Simulation of hypervelocity penetration in limestone

A parameter study was performed to examine the (shock) damage obtained with long-rod and spherical mono-material penetrators impacting two varieties of limestone. In all cases, the impacts were assumed to be normal to the plane of the rock and at zero angle of attack (in the case of the rods). Impact velocities ranged to 15 km/s but most calculations were performed at 4 and 6 km/s and the penetrator mass was fixed at 1000 kg. For unlined underground structures, incipient damage was defined to occur when the peak stress, σpk, exceeds 1 kb (100 MPa) and the applied impulse per unit area, Ipk, exceeds 1 ktap (1 kb μs). Severe damage was assumed to occur when σpk exceeds 1 kb and Ipk exceeds 1000 ktaps. Using the latter definition it was found that severe damage in hard, non-porous limestone with spherical impactors extended to a depth of 9 m on-axis for an impact velocity of 4 km/s and 12 m at 6 km/s. Cylinders with length-to-diameter (L/D) ratio of 8.75 achieved depth to severe damage of 23 and 40 m, respectively, under the same conditions. For a limestone medium with 2% initial gas porosity, the latter numbers were reduced to 12 and 18 m.