This paper deals with an experimental study of the effect of induced microcracking by temperature in a granite. A relative large range of temperature has been used (20°c to 250°c). Monotonic and cyclic tests with acoustic emission measures have been carried out. The results show that mechanical characteristics decrease with the increasing of the induced microcracking, particularly the young modulus for temperatures greater than 150°c. It is also observed by acoustic emission that the mechanical damage is more important when the thermal microcracking increases. Finally, cyclic tests with acoustic emission revealed subcritical microcracks growth phenomenom.


In dieser Arbeit wird eine experimentelle Studie zur thermischen Mikrorissbildung in Granit vorgestellt. Die betrachteten Temperaturen variieren zwischen 20°C und 250°C. Zwei Testserien wurden ausgefuehrt: Monotoner und zyklischer Zugversuch sowie Zugversuch mit Messung der akustischen Emission. Die Resultate zeigen eine Minderung der mechanischen Steifigkeit bei wachsender induzierter Mikrorissbildung. Mittels der akustischen Emission wird nachgewiesen, dass diese Minderung bei wachsender Mikrorissbildung zunimmt. Desweiteren wird die Existenz subkritischer Rissausbreitung waehrend der Versuche nachgewiesen.


Cet article presente une etude experimentale de l'effet de la microfissuration thermique induite dans un granite. Les temperatures etudiees varient de 20°c à 250°c. Deux series d'essais ont ete effectuees: essais de traction monotone et cyclique, essais de traction avec mesure d'emission acoustique. Les resultats montrent une diminution des caracteristiques mecaniques avec le degre de microfissuration thermique induite. L'emission acoustique permet de montrer que la degradation sous chargement est d'autant plus importante que la microfissuration thermique est elevee. Enfin, on montre l'existence d'une propagation subcritique de fissures au cours des essais.


Many pratical problems in geotechnical engineering like radioactive waste disposal require adequate understanding of thermal effects on rock behaviour. Because non linear deformations in rock materials are greatly due to microcracks evolution, it is deemed useful to investigate the effects of temperature through Induced microcracking..With the recent development of sophisticated rheological models (for example damage laws), the study of microcracking process and its consequences have focused attention of several researchers in rocks mechanics (Nur & Simmons, 1970; Tapponier & Brace, 1976; Wong, 1982). In recent years, studies of thermal microcracking have been conducted by several authors (Simmons, Cooper 1978; Bauer, Johnson 1979; Homand-Etienne, Houpert 1989; Rotonda 1991). Homand-Etienne & Houpert (1989) for example, have investigated two granite samples thermally cracked at temperatures ranging from 20 to 600°c. SEM observations allowed them to quantify crack densities and to show that during thermal treatement, the crack lengths remains quasi constant whereas it's width increases with cracking temperature intensity. More recently, Rotonda (Rotonda 1991) concluded that deformability properties are markedly reduced by microcracking whereas strength is less influenced. He has also observed that the effect of microcracking is felt more under tensile loading and less in uniaxial compressive loading. We must notice some other works on thermal effects (Perami, 1971; Guessous, 1981). The aim objective of this study is to quantify thermally induced microcracking on a granite behaviour under tensile loading. A particular attention will be devoted to damage effects.

2.1 Artificially microcracking

Granite cylindrical samples, 37mm in diameter and of height to diameter ratio of about 2 were tested. In order to avoid thermal shock, the specimens were microcracked by slowly heating, at a rate of 50°c/hr. The predetermined temperatures are chosen equal to 20, 50, 80, 120, 150 and 250°C. In each case, the maximum temperature was maintened for 5 hr. The cooling of the samples has been done at a rate similar to the heating one.

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