The two mechanisms of particle impact breakage and the velocity effect

Abstract

This paper describes a two-dimensional computer simulation study of the impact induced breakage of brittle solid particles. The work started as an attempt to understand the effect of impact velocity on the induced breakage and in particular to explain why there is a shift in the slope of the size distribution at large impact velocities. The results indicate that the observed breakage pattern is the result of two breakage mechanisms. When an unbroken particle experiences an impact, tensile stresses are generated along any line projecting radially outward from the contact point. This will generate cracks along those lines which we refer to as Mechanism I breakage and will be the dominant mode of breakage while the particle's center of mass is still approaching the plate. However, before breakage is complete, cracks will form perpendicular to the Mechanism I cracks. This we refer to as Mechanism II breakage. As no tensile stresses in directions perpendicular to Mechanism I cracks would appear in an unbroken particle, the stresses that lead to Mechanism II breakage must only appear as a result of Mechanism I breakage. The simulations show that the Mechanism II breakage appears to be a result of the buckling of the fragments remaining after the Mechanism I breakage. Finally, the velocity effect appears to be a result of the tradeoff between these two modes of breakage. For high velocity impacts the Mechanism I stresses are large and thus Mechanism I breakage is dominant. At smaller velocities Mechanism II plays a larger role in the overall breakage.