Initial damage states of hypervelocity impact for tungsten projectile on aluminum target

Abstract

Hypervelocity impact is a highly nonequilibrium process because the states traversed by the system cannot be described in terms of constitutive parameters that represent the system as a whole. Spontaneous changes of local material elements defy equilibrium and hence homogeneity. A consistent description of the hierarchy of damage states is made possible only by synchronizing the thermal fluctuation with mechanical deformation. This involves considering nonequilibrium dissipative effects that are beyond the scope of classical continuum mechanics and physics. The damage states of a projectile impacting a target at hypervelocity are predicted by application of the isoenergy density theory. In addition to failure by fracture and fragmentation, the analysis also includes local phase transitions where a portion of the solid may transform to liquid and/or gas. Changes in local strain rates and strain rate history are derived rather than preassumed. This enables a realistic evaluation of intense deformation, heating, melting and vaporization that occur nonhomogeneously in the projectile/target system during the course of impact. The case of a tungsten projectile impacting on an aluminum target at 9000 m/s is presented as an example. The sequence of nonequilibrium damage states is traced in nanoseconds. Within approximately 15 ns, the local strain rate in the aluminum target increased to 10 4 s −1 for the solid phase and 10 5 s −1 for the liquid phase. Phase transformation has already occured locally. The solid/liquid interface is highly unstable with a strain rate of the order of 10 8 to 10 9 s −1. The average strain rate in the tungsten projectile is 10 3 s −1. The size and speed of debris splashing into the empty space are also predicted. The velocity of the debris is found to be more than eight times the initial impact velocity of the projectile, a result that agrees with past observations.