  Context. We investigate the effects of including material strength in
multi-material planetesimal collisions. Aims. The differences between
strengthless material models and including the full elasto-plastic model for
solid bodies with brittle failure and fragmentation when treating collisions of
asteroid-sized bodies as they occur frequently in early planetary systems are
demonstrated. Methods. We study impacts of bodies of Ceres-mass with a solid
rock impactor and a target with 30 weight-% water content as surface ice.} The
initial impact velocities and impact parameters are varied between the escape
velocity $v_\mathrm{esc}$ to about 6 $v_\mathrm{esc}$ and from head-on
collisions to close fly-bys, respectively. We simulate the collisions using our
own SPH code utilizing both strengthless material and the full elasto-plastic
material model including brittle failure. Results. One of the most prominent
differences is the higher degree of fragmentation and shattered debris clouds
in the solid model. In most collision scenarios however, the final outcomes are
very similar and differ primarily by the about one order of magnitude higher
degree of fragmentation in the solid case. Also, the survivors tend to be of
less mass in the solid case which also predicts a higher water loss than the
strengthless hydro model. This may be an effect of the relatively low-energy
impacts that cannot destroy the solid material instantly. As opposed to giant
impacts we also observe an indication that some water ice gets transferred
between the bodies.
