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 blunnt crack tip can form a inverse piled-up or double piled-up group, depending upon the applied stress intensity factor Km, lattice friction stress t 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 a =-64