To reveal the dynamic mechanical response and energy dissipation behavior of rockburst-prone coal samples under impact loading, the compressive experiments on Xinzhouyao coals (prone) and Machang coals (nonprone) under different impact loadings were carried out using the Split Hopkinson Pressure Bar (SHPB). The dynamic mechanical properties were studied, including dynamic elastic modulus, strain rate, peak stress, peak strain, dynamic increment factor, and energy dissipation. The results show that the dynamic elastic modulus, peak stress, and peak strain of both prone and nonprone coals perform an obvious correlation with the increase of strain rate. The strain rate strengthening effect on the dynamic elastic modulus and compressive strength of rockburst-prone coal samples are more significant, reflected by the greater increment with the increase of strain rate, while the dynamic increment factors of both prone and nonprone coals show apparent strain rate strengthening. The incident, reflected, and transmitted energy of both two coals linearly increases with the impact velocity, although the increased rate may be different. The dissipated energy of rockburst-prone coal samples increases faster, while the rate of the increase of the dissipated energy is more stable with strain rate. The results may provide an important reference for revealing the failure law of engineering-scaled coal mass suffered by rockburst.
Coal and rock mass buried in deep is subjected to a complex geological environment with the depletion of shallow resources and the increase of coal mining depth [
Kolsky first proposed the Split Hopkinson Pressure Bar (SHPB) system in 1949 to investigate the dynamic responses [
The above research has an important role in promoting the study of mechanical properties of coal and rock under impact loading. Extensive suites of dynamic SHPB tests have been carried on rock and common coal materials, but only a few such dynamic tests have been completed on rockburst-prone coals. Most of these experiments only obtained the dynamic stress-strain curve. Here, we supplement this dearth of observations by recovering a full suite of dynamic failure characteristics and energy dissipation laws to contrast the response of rockburst-prone coals (Xinzhouyao coals) against a control sample of rockburst resistant coals (Machang coals) by using the Split Hopkinson Pressure Bar (SHPB) experimental system to reveal the mechanism of coal mine dynamic disasters induced by impact load.
One group of coals with rockbust proneness was taken from Xinzhouyao Coal Mine, Shanxi province. For comparison, another group of coals with nonproneness were collected from Machang Coal Mine, Guizhou province. The collected coal blocks are drilled and polished in the Mining Engineering Laboratory of Liaoning Technology University. According to the recommendation of the International Society of Rock Mechanics (ISRM), the sample size is
Static mechanical properties of coal samples.
Sampling location | Uniaxial compressive strength (MPa) | Uniaxial tensile strength (MPa) | Elastic modulus (GPa) | Poisson’s ratio | Cohesion (MPa) | Friction angle (°) | Remarks |
---|---|---|---|---|---|---|---|
Xinzhouyao coal | 30.19 | 1.34 | 3.66 | 0.20 | 3.67 | 29.89 | Rockburst-prone |
Machang coal | 17.73 | 0.70 | 0.85 | 0.15 | 0.51 | 37.55 | Rockburst nonprone |
The SHPB system is composed of bullet, bullet velocimetry, incident bar, transmission bar, buffer bar, and damper, shown in Figure
Photograph of the used Split Hopkinson Pressure Bar (SHPB) system.
In the experiment, high pressure nitrogen is used to provide impact loading for the bullet. The bullet moving velocity is controlled by the pressure on the incident bar. When the elastic stress wave propagates to the interface between the specimen and the incident bar, the stress wave will reflect and transmit. At the same time, an electrical signal is formed. All electrical signals are collected by strain gauges at both ends of the incident and transmission bars and delivered to the data acquisition system.
Based on the one-dimensional stress wave theory, the collected strain signal is processed by the three-wave method to obtain the dynamic stress-strain relationship of coal samples [
Let the cross-sectional area of the two bars be the same bar, the relation between the incident, reflected, and transmitted waver induced strain can be obtained:
Substituting equation (
The SHPB experiments involve five kinds of energies, including bullet carried energy, incident energy, reflected energy, transmitted energy, and dissipated energy. The incident, reflected, and transmitted energy of the test system can be expressed as follows [
According to the mass conservation law, the energy dissipation in the failure process of coal sample (
The nitrogen pressure provided by this experiment is 0.200 MPa, 0.225 MPa, 0.250 MPa, 0.275 MPa, 0.300 MPa, and 0.325 MPa. Repeat three times of each nitrogen pressure to ensure that the strain rate of the two groups of coal samples is consistent in the experiment. The variation law of incident velocity under different nitrogen pressures is shown in Table
Variation of strain rate under different pressures.
