This paper presents the results of a large-scale atomistic simulation study of the process of emission of multiple dislocations in Al. We use embedded-atom method potentials based on ab-initio data and molecular statics and dynamics techniques to study the configuration of the crack tip as the dislocation emission process evolves. In the configuration studied, the crack is oriented in a {111}-type plane with a [110]-type crack front and the dislocations are emitted in adjacent inclined {111}-type planes. The dislocations are Shockley partials and they form a twinned region. The number of dislocations emitted increases with increasing applied stress intensity and is limited if the dislocations are not allowed to reach their equilibrium positions The shielding effect of the emitted dislocations decreases the total stress intensity factor at the crack tip but also causes a net decrease in the mode-II stress intensity factor projected on the slip plane of the emitted dislocations. Most importantly, this lower stress intensity along the slip plane limits the emission of new dislocations and, after a number of dislocations are emitted, the crack advances by cleavage for several lattice periods. The process is then repeated, resulting in a combined dislocation emission-crack propagation process. These results suggest a mechanism for the brittle-to-ductile transition that depends strongly on dislocation mobility and pinning behaviour.