Research activities have focused on investigating the mechanical behavior of micro-cracked solids on the material and system levels. A plane-strain apparatus has been designed and built for determining the constitutive response of rock-like materials. The biaxial device (U.S. Patent number 5,063,785) is unique because it allows the failure plane to develop and propagate in an unrestricted manner, as opposed to conventional systems where the material is constrained by the testing apparatus. In addition, homogeneous deformation is promoted by the use of a stearic acid-based lubricant.
The failure response of structures composed of rock-like materials is influenced by the localization of deformation in the form of a process zone. This region, which is the seat of energy dissipation, has a fundamental importance for defining the behaviour of brittle materials in terms of the post-peak instability (a qualitative size effect) and the maximum stress (a quantitative size effect). The importance of defining the "strength" of brittle materials is related to the increased use of high-strength concrete and the urgency of storing wastes within rock. Recent developments include (1) the use of acoustic emissions to identify the zone of localized deformation; and (2) an explanation of the size effect, based on a characteristic length of the material.
It is evident from experimental observations of most rock that an analysis of the structural behavior and in particular, an evaluation of the nominal strength, requires a knowledge of the evolution of microcracking as a function of applied load. Among the methods used to examine development of microcracks within a test specimen is the acoustic emission (AE) technique, which is based on the recording of transient elastic waves resulting from the sudden release of energy due to microcracking.
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Zeng, F., Folta, B.L., Labuz, J.F. (2019). Strength testing of sandstone under multi-axial stress states. Geotechnical and Geological Engineering, 37(6), 4803-4814. doi: 10.1007/s10706-019-00939-5
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