High‐purity monocrystalline aluminum disks of three crystallographic orientations were subjected to carefully controlled planar impact producing low levels of spall damage. This damage was observed by optical and scanning electron microscopy of sections through the recovered disks, and was found to consist of voids of essentially octahedral form having {111} planes as faces. To describe the growth of these voids we propose a kinematical model based on the motion of edge dislocations. Dynamical equations describing the rate of growth of an individual void are obtained by applying established concepts of dislocation mechanics to the kinematical model. Finally, the dynamical void growth model is combined with an empirically established nucleation model to yield equations for calculating the total volume growth rate in a spalling sample. Extension of these results to other ductile fracture phenomena is suggested.

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Because the introduction of a screw dislocation into a crystalline body does not alter its volume, this type of dislocation cannot contribute to void growth. In this article screw dislocations are ignored entirely; the word “dislocation” means “edge dislocation”.
6.
One can easily develop a model in which void growth is solely or partially due to the nucleation of dislocations along the equatorial edge. This model is not discussed because it seems less reasonable than the one involving motion of existing dislocations. A model based solely on nucleation indicates that voids grow at a rate proportional to the two‐thirds power of their present size.
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D. S. Wood (private communication).
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For the present purpose we define low‐level spall damage to be that for which the voids are sufficiently few in number and small in size that they are noninteracting and do not cause gross perturbations in the applied stress fields.
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10.
Any expression relating v to τ (and perhaps also the temperature) can be used in place of Eq. (5). Other expressions currently in use are v = vmax exp(−B/τ) and v = vmax(τ/τ0)n, and a modification of Eq. (5) yielding a limiting dislocation velocity for large stresses.
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© 1972 The American Institute of Physics.
1972
The American Institute of Physics
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