normal stress decreases the 20 GSFH, but is not useful for general mechanics problems. The existence of the inelastic displacement can lead to incorrect measurements of the GSFE and GSFH in finite systems loaded by an applied strain. Application of the GSFE and the inelastic normal displacement to both fcc dissociation distance versus applied normal stress and crack tip dislocation emission under mixed Mode II/I loading show very good agreement with direct simulations. In general, "opening softening" effects 25 are not universal, and so the analysis of any particular nanomechanics problem requires precise implementation of the combination of GSFE and inelastic normal displacement rather than the GSFH.

Dislocation emission from a crack tip is a necessary mechanism for crack tip blunting and toughening. In fcc metals under Mode I loading, a first partial dislocation is emitted, followed either by a trailing partial dislocation ("ductile" behaviour) or a twinning partial dislocation ("quasibrittle"). The twinning tendency is usually estimated using the Tadmor and Hai extension of the Rice theory. Extensive molecular statics simulations reveal that the predictions of the critical stress intensity factor for crack tip twinning are always systematically lower (20-35%) than observed. Analyses of the energy change during nucleation reveal that twin partial emission is not accompanied by creation of a surface step while emission of the trailing partial creates a step. The absence of the step during twinning motivates a modified model for twinning nucleation that accounts for the fact that nucleation does not occur directly at the crack tip. Predictions of the modified theory are in excellent agreement with all simulations that show twinning. Emission of the trailing partial dislocation, including the step creation, is predicted using a model recently introduced to accurately predict the first partial emission and shows why twinning is preferred. A second mode of twinning is found wherein the crack first advances by cleavage and then emits the twinning partial at the new crack tip; this mode dominates for emission beyond the first twinning partial. These new theories resolve all the discrepancies between the Tadmor twinning analysis and simulations, and have various implications for fracture behaviour and transitions.

Bearing performance depends on the ability of steel to cope with a large number of stress cycles. Long bearing lives are possible because the microstructure of bearing steel has excellent resistance to Rolling Contact Fatigue (RCF). Nevertheless, it is observed that the microstructure suffers changes in the region where the maximum Hertzian contact stress occurs. Here we give an overview of the present knowledge in the area of the formation of dark etching regions (DERs) during RCF. Factors that influence the formation of DERs, various types of characterisation techniques, and the observations made in the literature are discussed. In addition, the applicability of several proposed simulation models of the formation of DERs is discussed.

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