Breakage Characterization of Magnetite under Impact Loads and Cyclic Impact Loading

Investigation of the damage characteristics of rock material under the combined effect of cyclic load and impact load is extremely crucial for many mining engineering applications. Based on energy dissipation theory, we considered factors such as circulation times, the cyclic stress of a uniaxial cyclic load test, and the impact pressure (strain rate) of a split Hopkinson pressure bar (SHPB) test, studying the damage mechanism of red sandstone under the combined action of a uniaxial cyclic and impact loads. We found that when the load stress is 60%, 70%, and 80% of the uniaxial compressive strength (UCS) of red sandstone, the stress can still promote the development of microcracks and the generation of new cracks in the rock, increasing the inner damage of the rock so that it reduces the rock strength. As the cyclic time increases, the energy dissipation ratio presented a trend of decreasing dramatically and then maintaining basically no change, and the peak strain and the damage variable of the rock both increased first and then tended to remain relatively constant. The damage variable of the rock specimen under the combined action of the uniaxial cyclic loading and impact load had a significant corresponding relationship with the initial damage. As the rock specimen initial damage increases, the damage variable and the peak stress of the rock specimen both decreases almost linearly with initial damage. Meanwhile, the average particle diameter of the rock specimen after breakage gradually increased, showing a positively correlated tendency. The ratio between the fragment quality of the bigger particle diameter in the total rock specimen quality gradually increased. Under the conditions of the same initial damage, a higher impact pressure resulted in smaller rock fragment average particle diameters.

To investigate the dynamic mechanical properties of water-saturated fissure rock at different strain rates, prefabricated sandstone specimens with a 45° dip angle were treated with water saturation and the impact compression test was performed with a Split Hopkinson Pressure Bar (SHPB) test device at different impact pressures. The results show that the clusters of dynamic stress–strain curves of water-saturated and natural sandstone specimens with a 45° dip angle of prefabricated fissures are basically similar under different impact air pressures. A distinct strain rate effect was observed for dynamic strain and dynamic compressive strength, both of which increased with increasing strain rate. From the failure pattern of the specimen, it can be seen that cracks appeared from the tip of the prefabricated fissure under axial stress, spreading to both ends and forming wing cracks and anti-wing cracks associated with shear cracks. As the strain rate increased, the energy dissipation density of the specimen gradually increased, and the macroscopic cracks cross-expanded with each other. The fracture form of the specimen showed a small block distribution, and the average particle size of the specimen gradually decreased. The specimen crushing energy dissipation density was negatively correlated with fracture size, reflecting a certain rate correlation. The sandstone fragments’ fractal dimension increases with the increase in crushing energy dissipation density, and the fractal dimension may be applied as a quantitative index to characterize sandstone crushing.

This study systematically investigates the failure patterns, energy dissipation, and fracture behavior of rock specimens containing a vertical hole under impact loads. First, an improved damage calculation equation suitable for the analysis of rock specimens with a vertical hole is obtained based on the one‐dimensional stress wave theory and the interface continuity condition. After that, the Hopkinson pressure bar (SHPB) device was used to conduct cyclic impact tests with different impact pressures and impact modes (impact pressures with equal amplitude and unequal amplitude). The experimental results suggest that, under the equal‐amplitude high pressure and unequal‐amplitude pressure, the degree of damage of the rock significantly increased, the bearing capacity greatly reduced, and the rock gradually transitions from having good ductility to experiencing brittle failure. The cumulative specific energy absorption value gradually increases with the increase in the cyclic impact. Compared to that of the equal impact condition, the degree of damage to the rock is more severe for the case of equal‐amplitude high pressure and unequal impact, and the failure mode undergoes a transformation from transverse tensile failure to transverse tensile failure‐axial splitting failure combination and axial splitting failure. Through the analysis of rock energy changes and rock failure patterns during cyclic impact, it will be helpful to predict and control the fracture caused by local stress concentration during excavation, thus can reduce the cost of support and reinforcement in excavation and improve the stability of surrounding rocks.

