| 摘 要： |
A high degree of alteration causes swelling, disintegration and argillization of granite and thus weakens its physical and mechanical properties. Very few experimental studies, however, have been conducted to quantitatively characterize the strong-to-weak transition of the mechanical behaviours of granite affected by varying degrees of alteration. A new miniature tensile instrument system, which can visualize the failure process and simultaneously record the stress-strain curves, was utilized to test the tensile strength of dog-bone sliced samples of altered granite. Meanwhile, an improved quantitative method was proposed to characterize the alteration degree. The results demonstrate that the tensile strength of granite decreases significantly with increasing alteration rate, while the strain at the peak strength and fracture angle increase. Additionally, the orientation of weaker crystals and the mineral size distribution significantly affect fracture propagation. But the effect will be weakened as the alteration rate increases. On the basis of fracture angle, tensile peak strength, strain at peak strength, and microscopy images of crack initiation and propagation, three major classes of fractures are identified from the test results: (1) extension fracture at a low alteration rate: crack initiates and propagates along biotite-brittle mineral interfaces; (2) shear fracture at a high alteration rate: intragranular crack initiates within altered feldspar and propagates along the interior of altered mineral grains; (3) hybrid fracture at a moderate alteration rate: the above two failure phenomena coexist. A simple model that considers the fracture process of altered granite was proposed to explain the transition from extension fracture to shear fracture. |
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