Laboratory investigation on mechanisms of stress wave propagations in porous media

Abstract

A number of porous models having the similar statistical characteristics of pores and physical properties with natural sandstones have been produced using reactive powder concrete (RPC) and polystyrenes. Spit-Hopkinson-Pressure -Bar tests and CT scans have been carried out on the models with the various porosities to probe the performance of wave propagations and the responses of pores and the matrix during wave propagations. It is shown that porosities significantly influence wave propagations. For an identical impact strain rate, the greater the porosity is, the larger the amplitude of the reflected wave appears, the more the peak in the reflected wave presents, and the smaller the amplitude of the transmitted wave turns out. A single peak emerges in the reflected wave when the porosity falls down to 5%. The larger the impact strain rate, the much remarkable the phenomena. The energy-dissipated ratio of porous models, i.e., W J /W I , linearly increases with the increment of porosities. The ratio is sensitive to the impact strain rate. Differences in the performance of wave propagations and energy dissipation result from the varied mechanisms that pores response to impacts. For the porosity less than 10%, the mechanism appears to be a process fracturing the matrix to generate new surfaces or pores. Energy has primarily been dissipated in creating new surfaces or pores. No apparent pore deformation takes place. The impact strain rate takes little effect on pore geometry. For the porosity of 15% or more, the mechanism works depending on the impact strain rate. When a low impact strain rate applies, the mechanism still appears to crack the matrix to generate surfaces or pores, but the amount is lower as compared to the case with a low porosity. If a large impact stain rate applies, the mechanism combines both fracturing the matrix and deforming the pores, with the deforming pores predominating. The vast majority of energy has been dissipated to deform pores. Only high porosity and impact strain rate can bring significant deformation to the pores. The proposed eccentricity of pores is capable of characterizing the geometry of pores and its change during wave propagations.

Numerical simulation of mechanisms of deformation, failure and energy dissipation in porous rock media subjected to wave stresses

“…Figure 13 shows the equipment and the loading method for SHPB tests. It should be noted that in SHPB tests, two-dimensional dispersion effect and friction effect between contact surfaces during wave propagation can hardly be eliminated, and the assumptions in the one-dimensional wave theory can seldom be satisfied [21,[34][35][36][37][38]. The experimental and theoretical results on stress wave propagation in rock can be very different.…”

“…The incident wave was a triangular pulse, with cycle of 0.3 ms, wavelength of 1.56 m, elements were used to simulate bars. In order to reflect the non-linear deformation and failure of porous medium under impact loading, based on the impact tests on natural sandstone and porous models [21,39], the HJC (HolmquistJohnson-Cook) constitutive model was employed for the matrix. The material parameters of the HJC constitutive model are shown in Table 2.…”

“…Due to the large population and complex and disordered spatial distribution of pores, it is extremely difficult to theoretically correlate the stress motion, constitutive equations and energy accumulation and dissipation in rock or rock masses with lithology, pore structure characteristics and deformation properties [13][14][15][16][17][18][19][20]. Alternatively, laboratory or field tests and numerical simulation are more commonly adopted to analyze stress wave propagation in natural porous rock or rock masses [21][22][23][24][25][26][27][28][29][30][31][32]. Undoubtedly, the studies are valuable and helpful for better understanding the law of wave propagation and attenuation in rocks, deformation and failure mechanisms, and triggering mechanism of dynamic disasters.…”

“…In most cases, people can only observe the apparent mechanical properties of the "black box" under stress waves and seldom learn or quantitatively describe the change in pore structures and its mechanism. It is worth noting that in recent years, some researchers have tried to apply numerical modeling and experimental methods such as CT scanning to analyze the effect of pore structures or spatial characteristics on stress transfer and wave propagation in porous rock-like media, and have achieved preliminary results [16,21,[25][26][27][28][29]33]. However, it is still far away from unveiling the effect of pore structures in the "black box" subject to stress waves.…”

Computation of fractal dimension of rock pores based on gray CT images

“…Hence steady progress has been made in two areas, specifically the mechanism of evolving defects or cracks and the elucidation of damage constitutive relationships. Ju et al [20][21][22] investigated the geometric features and distribution properties of pores in rocks by means of CT scanning tests of sandstone and foam concrete, and then constructed a statistical model of rock pore structure that was used in numerical simulation by finite-element software. The results indicated the important role of CT technology for researching microscopic damage of rock materials.…”