The investigation of projectile impacts on granites

Abstract

It is essential to further research extensively in rock fracture mechanics due to widely used granite in the construction as well as safety industry. Less stiff projectile has higher impact duration with its target, thus more kinetic energy from the projectile is expected to be transferred in damaging the target. Granite rock plate specimens having 18 mm thickness were subjected to 9.5-mm diameter steel, copper and lead spherical projectile impacts. Thus, the research was conducted to study the effect of varying projectile stiffness on the damage size generated in the granite target. Two experimental configurations were adopted for impact on rock specimen, namely confined surface impact and unconfined edge impact. In the first category, the impact was conducted on the flat surface of the rock specimen while in the second category, the impact was conducted on the specimen edge. Each configuration consisted of three modes of impact, which are steel-on-rock (SOR), copper-on-rock (COR) and lead-on-rock (LOR) where the projectile stiffness increases from lead to steel materials. The experimental outcomes were then numerically predicted by using mesh-based elements and mesh-free smooth particle hydrodynamics (SPH) methods with power law plasticity and Johnson-Holmquist material models were utilized to model the ductile and brittle material behavior respectively. Steel projectile was found to cause higher damage on the impacted area, while copper projectile produced larger radial cracks in confined surface impact and extended crater in unconfined edge impact. Longer contact time between copper and rock specimen allowed more kinetic energy to be transferred to generate higher damage in the rock target. The least stiff lead projectile however, produced the smallest damage because of its low yield strength instigated the projectile to absorb most of the kinetic energy in its massive deformation. Edge impact on the other hand, produced mainly double-layered crater formation subjected to SOR and COR modes of impact. Experimental findings revealed the copper projectile produced significantly larger craters compared to SOR impact, showing the superiority of copper over steel projectile in causing higher damage and fracture of rock specimen. Nevertheless, lead projectile did not follow the trend of projectile stiffness as demonstrated by the copper projectile. Therefore, threshold value of projectile stiffness was proposed for the extent of damage failure to be strongly dependent upon the projectile stiffness. Explicit finite element was used to simulate the experimental work. Crater dimensions were modeled efficiently by using refined mesh models for rock specimen. SPH method was capable to predict the formation of double-layered craters on impacted edge and had close proximity with experimental evidence. Mesh-based elements method, on the other hand was able to represent irregular crater formation obtained from the impact tests.