Mechanics And Sound Wave Propagation Of Granular Solid Under Direct Shear

Natural disasters such as debris flow, avalanche and landslide are essentially rel ated to quasi solid-liquid phase transition induced by the particle system instability. H owever, the billions number of these units in actual granular samples and the non-equi librium state dissipative characteristics challenges either theoretical research or experi mental studies. In this dissertation we mainly employed the characteristics of propagat ing pulsed sound waves in sheared granular solid under normal force to study the micr oscopic structure of the granular solid in quasi solid-liquid phase transition induced by direct shear.Wave propagation is the most basic physical phenomenon in all media. The aco ustic wave velocity is directly related to the compressibility and density of the system. In contrast, wave properties in granular materials are complicated due to the heteroge neous nature of these systems. We apply the time-of-flight method to study the pulse wave propagating characteristics in granular solid under normal force. It is found that the transmitted temporal signal consists of a primary low-frequency stable E wave and a strongly fluctuating unstable S wave. The width of the pulse wave, the size of the re ceiver and the particle size of the sample all affect the intensity and duration of the E-wave and S-wave. Surprisingly we observed in the experiment that the sound velocity increases with the thickness of the sample.The existence of force chains in granular solid is one of the key element of the mechanics of a granular system. Mesoscopic effect appears. Direct shear is one of the most commonly performed experimental methods for studying the quasi solid-liquid p hase transition. However For a long time, engineers neglect the discreteness and heter ogeneous nature of the granular system. We use high precision spherical glass beads t o exclude effects due to properties of the material, such as fineness, surface friction co efficient, the coefficient of restitution, dispersiveness and other factors affecting the e xperimental accuracy in direct shear test. The results show that not only the ratio of th e width of the container and the particle size, but also that of the thickness of the samp le and its width will make a difference to experimental results. According to our exper imental observations we suggest the current standard of such a ratio for the direct shea r test shall be modified.According to the acoustics theory of continuous medium, the sound wave in sol id has longitudinal mode and transverse mode, in contrast to the fluid, in which only t he longitudinal wave exists in. Acoustic signal is used to study the mechanical propert ies of dense granular system under direct shear. During the process of direct shearing, it is found that amplitude and velocity of the sound wave decrease rapidly with the inc rease of shear stress. After the yield point of stress with increasing strain, the velocity of the propagating wave decreases gradually and attains a constant. The well-known effective Medium Theory and Granular Elastic Theory are employed for explaining th ese results. It confirms that the change of sound speed is produced by the changing of shear stress and volume expansion in sample, not by dissipation in the shear band.