An experimental method to study the dynamic tensile failure of brittle geologic materials

Abstract An experimental method was developed to study the tensile failure of brittle geologic materials at strain rates of approximately 10 to 20/s. In these experiments, a cylindrical rod specimen is first loaded in static triaxial compression, then the axial pressure is released from each end simultaneously and very rapidly. The resulting rarefaction waves interact in the center of the rod to produce a dynamic tensile stress equal in magnitude to the original static compression. The pressure acting on the radial surface is approximately constant during the experiment. As an application of this method, several experiments were performed on concrete. Transient measurements were made of the axial load at each end, the confining pressure, and axial and circumferential surface strains at several locations along the length of the rod. Usually a single fracture occurred near the midpoint of the rod. In some experiments multiple fractures occurred. Assuming the peak observed strains in these experiments to be elastic the unconfined tensile strength of the concrete at a strain rate of 10 to 20/s was, on average, approximately 40% higher than the static splitting tensile strength. At the same strain rate, the tensile strength with 10 MPa confining pressure averaged approximately 100% higher than the static splitting tensile strength and 40% higher than the unconfined tensile strength at 10 to 20/s. Nonlinear analyses indicate that these estimates are reasonable, but that in general the assumption of elastic response is not valid. To match the measured strain histories with calculations requires that the rod be modeled inelastically.