Paper presents the results of mechanical tests of carbonate rock samples under quasistatic uniaxial compression. The main objective of the work was to identify two different dissipative processes following failure of rock samples in pre and post critical loading range. Two micro mechanical processes fracture of rock and grain slips were identified as damage and plastic dissipation. These micro mechanical failure processes and related energy changes were presented for various strain rate loading conditions.

1 Damage and Plastic Dissipation as a Micro and Macro Mechanical Processes Effects

Rock damage is a process of nucleation and propagation of initial flaws and microfractures of rock leading to macroscopic failure of samples. This process is strongly related to initial heterogeneous and discontinuous structure of rock. Microfractures initiate from local stress concentrations at flaws tips, grain boundaries (Tapponnier & Brace 1976, Olsson & Peng, 1976) or pores (Sprunt & Brace, 1974, Kranz, 1979), and propagate in a direction of the major principal compressive stress (Wawersik & Brace 1971, Kranz 1979, Wong 1982). In the case of uniaxial compression, in the cracks and fractures oriented in accordance with the loading direction shear slips1 occur, with result of tensile stress concentrations located at the crack tips. Propagation of cracks in a form of the so-called wing cracks on the direction of the loading is the consequence of this state (Ashby & Sammis 1990, Horii & Nemat-Nasser 1985). Cracks whose direction is perpendicular to the direction of the loading remain "deactivated" and do not take part in the process of the damage growth.

In the uniaxial compression conditions rock samples behave brittle, often with axial splitting failure mode in the post critical loading range. Despite this phenomenon in the pre and post critical loading range after unloading of rock sample, the permanent strains, and the hysteresis on stress strain characteristics are clear visible (Fig. 1). Permanent strain in the uniaxial compression and hysteresis are a plasticity effects which can be explained on the basis of micromechanical analysis (Jaeger et al. 2007).

Jaeger, Cook and Zimmerman explained the hysteresis and permanent strain as a effect of frictional resistance along the microcracks faces and the energy dissipation trough frictional sliding. These micromechanical phenomena observed in the pre critical loading range on grain boundaries and microfractures (Tapponnier and Brace 1971) are also observed in the post critical loading regime. Following the macrofracture formation the permanent strain and hysteresis in this range may be explained as a result of frictional sliding on the boundary of broken pieces of rock sample.

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