The experimental study of fatigue damage to coal under cyclic loading is important for guiding the design of pillars in underground coal mines where the pillars may be affected by repeated mining activity. In this paper, the strength, deformation, energy dissipation, and fatigue of samples of coal from a mine in China are studied using cyclic loading with a servo-controlled rock mechanical test system. The results indicate that coal is more likely to suffer fatigue damage than other, harder, rock lithologies. Under uniaxial cyclic loading, the fatigue failure “threshold value” for the coal samples studied is less than 78% of its uniaxial strength, but there is also a certain amount of fatigue damage when the cyclic loading/unloading experiments are carried out below the threshold value for fatigue failure. Axial deformation during the tests can be divided into three stages: initial deformation, constant steady deformation, and accelerated deformation. Transversal deformation can be divided into two stages: stable deformation and accelerated deformation. During cyclic loading experiments, imminent sample failure is signaled when transversal deformation increases significantly and quickly and the deformation recovers little when the load is removed. With an increasing number of loading/unloading cycles, a graph of energy dissipation per unit volume versus number of cycles presents an L-shaped curve when the coal samples do not suffer fatigue failure. However, for the coal samples that do rupture due to fatigue, the curve is U-shaped. Under cyclic loading, the evolution of compaction, strain hardening, strain softening, and failure of coal can be revealed in great detail by fatigue damage experiments. 相似文献
Tropical cyclone (TC) disasters have frequently caused casualties in the coastal areas of China. According to the statistics of dead and missing people due to TCs from 1951 to 2014, the number of fatalities has been significantly decreasing over time. However, deadly TC events have still caused great losses of life in recent years, which are characterized as significant abrupt fluctuations superimposed along the downward trend of the long-term fatality time series. The numbers of fatalities caused by TC disasters are influenced by variables such as the intensity of TC hazards, the population exposed to TCs and the vulnerability of people to TC hazards. It is thus of great significance to analyze their temporal characteristics and understand the forces driving these changes. First, the time series of the TC wind, precipitation, spatial distribution of population, fatality and disaster risk reduction (DRR) measures of China from 1951 to 2014 are reconstructed. Second, the improved power dissipation index, total precipitation, integrated intensity and index of exposed population are calculated, and the population vulnerability indices, including mean and relative fatality rates, are derived. Third, the change trend of each index is detected using the Mann–Kendall test. Finally, the main driving factors of the long-term change trend and fluctuations of the TC fatalities are analyzed by a negative binomial regression model and standard deviation statistics. It is found that the decrease in vulnerability based on the improvement in structural and non-structural measures is the main driving force of the decreases in fatalities over the past six decades. Although the total population and exposure have increased dramatically in the coastal areas of China, their contributions to the increase in the fatality risk were counteracted by the decrease in vulnerability. Abrupt and catastrophic disasters were mostly caused by TCs with hazards of high intensity that surpassed the capacity of structural measures; the lack of forecasting or early warning, as well as improper emergency response actions, may also have triggered the great loss of lives. To reduce the fatalities of future TCs, especially those that may exceed the capacity of structural measures, the enhancement of non-structural measures and the adaptation of resilience strategies should be priorities for future people-centered disaster management.
Natural clays usually show anisotropic stiffness due to their deposition process and anisotropic in situ stress state. The stiffness anisotropy depends on both of the stress anisotropy and fabric anisotropy, while the latter can be quantified by the stiffness anisotropy at isotropic stress states. This paper measures the K0 value (i.e., stress anisotropy) and elastic shear stiffness anisotropy of natural Shanghai clay in a triaxial apparatus with horizontal and vertical bender elements. The results show that the K0 value of Shanghai clay lies in the range of 0.40–0.66, and an empirical equation is proposed to estimate the K0 value based on the plasticity index and initial void ratio. The fabric anisotropy of natural Shanghai clay lies in the range of 1.2–1.4 with a stronger fabric in the horizontal plane. Moreover, the experimental data of the stiffness anisotropy and fabric anisotropy of different clays in the literature are reviewed and analyzed. It reveals that the stiffness anisotropy generally increases, while the fabric anisotropy remains nearly the same during K0 consolidation. For normally consolidated clay, the fabric anisotropy generally lies in the range of 1.1–1.7. For overconsolidated clays, the fabric anisotropy generally increases as the overconsolidation ratio increases. Empirical equations are proposed to approximately estimate the fabric anisotropy of clays based on its stress normalized elastic shear stiffness.
