Design of Pressure Energy-Absorbing FRP Anchors and Numerical Analysis of Mechanical Properties

Abstract

Conventional FRP anchor rods have low elongation and poor impact resistance, both of which do not meet the support requirements of rock burst roadways. Therefore, a pressure energy-absorbing FRP anchor rod composed of an FRP rod body, tray, energy-absorbing sleeve and round table nut was designed. Numerical simulations were carried out to study the mechanical properties of the FRP anchor rod in static tension and impact tension, and to compare its mechanical properties with those of conventional FRP anchor rods. The results show that the pressure energy-absorbing FRP anchor rod is stretched in four stages: the front-elastic stage, constant resistance to compression, the back-elastic stage and damage, with an additional constant resistance to compression stage compared with conventional FRP anchors. The elongation, energy absorption and impact resistance time of the pressure energy-absorbing FRP anchor rods are greater than those of conventional FRP anchor rods, and the mechanical properties of the pressure energy-absorbing FRP anchor rods are better than those of conventional FRP anchor rods. As the impact velocity increases, the energy absorption rate of the pressure energy-absorbing FRP anchor increases non-linearly. The impact energy and impact velocity have less influence on the breaking load, elongation and energy absorption of pressure energy-absorbing FRP anchor rods. The research results can provide a theoretical basis for the application and parameter design of the pressure energy-absorbing FRP anchor rod, and provide support for the safe and efficient mining of the mine.

1. Introduction

As an active support method, anchor support is widely used in coal mine roadways [1,2]. The underground recovery gang is mainly supported by threaded steel anchor rods. When recovering, the coal mining machine rubs against the threaded steel anchor rods, easily causing sparks and the possibility of gas and coal dust explosion accidents [3]; at the same time, rebar anchor rods mixed with coal can adversely affect personnel and equipment such as conveyors during transport, and accidents such as chain jams on scrapers and scratches on coal belts can occur [4].

FRP (fiber-reinforced plastic) anchor rods are the most commonly used non-metallic anchor rods [5,6]; they are easy to cut with coal mining machines [7,8,9] and can solve the above problems. Scholars at home and abroad have conducted a lot of research on FRP anchors and have produced extensive research results. Wan Hao et al. [10] analysed the deformation of the surrounding rock of the roadway and the force characteristics of the anchor rods after the FRP anchor rods and the threaded steel anchor rods were supported separately in a high-stress soft rock roadway. Li Yingming et al. [11] conducted tensile tests on FRP anchor rods using a specially designed pull-out test device. Wang Gangfeng et al. [12] selected parameters such as the length of FRP anchors and analysed the effect of each parameter on the support effect. Guo Unite [13] compared the deformation and damage characteristics of a soft laminated roof slab under the support of pipe slit anchor rod and FRP anchor rod using numerical simulation. Wang Wenjie et al. [14] established a dynamic response model for full-length mortar-anchored FRP anchor rods based on the exploding spherical wave theory. Ye Yicheng et al. [15] carried out a roadway convergence monitoring test for FRP anchor rod support. Ascione F et al. [16] evaluated the effect of anchor diameter on the load-bearing failure strength of FRP anchor rods at different load inclination angles. Ngo T T et al. [17] tested the performance of FRP anchor rods, and the test results showed that FRP anchor rods can effectively replace steel anchor rods. Ma Jianjun et al. [18,19,20] proposed a coupled thermal-elastic-plastic damage model for concrete subjected to dynamic loading, and studied the effects of karst caves on tunnel stability using the distinct lattice spring model (DLSM).

With the gradual depletion of shallow coal resources, coal resource development continues to move deeper into the earth, and deep coal resource mining in kilometre-deep shafts is gradually becoming the new normal for coal resource development [21,22,23,24]. Entering 1000–2000 m deep mining, coal mine impact ground pressure and other dynamic disasters are more significant [25,26,27,28]. In recent years, practice has proved that the use of energy-absorbing support can effectively control the deformation of roadway-surrounding rock and can effectively prevent and control roadway impact ground pressure [29,30,31]. However, conventional FRP anchors (hereinafter referred to as conventional anchors) have low elongation and poor impact mechanical properties due to low energy absorption, and these do not meet the requirements of impact roadway support. Therefore, the use of conventional anchor rods in impact pressure roadway support has been limited. In order to meet the requirements of impact ground pressure roadway support, a kind of pressure energy-absorbing FRP anchor rod (hereinafter referred to as a pressure-absorbing anchor rod) was designed to have high energy absorption capacity and elongation. Numerical simulations were carried out to investigate the mechanical properties of the pressure-absorbing anchor rods using static and dynamic analysis.

