In situ tensile tests in a transmission electron microscope (TEM) show that dislocations emitted from a mode II crack tip will form a inverse piled-up group after equilibrium or a double piled-up group when they meet a obstruction, e.g., grain boundary or second phase. A microcrack can initiates in front of the piled-up group of dislocations. Micromechanics analysis shows that dislocations emitted from a mode II blunt crack tip can form a inverse piled-up or double piled-up group, depending upon the applied stress intensity factor K_(lla), lattice friction stress τ_ f and the distance of the obstruction from the crack tip L. The maximum normal stress in front of the double piled-up group which is located at the direction of α = -64° increases with the increase in the stress intensity K_(lla) and the obstruction site L, and the decrease in the friction stress τ_f When it increases to equate the cohesive strength, a microcrack will initiate in front of the piled-up group. In situ tensile tests in a transmission electron microscope (TEM) show that dislocations emitted from a mode II crack tip will form an inverse piled-up group after equilibrium or a double piled-up group when they meet a obstruction, eg, grain boundary or second phase. A microcrack can initiates in the front of the piled-up group of dislocations. Micromechanics analysis shows that dislocations emitted from a mode II blunt crack tip can form a inverse piled-up or double piled-up group, depending upon the applied stress intensity factor K_ (lla), lattice friction stress τ_f and the distance of the obstruction from the crack tip L. The maximum normal stress in front of the double piled-up group which is located at the direction of α = -64 ° increases with the increase in the stress intensity K_ (lla) and the obstruction site L, and the decrease in the friction stress τ_f when it increases to equate the cohesive strength, a microcrack is in the front of the piled-up group.