Short Cantilever Rock Beam Structure and Mechanism of Gob-Side Entry Retaining Roof in Reuse PeriodRead the full article
Shock and Vibration publishes papers on all aspects of shock and vibration, especially in relation to civil, mechanical and aerospace engineering applications, as well as transport, materials and geoscience.
Chief Editor, Dr Thai, is based at the University of Melbourne and his current research focuses on high strength materials for sustainable construction of buildings, bridges and other infrastructure.
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Experimental Investigations on Electrical Charge of Precracked Rock Specimens under Uniaxial Compression
The electrical charge characteristic of rock materials under compression is an important index for predicting the development of rock fractures and the failure of engineering structures. However, the charge behaviours of a preexisting rock sample have not been studied in depth. In this study, sandstone samples with a single fabricated precrack at different angles of inclination are prepared. The uniaxial compression tests are performed to study the charge behaviours associated with the initiation and propagation of secondary cracks, the mechanical properties, and the progressive failure of stressed rock samples. An improved analytical model based on the maximum tensile stress failure theory for brittle materials is also proposed for determining the crack growth paths of the single precrack rock samples under uniaxial compression. The friction factors of crack surfaces are computed. The results show that the step functions on the curves of charge accumulation over time correspond to the fluctuation of stress, indicating the initiation of microcracks. The sample with a crack inclination angle of shows the largest amount of both the first charge and the total accumulation. The analytical model shows a positive relationship between the crack face friction factors and the charge accumulation. The analytical solution of the crack development angles shows good agreement with the experimental results. This work may provide reference for the similar studies regarding the correlation of charging behaviours to the compressed rock materials.
Mechanical Model of the Initial Leg Pressure Increment of the Shield and Its Application to Monitor Roof Load in Longwall Panels
The shield pressure cannot always be used to represent the upper load of longwall panels, since its value is steady or even decreases by the yielding action. However, the leg pressure increment of the shield (LPIS) at the initial stage is not influenced by yielding and could therefore be an important factor to judge the state of overlying loads. In this study, a mechanical model is established to analyze the relationship between the overlying loads of the main roof and LPIS after cutting. There is a linear positive correlation between leg pressure increment and overlying loads and a second-order relationship between leg pressure increment and length of main roof cantilever in the proposed model. Therefore, it can be used to determine the magnitude of roof weighting strength in different periods as well as the length of the main roof cantilever in a period. Finally, the mine pressure difference between the period of fully mechanized mining and the period of fully mechanized caving mining in the MinDong-1 coal mine serves to verify the rationality of the proposed model.
Numerical Simulation of Impact Loads of Main Fan Blades in Gas Explosion
The shock wave generated by a severe gas explosion accident can damage the main fan, and the toxic and harmful gases in the well cannot be discharged in time, leading to the expansion of disaster accidents. Therefore, it is meaningful to study impact loads of main fan blades in gas explosion. In this paper, a full-scale three-dimensional numerical simulation model has been established based on No. 2 Yangchangwan Coal Mine in China. The propagation of shock wave in shaft and air tunnel and the dynamic process of main fan blade subjected to shock load when gas explosion of different volume occurs in heading face have been simulated. The overpressure on the blade at different times, the overpressure distribution on the blade, and the relationship between the overpressure and the explosion intensity have been obtained. The results showed that the time when the explosion shock wave reached, each blade of the wind turbine was basically the same, and the time when each blade reached, the maximum overpressure was basically the same. With the increase of gas explosion volume, occurrence time of overpressure and maximum overpressure time on the fan blade were shortened, and the time interval between them was also shortened. There is a little difference in the overpressure of each blade. Fan blade directly above the hub was subject to the highest overpressure, and fan blade directly below the hub was subject to the lowest overpressure. The overpressure of the maximum overpressure blade was 5.44% to 6.77% higher than that of the minimum overpressure blade. The distribution of overpressure on each fan blade was uneven, and the overpressures on blade edges were the lowest. The overpressure on the fan blades showed a corrugated distribution along the radial direction. There was 12.06% to 15.40% difference between the maximum and minimum overpressure section on the fan blade.
