Research on FRP bolt-end failure mechanism

M INING SCIENCE AND TECHNOLOGY Mining Science and Technology 19 (2009) 0522–0525

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Research on FRP bolt-end failure mechanism LI Ying-ming1,2, MA Nian-jie3, YANG Ke1, SHI Jian-jun4 2

1 School of Energy and Safety, Anhui University of Science and Technology, Huainan, Anhui 232001, China MOE Key Laboratory for Coalmine Safety and High Efficiency Mining, Anhui University of Science and Technology, Huainan, Anhui 232001, China 3 School of Resources and Safety, China University of Mining and Technology, Beijing 100083, China 4 School of Safety Engineering, North China University of Science and Technology, Beijing 101601, China

Abstract: Given the fact that FRP bolts for roadway support are often under a certain amount of eccentric load, we studied the problems of failure of FRP bolt-ends using mechanical analysis, numerical simulation and a laboratory experiment to reveal the FRP bolt-end failure mechanism. The results show that bolt-end stress increases rapidly, making the maximum stress under an eccentric load to be 5 to 7 times greater than that under a normal load, resulting first in the formation of some fractures at the bolt-end, which then spreads to the entire cross-section of the bolt. Keywords: FRP bolt; bolt-end; eccentric load; failure mechanism

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Introduction

FRP (fibre reinforced plastic) bolts made in England and France are generally made of pure FRP. However, they are too expensive to be used in China[1]. In order to cut down on the of cost FRP bolts, many scientists during the 1970s to the 1990s, have been engaged in studying FRP bolt-end structures. Most of these FRP bolts made in China are characterized by a FRP rod with metal components. Then, at the beginning of the 21st century, FRP bolts made of complete non-metal material were developed by using foreign technology in combination with independent R & D. Present FRP bolt-end structures can be divided into two classes: a metal sleeve type and one made of complete non-metal material. According to special features of each of these structural types, both classes Table 1 Material

can be further divided (Table 1). However, FRP bolts are often broken at the end in practical applications, quite possibly caused by rib breaking[2–3] (Fig. 1). Many factors may cause FRP bolt-end failure. In our study, we have investigated the FRP bolt-end failure mechanism largely in situations of bolts under eccentric loads.

Fig. 1 Photographs of FRP bolt failure for coal side support under practical conditions

Kinds of FRP bolt-end structures

Connection way

Typical products

Cementation Metal sleeve type

Mechanical Cementation-mechanical connection

Full non-metal

FRP bolts made by Hangzhou Environment Protection Research Department in 1994 FRP bolt made by China University of Mining and Technology (Beijing) in 2000

Thread cutting Injection FRP bolt

FRP bolt made by the Hebei Jikai Group Company in 2004

Received 12 December 2008; accepted 03 March 2009 Project 08040106839 supported by the Excellent Youth Foundation of Anhui Province Corresponding author. Tel: +86-554-6668771; E-mail address: [email protected]

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Research on FRP bolt-end failure mechanism

inscribed circle of the bolt nut[4]. The calculation model is shown as Fig. 3.

Mechanical analysis

Because the surface of the rib is not perpendicular to the drilling hole, as shown in Fig. 2, the screw of the bolt-end frequently brings about an eccentric load when the bolt works normally. In practice, the boltend is always affected by the eccentric load. So, the FRP bolt-end under this eccentric load is analyzed theoretically as follows. Similar to that stated by Kong et al., we postulated the hypothesis that the bolt-end is fully affected by the eccentric load which is a concentrated force at the

