Experimental study of granular material behaviour and microstructure in a photoelastic disk assembly
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AbstractIn this thesis, both micro and macro mechanical studies were conducted on an idealized granular assembly under different kinematic and static conditions. The experimental approach involved the use of an analogue granular material composed of an assembly of photoelastic disks in order to permit the visualization of force transmission networks through fringe analysis. A comprehensive 2-D biaxial testing apparatus was developed along with four high precision actuators that can load a specimen consisting of an assembly of photoelastic disks in either stress or strain control mode. Two different disk cross-sections were used: namely pentagonal and circular, while between 200 and 400 disks were used to constitute a specimen of approximately 100 mm x 100 mm in size. A series of tests was conducted along proportional strain paths so that the specimen responses in the complete forced compression and dilation regimes could be explored. The specimens were sheared under different static and kinematic conditions with a special focus on the dilation regime, given its importance in granular mechanics. Initially, tests were conducted on isotropically consolidated specimens of pentagonal disks to observe typical force-displacement responses as well as force response paths. Microstructural features such as force transmission and its evolution were studied through image analysis and photoelasticity. In particular, a micro-mechanical description of the deformational processes that occur during the 'static liquefaction' of a specimen under forced dilation was established. Furthermore, strain probing tests were conducted on a specimen at the same amount of preshearing, but with different stress histories. It was found that stress history effects tend to fade away with increasing levels of straining. The effect of intergranular friction was also investigated, and it was found that it only plays a role towards providing the stability of fabric, once the latter is formed. In order to study the constitutive nature of the granular assembly, force response envelopes were determined at different probing strain levels. Both the shape and orientation of the force response envelopes elucidated the effect of pre shearing stress history as well as probing strain level on the material behaviour. Furthermore, a series of force probing tests was conducted on the same above-mentioned specimens so that the strain response envelopes could be determined at different probing force levels. Both the force and strain response envelopes gave identical behaviours in terms of the material response as a function of fabric and its evolution. One of the important findings that emerged from the force probing tests is that a plastic flow rule was established, and hence confirmed the applicability of the theory of plasticity in the modelling of granular materials. A material stability analysis was also performed on the proportional strain path test results by using Hill's criterion for the sign of the second order work. The threshold dilation rates at which instability occurs were determined for specimens subjected to different initial confinements. In some cases, material softening was observed before material instability had occurred. The amount of dilation and dilation rates that the material can sustain before instability was investigated for different initial confinements. The strain probing tests were further performed with circular disks in order to study the effect of kinematic constraints imposed by disk cross section and diameter on material behaviour under forced dilation. For the same initial confinement and strain path, significant differences were observed in the specimen strength as the disk cross section and diameter were changed. The microscopic force transmission and its evolution in the specimen were presented from the photoelastic observation of pictures taken at different characteristic points of loading history in order to explain the salient failure mechanisms.
Bibliography: p. 202-208