Three-dimensional stress-strain behavior of cohesionless material subjected to high strain rate

Experimental methods used to subject drained triaxial compression axisymmetric test specimens composed of cohesionless material to very high strain rates (up to ∼1,800%/sec) and analysis of resulting data are presented. Included is the use of a custom gravity drop-frame loading system to generate high strain rates and a custom square triaxial cell to minimize optical distortion. Methods to collect and analyze the experimental results are presented. High-speed film photography coupled with digital image analysis techniques are used to capture specimen deformations. Experimental issues such as image analysis of specimens, optical corrections, time synchronization of photographic images with data acquisition, inertial effects, global versus local strain measurements, axisymmetric shape of the specimen during deformation and membrane compliance are examined by analyzing test results that were performed at high strain rates.; Experiments were performed on dry, vacuum evacuated specimens at two different effective confining pressures (98 and 350 kPa) and two different densities (loose and medium). Using digital image analysis techniques, strains were measured locally near the center of the specimen to obtain values that were least affected by frictional and boundary conditions. Three-dimensional stress-strain relationships are presented and discussed. The following effects were observed with increasing strain rates: the elasto-plastic stiffness increased significantly; the failure strength increased moderately; the axial strain at peak stress decreased significantly; volumetric strains became more dilatant. Unusual behavior was observed at very high strain rates. Examples include: the peak stress was not always associated with the maximum dilatancy rate and shear band inclination angle was noticeably reduced.; Testing constraints are presented in relation to high strain rate triaxial testing. Differing specimen height to diameter (H/D) ratios and drop heights were used to investigate such issues. Results indicate an upper experimental threshold of applied strain rate for triaxial testing using the loading methods presented herein. Beyond such, the specimen appears to lack a measurable region where uniform stresses and strains occur.