Electron microscope studies of metal fracture surfaces have shown the existence of fine-scale plastic tearing during the early stages of crack growth for a large number of specimens. “Running” fractures in many structural metals (steels, aluminum alloys, and titanium alloys) propagate by the satellite nucleation of one or more different fracture mechanisms with the resultant submerged cracks linking with one another and with the major fracture to form incremental advances of the fracture front. The transition of macroscopically flat fracture to shear lips may occur over distances of about 10 to 100 microns, with the fine-scale fracture mode often changing from that of the fast fracture mode to tearing, normal rupture, and shear rupture. Shear lips always exhibit shear rupture dimples at high magnifications. Fatigue cracks are usually propagated by extremely fine-scale plastic rupture processes.

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