2. Design of Pressure-Absorbing Anchor Rods

2.1. Structural Design of the Pressure-Absorbing Anchor Rod

The pressure-absorbing anchor rod consists mainly of an FRP rod, a tray, an energy-absorbing sleeve, and a round table nut. The pressure-absorbing anchor rod is shown in Figure 1. The right end of the FRP rod body is provided with a thread. The round table nut has a hole diameter in the centre, and the hole diameter is provided with a thread, which is convenient to connect with the external thread at the right end of the FRP rod body. The tray and the energy-absorbing sleeve pass through the FRP rod body and are tightly anchored by the round nut.

The energy-absorbing sleeve is a thin-walled round tube generally designed to be between 80-150 mm in length. The left half of the round table nut is round table-shaped and the right half is cylindrical; the diameter of the finer end of the round table is slightly smaller than the diameter of the energy-absorbing sleeve, while the diameter of the thicker end of the round table is the same as the diameter of the cylinder and larger than the diameter of the energy-absorbing sleeve. The bearing capacity of the energy-absorbing sleeve is designed to be 85% of the breaking load of the FRP rod.

2.2. Principle of Operation of the Pressure-Absorbing Anchor Rod

After installation of the pressure-absorbing anchor rod in the roadway, the force acting on the pressure-absorbing anchor rod gradually increases with the deformation of the roadway-surrounding rock, and the FRP rod starts to undergo elastic deformation.

When the force on the pressure-absorbing anchor rod increases to the load-bearing capacity of the energy-absorbing sleeve (85% of the FRP rod-breaking load), the FRP rod stops deforming elastically, the round table nut slides along the inner wall of the energy-absorbing sleeve, and the energy-absorbing sleeve starts to deform. The plastic deformation of the energy-absorbing sleeve directly absorbs the deformation energy of the surrounding rock, while the energy-absorbing sleeve gives way to the space to indirectly dissipate the deformation energy of the surrounding rock. The energy-absorbing sleeve gives way to absorb energy, which effectively improves the impact resistance of the anchor rod, thus improving the stability of the “anchor rod-roadway rock” support system.

After the deformation of the energy-absorbing sleeve has been completed, the force on the pressure-absorbing anchor rod continues to increase as the tunnel envelope continues to deform and the FRP rod begins to deform elastically again.

As the deformation of the tunnel rock continues, the force on the pressure-absorbing anchor rod increases to the point where the compression anchor is broken and the pressure-absorbing anchor rod fails.

3. Static Characteristics Analysis

The finite element models of conventional anchor rods and pressure-absorbing anchor rods were established separately using the finite element software ABAQUS, and the mechanical properties of pressure-absorbing anchor rods under static tension were investigated through comparative analysis.

3.1. Model Building and Parameter Setting

The main components of the conventional anchor rod model include the rod, tray and nut. The rod is 2500 mm long and has a diameter of 20 mm; the tray is 150×150×10 mm; the diameter of the central hole in the tray is 22 mm (Figure 2a). The main components of the pressure-absorbing anchor rod model include the rod, tray, energy-absorbing sleeve and round table nut. The length of the rod is 2500 mm and the diameter is 20 mm; the height of the round table nut is 38 mm, the diameter of the thin end is 28 mm, the diameter of the thick end is 48 mm and the taper angle of the end is 26.5°; the inner diameter of the energy-absorbing sleeve is 40 mm, the wall thickness is 4 mm and the height is 150 mm; the tray size is 150 × 150 × 10 mm. The diameter of the round hole in the centre of the pallet is 22 mm (Figure 2c).

