Abstract
Electron microscope studies have been made of the propagation of shear cracks and the distribution of dislocations in the plastic zone during in situ tensile deformation of stainless steel single crystals. In thin-foil specimens under plane stress conditions, the plastic zone was coplanar with the crack and it represented the slip system of maximum resolved shear stress. The dislocations in the plastic zone were in the form of an inverse pile-up. Contrast analysis has shown that these dislocations were of pure screw type which were extended into two Shockley partials. The crack opening displacement was measured directly by counting the number of dislocations in the plastic zone and it was found to be approximately equal to the foil thickness. These observations are in good agreement with the model of shear cracks proposed by Bilby, Cottrell and Swinden (1963). The size of the plastic zone was much shorter than the value predicted by the theory and possible reasons for this discrepancy are discussed.

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