Dynamic fragmentation phenomena involve two important mechanisms: atomic debonding and internal impacts among formed fragments. While the former, i.e. energy dissipation due to crack propagation, has been frequently studied both theoretically and numerically, the same does not apply to the latter due to the inherent difficulty of tracking contact occurrences. In fact, in order to simplify computations, fragment to fragment interactions are often neglected when dealing with tensile expansion. We study the validity of this assumption using a simple 1D finite-element model. We consider a brittle bar subjected to tensile loading, and model material failure with cohesive elements. We show that neglecting internal contacts has little influence at high loading rates. However, our results reveal that even in initially pure tensile cases, for low strain rates and brittle materials, fragment interactions drastically change the fragmentation process as well as the fragments’ residual velocities.