Optimal Semiactive Damping Control for a Nonlinear Energy Sink Used to Stabilize Milling
Improving product quality of machining components has always met with problems due to the vibration of the milling machine’s spindle, which can be reduced by adding a vibration absorber. The tuned vibration absorber (TVA) has been studied extensively and found to have a narrow bandwidth, but the cutting force possesses wide bandwidth in the process of machining parts. Introducing nonlinearity into the dynamic vibration absorber can effectively increase the bandwidth of vibration suppression and can significantly improve the robustness of the vibration absorber. In addition, a semiactive TVA has proved to be more effective than a passive TVA for many applications, so the main purpose of this study is to find some appropriate semiactive control methods for a nonlinear energy sink (NES), a nonlinear vibration absorber, in structural vibration applications. Two semiactive control methods are considered in this study: continuous groundhook damping control based on velocity and on-off groundhook damping control based on velocity. To fairly compare these vibration absorbers, the optimal parameters of a passive TVA, a passive NES, and two semiactive NESs are designed using numerical optimization techniques to minimize the root-mean-square acceleration. Two cutting forces are introduced in this study, a periodic force and an aperiodic force, and the four vibration absorbers are compared. When the primary structure is excited with aperiodic cutting force, the amplitude of the primary structure decreased by 17.73% with the passive TVA, by 72.29% with the passive NES, by 73.54% with the on-off NES, and by 87.54% with the continuous NES. When the primary structure is excited with periodic cutting force, the amplitude of the primary structure decreased by 49.01% with a passive TVA, by 86.93% with a passive NES, by 96.38% with an on-off NES, and by 99.23% with a continuous NES. The results show that the passive NES is better than the passive TVA; the semiactive NES provides more effective vibration attenuation than the passive NES, and the continuous control is more effective than the on-off control.
Simulation and Optimization of Acoustic Package of Dash Panel Based on SEA
The instrument panel assembly is an important structure to shield powertrain noise and front wheel noise, and the instrument panel sound insulation system plays a key role in it. Therefore, the sound insulation characteristics of the instrument panel sound insulation components appear particularly important. In the existing technology, the acoustic design of instrument panel mostly adopts the reverse design method and pays little attention to the forward design of acoustic system, which tends to lead to the shortcomings such as inaccurate acoustic design and poor acoustic design quality. The reverse design also restricts the development cycle, so the control and design of acoustic performance cannot be realized in the initial stage of design. Based on the above problems, the statistical energy model of the dashboard model was established by combining the statistical energy flow method and the proxy model method, and the influences of different acoustic cladding layers, different thickness, leakage, and new material microfibers on the sound insulation parameters of the front coaming were simulated. The general rules that affect the sound insulation performance of the structure are obtained. On this basis, multiobjective genetic algorithm and proxy model method are used to optimize the insertion loss of the front panel acoustic pack and the weight of the dashboard by introducing multiobjective variable and experimental design method, so as to obtain the best solution to meet the requirements of insertion loss and lightweight acoustic pack of the dashboard. It is of great engineering significance for the development of acoustic components for forward design instrument panel.
Analysis of the Characteristics and Influencing Factors of Gas Explosion in Heading Face
In order to accurately grasp the characteristics and influencing factors of gas explosion in heading face, the mathematical model of gas explosion was determined. According to the actual size of a heading face of a coal mine, a 3D geometric model with a length of 100 m was established, and the effects of ignition energy and gas explosion equivalent on the gas explosion characteristics of the heading face were analyzed. The results show the following. (1) The mathematical models for numerical simulation of gas explosion can accurately simulate the gas explosion and its propagation process. The time-space step size has a great influence on the simulation results. The grid spacing for numerical simulation of mine gas explosion is determined to be 0.1 m and the time step length is determined to be 0.001 s. (2) The ignition energy has a limited effect on gas explosion characteristics. It only has a certain influence on the gas explosion process, but has little influence on the overpressure of shock wave. The larger the ignition energy is, the faster the explosion reaction speed is, and the maximum overpressure increases slightly. When the ignition energy increases to a certain value, the time of peak shock wave and the maximum overpressure both tend to be stable. The ignition energy has little effect on gas explosion characteristics when an explosion accident occurs underground with a large amount of gas accumulation. (3) The gas explosion equivalent has a great influence on the overpressure of gas explosion shock wave. The higher the explosion equivalent is, the greater the pressure is, and the peak value of the shock wave overpressure increases with the explosion equivalent as a power function. The research results have important guiding significance for the research and development of new technology for prevention and control of gas explosion.