An experimental investigation was conducted to study the response of soft, porous rock, green shale, under dry and liquid-filled conditions to normal impact of hemispherically-tipped cylindrical steel projectiles with diameter and mass of 6.35 mm and 31 g, respectively. Disk-shaped green shale specimens with nominal thickness and diameter of 32 mm and 127 mm, respectively, were tested in the initial projectile velocity range of 18 to 42 m/s. For the liquid-filled tests, the green shale specimens were saturated with kerosene. A total of eight impact tests were conducted. Information on the extent and pattern of the damage in the rock and velocity histories of the projectiles were collected. It was found that while the penetration depth was quadratically related to the initial projectile kinetic energy for the dry case, the penetration depth for the liquid-filled case was directly proportional to the initial projectile kinetic energy. The results of the impact tests under dry and liquid-filled conditions conformed well to a linearly viscous model and a Coulomb frictional model, respectively. The major deformation processes in the dry specimens were found to be compaction and equivolumnial distortion of the material. In the liquid-filled case, however, formation of cracks and equivolumnial distortion with less material compaction were observed. Furthermore, craters were formed as a result of the crack formation in the liquid-filled specimens.
Numerous studies have been conducted in an attempt to understand the behavior of soft, porous materials. The response of porous materials to load has been studied extensively, primarily from analytical considerations of continuum mechanics (Bhatt et al., 1975). In addition, the compressibility of porous rocks under quasi-static loading has been studied experimentally (Stephens et al., 1970). Wave propagation problems in a porous material has also been investigated theoretically (Carroll et al., 1973). A series of studies on the penetration of projectiles into soft materials, in which large deformations are generally experienced, resulted in the development of finite-difference or finite-element procedures for either visco-elastic or elastic-plastic targets subjected to large deformations (Johnson, 1976). In the present investigation, projectile impact on soft, porous green shale under dry and liquid-filled conditions was studied experimentally. This is a substantial extension of a previous effort in which the impact of 6.35 mm diameter spherical projectiles on dry green shale was studied (Kabo et al., 1977). The current effort aims, in part, to extend the available data base by measurement of the velocity histories and characterization of the damaged region in the green shale under both dry and liquid-filled conditions. The major objective here, however, is to study the effect of the presence of liquid in the rock on the response of green shale to impact. The purpose of the present study also includes the construction of simple models describing the relation between the final penetration depth and the initial projectile kinetic energy. Comparison of the test data obtained under dry condition to the predictions of a more comprehensive theoretical model was also included in the present investigation.