A laboratory method for determining the dissipated energy

Abstract

It is well known, that the strength of rocks depends on the strain or stress rate. Increasing the stress or strain rate, the strength of the rock material is increasing. The goal of this paper is to present a theoretical background of this behavior. A laboratory method is suggested to determine the critical dissipated energy density (CDE) of homogeneous-isotropic rock. In case of uniaxial compression it can be calculated easily with the difference of the work done by the external force and the energy connected to the change of structure. These energies can be measured by applying different stress or strain rates, if they tend to infinite and zero. It was assumed, that the Poynting-Thomson (standard) model can be used for modeling the rheological behavior of the intact rock, and this model was applied determining the critical dissipated energy density. The uniaxial compressive tests were carried out for both “infinite” and “zero” stress rates, and the measured stress-strain curves were compared and analyzed. According to the results, the critical dissipated energy per unite volume can be used as a material constant: it is influenced by the texture of the rock, internal bonds, the cohesion, the strength of the minerals, the porosity, etc. Using seven different types of rocks the critical dissipated energy per unite volume was determined and the relationship with the compressive and tensile strength, ultrasonic wave velocity and porosity was calculated. Knowing this critical dissipated energy, probably the tunnel stability can be calculated easier, or it can be used in the calculation of the earthquakes damage, among others.