Micromechanisms of deformation and fracture in shearing aluminum alloy sheet

This paper presents the microscopic observations and provides insight into the results and phenomena reported in the previous paper (Li M. An experimental investigation on cut surface and burr in trimming aluminum autobody sheet. International Journal of Mechanical Sciences 2000;42(5):889-906). It is found that the localization zone is curved for the traditional 0 degrees cutting and does not extend from the top blade tip to the bottom blade tip. Instead, it extends to the initial indentation point by the bottom blade tip. However, the localization zone appears to be straight for angled cutting and does extend from the top blade tip to the bottom blade tip. This explains the mechanisms of burr formation and cut surface profiles as well as the formation of the secondary burnish areas. It is observed that for aluminum alloys cracks initiate and propagate well ahead of the moving blade tip regardless of the blade sharpness. Following the formation and propagation of the localization zone, the initiation, growth and coalescence of microvoids occur far ahead of the macroscopic crack tip. At the locations where the fracture mode switches from "shear" to "shear and tear", large second-phase particles are present which trigger the unsteady progress of the fracture process. Finally, a few non-dimensional parameters are discussed with respect to analyzing shearing ductile metals and alloys. (C) 2000 Elsevier Science Ltd. All rights reserved.