Damage characteristic of aluminum nanorod under hypervelocity impact

Understanding the dynamic behavior of materials under hypervelocity impact is of great importance to develop new materials or structures for protective applications. The present work gives insight into the damage characteristic of aluminum nanorod under hypervelocity impact based on atomistic simulations. First of all, the propagation of impact wave is found to experience a rapid decaying because of its release from the side surface, which leads to a complex three-dimensional stress wave and two tension regions inside the nanorod. The damage mode under this tension state is found to be very different from the classical spallation. Due to the interaction of two release waves from the side and end surfaces, a temporary spall damage is observed and its initial tensile strength is close to that of bulk material. However, that early spall damage does not develop into a complete spall fracture. More importantly, all generated voids are found to be closed eventually after their coalescence. Furthermore, the mass continues expanding outward from the impact plane and finally causes a radial annular fragmentation. The annular fragmentation shows a clear crystalline direction dependence for low impact velocities. The number and the size of final fragments are found to follow a power law relationship for all impact velocities.

Statistics of energy dissipation in the hypervelocity impact shock failure transition

In the hypervelocity impact event, shock waves subject material to failure transitions with the attendant dissipation of the imparted energy. Under shock compression, failure and dissipation entail intense compression, inelastic shear and compaction. Through shock interactions, states of dynamic tension are achieved and further failure dissipation involves fracture and fragmentation. The nature of failure of solids in the shock environment has encouraged considerable experimental effort through the past several decades. Such efforts have yielded results that suggest universality in the shock failure response over significant spans of shock intensity. Examples include the fourth-power relation between pressure and strain rate in solid-material compressive shock waves, and power-law relations capturing spall fracture strength and fragmentation size scale in dynamic tensile failure. Comparable power-laws also describe the shock compaction of distended solids. The present paper explores a statistical perspective of the underlying micro failure dynamics for the purpose of achieving better understanding of the macro failure trends noted above. A statistical correlation function description of the random micro velocity field is introduced. Through the attendant kinetic dissipation, the statistical fluctuation-dissipation principle is applied to the shock failure transition. From this statistical approach, power-law relations for compressive and tensile shock failure emerge that replicate the reported experimental behaviors.