Existing models for predicting penetration depth of munitions and explosives of concern (MEC) are inaccurate and insufficient from a user (range manager, U.S. Army Corps of Engineers (USACE) project manager, or environmental consultant) operability perspective for current needs. The researchers attribute poor model performance to (1) a heavy dependence on empirically derived parameterizations poorly linked to the physical properties of the target material or (2) physics based models that inadequately capture the salient mechanical processes, especially in the first meter of penetration. To address these shortcomings the objective was to develop improved constitutive behavior models by using a micromechanical approach that explicitly accounted for material properties such as soil moisture content, grain size, shape, and density.
The technical approach involved the development of a micromechanical-based model using a hybrid discrete element model (DEM)/finite element model (FEM) capable of a detailed treatment of near-surface soil properties. DEM model particle configurations were generated from three-dimensional microCT imaging of a soil sample. To examine the effects of varying levels of moisture on the dynamic behavior of a soil, researchers fabricated a small-scale triaxial shear test to inform the DEM contact model development and calibration. Projectile-drop tests into sand were conducted with a scale version of a 57-mm projectile where researchers measured projectile penetration for comparison with model results.
An improved constitutive model framework has been developed that improves projectile penetration by relying only on the parent material characteristics (elastic modulus, Poisson’s ratio), grain geometry, friction coefficient, and the volume of pore water. Of these parameters, the friction coefficient was the least known for a particular soil type. The triaxial shear test experiments, which characterized the dynamic behavior and transfer of energy through the soil fabric, served as a good source of data to calibrate the friction coefficient. The preliminary numerical model results compared well in a qualitative sense with the drop-test measurements.
Application of these improved constitutive soil behavior models allows for probability estimates of specific munition types to the depth of interest that will lead to accurate time and cost projections of MEC cleanup during project planning. This will provide the MEC recovery team with tighter bounds on the uncertainty associated with apparent MEC at depth identified during geophysical surveys and will limit the over-excavation of apparent MEC at depth, thus reducing the Department of Defense cleanup costs for the Formerly Used Defense Sites and Base Realignment and Closure sites under the Military Munition Response Program.