An explicit algorithm was used to simulate the stretching process. For the conventional anchor rod model, set the rod, pallet and nut density to 2200 kg/m³, the modulus of elasticity to 43 GPa, the Poisson’s ratio to 0.3, and set the fracture stress to 530 MPa and fracture strain to 0.015. For the pressure-absorbing anchor rod model, set the rod, pallet and nut density to 2200 kg/m³, the modulus of elasticity to 43 GPa, the Poisson’s ratio to 0.3, and set the fracture stress to 530 MPa and fracture strain to 0.015. The density of the energy-absorbing sleeve was 7850 kg/m³, the modulus of elasticity was 210 GPa, the Poisson’s ratio was 0.3, the yield strength was 400 MPa, and the tensile strength was 588 MPa. The energy-absorbing sleeve itself was set to be in self-contact, and the other surface contact methods were set to be surface-to-surface contact; the contact properties were the software defaults, the contact method was the penalty contact method, and the friction coefficient was 0.3. The boundary conditions of the model were as follows: the left end of the rod was completely fixed, and the end of the round nut was only allowed to move axially. displacement. A rigid plate with a diameter greater than the diameter of the rod is displaced 200 mm from the left side of the pallet to the right, and the mesh module uses C3D8R cells for each component; the mesh shape is hexahedral, the pallet and rod mesh size is set to 5, the thin-walled circular tube mesh size is set to 2, the cell type of the rod is set to hourglass control for stiffness, and the cell is set to delete.

3.2. Numerical Model Validation

The stress-strain curves of the simulated tensile process of the conventional anchor were compared with those in reference [32], and the comparison of the stress-strain curves is shown in Figure 3. The overall trend of the stress-strain curves was consistent, verifying the validity of the model and parameters.

3.3. Static Tensile Analysis

The stress clouds during the tensile deformation of the two types of anchors under static forces are shown in Figure 4, the force-displacement curve is shown in Figure 5 and the energy absorption-displacement curve is shown in Figure 6.

From the data in Figure 4, Figure 5 and Figure 6, the following can be found:

(1) The breaking load of the conventional anchor rod is 167 kN; the breaking load of the pressure-absorbing anchor rod is 166 kN, and the load-bearing capacity range of the compression deformation process of the energy-absorbing sleeve is 140.6–142.1 kN, indicating that the breaking load of the conventional anchor rod and the pressure-absorbing anchor rod are basically the same. The compression deformation process of the energy-absorbing sleeve can provide a constant load-bearing capacity.

(2) The conventional anchor rod static tension process goes through the elastic phase and the damage phase. The static tensioning process of the pressure-absorbing anchor rod goes through the front-elastic stage, constant resistance to compression, the back-elastic stage and damage, with an additional constant resistance to compression stage compared with the conventional anchor rod.

(3) The displacement at breakage of the conventional anchor rod and the pressure-absorbing anchor rod was 38.24 mm and 180.4 mm, respectively, and the elongation rates were 1.53% and 7.22%, respectively. The elongation of the pressure-absorbing anchor rod was 4.72 times that of the conventional anchor rod, and the elongation of the let-put anchor was significantly higher than that of the conventional anchor rod.

(4) During the static tensioning process, the energy absorption of the conventional anchor rod and the pressure-absorbing anchor rod was 3.65 kJ and 21.89 kJ, respectively, and the energy absorbed by the pressure-absorbing anchor rod was six times higher than that of the conventional anchor rod. The pressure-absorbing anchor rod had a higher impact resistance than the conventional anchor rod.

4. Dynamic Characterization Analysis

To compare the dynamic performance of the pressure-absorbing anchor rod with that of the conventional anchor rod, numerical simulations of the two anchors subjected to impact tension were carried out using ABAQUS numerical simulation software. Due to the deformation of the surrounding rock of the roadway, the deformation of the surrounding rock can directly act on the pallet of the bolt. A rigid body with kinetic energy was used to impact the anchor pallet, and the effect of impact energy and impact velocity on the mechanical properties of the let-put anchor was analysed by controlling the mass and velocity of the rigid body. The parameters for the impact tensioning of the pressure-absorbing anchor rod and the conventional anchor rod are the same as for the static tensioning.