In order to study the dynamic compression mechanical properties of engineering rock under high strain rate （100~102 S−1）loads, dynamic compression tests of three common engineering rocks (marble, sandstone and granite) taken from the Qinling Mountain are studied subjected to five different kinds of shock air pressure using Φ 100 mm split Hopkinson pressure bar test system improved with purple copper waveform shaper. The dynamic compression stress-strain curves, dynamic compressive strength, peak strain, energy absorption rate and elastic modulus of three rocks variation with strain rate are researched. The dynamic compression failure modes under different strain rates are analyzed. Then the three-dimensional numerical simulations of waveform shaper shaping effects and stress wave propagation in the SHPB tests are carried out to reproduce the test results. The research results show that the dynamic compression stress-strain curves show certain discreteness, and there is an obvious rebound phenomenon after the peak. With the increase in strain rate, the dynamic compressive strength, peak strain and energy absorption rate are all in a certain degree of increase, but the elastic modulus have no obvious change trend. Under the same strain rate, the dynamic compressive strength of granite is greatest while of sandstone is least. With the increase in strain rate, the margin of increase in peak strain and energy absorption rate of granite is greatest while of sandstone is least. The failure modes of the sample experience a developing process from outside to inside with the increase of strain rate.

Mechanical behavior and constitutive model of frozen soil subjected to cyclic impact loading

To investigate the failure characteristics and high-strain-rate mechanical response of polyvinyl alcohol-modified alkali-activated materials (PAAMs) under static and dynamic impact loads, quasi-static and uniaxial impact compression tests were performed on AAMs with varying PVA content. These tests employed a universal testing machine and an 80 mm diameter split Hopkinson pressure bar (SHPB). Digital image correlation (DIC) was then utilized to study the surface strain field of the composite material, and the crack propagation process during sample failure was analyzed. The experimental results demonstrate that the compressive strength of AAMs diminishes with higher PVA content, while the flexural strength initially increases before decreasing. It is suggested that the optimal PVA content should not exceed 5%. When the strain rate varies from 25.22 to 130.08 s−1, the dynamic compressive strength, dissipated energy, and dynamic compressive increase factor (DCIF) of the samples all exhibit significant strain rate effects. Furthermore, the logarithmic function model effectively fits the dynamic strength evolution pattern of AAMs. DIC observations reveal that, under high strain rates, the crack mode of the samples gradually transitions from tensile failure to a combined tensile–shear multi-crack pattern. Furthermore, the crack propagation rate rises as the strain rate increases, which demonstrates the toughening effect of PVA on AAMs.

Dynamic mechanical response and fracture mechanism of granite with structural plane containing planar and circular filling simultaneously: The influence of extension length

To investigate freeze–thaw cycle effects on the energy dissipation and strength characteristics of cement soils under impact loading, impact compression tests were carried out using a split Hopkinson pressure bar on cement soils under various freeze–thaw cycles (0, 1, 3, 6 and 10 times). The Zhu–Wang–Tang (ZWT) model was modified to predict the relationship between deformation and strength in cement soils under various test conditions. The obtained test results revealed that the freeze–thaw cycle number and impact pressure had significant effects on the fractal dimension, strength and absorbed energy of cement soils and there existed a critical freeze–thaw cycle number. It was found that the increase of the freeze–thaw cycle number gradually decreased strength and absorbed energy and increased the fractal dimension. When freeze–thaw cycle number was between 0–6, strength, fractal dimension and absorbed energy were significantly changed. For freeze–thaw cycle numbers greater than 6, the effects of the above factors were gradually alleviated. A modified constitutive model was able to accurately describe cement soil mechanical responses under high strain rate conditions, and the relative error between the predicted and experimental results was in the range of ±7%.

Experimental study on dynamic compression behavior characteristics and stress wave propagation law of rigid-flexible combinations under cyclic bi-directional impact loading

