Ultrasonic imaging and acoustic emission monitoring of thermally induced microcracks in Lac du Bonnet granite

Abstract

Concurrent ultrasonic tomography and acoustic emission monitoring were employed to study thermally induced microfracturing in an unconfined, 15-cm cube of Lac du Bonnet granite from Atomic Energy of Canada Limited's Underground Research Laboratory. An electrical resistance cartridge heater, placed in a central vertical borehole, was used to cycle the sample to progressively higher peak temperatures between 75øC and 175øC. Tomography data were collected, at room temperature, before and after each thermal cycle. Acoustic emission monitoring proceeded during both heating and cooling phases of each thermal cycle. Microfractures opened above 80øC and eventually coalesced into a macroscopic fracture plane. The macroscopic fracture originated at the outer edges of the sample and then extended inward, parallel to the fast velocity direction, and eventually intersected the borehole. Both acoustic emission locations and slowness difference tomography clearly delineated the fracture plane. We attribute the development of the macroscopic fracture to a thermal gradient cracking mechanism acting upon a brittle, anisotropic medium. 22,231

Damage and Changes in Mechanical Properties of a Gabbro Thermally Loaded up to 1,000°C

“…DAVID et al (1999) summarized some laboratory studies that clearly showed how thermal cracking modifies elastic moduli (BARBISH and GARDNER, 1969;HEARD and PAGE, 1982;NASSERI et al, 2007), permeability (DAROT et al, 1992;MENÉNDEZ et al, 1999), and mechanical strength (HOMAND and TROALEN, 1984;HOMAND and HOUPERT, 1989). JANSEN et al (1993) investigated the development of thermal cracks by continuously monitoring the acoustic emissions (AE) generated during thermal cycles in a granite specimen. They showed that, at high temperatures, microcracks open and coalesce to form a macroscopic fracture.…”

Changes of Selected Structural and Mechanical Properties of the Strzelin Granites As Induced By Thermal Loads / Wpływ Obciążeń Termicznych Na Zmiany Niektórych Strukturalnych I Mechanicznych Właściwości Granitów Strzelińskich

“…Homand-Etienne and Houpert (1989), in turn, demonstrated how the porosity of the fractures in selected rocks changes with an increase in the temperature of sample heating, and David et al (1999) analyzed the impact of temperature-induced fractures on the microstructure and selected physical properties of the La Peyratte granite. Development of thermal fractures in granite was also observed -by means of an ongoing monitoring of acoustic emission -by Calleja and Ruiz de Argandona (1984) and Jansen et al (1993). The results obtained by these researchers showed that microfractures can be observed as early as when the temperature value exceeds 80°C, and they can subsequently join together, forming structures that are macroscopic in their nature.…”

Bonded-particle modeling of thermally fractured granite

“…Using a heater placed in a central borehole, a sample of LdB pink granite was thermally cycled to progressively higher peak temperatures. Six heating cycles were performed with peak temperatures varying from 75 to 245 1C while AE data were collected; a detailed description of the experiment is given in [5,24]. The rock sample, supplied by Underground Research Laboratory (URL), Canada, was a 22 cm long cylinder, with a diameter of 30 cm and a through-going axial borehole with a diameter of 3 cm.…”

“…This paper describes the results of numerical simulations that were conducted in order to improve our understanding of the mechanical damage caused by thermal loading. These simulations were based on numerical models that were parameterized and validated by a previous experiment by Jansen et al [5].…”