1975 年 24 巻 261 号 p. 520-525
The process of crack propagation was continuously studied by extending thin foils of pure Al with pre-existing cell structures in an electron microscope. The dislocation structures near the fractured part of pure Al sheet specimens of 100μ thickness were also observed.
The results are summarized as follows;
(1) When a crack propagates through the cell structures, thinning occurs in the cells lying in the direction of crack propagation due to the dislocations generated from the crack tip. However, pre-existing cell walls still remain in the thinned zone and become sharpened, and finally these cell walls attain the shape of low angle boundaries. Dislocations moving in the thinned zone are stopped at both sides of the zone and the cell walls with high dislocation density are formed there.
(2) The possible mechanisms of crack propagation in the thinned zone, in which the sharp boundaries and the dense dislocation arrays are remaining, are as follows; (i) shear rupture by the generation of dislocations from the crack tip, (ii) coalescence of the microcracks with the main crack, and (iii) failure along the sharp boundary.
(3) Fracture teeth of less than several thousands angstroms thick are formed at the fractured parts of the sheet specimen. Well-defined low angle boundaries remain in the fracture teeth. In the still thicker parts near the root of fracture teeth, dislocation tangles or fine cell structures exist. The processes of formation of these structures can be explained from the direct observations of crack propagation.