Numerical simulations with Discrete Element Method are used to study agglomerate breakage under two different kinds of dynamic solicitations: normal impact and tangential friction. Simple mechanistic models based on energy balance are developed for each case, and show good agreement with the results of the simulations. As far as impact is concerned, damage ratio, defined as the ratio of the number of broken bonds to the initial number of bonds, is found to depend on a dimensionless number I=kv2/c2, where v is the impacting velocity, and k and c are microscopic parameters describing material stiffness and cohesiveness. In the case of tangential friction, damage ratio is shown to depend on J=kv/c2. This model appears to be consistent with some available experimental data on impact and abrasion wear tests.