EXPERIMENTAL STUDY ON RAIL PAD STIFFNESS OF FASTENER IN URBAN RAIL TRANSIT

Xueying LENG，Hong XIAO，Yang HE，Tao XIN，Lei HUO

School of Civil Engineering，Beijing Jiaotong University，Beijing 100044

Abstract：The fastener system is one of key structures of ballastless track，in which，the rail pad provides most elasticity for ballastless track structure.Reasonable track stiffness is important practical significance for extending the service life of the track structure，reducing the site maintenance workload and improving the economic benefits of the line.This paper develops a kind of new test equipment for site test，rapidly measuring the pad stiffness，to research the pad stiffness characteristics and its change rule with the active time，randomly selects several groups of active rail pads，tests their stiffness and compares with the stiffness of the same model of new pads，studies the time-variant characteristics of the rail pad stiffness and discusses the determination of the track stiffness scope of the high speed railway in China.

Keywords：rail pad stiffness，ballastless track，fastener

Email：12231148@bjtu.edu.cn

1　Introduction

With the rapid development of urban rail transit，the traffic volume and axle load are increasing and failure phenomena of the fastener as an important component of the track structure are gradually increasing.Many metro lines in Beijing encounter fastener fracture.For example，in DTⅥ2 fastener adopted in a metro line，476 elastic strips are broken and 275foundation bolts are broken in a year[1].With the increase of the number of the failed fasteners，the interaction force between the wheel rails as well as the displacement and acceleration of the track components will be increased sharply，greatly affecting dynamic derailment of the train，accelerating track structure damage and even endangering the traffic safety[2].

The fastener system is one of key structures of ballastless track，in which，the rail pad provides most elasticity for ballastless track structure.The rail pad is made of viscoelastic high polymer material.With the line operation，the rubber rail pad will be aged and hardening，which will result in stiffness changes，thereby affecting the loading features of the wheel-rail system[3].Therefore，it is of important significance for line maintenance to study the change rule of the pad stiffness with the operation time and thereby study the change rule of the wheel-rail system random vibration response on the ballastless track with the time[4].

This paper develops a kind of new test equipment for site test，rapidly measures the pad stiffness to research the pad stiffness characteristics and its change rule with the active time；randomly selects several groups of active rail pads，tests their stiffness and compares with the stiffness of the same model of new pads，studies the time-variant characteristics of the rail pad stiffness and discusses the determination of the track stiffness scope of the high speed railway in China[5].

2　Static Stiffness Test Equipment of Rail Pad

2.1　Test equipment research and development

Currently，Chinese scholars have carried out some tests to determine the stiffness.The universal tester is mainly used as the loading equipment in the tests，which is controlled by computer，driven by servo motor，loaded by precision ball screw machine[6].The signal is measured by the sensor，converted into digital signal and then collected by computer.The displacement measurement accuracy can reach 0.001 mm.The determination method is accurate，but the large equipment and complex operation are not suitable for site test to obtain the pad stiffness quickly.To this end，the author develops a kind of new equipment，as shown in Figure 1 and Figure 2.This equipment，with simple operation，accurate result and short test time，is suitable for rapid measurement of pad stiffness in the site test and improves the efficiency of the whole test.

2.2　Test equipment and test piece preparation

The hydraulic jack is used as the loading equipment.The test pieces are divided into two groups，i.e.unused pads（“new pads” for short）in a group and the used pads（“old pads” for short）on site in the other group.At least three pads are tested in each group to ensure test data effectiveness.

3　Static Stiffness Test of Rail Pad

The rail pad stiffness is tested in the track engineering laboratory in Beijing Jiaotong University.In the test，the vertical load P is exerted on the rail head of the single rail support through the jack.The vertical displacement of the corresponding pad is measured by dial indicator and dial gage，so as to inversely calculate the static stiffness of the pads.

Set five test schemes according to the site condition requirements：

Condition 1：Test three old rubber pads Ⅰ，three new rubber pads Ⅰ，three old rubber plates Ⅱ and three new rubber plates Ⅱrespectively.

Condition 2：Ⅰ new ×Ⅱ new ×3 groups.

Condition 3：Ⅰ old ×Ⅱ old ×3 groups.

Condition 4：Ⅰ new ×Ⅱ old×groups.

Condition 5：Ⅰ old ×Ⅱ new×groups.

The measuring points are arranged as shown in Figure 3.The vertical force exerted by the jack in the test is shown in Table 1.

In the official test，load continuously at the speed of 0-1kN/s and the loading force of F.Stop 30 s after loading each time and record the displacement D respectively.The time of a test process is 3 min.

The static stiffness is calculated according to Formula（1）：

K=F/D　　（1）

where，F is load exerted to the standard test piece of tested rubber pad；D is displacement of the tested rubber pad test piece when loaded to F，mm；K is static stiffness of pad，kN/mm.

4　Test Results and Analysis

In the test，each condition combination state is tested for three times and the average value is taken.The linear changes are shown in Figure 4and Figure 5 and the test results are shown in Table 2.

Table 2 shows that the stiffness of single-l ayer old rubber pads is 44.729 kN/mm and 46.633 kN/mm and of the new rubber pads is 47.223 kN/mm（there may be data error）and 37.892 kN/mm and that the stiffness of the old rubber pads is obviously large；the test stiffness of the double-layer rubber pads is 22.65 kN/mm，23.515 kN/mm，23.978 kN/mm and 24.451 kN/mm respectively in different conditions and is half of that of single-layer pads；each linear scale is about 0.99.

It can be seen that the greater the vertical load P of the rubber pad is，the larger the vertical displacement is，and the rubber stiffness on site shows significant linear changes；with the increase in the active time，the pad stiffness will be increased and the stiffness of the double-layer pads is half of the single-layer pads；the double-layer or three-layer composite rubber pads are beneficial to reduce the track stiffness and improve the track elasticity.

5　Conclusions

（1）A set of test equipment is researched and developed for the static stiffness test of the fastener rail pad in this paper.The equipment，characterized by easy carrying，simple operation，short test time and easy site test，improves the efficiency of the whole test research and is of good promotion value through thespecific test verification.

（2）The test results show that the rubber stiffness on site shows significant linear changes；with the increase in the active time，the pad stiffness will be increased and the stiffness of the double-layer pads is half of the single-layer pads.

（3）The elastic layer thickness may be selected as 120-160 kN/mm for new heavy haul ballast track line，100-140 kN/mm for reconstructed heavy haul line of existing ballast track and 40-60 kN/mm for heavy haul ballastless track line of stiff ballast bed.

Acknowledgement

This work was financially supported by the National Natural Science Foundation of China（No.51578055），the Fundamental Research Funds for the Central Universities（No.2015JBZ004）.

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