During deep unconventional resource extraction, high temperatures and dynamic loads exert a substantial impact on the stability of geotechnical structures. In this study, an improved Split Hopkinson Pressure Bar (SHPB) system was employed to conduct dynamic impact tests on high-temperature-treated red sandstone specimens at different impact velocities. A systematic analysis was carried out to investigate the effects of high temperature and impact velocity on the changes in dynamic mechanical properties and failure modes of the sandstone. Based on sieve test results, the size distribution of fragmented particles was quantified using fractal dimension analysis. Furthermore, by integrating thermal gravimetric analysis (TGA) and scanning electron microscopy (SEM), a clear correlation between the thermal damage evolution of sandstone and its microstructural degradation was established, thus providing novel insight into the dynamic failure mechanism. The results indicate that the dynamic mechanical properties of red sandstone show a significant temperature-dependent effect, following a decreasing trend described by a quadratic function. An increase in temperature significantly decreases the difference in dynamic strength of rocks under varying impact velocities. The particle size distribution of sandstone fragments displays statistical self-similarity, remaining unaffected by changes in temperature or loading velocity. At high loading rates, strain rate effects lead to a reduced influence of temperature on the fractal dimension. These findings offer valuable insights for the optimization of wellbore stability and enhancement of rock fragmentation efficiency.

UR50 ultra-early-strength cement-based self-compacting high-strength material is a special cement-based material. Compared with traditional high-strength concrete, its ultra-high strength, ultra-high toughness, ultra-impact resistance, and ultra-high durability have received great attention in the field of protection engineering, but the dynamic mechanical properties of impact compression at high strain rates are not well known, and the dynamic compressive properties of materials are the basis for related numerical simulation studies. In order to study its dynamic compressive mechanical properties, three sets of specimens with a size of Φ100 × 50 mm were designed and produced, and a large-diameter split Hopkinson pressure bar (SHPB) with a diameter of 100 mm was used to carry out impact tests at different speeds. The specimens were mainly brittle failures. With the increase in impact speed, the failure mode of the specimens gradually transits from larger fragments to small fragments and a large amount of powder. The experimental results show that the ultra-early-strength cement-based material has a greater impact compression brittleness, and overall rupture occurs at low strain rates. Its dynamic compressive strength increases with the increase of strain rates and has an obvious strain rate strengthening effect. According to the test results, the relationship curve between the dynamic enhancement factor and the strain rate is fitted. As the impact speed increases, the peak stress rises, the energy absorption density increases, and its growth rate accelerates. Afterward, based on the stress–strain curve, the damage variables under different strain rates were fitted, and the results show that the increase of strain rate has a hindering effect on the increase of damage variables and the increase rate.

Research on confining pressure effect of pore structure of coal-rich in coalbed methane under cyclic impact

Static and dynamic uniaxial compressive strength of shale rock is studied in the present study. Shale rock is exposed to explosive loading/ impact loading during hydraulic fracturing. Which necessitate that strength of the shale rock under impact loading should be studied before performing hydraulic fracturing. For this purpose, outcrop shale rock from Khyber Pakhtunkhwa, Pakistan has been investigated under high strain-rates of order of 10 2 s -1 , using Split Hopkinson Pressure Bar (SHPB). Two specimens were cored out from a single sample of shale rock for dynamic testing and static testing. Static compressive strength 11.84 MPa, was investigated using Universal Testing Machine (UTM). Average dynamic compressive strength 38.83 MPa was observed during testing. The compressive strength increased 3.27 times that of the static compressive strength.

To investigate the effect of fracture distribution on the mechanical responses and failure forms of coal–rock combined masses under impact loading, dynamic impact tests on coal–rock-like combined bodies containing pre-existing cracks were carried out using split Hopkinson pressure bar. The dynamic mechanical properties and the failure and fractal characteristics of specimens under different impact velocities were investigated. The obtained results showed that the strength of fractured combined bodies was influenced by the positions and angles of cracks. It was found that fractures in coal sections had great influence on specimen strength and the maximum influence was obtained for the angle of 30°. An obvious strain rate effect was also observed on the dynamic strength of RCB (combined body with fractures in rock section) specimens. Fractal dimensions of the coal sections of cracked combined bodies were close to those of intact ones when at angles 60° and 90°, while they were smaller at angle 30°. The existence of cracks changed stress concentration in specimens under impact loading, changing their dynamic mechanics and failure characteristics, where fractures in coal sections with lower strength had a greater effect on the overall stability of combined bodies. Therefore, strengthening supporting work on the weak part of combined or layered rock masses has to be conducted in engineering practice and the failure of rock masses due to crack propagation in adjacent rock masses should be prevented.

Crack propagation and dynamic properties of coal under SHPB impact loading: Experimental investigation and numerical simulation