Fig. 3

M zo = QyQ ) which act on the XZ and XY planes

respectively. Given the mechanisms of material, the stress at any point (y, z) in the cross section of the bolt is as follows[5]: z z y y· Q M z M y Q§ σ = + y + Z = ¨1 + Q2 + Q2 ¸ (1) ¨ A Iy Iz A© iy iz ¹¸ where A is the applicable sectional area of the bolt-end and iy and iz are the inertial radii for the Y and Z axes. For any point (Y0, Z0) on the neutral axis, the equation of the neutral axis is derived as:

zQ z iy

2

+

yQ y iz

2

=0

Fig. 2

Working state of a FRP-end in the mine

Mechanics model of bolt-end under eccentric load

The axis of the bolt serves as the X axis, while the two main inertial axes centered in Figs. 3b and 3c are the Y and Z axes. It is assumed that the eccentric tension Q is parallel to the axis of the bolt body, whose working point is in the first quadrant and its coordinates are yQ and zQ, as shown in Fig. 3b. The eccentric tension Q is simplified from the point (yQ, zQ) to (yQ, 0) and then to the centre of the figure (0, 0). After that the eccentric tension Q is transformed into a couple of bending moments ( M yo = QzQ and

1+

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(2)

From Eq.(2), the neutral axis is a beeline which does not cross the section centre O. Providing y0˙0 or z0˙0, we can easily obtain the intercept of the neutral axis with the Y and Z axes such that: iy 2 i2 (3) a y = − z , az = − yQ zQ

In order to determine the magnitude of direct stress

at the most dangerous point on the bolt-end cross section, we assumed that Q acts on point Pƍ which is in Z axis and in the inscribed circle of nut. All points on the circle of the bolt-end cross section are dangerous points of which point D, where Y is 0, is the most dangerous point. From Eq.(1), the direct stress of point D is s d Q⋅ ⋅ Q 2 2 = Q ⋅ (1 + 4 s ) σD = + (4) ʌd 4 A A d 64 where s is the diameter of the inscribed circle of the nut and d the nominal diameter of the bolt-end screw thread. To nuts M16, M18, M20 in which each number refers to d in Eq.(4), their corresponding inscribed diameters (s) are 24, 27 and 30, where s/d is usually 1.5[6]. We obtained σ D = 7σ t where ıt is stress on the bolt-end. If we ignore the effect of s, then σ D = 5σ t . It can be shown that, if the eccentric load of the bolt-end is considered as a concentrated load, the magnitude of direct stress ranges from 5 to 7 times the general load without the eccentric load. Obviously, the eccentric load is worse at the bolt-end.

3 Numerical simulation In order to obtain the stress distribution characteristics of injecting the FRP bolt under an eccentric load, we used ANSYS software to establish the injection FRP bolt model with a body model[7–10]. First, a geometry model, which can express the shape of the model, was created and then the model was meshed to generate customized points and units. An

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established model is shown as Fig. 4. In the course of the analysis of our calculation, three dimension body units with 10 points were applied to the model in the use of the structural analysis module of the ANSYS software. According to structure and load of the bolt, there were 35470 units and 52610 points in the model. Some parts (threads) are shown in detail (see Fig. 4b).

(a) Model map of bolt-end

and verify the theoretical and simulation conclusions, we conducted a simulation experiment of an eccentric load (Figs. 6 and 7) and provide the results of the experiment. As described in Fig. 7, cracks first appeared on the injection sleeve when the eccentric load reached 35 kN. With an increasing load, cracks rapidly expanded and the bolt-end broke over the entire cross-section when the load reached 42.4 kN (Fig. 7).

(b) Discrete model map of bolt-end

An established model

For the discrete model holds that, it is restricted by practical loads to the bolt. The mechanics of the model of the bolt is simplified as follows: the bolt is anchored at the front end of the bolt, i.e., the displacement of the end in the coal is restricted; the displacement of the bolt-end is also restricted. Finally, an eccentric load, which is a 200 N centralized load on the nut, is applied to the bolt-end. We then selected the dangerous section in order to analyze the stress distribution of the bolt-end. Fig. 5 shows the color contour of the stress distribution of the dangerous section. From Fig. 5 it is seen that, as the stress distribution characteristics of the crosssection of the stress become larger from the section centre to its perimeter, there are several areas of stress. On the whole, stress near the applied load side is much larger than elsewhere;, stress is least in the centre of the cross-section, most stress is concentrated at the applied load side where the stress is 3~5 times larger than at the other side. These results suggest that, if the eccentric load were a centralized load on the nut, the most stressed region of the bolt would be the bolt-end thread of the applied load nut. Therefore, our numerical simulation results are highly consistent with the conclusions of our theoretical analysis.