4.1. Impact Energy Impact Analysis

The mechanical properties of the anchor rods under three different energy impacts of 2 kJ, 15 kJ and 30 kJ were investigated by the energy absorption of the conventional anchor rod and the pressure-absorbing anchor rod during the static tensioning process.

The stress cloud diagrams of the two anchors during tensile deformation at different impact energies are shown in Figure 7 (with 30 kJ impact as an example); the force-displacement curve of the conventional anchor rod is shown in Figure 8, the force-displacement curve of the pressure-absorbing anchor rod is shown in Figure 9, the energy absorption-displacement curve of the two anchors is shown in Figure 10, and the mechanical properties of the two anchors are shown in

Table 1. Mechanical properties of two anchors at different impact energies.

Type of Anchor Rod	Impact Energy (kJ)	Impact Velocity (m/s)	Breaking Situation	Breaking Load (kN)
Conventional anchor rod	2	2	unbroken	none
Conventional anchor rod	15	2	broken	164.3
Conventional anchor rod	30	2	broken	164.7
Pressure-absorbing anchor rod	2	2	unbroken	none
Pressure-absorbing anchor rod	15	2	unbroken	none
Pressure-absorbing anchor rod	30	2	broken	165.4

.

From the data in Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10 and Table 1, the following can be found:

(1) In impact tension, conventional anchor rods and pressure-absorbing anchor rods undergo the same deformation phases during tension as they do in static tension, and pressure-absorbing anchor rods have a stable and repeatable deformation damage pattern. At 2 kJ impact, both the conventional anchor rod and the pressure-absorbing anchor rod were not broken; at 15 kJ impact, the conventional anchor rod was broken and the pressure-absorbing anchor rod was not broken; at 30 kJ impact, both the conventional anchor rod and the pressure-absorbing anchor rod were broken. This means that the compression anchor can withstand greater energy impact when stretched and has better impact resistance.

(2) The breaking loads of conventional anchor rods under static force and 15 kJ and 30 kJ impact were 167.0 kN, 164.3 kN and 164.7 kN, respectively; the breaking displacements were 38.24 mm, 36.98 mm and 36.51 mm, respectively; the elongations were 1.53%, 1.48% and 1.46%, respectively; and the absorbed energies were 3.65 kJ, 3.00 kJ and 2.96 kJ, respectively. This indicates that the impact energy has a small effect on the mechanical properties of conventional anchor rods. The change inthe breaking load, breaking displacement, elongation and energy absorption of conventional anchor rods in impact tension compared to static tension is small, and the effect of impact energy on the mechanical properties of conventional anchor rods can be ignored.

(3) The breakage loads of the pressure-absorbing anchor rods were 165.3 kN and 165.4 kN under static force and 30 kJ impact, respectively; the breakage displacements were 180.4 mm and 173.6 mm, respectively; the elongation was 7.22% and 6.94%, respectively; and the absorbed energy was 21.89 kJ and 21.51 kJ, respectively, indicating that the impact energy had a small effect on the mechanical properties of the pressure-absorbing anchor rods. The change in the breaking load, breaking displacement, elongation and energy absorption during impact tensioning compared to static tensioning is small, and the effect of impact energy on the mechanical properties of pressure-absorbing anchor rods can be ignored.

(4) At an impact energy of 30 kJ, the breaking loads of conventional anchor rods and pressure-absorbing anchor rod anchor rods were 164.7 kN and 165.4 kN, respectively; the breaking displacements were 36.51 mm and 173.6 mm, respectively; and the elongation rates were 1.46% and 6.94%, respectively. The elongation of the pressure-absorbing anchor rod is 4.75 times that of the conventional anchor rod, indicating that the pressure-absorbing anchor rod has better elongation performance.

4.2. Analysis of Impact Velocity Effects

As can be seen from Table 1, both the conventional and the pressure-absorbing anchor rod were pulled off when the impact energy was 30 kJ. Therefore, the simulations were carried out to study the tensile properties of the pressure-absorbing anchor rods at a constant impact energy of 30 kJ and impact velocities of 2 m/s, 3 m/s, 4 m/s, 5 m/s and 6 m/s, respectively.