Sampling location | Numbering | Average length (mm) | Diameter (mm) | Average nitrogen pressure (MPa) | Average incident velocity (m/s) |
---|---|---|---|---|---|
X-1 | 50.00 | 50 | 0.200 | 6.31 | |
X-2 | 50.15 | 50 | 0.225 | 8.07 | |
X-3 | 50.20 | 50 | 0.250 | 8.71 | |
X-4 | 50.00 | 50 | 0.275 | 9.72 | |
X-5 | 49.85 | 50 | 0.300 | 10.81 | |
X-6 | 50.10 | 50 | 0.325 | 11.64 | |
Y-1 | 50.12 | 50 | 0.200 | 6.39 | |
Y-2 | 51.05 | 50 | 0.225 | 7.97 | |
Y-3 | 50.10 | 50 | 0.250 | 8.77 | |
Y-4 | 50.13 | 50 | 0.275 | 9.75 | |
Y-5 | 49.60 | 50 | 0.300 | 10.87 | |
Y-6 | 50.00 | 50 | 0.325 | 11.70 |
In order to ensure the stable transmission of the transmitted wave, the vaseline reagent was evenly smeared on both ends of the coal sample to keep it fully lubricated and coupled with the two bars, and the transverse strain caused by the difference in Poisson’s ratio between the bar and the coal sample was eliminated. The coal sample is placed between the incident bar and the transmission bar, as shown in Figure
Picture of the placed coal sample and pressure bars.
Figure
Relationship between impact velocity and strain rate.
Because the stress-strain curves of coal samples show good similarity under the same strain rate, a typical stress-strain curve is selected for each strain rate in order to compare the stress-strain curves of coal samples at different strain rates more intuitively as shown in Figure
Dynamic stress-strain curve of coal samples under different strain rates. (a) Rockburst-prone coal samples. (b) Nonprone coal samples.
Static stress-strain curve of coal sample.
In order to reflect the difference of each coal sample and the change rule of the elastic modulus of the coal sample with the strain rate, the elastic modulus obtained from three repeated experiments at each strain rate is retained in the image, and the fitting curve of the average elastic modulus and the strain rate is made, as shown in Figure
Relationship between dynamic elastic modulus and strain rate. (a) Rockburst-prone coal samples. (b) Nonprone coal samples.
The dynamic elastic modulus of the two groups of coal samples increases with the strain rate, and the correlation between them is significant. The dynamic elastic modulus of rockburst-prone coal samples increases rapidly from 7.42 GPa to 27.56 GPa, with the strain rate from 87.76 s−1 to 116.83 s−1, increased by 3.71 times, while the static elastic modulus of coal samples is 3.66 GPa, increased by 7.53 times. The dynamic elastic modulus of the nonprone coal samples increases from 1.14 GPa to 2.34 GPa, increased by 2.05 times, and the static elastic modulus is 0.85 MPa with 2.75 times of increment.
The relation between dynamic elastic modulus and strain rate of two groups of coal samples is fitted as follows:
The relationship between strain rates and peak stress is shown in Figure
Relation between peak stress and strain rate. (a) Rockburst-prone coal samples. (b) Nonprone coal samples.
The relationship between peak strain and strain rate is shown in Figure
Variation of peak strain with strain rate. (a) Rockburst-prone coal samples. (b) Nonprone coal samples.
Dynamic increment factor (DIF) is the ratio of dynamic compressive strength to static compressive strength [
The curves of dynamic increment factors of two groups of coal samples at different strain rates are shown in Figure
Relationship between dynamic increment factor and strain rate of coal samples.
The variation of stress wave energy of coal samples with impact velocity is shown in Figure
Relationship between stress wave energy and impact velocity. (a) Rockburst-prone coal samples. (b) Nonprone coal samples.
The curves of dissipated energy versus strain rate of both rockburst-prone and nonprone coal samples are shown in Figure
Variation of dissipated energy of coal samples with strain rate. (a) Rockburst-prone coal samples. (b) Nonprone coal samples.
The strain rate of both two groups of coal samples increases significantly with the increase of impact velocity, but there is a turning point in the strain rate change of the nonprone coal sample. The static stress-strain relationship and the dynamic stress-strain relationship show obvious differences. The dynamic stress-strain curve can be divided into four stages: the initial nonlinear compaction stage, the linear elastic stage, the microfracture evolution stage, the plastic fracturing stage, and the rapid unloading stage.
The dynamic elastic modulus of coal samples increases with the increase of strain rate. However, the dynamic elastic modulus and strain rate strengthening characteristics of rockburst-prone coal samples are more significant. The peak stress and peak strain of both rockburst-prone and nonprone coal samples show an obvious increase effect with strain rate under impact loading.
The dynamic increment factor (DIF) of all coal samples presents a significant strain rate effect. At a low strain rate, the rate correlation between rockburst-prone and nonprone coal samples is similar. However, with the increase of strain rate, the strain rate strengthening of rockburst-prone coal is more significant.
With the increase of impact velocity, the incident, reflected, and transmitted energy of both rockburst-prone and nonprone coals increase with varying degrees. The energy of the rockburst-prone coal sample is more likely to be concentrated than that of nonprone coal sample, and the dissipated energy is greater with obvious strain rate correlation.
The experimental data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
This work was financially supported by the State Key Laboratory of Mining Disaster Prevention and Control, co-founded by Shandong Province and the Ministry of Science and Technology (MDPC201806); the National Natural Science Foundation of China (51874159 and 52074146).