Modeling of deformations of Roma Plastilina # 1 clay in column-drop tests by incorporating the coupled strain rate and temperature effects

Abstract

Roma Plastilina (RP) # 1 clay has been used as a backing material in ballistic testing of body armor. Column-drop tests are conducted to assess the suitability of RP # 1 clay for ballistic tests. In the current study, deformations of RP # 1 clay induced by projectiles employed in column-drop tests are simulated using finite element models based on the Johnson-Cook (J-C) material model and a new constitutive model. Both the J-C and new models account for strain hardening, strain rate hardening and temperature softening effects exhibited by RP # 1 clay. However, the three effects are treated separately in the J-C model, while the coupling between the temperature and strain rate effects is considered in the new model, which contains seven material constants. The simulations are conducted using the explicit dynamic solver LS-DYNA, and a user-defined material module along with a computational algorithm is developed for the new model and implemented in LS-DYNA. The effect of the hydrostatic pressure is incorporated in the current simulations through using an equation of state (EOS) (with three different types), unlike in existing studies. The numerical results based on both the J-C and new constitutive models reveal that the hydrostatic pressure effect is insignificant for RP # 1 clay under the dynamic loading conditions in the column-drop tests. In addition, it is found that for RP # 1 clay tested at 37 °C the simulation results for the final penetration depth based on both the J-C and new models agree well with the experimental value. However, for RP # 1 clay tested at 23 °C (the room temperature), the permanent depth of penetration predicted using the J-C model differs significantly from what was measured experimentally, while that predicted by the new constitutive model is very close to the experimental value.

Keywords:

• Roma Plastilina # 1 clay
• column-drop test
• depth of penetration
• Johnson-Cook model
• equation of state
• hydrostatic pressure
• strain rate
• plasticity

Acknowledgement

The support by the U.S. Army is gratefully acknowledged. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the U.S. Army. In addition, the authors would like to thank Drs. Steven Mates and Amanda Forster of NIST in Gaithersburg, MD for their helpful comments.

Additional information

Funding

The work reported here was partially funded by the U.S. Army under Contract No. W91CRB-18-C-0003.