The stress clouds during the tensile deformation of the two anchors at different impact velocities are shown in Figure 11 (with 4 m/s impact as an example). The force-displacement curve of the conventional anchor rod is shown in Figure 12, the force-displacement curve of the pressure-absorbing anchor rod is shown in Figure 13, the force-time curve of the conventional anchor rod is shown in Figure 14, the force-time curve of the pressure-absorbing anchor rod is shown in Figure 15, the energy absorption-displacement curve of the conventional anchor rod is shown in Figure 16, the energy absorption-displacement curve of the pressure-absorbing anchor rod is shown in Figure 17, the impact time-velocity curve of the two anchors is shown in Figure 18, the mechanical properties of the two anchors are shown in

Table 2. Mechanical properties of two anchor rods at different impact energies.

Type of Anchor Rod	Impact Velocity (m·s−1)	Breaking Load (kN)	Breaking Displacement (mm)	Elongation	Energy Absorption (kJ)	Impact Resistance Time (ms)	Energy Absorption Rate (J·ms−1)
Conventional anchor rod	2	164.9	36.51	1.46%	2.96	18.63	158.9
Conventional anchor rod	3	166.5	33.72	1.35%	3.23	11.35	284.6
Conventional anchor rod	4	162.3	33.92	1.36%	2.88	8.34	345.3
Conventional anchor rod	5	162.4	33.88	1.36%	3.08	7.26	424.2
Conventional anchor rod	6	158.0	27.21	1.09%	3.08	4.53	679.9
Pressure-absorbing anchor rod	2	165.4	173.6	6.94%	21.51	103.70	207.4
Pressure-absorbing anchor rod	3	164.7	175.7	7.03%	22.14	70.05	316.1
Pressure-absorbing anchor rod	4	157.5	176.5	7.06%	21.56	52.88	407.7
Pressure-absorbing anchor rod	5	161.5	174.7	6.99%	22.03	42.02	524.3
Pressure-absorbing anchor rod	6	159.9	174.1	6.96%	21.30	35.29	603.6

, and the energy absorption rate-impact velocity curve of the pressure-absorbing anchor rod is shown in Figure 19.

From the data in Figure 11, Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17, Figure 18 and Figure 19 and Table 2, the following can be observed:

(1) The breaking loads of conventional anchor rods at impact velocities of 2 m/s, 3 m/s, 4 m/s, 5 m/s and 6 m/s were 164.9 kN, 166.5 kN, 162.3 kN, 162.4 kN and 158.0 kN, respectively, with an average breaking load of 162.82 kN. The breaking displacements were 36.51 mm, 33.72 mm, 33.92 mm, 33.88 mm and 27.21 mm, with an average breaking displacement of 33.048 mm, an elongation of 1.46%, 1.35%, 1.36%, 1.36% and 1.09%, with an average elongation of 1.32%, and an energy absorption of 2.96 kJ, 3.23 kJ, 2.88 kJ, 3.08 kJ, 3.08 kJ, respectively. The average energy absorption was 3.05 kJ. There are small changes in the breaking load, breaking displacement, elongation and energy absorption of conventional anchor rods with increasing impact velocity, indicating that the impact velocity had a small effect on the mechanical properties of the conventional anchor rods.

(2) The breaking loads of pressure-absorbing anchor rods were 165.4 kN, 164.7 kN, 157.5 kN, 161.5 kN and 159.9 kN at impact velocities of 2 m/s, 3 m/s, 4 m/s, 5 m/s and 6 m/s, respectively, with an average breaking load of 161.80 kN. The breaking displacements were 173.6 mm, 175.7 mm, 176.5 mm, 174.7 mm and 174.1 mm, respectively, with an elongation of 6.94%, 7.03%, 7.06%, 6.99% and 6.92 mm, and elongation of 6.99%, 7.03%, 7.06%, 6.99% and 6.1 mm. 176.5 mm, 174.7 mm and 174.1 mm, respectively, with an average breaking displacement of 174.92 mm, elongation of 6.94%, 7.03%, 7.06%, 6.99% and 6.96%, and an average elongation of 6.99%. There absorbed energy measurements were 21.51 kJ, 22.14 kJ, 21.56 kJ, 22.03 kJ, and the average absorbed energy was 21.71 kJ. There are small changes in the breaking load, breaking displacement, elongation and energy absorption for pressure-absorbing anchor rods as impact velocity increases, indicating that the impact velocity had a small effect on the mechanical properties of the pressure-absorbing anchor rod.

