Nucleation, blunting or propagation of a nanocrack in dislocation-free zones of thin crystals

Nucleation, blunting and propagation of nanocracks in dislocation-free zone s (DFZs) ahead of crack tips in ductile and brittle metals have been invest igated by tensioning in situ with a TEM, and analysed using microfracture m echanics. The results show that in either ductile or brittle metals, many d islocations could be emitted from a loaded crack tip and a DFZ formed after equilibrium. The stress in the DFZ may be up to the cohesive strength of t he material, and then a nanocrack is initiated in the DFZ or directly from the crack tip. In ductile metals, the nanocrack is blunted into a void or n otch during constant displacement. In brittle metals, the nanocrack propaga ted as a cleavage microcrack rather than being blunted.