ABSTRACT:

Rock and rock masses are composed of discrete elements of microstructures such as different grains and microcracks. In this study, first we show experimental results that a propagation, coalescence and increased density of microcracks cause mesoscale and macroscale cracks, and that elastic and plastic properties of rocks are significantly affected by these microstructures. Next, a homogenization theory to model the elastoplastic behavior of the material with microscale structure is presented

RESUME

Les roches et les blocs de roche sont constitues d'elements discrets à microstructure tels que les grains differents et les microfissures. Dans cette etude, nous avons tout d'abord demontre les experimentaux par lesquels la propagation, la coalescence et I'accroissenment de densite des microfissures provoquent des fissures de mesolimenson et de macrodimension et que par ailleures, il est significatif que les proprietes elastiques et plastiques des roches sont affectees par ces microstructures. Ensuite, une thêorie d'homogeneisation modelant Ie comportement eleastoplastique de la matière avec une structure de microdimension a egalement ete presentee.

ZUSAMMENFASSUNG:

Gesteine unt Gesteinensmassen setzen sich aus unterschiedlichen Bestandteilen mit Mikrostruktur zusammen, die zum Beispiel unterschiedliche Körnungen und Haarrisse aufweisen. In dieser Untersuchung zeigen wir zuerst die Versuchsergebnisse, daß Ausdehnung und Zusammenziehung, sowie eine vemehrte Dichte von Haarrissen Risse im Mittel-und Makrobereich verursachen, und daß die elastischen und plastischen Eigenschaften von Gesteinen durch diese Mikrostrukturen wesentlich beeinflußt werden. Daran anschließend wird eine Theorie zur Vereinheitlichung vorgestellt, mit der das elasto-plastische Verhalten von Materialien mit Mikrostukturen modellmaßsig dargestellt wird.

1 INTRODUCTION

Mechanical properties of rock are affected by Its microstructure such as constituent minerals and microcracks. Microscopic studies of cracks in post- loaded samples were made by using a scanning electron microscope (Friedman et al. 1970; Wong 1982; Mardon et al. 1990). For damage propagation,. Peng et al. (1972) loaded Chelmsford granite specimens to various levels, unloaded, cut lengthwise into halves and then observed. These experimental studies showed that macroscopic fractures grow from grain-scale microcracks. Whereas, these works have a drawback that they were performed under zero stress condition using thin or cut section after experiment or under artificially fractured condition. We developed a new testing equipment to observe the micro-damage behavior continuously during the deformation without unloading for the same specimen. The experiments were currently carried out under uniaxial compression. The effect of microscale structure for fracturing process of rock is not simple. In this study, an elastoplastic homogenization theory is shown to analyze the stress state for rock specimen with microscopically distributed minerals.

2 EXPERIMENTS

The procedure of microcrack formation and subsequent damage propagation were observed in a series of compression test by using granite specimens under nominally dried condition at room temperature.

2.1 Experimental procedure

The specimens of coarse-grained granite from Geochang, Korea are consisted mainly of 36.5% quartz, 56.3% feldspar and 7.2% biotite (a modal test results). The bulk density and apparent porosity of the granite are 2.58 g/cm3 and 0.83%, respectively. The dimension of specimens is shown in Figure l. The experimental system shown in Figure 2 consists of three subsystems: a) loading system, b) data-recording system and c) observation system.

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