Spalling behaviors of Pb induced by ramp-wave-loading: Effects of the loading rise time studied by molecular dynamics simulations

Spalling processes under ramp-wave loading are investigated by molecular dynamics simulations. The simulations reveal that spalling is greatly influenced by the loading rise time of the ramp-wave. It is discovered that, in spalling induced by ramp-wave loading, whether the surface layer is in solid state or in liquid state is tightly related to the rise time of the loading wave. In cases of shorter rise time, the surface layer may be in liquid state. In cases of longer rise time, the surface layer keeps in solid state even when the peak loading pressure exceeds the threshold of melting in compression. The formation of solid surface layer is due to that the maximum compression pressure experienced by materials near the free surface is smaller than the threshold of melting as the rise time increases to be long enough. The result provides a reasonable explanation for some controversy about the state (liquid or solid) of the surface layer when the peak pressure exceeds the threshold of melting. The rise time effects on the tensile stress evolution and the cavitation process are also discussed. Comparisons with experiments and other computer simulations are made where necessary. (C) 2016 Elsevier B.V. All rights reserved.