Ls Dyna Version 971 Crack

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The plastic deformation on the CSCM concrete model indicates the strain localization according to different loading stages where failure propagates as shown in Figure 16. Also, the cracks are initiated and progressed until the ultimate load level is reached in Figure 17. Crack patterns are obtained by use of Winfrith concrete material option. The numerical analysis indicates crack growth initiating from the midspan at the bottom of the reinforced concrete beams and propagates upward the cross section as the applied load is reached to ultimate state.

During the blasting excavation of deep underground caverns, the effects of the structural surface on crack propagation are usually considered in addition to the clamping effects of high in situ stress. Based on the notched borehole and timing sequence control (TSC) fracture blasting method, this paper studies the effects of different borehole shapes on the degree of damage of the surrounding rock and profile flatness of the rock anchor beams and the effects of different filled joint characteristics on the blasting crack propagation rules. The results show that the damage depth of the surrounding rocks by round hole smooth blasting is approximately twice that by notched hole smooth blasting, by which the profile formed is flatter. The notched primary borehole (PBH) remains a strong guidance for crack propagation in a rock mass with filled joints, while the stress concentration effects of the round target borehole (TBH) cannot fully guide the cracks until they fall within a certain distance between the PBH and TBH. It is favourable for cracks to propagate along the lines between boreholes with larger filled joint strengths and larger angles between boreholes.

Previously, the main blasting circuit optimization method was to adjust the borehole distance and charging structure. Li18 proposed a TSC fracture blasting method, where adjacent boreholes were divided into PBHs and TBHs (Fig. 1). The TBH works as an empty hole before initiation and greatly guides the stress wave19,20, which can effectively control the crack propagation21. Liu22 used the empty hole as a swelling space and surface to study the effect of the distance of the hole to the crack propagation. Cho23 studied the guidance of empty holes on cracks via delayed borehole initiation in the model experiment. Yi24 studied the superimposed effects of stress waves under a delayed initiation of adjacent boreholes. Through the laboratory delayed borehole initiation test, Johansson25 analysed the mutual interaction form and stress distribution of stress waves between PBHs and TBHs and proposed the calculation method of the optimal delayed initiation. Khandelwal26 discussed the advantages of the delayed control blasting technology over the surrounding rock vibration reduction through tests.

Although the TSC fracture blasting method can improve the blasting excavation efficiency and cracking results, it is not favourable for round boreholes when attempting to form a good excavation profile in a rock mass with joints, while notched boreholes can greatly improve the profile forming results of a jointed rock mass27,28. This paper studies the blasting excavation of a rock mass with filled joints at the rock anchor beam in the deep underground caverns of the Baihetan Hydropower Station based on the TSC fracture blasting method for a PBH notch, to examine the propagation behaviour of cracks in a rock mass with filled joints.

The principle of rock mass damage depends on the property of rock mass as well as the practical force conditions. The pressure of rock mass, taking the Mises damage rule, forms the crushing area of rock mass blasting, while the cracks area is the result of the damage of tensile force. The damage rule of rock mass is as follows:

where, \(\sigma_{VM}\) is the von Mises effective stress of any point in rock mass; \(\sigma_{ij}\) (i, j = 1, 2, 3) is the stress components of rock mass. \(\sigma_{t}\) is the tensile stress of explosion load of any point in rock mass; \(\sigma_{cd}\), \(\sigma_{td}\) are known as uniaxial dynamic compressive strength and tensile strength of rock mass respectively.

The above research describes that the notched hole also well guides detonation cracks in rock masses with filled joints. Since it takes approximately twice as long to drill a notched hole than a round hole, the construction process of blast excavating the underground powerhouse will be affected. Based on the TSC fracture blasting excavation method proposed by Li48, only PBHs are notched in the rock mass model with filled joints; the distance between adjacent boreholes is taken as approximately 19 times the diameter of the boreholes, i.e., 800 mm; the delayed initiation time between PBH and TBH is taken as 1 ms. To compare and analyse the effects of the round target borehole with and without stress concentration effects on the crack propagation, the method in Fig. 10 is used to connect the smooth blasting borehole circuit.

Based on the in situ stress of the rock anchor beam of the underground powerhouse at the Baihetan Hydropower Station, the in situ stress is approximately taken as 20 MPa. The crack propagation through the results of rock mass models with different filled joint characteristics and directions are shown in Fig. 12.

With identical joint angle, when the filled joint strength is 0.5 MPa and 1 MPa, there are many secondary cracks near the TBHs. When the filled joint strength is 2 MPa and 3 MPa, the joint surface has a reduced reflecting force on the blasting stress wave, and the number of secondary cracks between boreholes is also reduced to reduce the degree of damage of the surrounding rocks and be favourable for forming a flatter excavation profile.

With identical joint strength, when the joint angle is 15° and 30°, severe damaged areas occur near the middle TBH. However, with an increasing joint angle, the reflection effects of the stress wave on the rock mass between adjacent joints gradually decrease, and the secondary cracks near the TBH are accordingly reduced to decrease the degree of damage of the surrounding rocks and help form a flat excavation profile between boreholes.

In the TSC fracture blasting excavation method, the TBH with obvious stress concentration effects caused by PHB notching more easily produces penetrating cracks along the line between boreholes. By controlling the number of adjacent TBHs, we can decrease the degree of damage of the surrounding rocks while improving the blasting excavation efficiency.

When the strength of the filled joints and angle of joints with a line between boreholes increase, it is more favourable for cracks to propagate along the line between boreholes during the jointed rock mass blasting excavation.

In this paper, the crack propagation law under different hole shapes and detonation timing sequences is mainly based on numerical simulations. But the mechanical properties of rock mass are more complicated in actual engineering. So more advanced test methods are needed to get more accurate mechanical properties of rock masses to verify the numerical simulation results, and optimize the new method proposed in this paper in the underground cavern blasting excavation process. 2b1af7f3a8