Abstract
The plastic zone created at the crack tip is observed microscopically by using a scanning electron microscope. CT specimens of 2017-T4 and 7075-T6 aluminum alloy are loaded to or over the JIC level and the crack tip regions are observed, so that size of the process zone (PZ) and the fracture mechanism in it are estimated. The results are as follows : (1) In the process zone, the number of nucleated voids and their diameters are decreased along the propagation direction of crack. The size of PZ is about 100∼130 μm for 2017-T4 and 50∼150 μm for 7075-T6. (2) The diameter of the void which is nucleated at the inclusion in the process zone, increased with the increase in J value and saturated just before separation, that is, at the nucleation of the dimple. (3) Coalescence of voids seems to occur in two ways. (4) For ductile fracture as in 2017-T4 and 7075-T6, the crack tip is always blunted both on the blunting line and the R-curve.
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