Interface Crack Propagation in Abaqus, using Cohesive Interaction

Hi

I am trying to model interface crack propagation in Abaqus using Cohesive Interaction (default module in Abaqus). My model consists of two rectangular domains attached to each other by the longer edge. I am defining two surfaces on each of these domains and defining traction separation laws for these surfaces. There is a small portion in the middle for which I am not defining any traction separation law (thereby making it my notch). I am then applying displacement controlled loading on the remaining two longer edges. I am having two set of problems:

1. Since I am interested in studying Dynamic Crack Propagation at the interface, I am defining a Dynamic Explicit Step for my Analysis. But somehow the analysis is not working and the job is getting aborted due to errors. Also since my shear wave velocity is pretty large (~1km/s) the corresponding time increment required would have to be extremely small, which makes it a bit difficult for the analysis to run on my machine.

2. I am employing a Dynamic Implicit Analysis to overcome (1), but to my surprise here, the two rectangular domains are separating rightaway. Ideally the crack should propagate throguh the interface over a finite time, instead of just snapping throughout the interface, like I'm getting in my analysis. 

Can anyone help ?