A modification to radial strain calculation in rock testing

The complete uniaxial or triaxial stressâ€"strain behavior of a rock sample can be achieved in compressive testing using a servo-controlled testing system. Initially, the load-deformation data is recorded during an experiment, and then to eliminate the scale dependency of force, this data are converted to the stressâ€"strain curve for final reporting. It has been found that because of the particular design of the current sample instrumentation system used to measure the circumferential or lateral deformation during the uniaxial or triaxial compressive tests, a modification is required to be included in the conventional method of radial strain calculation. As a result, a new analytical solution has been developed to improve the accuracy of the radial strain calculation. The modification procedure differs depending on whether the test-sample diameter is greater or less than 50 mm. A comparative study was conducted based on visual observation and quantitative analysis to show the variation in a number of rock parameters including elastic modulus, Poisson’s ratio, peak stress, critical strain, residual stress strain, softening fracture energy, and residual stress when this modification was implemented. For a 50-mm-diameter sample, which is the suggested size for rock testing according to the International Society for Rock Mechanics, the effect of this modification on the parameters obtained from the post-peak region, such as critical strain, residual stress strain, and softening fracture energy varied from 5 % to 9 %. It was concluded that the impact of this modification procedure is more significant with smaller-diameter samples used in rock testing.