Dynamic Instability in Intergranular Fracture

A molecular-dynamics model for crack propagation under steady-state conditions is used to study dynamic instabilities along a grain boundary in aluminum that occur when the crack speed approaches of the material’s Rayleigh wave speed. Instead of crack branching, as is characteristic for a crack propagating in a homogeneous environment, the instability of an intergranular crack results in a periodic series of dislocation bursts. These bursts limit the crack speed and produce velocity oscillations with a large increase in energy dissipation that increases the grain boundary toughness.

Figure 1

Atomistic snapshot giving the crystallography and structure of the MD system. See text for details.

Figure 2

A series of snapshots taken at various times $t$ from the crack initiation given in ps. Four types of atoms are identified (see text): fcc, hcp, disordered, and surface atoms. Only the non-fcc atoms are shown. The surface atoms are shown in gray, while hcp and disordered atoms are shown in black. Thus, the straight lines of black atoms away from the crack are either twin boundaries (1) or stacking faults (2) in a partial (3) or perfect (4) dislocation.

Figure 3

(a) Energy density $\Phi$ around the crack tip vs time—$\Phi$ is the sum of potential and kinetic energy taken over a volume of $[557]a_0\times [7710]a_0\times h=66{\mathrm{nm}}^3$ around the crack tip. (b) The speed of crack growth in units of the Rayleigh wave speed $c_R=3180\mathrm{m}/\mathrm{s}$ for this model. The letters (a) through (i) relate to the snapshots in Fig. 2.

Figure 4

Stored potential energy vs increasing crack length. Open circles relate to the potential energy of the MD system (excluding the surface and GB energy), and the solid line relates to the elastic energy of the FE system. As the FE system is more idealized by not including the GB elastic zone and having no GB and surface energies, the data are normalized by the energies of the two systems with no crack for a better comparison.