Fig. 5

No.4

Color contour of stress distribution of dangerous cross-section

4 Experimental research In order to study the deformation and destructive characteristics of FRP bolts under an eccentric load

Fig. 6

Eccentric experiment

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Fig. 4

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Fig. 7

Tensile properties curve of FRP bolt under eccentric load

A FRP bolt rod has a large extension strength, i.e., 600~700 MPa, but no capacity of plastic deformation and extensibility is low, just 1%~1.5%. Our experimental results show that the stress of the bolt-end increases rapidly under an eccentric load, with the result that it first forms several fractures at the bolt-end, which then spread to the entire crosssection[11].

5

Countermeasures for FRP bolt failure

Given the conclusion of the FRP bolt-end failure mechanism reached, there are two technological ways to prevent the bolt from eccentric failure. The first is to avoid or reduce the eccentric load; the second is to enhance the anti-eccentric-load capacity of the bolt. At present, the first method is taken to prevent the bolt from eccentric load failure in the field. Specific measures are as follows: 1) Place the emphasis on the management of bolt construction and design. It is found in the field that the main cause of eccentric failure is that the bolt or drill is not perpendicular to the roadway rib. Therefore, more attention in the field should be paid to the technology of bolt construction. A specific demand is to make rib

LI Ying-ming et al

Research on FRP bolt-end failure mechanism

perpendicular as much as possible, to the direction of the bolt. It is necessary that the error of the angle be not more than 5° and the plate should be close to the rib. 2) Net appended for rib integrity Sometimes part of a rib caves in. As a result, the bolt plate is loaded only on part of the plate, so that the plate is no longer perpendicular to the bolt, which brings about the eccentric load. In order to avoid this occurrence, a metal or plastic net is appended to the rib, in support of maintaining rib integrity. The measures mentioned above are designed to prevent bolts from eccentric failure. But it is difficult to construct bolts with an exact angle, given present day technology. In addition, coal seams are soft and ribs are not flat and of low integrity, which makes it difficult to avoid eccentric failure, despite these two preventive measures. Therefore we should place more emphasis on the study of FRP bolt-end structures in order to enhance the anti-eccentric-load capacity of bolts. In the end, new bolt-end structures which can avoid eccentric failure need to be developed.

6

Conclusions

1) It often happens that FRP bolts are broken at the end under eccentric load in the course of practical applications 2) From the mechanics of material, it has been shown that if the eccentric load of a bolt-end is considered as convergence, the magnitude of the direct stress ranges from 5~7 times the general load without an eccentric load. 3) ANSYS software was used to simulate a bolt-end under an eccentric load. The results show that stress is least in the centre of the cross-section of the bolt-end and largest at the applied load side, where the stress is 3~5 times larger than at the other side. 4) A simulation experiment of an eccentric load shows that cracks first appear on the injection sleeve when the eccentric load is 35 kN. With an increasing load, cracks rapidly expanded and in the end the bolt-end broke over the entire cross-section the when the load reached 42.4 kN. 5) FRP bolt-end failure can be summarized as follows: FRP bolt rods have great extension strength of 600~700 MPa. However, FRP bolts have no capacity of plastic deformation and extensibility is low, just 1%~1.5%. Bolt-end stress increases rapidly under an eccentric load, which leads to a break at this point and then spreads to the entire cross-section. 6) There are two technical ways to prevent bolts from eccentric failure: one is to avoid or reduce the

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eccentric load; the other is to enhance their antieccentric-load capacity. 7) It is difficult to avoid eccentric loads. Therefore we should place more emphasis on the study of FRP bolt-end structures in order to enhance the antieccentric-load capacity of bolts.

Acknowledgements Financial support for this work was provided by the Excellent Youth Foundation of Anhui Province (No.08040106839) and is gratefully acknowledged. In addition, we would like to thank the anonymous reviewers who have helped to improve the paper.

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