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[Display omitted] • Atomistic simulation and modeling of free surface effect on nanocrack initiation. • Nanocrack initiation and intergranular fracture due to nanocrack growth are captured. • Theoretical description of nanocrack initiation induced by GB dislocation pileup. Free surfaces have significant effects on plastic deformation and fracture of nanocrystalline materials (NMs), especially for nano- or micro- scale specimens. To reveal the influence mechanism of free surfaces on crack initiation and growth, molecular dynamics (MD) simulations are carried out. The results show nanocrack initiation at triple junctions near free surfaces and the intergranular fracture due to nanocrack growth and coalescence. Based on the mechanism captured by MD, a theoretical model is established to describe the process of grain boundary (GB) dislocation evolution and nanocrack initiation. The critical stress for nanocrack initiation and the critical magnitude of GB sliding step are obtained for nanocrystalline nickel, aluminum and copper. Based on the proposed theoretical model, the fracture behaviors are discussed for the three nanomaterials. [ABSTRACT FROM AUTHOR]

• Theoretical solution of an elastic-plastic crack in a half plane is presented. • Theoretical modeling of crack-tip plasticity in a strain-hardening material. • Theoretical modeling of plasticity at the mixed-mode crack tip. • Crack plane displacement is predicted accurately by the theoretical modeling. The crack growth behavior is influenced greatly by the crack-tip plasticity. Most of the existing two-dimensional (2D) theoretical models of crack-tip plasticity are appropriate only for elastic perfectly-plastic materials under simple mode load, which limits the application range. This paper presents a theoretical model based on the distributed dislocation technique (DDT) that can be used at mixed loads with bilinear strain-hardening and perfectly-plastic materials. The problem of a half plane containing an edge crack under uniaxial tensile load or mixed loads is considered in both 2D limit cases: plane stress and plane strain. The theoretical modeling results are compared with finite element simulations. The results show the developed models are suitable for strain-hardening materials under mixed loads, which is a new progress in the theoretical modeling of crack-tip plasticity. [ABSTRACT FROM AUTHOR]