Experimental Study on Stress Uniformity and Deformation Behavior of Coals with Different Length-to-Diameter Ratios under Dynamic Compression

In this study, a uniaxial impact compression test was performed on coal samples with length-to-diameter L / D ratios of 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1 using a Φ 50 mm split Hopkinson pressure bar (SHPB) test system. This study researched the stress uniformity and deformation behavior of coal samples with different L / D ratios during dynamic compression, defined the stress equilibrium coefficient ξ , proposed a new method for determining whether a sample meets the stress uniformity hypothesis, and obtained the critical L / D ratio of 0.6 and the optimal L / D ratio of 0.3 or 0.4 for coal samples to obtain the stress equilibrium. The experimental results showed that the dynamic stress-strain curve of coal had an elastic stage, a plastic stage, and a failure stage. As the L / D ratio increased, the proportion of the elastic stage to the prepeak curve of the samples declined progressively; with an increase in the L / D ratio, the peak part of the curve also changed from “sharp” to “stagnated,” while an increase in the plasticity led to strain softening. As the L / D ratio of the samples increased, the average strain rate decreased approximately as a power function, and the decreasing trend was gradually reduced from 296.49 s−1 ( L / D =0.3) to 102.85 s−1 ( L / D =1), with a reduction of approximately 65.31%. With an increase in the L / D ratio, the peak strain gradually decreased exponentially. This study concluded that the SHPB test protocol design is of a certain reference value for low-density, low-strength, heterogeneous brittle materials, such as coal.

Influence of Specimen Thickness on the Acquisition of Al6061-T6 Material Properties Using SHPB and Verified by FEM

The Split-Hopkinson pressure bar (SHPB), which is used for acquiring material properties at high strain rates (102–104 s−1), requires proper specimen size selection. Under the same applied pressure, an increased S-S curve is obtained as the thickness of the specimen decreases. In this study, 1.5 t, 2.0 t, 3.0 t, 5.0 t, and 7.0 t specimens of Al6061-T6 material were tested under 1.0 bar to understand the influence of specimen thickness on the acquisition of material properties. To grasp the behavior of the SHPB test in real time, Finite Element Method (FEM) was performed using the LS-DYNA program. During the SHPB test, the impedance is increased due to the variation in the specimen area. Because of the influence of impedance, the transmitted pulse increases, and the reflected pulse decreases. As a result, the specimen is deformed in the high-strain rate region, and the S-S curve is increased as the thickness decreases. In addition, by performing the test under different pressure conditions that created similar strain rate regions, the material properties remained constant with thickness variations.

The dynamic characteristics of backfill made of mining waste at one-dimensional dynamic and static loading

Backfill is widely used in underground mines around the world for its effective reduction in environmental impact of mining operations by utilizing a part of mine waste as underground backfill material. The strength of backfill plays a critical role in improving stop stability and preventing surface subsidence. In this paper, a series of SHPB (Split Hopkinson Pressure Bar) tests with different strain rates and static axial pressures are conducted. The results show that: (1) The dynamic strength of the backfill specimen increases first and then decreases with the increase of static axial pressure. It reaches a maximum when the static axial pressure reaches 30% of the static compressive strength in the SHPB test. (2) The stress-strain curves of backfill specimens can be divided into three stages: elastic stage, yield stage and failure stage. The compaction stage is obscure. The backfill specimens are not sensitive at low strain rate. (3)With the increase of incident energy, the absorbed energy mounts. (4) The failure mode of the backfill specimen is tensile failure when static axial pressure is 0MPa in the SHPB test while it becomes compression shear failure when static axial pressure is higher than 0MPa. (5) The backfill specimen is very compressed when it is loaded with axial stress and confining stress simultaneously. This compression property of backfill specimen may be related to the nature of hydration products at different curing times, which requires further research in the future.

Experimental Study on the Dynamic Mechanical Properties of Large-Diameter Mortar and Concrete Subjected to Cyclic Impact

Rock bursts are typically accompanied by multiple shocks. In order to explore the dynamic characteristics of filling materials in rock burst roadways, we employ the split Hopkinson pressure bar (SHPB) test to analyze the dynamic mechanical response of mortar and concrete under cyclical impact. The SHPB test results of the large-size specimen indicate the improvement in the waveform shape and the reduction in the wave dispersion via the application of the rubber sheet as the pulse shaper. Under cyclic impact, the peak stress and energy utilization ratio of mortar and concrete specimens were reduced, demonstrating obvious fatigue characteristics. The mortar peak stress and energy utilization ratio were observed to be sensitive to the impact times, while those of concrete were sensitive to impact pressure. The damage evolution of mortar and concrete exhibited very similar trends under the cyclic impact load, whereby the impact pressure had minimal effect on the damage evolution.

Analysis of Dynamic Damage-Induced Porosity Changes of Granites in Leaching Mining Technique Based on SHPB Test

In situ leaching was a very important technical approach in ion-adsorption rare earth mining, because it can effectively avoid the geological hazards, protect the environment, and reduce the mining costs. The critical issue of this mining technique was to increase the permeability of ion-adsorption rare earth deposits. Due to the close relationship between the permeability and the porosity, in this paper, several experiments were conducted to find the dynamic characteristics of the granite deposit and the relationship between the dynamic characteristics and the porosity. Moreover, the SHPB test system was equipped to conduct the dynamic test, and the ultrasonic wave detector with high precision was employed to obtain the damage factor of granites. The test results showed that the failure mode under dynamic load and static load was close, and they both had splitting failure. Besides, when cyclic dynamic loading velocity was between 5.8 m/s and 8.4 m/s, the specimen was not a failure, but it caused the damage and changed the porosity. And the dynamic thresholds of failure stress and damage stress were found. Finally, a linear relationship between the porosity and the damage factor was found, which would help to analyze and predict the change of porosity under different dynamic loading velocities.

Physics and Dynamics Characteristics and Energy Analysis of Freeze-Thaw Limestone

In order to study the physical and dynamic properties of rock after damage, an open-type saturated water freeze-thaw test at ±20°C was carried out on the limestone specimen, the size, quality, and longitudinal wave velocity with measured after freeze-thaw cycles for 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 times, and the SHPB test device was used to carry out the impact compression test with eight kinds of loading rate. This text analyzes the damage evolution characteristics on the physical properties of limestone of cycle times of freeze-thaw and discusses the dynamic compression mechanical characteristics and energy dissipation law of limestone specimens after freeze-thaw cycles. The test results show that the mass and longitudinal wave velocity of the specimen decreased and the volume and density increased. The damage factors have the quadratic function positive correlation with the cycle time of freeze-thaw. Moreover, the dynamic compression stress-strain curves of the specimens under different loading rates are similar in shape, and the curve shows an upward trend with increasing loading speed. In addition, with the loading rate increasing, the dynamic compressive strength and dynamic elastic modulus of the specimen increased and the dynamic strain decreased. In the SHPB test, the reflected energy, transmitted energy, and absorbed energy all increased linearly with incident energy. The dynamic compressive strength and absorbed energy increase as a power function, and the strain rate and absorbed energy increase as a quadratic function.