SPH particles solver was developed based upon Lagrangian algorithm. Lagrange/Lagrange Interaction in Autodyn (Contact/Body Interaction in WB Explicit Dynamics GUI) must be defined between the Lagrange projectile and the SPH target.
Furthermore, gap contact is recommended to be used with Lagrange/SPH contact. Thus, a non-zero gap size is needed for the contact calculation between the Lagrange projectile and SPH particles.
Yes, it is OK to model both projectile and target in SPH particles for most of impact/penetration simulations.
However, if you are modeling an oblique impact and the angle between the line of impact direction and the normal direction of the target is large (for example, 60 degrees), you may observe the sticking particles between the projectile and the target. In this situation, it would be a good idea to model the projectile as Lagrange part, rather than SPH particles.
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