Modelling collisions of soft agglomerates at the continuum length scale

Growth and breakdown mechanisms in granulation processes may involve collisions between soft plastically deforming agglomerates. It has been established previously that the flow stress in such collisions increases with the strain rate, which is dependent on the impact velocity and the size of the agglomerates. In the current paper, a scaling model is described that is based on a continuum constitutive relationship and formulated in terms of accessible experimental parameters. It is an extension of an existing contact mechanics model for elastoplastic nonadhesive collisions and therefore limited to deformations in which the contact radius is less than about 40% of the agglomerate radius. In addition, it is assumed that the elastic strains are small compared to the maximum value. Finite element simulations were carried out for a range of impact velocities and material parameters associated with an elastoviscoplastic constitutive relationship of the type used in the scaling model. The results were employed to validate the scaling model. Within the specified limits of applicability, it was found that the coefficient of restitution, contact area, loading and unloading curves and also the time evolution of the compressive displacement could be calculated with relatively high accuracy. Moreover, it was found that for a viscoplastic material the rate of decrease of the coefficient of restitution with increasing impact velocity is greater than for plastic deformation. The model should prove useful in understanding collision processes in granulation systems, particularly those occurring at relatively high impact velocities when the main energy dissipation process arises from viscoplastic deformation. Under this circumstance, the influence of adhesion is negligible and coalescence may be taken to occur when the coefficient of restitution is small. (C) 2004 Published by Elsevier B.V.