*Article* **The Strain Rate Effects of Coral Sand at Different Relative Densities and Moisture Contents**

**Kai Dong †, Kun Jiang \*,† and Wenjun Ruan**

School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, China; dongkai@njust.edu.cn (K.D.)

**\*** Correspondence: jkeddy@163.com

† These authors contributed equally to this work.

**Abstract:** A 37-mm-diameter split Hopkinson pressure bar (SHPB) apparatus was used for impact loading tests to determine the effects of the relative density and moisture content on the dynamic properties of coral sand. The stress–strain curves in the uniaxial strain compression state were obtained for different relative densities and moisture contents under strain rates between 460 s−<sup>1</sup> and 900 s−1. The results indicated that with an increase in the relative density, the strain rate becomes more insensitive to the stiffness of the coral sand. This was attributed to the variable breakageenergy efficiency at different compactness levels. Water affected the initial stiffening response of the coral sand, and the softening was correlated with the strain rate. Strength softening due to water lubrication was more significant at higher strain rates due to the higher frictional dissipation. The volumetric compressive response of the coral sand was investigated by determining the yielding characteristics. The form of the constitutive model has to be changed to the exponential form, and different stress–strain responses should be considered. We discuss the effects of the relative density and water content on the dynamic mechanical properties of coral sand and clarify the correlation with the strain rate.

**Keywords:** coral sand; strain rate; moisture content; relative density; volumetric compressive
