Computational modeling of the impact response of Roma Plastilina across a wide range of strain rates

Abstract

It has long been known current helmet test methodology suffers from a missing connection between helmet test standards and the relevance to injury prevention. One of the tests in the protocol consists of impacting the helmet material plates with a high-velocity projectile and the performance assessment is based on the permanent deformation of the backing-material, Roma Plastilina clay. This work focuses on the development of a computational framework to develop a deeper understanding of the mechanical response of Roma Plastilina clay. Prior work has focused on the development of a clay model based on Cam-clay theory. In this work, it is shown this model failed to adequately capture the mechanical response across the range of strain rates of interest. To address this deficiency, the previous model formulation is extended to a more general rate-dependence model of the power-law type. Three impact tests are used to calibrate the modified constitutive model for the clay: one low-velocity test and two high-velocity tests. The low-velocity test is a drop test used to ensure the clay is well-conditioned for the high-velocity tests in which a high-velocity projectile impacts a plate with a clay backing. The final clay deformation for all three tests is compared against experimental data to ensure the accuracy of the clay model. Finally, to improve simulation efficiency, scalability of the computational framework is tested. It is concluded the computational framework is an effective tool for modeling Roma Plastilina clay. The constitutive model for the Roma Plastilina clay is validated, tested and final material parameters are determined that characterize the clay behavior over a large range of impact rates. The modified clay model is used to explore the phenomenon of separation between the plate and clay which was previously believed to only occur with hard plate materials.