Porous compaction as the mechanism causing the Hugoniot Elastic Limit

Unlike in metals, stress and particle velocity histories of shock waves in ceramic materials show a typical ramping above the Hugoniot Elastic Limit (HEL). Under the assumption that the HEL signifies the beginning of failure of the material, this ramping has been described using a viscoplastic model. However, there is ample experimental evidence to rule out plastic flow at the HEL-level of stress. Alternatively, moduli degradation can be used to model the ramping, but it seems improbable that the moduli would degrade significantly under increasing pressure. The proposed micro-mechanical mechanism for the HEL, and for the ramping beyond the HEL, is based on the process of porous compaction due to pressure above a threshold pressure pcrush. Although the pore volume of high-grade ceramics is quite small, it is sufficient to cause the ramping observed in Hugoniot measurements from the seventies. Additional evidence for the effect of porosity on the HEL stress has been given in recent years. Here, this experimental evidence is supported by simulations using a simple model for porous compaction. The simulations suggest that porous compaction is the main micro-mechanical mechanism causing the ramping and other features related to the HEL.