Impact Response of Coquina

Abstract

The Castillo de San Marcos Fort in St. Augustine, FL was built over 330 years ago and has endured numerous wars between the Spanish and the British. During these wars, cannonballs were fired at the fort walls and became embedded in the walls. The walls did not shatter, nor did large cracks form. The fort was constructed from a native rock called coquina, found on the east coast of Florida and the west coast of Australia. Coquina is a highly porous sedimentary rock, consisting of crushed shells, fossils, limestone, sand, minerals, and clay. There are no scientific studies illustrating coquina’s ability to withstand cannonball impacts. This research focused on testing coquina and a similar material (a commercial cellular foam) in uniaxial compression. The compression experiments revealed that coquina had two times the specific energy compared to a structural foam. The research revealed that the microstructure of coquina allows impact to be absorbed by progressive failure and hence possesses a high energy absorption capability.