The Impact Response of Coquina: Unlocking the Mystery Behind the Endurance of the Oldest Fort in the United States

Abstract

The Castillo de San Marcos fort in St. Augustine, FL, has withstood numerous wars and hurricanes for over 300 years. Interestingly, during military sieges it was discovered that the fort walls did not shatter upon impact, but instead absorbed the cannonballs. The endurance of this fort is attributed to a native rock, called coquina, from which it was constructed. Quasi-static and high strain rate uniaxial compression experiments were conducted on coquina and two other materials (sandstone and a structural foam) with similar structures and physical properties. Additionally, low-velocity ball impact experiments, analogous to impacts due to cannonballs, coupled with high-speed imaging were performed to investigate the impact response. The analysis explored the operative deformation mechanisms which gave rise to the fort’s endurance against impacting cannonballs. It was revealed that the microstructure of coquina does not lend itself to macroscopic brittle fracture, but absorbs impact energy by progressive crushing similar to a cellular structure, and thus allows for a high specific energy absorption.