Comment on "Modeling shock waves in orthotropic elastic materials" [J. Appl. Phys. 104, 044904 (2008)]

This comment identifies two main problems with the paper Vignjevic et al. [J. Appl. Phys. 104, 044904 (2008)] related to shock waves modeling in composites. (1) The authors claim that they have proposed two different stress decompositions based on the assumption that the stress tensor is split into two components: one component is due to volumetric strain and the other is due to deviatoric strain. Following this, the authors defined a pressure as the state of stress resulting only from volumetric deformation. However, neither the first nor second decomposition of the stress tensor proposed by the authors provides a procedure for separating the material volumetric compression from the deviatoric strain tensor which results in a state of stress corresponding to volumetric deformation. Furthermore, the uniquely correct decomposition of the stress tensor based on the same second order material tensors has already been published (see, for example, [Int. J. Plast. 24, 140 (2008)]). Also, the second decomposition of the stress tensor includes serious mistakes and inconsistencies. (2) In addition, the numerical simulation results proposed by the authors cannot be justified. An analytical calculation of the Hugoniot stress levels and the acoustic speed of sound through the thickness orientation for a selected carbon-fiber epoxy composite show that the stress decompositions and material properties reported by the authors do not agree with the experimental data for this material and, more importantly, contradict the shock wave stability requirements. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3483945]