Contribution of acoustic emissions for the study of fracture in rocks

Acoustic emissions (AE) are a tool commonly used in a wide variety of building materials for the quantification of structural damage. Given the potential of the application of acoustic emission techniques, a case study of limestones is presented in order to study the location of acoustic emissions of test specimens subjected to triaxial compressive tests (hollow cylinder test). There was a good agreement between the location/concentration of the estimated AE with the result obtained by the visual analysis of the fractures produced during the laboratory test (conducted until breaking). The determination of the energies associated to the acoustic events allowed to establish an energy profile for each load request applied during the test. Data shown in a table in pdf format.

The tests were carried out on limestone specimens chosen for their homogeneity at the scale of the block. The limestone specimens were placed in the Hoek cell which can be described as a 100 mm walled cylinder with a thickness of approximately 24 mm and a height of 120 mm. The Hoek cell adaptations necessary to perform this test are minimal in the sense that the modification falls only on the sleeve and the plungers. This guarantees the use of the cell in the standard triaxial tests. The first change consists of cutting four circular openings on the surface of the sleeve where four piezoelectric transducers were fitted. The test starts with a hydrostatic stress state, with the internal pressure being slowly increased. The stresses are transmitted to the specimen by means of a hydraulic press and by the outer sleeve of the Hoek cell, respectively axial and radial outer stress. The acoustic emission data is captured by four sensors distributed around the specimen. The times and amplitudes are recorded, allowing the subsequent transformation and analysis of the data. To aid in the processing of data, a velocity model is created. This is based on a discrete grid of points, which is designed through the sensor training technique. With this information an algorithm to pairing the events registered by the sensors is elaborated and the origin of the events is located. Throughout the test the internal and confining stresses are monitored.