In situ studies of fracture in solids

Electron microscope studies were made of the propagation of microcracks during in situ tensile deformation of stainless steel, molybdenum and magnesium oxide representing ductile, semi-brittle and brittle solids. As the stress is applied, the cracks are initiated at the edge of holes in the thinned foils. The extent of plastic activity around the cracks is measured in terms of the density and the distribution of dislocations and is sensitive to the ductile/brittle nature of the solid. The slip systems of the dislocations are determined by contrast analysis and stereoscopic observation. In stainless steel and magnesium oxide, the dislocations are relatively straight and they lie nearly perpendicular to the direction of the crack propagation, whereas in molybdenum the dislocations are in the form of irregular tangles which are stretched along the direction of the crack propagation. These observations are discussed in terms of the model of Dugdale concerning the formation of plastic zones under uniaxial loading in plane stress conditions