A series of non-intrusive testing methods-electronic speckle pattern interferometry (ESPI) and acoustic emission (AE)-are being adapted for investigating the process of failure for rock-like materials under wedge indentation. Furthermore, visualization devices are being used as an auxiliary tool to identify the deformation process and even count crack lengths from ESPI experiments. Numerical analyses, with the use of abaqus, may then be performed to simulate the failure process.
Optical methods, such as the EPSI, can be used to measure surface deformation of the specimen with very high resolution. For example, with ESPI, it is possible to follow the strain field and identify a displacement discontinuity at peak load as the region where the speckle pattern is broken. On the other hand, to study the development of the process zone and the progressive failure of rock-like materials, the AE technique is used to analyze source characterization and event location. The AE system contains eight receiving channels that are used to record waveforms coming from the sudden release of energy from microcracks in rock.
In the quantitative analysis of AE, each microseismic event is idealized as a point source of displacement discontinuity that can be modeled as the combination of body forces that yield the same elastic wave field as the defect. In addition, the assumption of a self-equilibrium state requires each unit body force to be paired with an equal and opposite force to form a dipole of forces that constitute the components of the seismic moment tensor. Furthermore, the seismic moment tensor can be evaluated and the source characteristics can be determined through the calibration of the eight receivers, where the arrival time of the signal is assumed to correspond to the first arrival of the normal displacement at the receivers. Therefore, combining the ESPI measurements and AE locations, two different, but in a certain sense, complementary pieces of information are obtained. With the AE locations, the geometrical features of the process zone are defined, whereas with the ESPI fringe pattern, the crack tip is identified.
Julie Bearden, Graduate Student Researcher
Fernanda Carvalho, Graduate Student Researcher
Li-Hsien Chen, Graduate Student Researcher
Sanchai Mitaim, Graduate Student Researcher
Previous work related to the use of the finite element software abaqus shows that the agreement between the numerical solution and experimental results is reasonable for the case of plane-strain compression. Because this software allows the user to solve various boundary value problems, to implement various material relations, and to automatically generate the finite element mesh, it can be used to predict the mechanical behavior and failure process of rock-like materials under different conditions, such as uniaxial compression, three or four point-bending, and indentation test.
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