Axial Friction Calibration For Molten Salt Triaxial-pressure Vessel And Experimental Simulation Of Fluid Effect On Crack Healing

The Wenchuan earthquake fault is a high-angle reverse fault, which dips at 50°-80°. High-angle reverse fault can hardly slip without presence of high-pressure fluid at depth. In this thesis, several mechanisms for fault weakening and fault healing are summarized. And finally the rock and fluid interaction is simulated under high temperature and high pressure.Fluid plays an important role in fault weakening, both in the physical and chemical aspects. On one hand, high pressure fluid can reduce the effective stress on the fault. On the other hand, the minerals may alter to some layered minerals which have very low friction coefficients with the participation of the fluid. And the reaction is reversible, while fluid environment changes after the earthquake, the layered minerals can also change to the strong minerals, which is helpful to heal the fault. Thus, fluid plays different roles during the earthquake cycle. When an earthquake occurs, the high pressure fluid makes the effective stress on the fault very low. After the earthquake, fault healing occurs with the fluid-assistant pressure solution and mineral phase change. During the interseismic period, fluid pressure increases with the fluid accumulation, and the fault weakens by fluid chemical effect.Before the simulating experiments, firstly, a series of experiments are carried out to calibrate the axial pressure. The calibrating experiments use the axial load cycle method based on the calibrated result of confining pressure and temperature by HAN (2009). The difference between the axial loads shown on the machine and the true axial pressure pressed on the sample is called total friction. It contains two kinds of forces: the frictional contact force and the squeezing force. The squeezing force makes the hit-point unclear,but does not affect the sample strength. While the contact force increases with piston in,and it makes the sample strength inaccurate. A series of experiments were carried out to determine the factors affecting the axial load. The confining pressure,temperature and strain rate are thought to be the main factors. The result shows that low confining pressure, high temperature and low strain rate give rise to the low axial friction. Using the molten salt cell assembly,the contact friction can be accurately determined. While in the solid salt assembly,it is difficult to determine the contact friction from the total friction. When the temperature is 200°C above the melting temperature of the confining pressure medium,the experiment results are most reliable. While when the temperature is below the melting temperature of salt,axial velocity highly affects the axial friction.The purposes of axial friction calibration are to determine the axial contact friction with axial displacement, and to determine the hit-point between the axial piston and the sample. The slope k of the stress with the displacement can be used to represent the friction force. The starting point where the stress goes up sharply is the hit-point, which means the Al2O3 piston begins to be connected with the sample. If the salt is not sufficiently molten, the hit-point is usually unclear. So it is important to make the experiment condition higher than the molting point of the salt, if a more accurate result is wanted.Finally, a few experiments are carried out to simulate the fault or crack's opening and healing. The samples are granite and amphibolite. Some of the samples are dried to remove the free surface water and part of the structure water, and some samples are dipped into water in order to increase water content. Both the mechanical data and the microstructure show that the dry samples contain main fracture through the whole sample, and it slips during the experiments, while the main deformation mechanism in the wet sample is cataclastic flow. The dark bands in the microcracks are found in the wet granite sample. The suture edges of the wet amphibolite grains show that pressure solution occurred during the slow rate loading. So it can be concluded that water can promote the water and rock reaction, or pressure solution, under a low strain rate.