5.1 Comparison of simulation results
The coal sample parameters have been obtained by uniaxial compression, and the parameters are imported into the model that established by FLAC3D, and the numerical simulation is started. According to the results of numerical simulation under different stress, the axial strain value of monitoring point A is derived. The axial creep curve of monitoring point A is plotted, and the numerical simulation curve under different stresses is compared with the creep experimental curve, as shown in fig6.
From the fig6, it can be seen that the experimental results are extremely similar to the numerical simulation results, whether it is the axial strain values and the trend of the axial creep curve. This also proved the rightness of the established model, and laid the foundation for the next step that analyzing the effect of different parameters on coal creep. From the stage of creep, the deformation appears at the moment of stress loading, and after the decay creep and isometric creep into the stable creep stage. However, analyzing the overall creep curve, the difference between the experimental and simulated curves appears in the initial creep stage, which is especially obvious when the stress is small, which will be discussed in the following.
5.2 Comparison of transverse creep at different locations
After the previous paper verified the rightness of the model built based on Fish language to simulate creep in FLAC3D software, the following section analyzes the effect that different parameters on the creep characteristics of coal samples through numerical simulations.
At the third level of stress , the transverse strain values are derived for point B and point C. B is located at the top of the model ,and C is located at the bottom of the model. The transverse creep curves of monitoring points B and C are plotted, and the transverse creep comparison is shown in Fig7(a).
As can be seen from Fig7(a), at different locations of the specimen in the vertical direction, they have different strains, and the top transverse strains of the specimen greater than the bottom. The instantaneous strain values at the top is 211, and the bottom is 194, the former was 1.08 times of the latter. The creep values from the top to the bottom of the specimen showed a decreasing trend, which reflected a transfer process that stress from the top to the bottom. So, in terms of transverse creep, the top was more affected by the stress and more prone to deformation.
5.3 influence of different bulk modulus on creep
According to the common rock mechanical parameters, the numerical simulations were set at the conditions of bulk modulus of 0.25 Gpa, 0.35 Gpa and 0.45 Gpa. A third level of stress is applied to the model, andthe axial displacement at point A (0, 0, 0) is monitored, and the creep simulation curves are shown in Fig7(b).
From Fig7(b), it can be seen that when the parameter of bulk modulus changes, the instantaneous strain and creep of the coal sample also change. When bulk modulus is 0.25Gpa, the instantaneous strain values is -56; when bulk modulus is 0.35, the instantaneous strain values is -72; when bulk modulus is 0.45Gpa, the instantaneous strain values is -101. It can be seen that the bulk modulus has a greater influence on the creep of coal samples, and the creep of coal samples decreases with the increase of bulk modulus. But , the coal samples with different bulk modulus have the same time from deceleration creep to steady creep, and their creep simulation curves have the same time to fluctuate. However, whether there is a linear relationship between the change of bulk modulus and creep of coal samples needs to be further investigated.
5.4 Influence of different shapes on creep
Set rectangular coal sample model with X=50mm,Y=100mm,Z=50mm , and all other data are the same as the cylindrical model established above. The third level of stress is applied to the model, and the displacement in the axial direction at point A (0, 0, 0) is still monitored. The creep is compared with that of the cylindrical coal sample, and the creep simulation curve is shown in Fig7(c) below.
From Fig.7(c), it can be seen that the instantaneous creep value of the cylindrical coal sample is -685; the instantaneous creep value of the rectangular coal sample is -423. The creep value of the cylindrical coal sample is much larger than the rectangular coal sample, and the former is 1.62 times of the latter, which indicates that the rectangular coal sample is more stable and can bear greater stress under the same conditions.
5.5 influence of specimen height on creep
To study the influence of specimen height on creep, the cylindrical specimens with heights of 95 mm, 100 mm, 105 mm and 110 mm were simulated numerically, applied the third level of stress and the displacement in the axial direction was monitored at point A (0, 0, 0). The creep simulation curves are shown in Fig7(d).
From Fig7(d), it can be seen that the axial strains produced by specimens of different heights are not the same under the same stress. When the height increases, the instantaneous creep value at point A increases accordingly. When the height is 90mm, the instantaneous creep value at point A is -666, when the height is 100mm, the instantaneous creep value at point A is -686, the latter is 1.03 times of the former; when the height is 100mm, the instantaneous creep value at point A is -686, when the height is 105mm, the instantaneous creep value at point A is -722, the latter is 1.05 times of the former; when the height is 105mm, the instantaneous creep value of point A is -722, and when the height is 110mm, the instantaneous creep value of point A is -842, the latter is 1.16 times of the former. The above three ratios are getting bigger, so the creep ratio of the same height difference also increases when the height of the model increases continuously. It can be seen that the change of the model height has a significant influence on the creep, and the creep increases with the increase of the model height, and the degree of creep is also increasing.