The average elongation of the pressure-absorbing anchor rod was 5.3 times higher than the average elongation of the conventional anchor rod, and the average absorbed energy of the pressure-absorbing anchor rod was 7.12 times higher than the average absorbed energy of the conventional anchor rod. This indicates that the elongation and energy absorption of pressure-absorbing anchor rods are greater, and have better elongation and energy absorption properties.

(3) At impact velocities of 2 m/s, 3 m/s, 4 m/s, 5 m/s and 6 m/s, the impact resistance time of the conventional anchor rods was 18.63 ms, 11.35 ms, 8.34 ms, 7.26 ms and 4.53 ms, respectively, while the impact resistance time of the pressure-absorbing anchor rods was 103.70 ms, 70.05 ms, 52.88 ms, 42.02 ms and 35.29 ms, respectively. As the impact velocity increased linearly, the impact time of both types of anchor rods decreased non-linearly, but the impact time of the pressure-absorbing anchor rod was longer at the same velocity, indicating that the pressure-absorbing anchor rod had better impact resistance.

(4) At impact velocities of 2 m/s, 3 m/s, 4 m/s, 5 m/s and 6 m/s, the energy absorption rates of pressure-absorbing anchor rods were 207.4 J/ms, 316.1 J/ms, 407.7 J/ms, 524.3 J/ms and 603.6 J/ms, respectively. This means that the energy absorbed per unit of time increased, indicating that the pressure-absorbing anchor rod has a good dynamic response.

(5) The load-bearing capacity of the energy-absorbing sleeve during deformation ranges from 134.9 to 148.1 kN at impact velocities of 2 m/s, 3 m/s, 4 m/s, 5 m/s and 6 m/s. This indicates that the energy-absorbing sleeve has a stable and repeatable deformation process at different impact velocities, and can provide a relatively constant load-bearing capacity.

5. Discussion of the Applicability of the Pressure-Absorbing Anchor Rod

The use of both conventional and pressure-absorbing anchors can solve the problem of sparks caused by friction between the coal miner and the anchor, but the advantages of pressure-absorbing anchors over conventional anchors are mainly in the following areas:

(1) The pressure-absorbing anchor can achieve flexible support. In the process of support under load, the amount of tension will gradually adapt to the increase in the deformation of the roadway-surrounding rock to further improve the stability of the surrounding rock.

(2) The energy-absorbing sleeve of the pressure-absorbing anchor absorbs part of the impact energy of the surrounding rock through the deformation of the expansion and has a constant resistance characteristic. The constant resistance energy absorption process improves the force situation of the anchor when subjected to impact loads, effectively improving the impact resistance of the pressure-absorbing anchor.

6. Conclusions

(1)

A pressure-absorbing anchor rod consisting of FRP rod body, tray, energy-absorbing sleeve and round table nut is designed, and the working principle of the pressure-absorbing anchor rod is given. It provides a design idea and theoretical basis for accomplishing anti-impact support of FRP anchor rods in roadways of rock burst mines.

(2)

In both static tension and impact tension, the pressure-absorbing anchor rod has a front-elastic stage, constant resistance to compression, a back-elastic stage and a damage phase, with an additional constant resistance to compression phase compared with the conventional anchor rod.

(3)

The elongation, energy absorption and impact resistance time of the pressure-absorbing anchor rod are greater than those of the conventional anchor rod, and the pressure-absorbing anchor rod has better elongation, energy absorption and impact resistance properties.

(4)

The impact energy and impact speed have less influence on the breaking load, elongation and energy absorption of the pressure-absorbing anchor rod. As the impact velocity increases, the energy absorption rate of the pressure-absorbing anchor rod increases non-linearly. The pressure-absorbing anchor rod can quickly absorb energy under impact loads, and has a better dynamic response and impact mechanical properties.