DEVELOPMENT OF AN EXPERIMENTAL TECHNIQUE AND RELATED ANALYSES TO STUDY THE DYNAMIC TENSILE FAILURE OF CONCRETE (BRITTLE, MICROCRACKING, FRACTURE, SOFTENING, ROCK)

The objective of this research was to develop and demonstrate experimental and analytical techniques to study the tensile failure of concrete and geologic materials at strain rates of about 10 per second. A new experimental method was developed, a set of experiments was conducted, and the experiments were interpreted with numerical calculations.; In the new experiment, a 5-cm-diameter rod is first loaded in static triaxial compression, then the axial pressure is released from each end simultaneously and very rapidly. The resulting relief waves interact in the center of the rod to produce a dynamic tensile stress equal in magnitude to the original static compression. Tensile failure occurs if the tensile stress exceeds the tensile strength for these conditions. The radial pressure is held approximately constant during the experiment. Several experiments of this type were performed on concrete. In every case the rod fractured near the midpoint; in some cases a second fracture also occurred several centimeters from the midpoint. 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.; Each experiment was interpreted with a set of one-dimensional finite difference calculations, using an elastic-fracturing (strain-softening) representation for the material. By trial and error, material parameters were chosen for each experiment so that satisfactory agreement was obtained between the calculated strains and the measured strains.; The calculations suggest that the response of the concrete in these experiments was inelastic even several centimeters from the locations of complete separation and that damage was concentrated at one or two locations about 2 or 3 cm from the central failure point. Furthermore, the calculations indicate that in some cases, the strain history measured a few centimeters from the central failure point is primarily a function of inelastic wave propagation from the fracture location to the strain gage and was less dependent on the behavior of the material right at the fracture.