Forced oscillations of delaminated sandwich structures are dominated by impacts. During motion the gap between the two adjacent parts of the structure opens and closes periodically. Each contact gives rise to an impact, which leads to energy loss. A 4 DOF beam model with concentrated masses allows discussion of principle on the influence of internal dissipation due to the impacts on the non-linear system's response and the evolution of impacts near resonance points. Experimental investigations confirm the numerical results.