The residual state of stress in old railroads for public transport by tram vehicles.

Ghita, Eugen ; Nagi, Mihai ; Carabas, Ioan Daniel 等

1. INTRODUCTION

The city of Timisoara is located in the south-west Romania's side ,closely to the border with Serbia and Hungary. The main public transport operator in the city of Timisoara is the Public Transport Administration Timisoara (R.A.T.T.) which continues a tradition of 130 years. Nowadays, R.A.T.T. provides 57,7% of the city's transportation, which means 52 millions people transported a year. The tram line network passes through narrow streets covering a distance of 90,2 km. The swampy soil is inadequate for an underground transportation, so the surface transportation is prevalent. Some of the railroads have a very old design. During the last years, the wear of the rail and the level of noise strongly increased.

2. WORKING PROCEDURE

The experimental strain gage method consists in drilling the railroad in different areas (head, basement and core of the rail profile).Usually the hole diameter is of 6 mm and the depth is 9 mm. The bending capacity of the railroad is not adulterated after the drilling procedure and it can be kept in service. As a consequence of the drilling process, the residual surface stresses vanish around the hole, the deformations on three directions are measured with a 3/120 rosette transducer and the values of the previous residual stresses will be calculated using a special software. In figure 1 you can see : 2a-the hole diameter; [[sigma].sub.r]-the radial stress; [[sigma].sub.t]-the circumference stress; [[sigma].sub.1]-the main maximum stress; [[sigma].sub.2]-the main minimum stress; r-the radial distance between the center of the hole and the point of interest; [theta]-the central angle. For a reference plate without any hole (a=0) ,the radial and circumference stresses are, (Heymann, 1986; Mocanu et. al., 1985):

[[sigma].sub.r]'=1/2 ([[sigma].sub.1] + [[sigma].sub.2]) + 1/2 ([[sigma].sub.1] - [[sigma].sub.2]) cos 2[theta] (1)

[[sigma].sub.[theta]]'=1/2 ([[sigma].sub.1] + [[sigma].sub.2]) - 1/2 ([[sigma].sub.1] - [[sigma].sub.2]) cos 2[theta] (2)

As a consequence of the drilling process, the residual surface stresses vanish around the hole and the deformations on three directions are measured with a 3/1200 rosette transducer (figure 1).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The deformations vary as it follows :

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

The main stresses [[sigma].sub.1], [[sigma].sub.2] and the central angle [theta] must be estimated. So, the state of surface stresses will be completely defined.

For that purpose, the deformations on the three directions are experimentally measured:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

where A and B are measurement constant values; E-Young's modulus.

The values of the previous residual stresses ([[sigma].sub.1] -maximum stress; [[sigma].sub.2]-minimum stress ) will be calculated using a special software:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (8)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (9)

3. RESULTS

Some experimental results (for the location of the rosette transducers in figure 2) are presented in table 1. The measured values of deformations and the calculated values of the main surface stresses are shown in the table 1.

4. CONCLUSIONS

The values of the residual main stresses in the rail cover a stress range between -280 MPa and 230 MPa. The state of stress due to the vehicle loading will be superimposed on the residual state of stress. The location of the extreme values of the maximum and minimum main stresses is on the rail head surface. High stress values and faults or cracks located in the same region may lead to dangerous effects on the safety and comfort of the passengers, (Safta et al, 1986).

In order to remove in depth the points of maximum stresses from the lateral rail head surface, the lubrication maintenance procedure is usually used. A mobile lubrication push-cart ensures the lateral lubrication of the linear portions of railroads on a distance of 200 meters. The device will be assembled on the front side of the experimental tram maintenance vehicle.

In association with the level of stresses, the proposed maintenance inspections include a periodical combined strain gage measurements and an ultrasonic procedure (in order to detect the cracks) every 6 months in different points on the route, especially the high crowded traffic areas. In order not to disturb the tram-traffic, these inspections are usually performed at night. Because of the high amount of measured data ,special automated measuring wagons have been introduced in order to inspect portions of 20-40 km of rail.

The rectification procedure of the rail head is allowed until a distance of 5 mm from the standard profile, because of safety reasons. The derailment coefficient at the wheel-rail contact (which means the ratio between the guidance lateral force from the rail and the vertical force on the wheel ) must be less than 1,2-1,3. , (Ghita, 1998; Ghita & Turos ,2006).

From financial reasons, the rectification and the lubrication are performed first. However, the safety measures impose the replacement of the railroad whenever the residual stresses reach 250 MPa and the cracks are longer than 1 mm.

The proposed method is preferred because that is a half-destructive method ,but the bending capacity of the rail is not adulterated after the drilling procedure.

The ultrasonic crack analysis may be performed with a portable ultrasonic crack detector ,a lightweight, compact and handy-portable flaw detector designed for use on large workpieces and in high-resolution measurements, (*** USM 25,2001).

5. ACKNOWLEDGEMENTS

This work was partially supported by the strategic grant POSDRU 6/1.5/S/13, (2008) of the Ministry of Labour, Family and Social Protection, Romania, co-financed by the European Social Fund--Investing in People.

6. REFERENCES

Ghita,E. (1998). Strength on wheel-rail contact, Mirton Publishing House,Timisoara,Romania,1998,ISBN 973-573-516-1

Ghita,E. & Turos,G. (2006). Dynamics of Railway Vehicles ,Eurostampa Publishing House, Timisoara,Romania,2006,ISBN (10) 973-687-400-1, (13) 978-973-687-400-0

Heymann, J. (1986). Experimentelle Festkorpermechanik, VEB Fachbuchverlag, Leipzig, Germany, 1986

Mocanu, D.R.., Modiga, M. & Iliescu, N. (1985). Experimental Stress Analysis (Strain Gage Measurements on the Models), Technical Publishing House, Vol. 2, 800-845, Bucharest, Romania,1985

Safta, V., Mocanu, D.R., Draghici, M., Ciorau, P. & Serban, VI. (1986). Materials Testing, Technical Publishing House,Vol. 3, 356-358,Bucharest,Romania,1986

*** USM 25-Technical Reference and Operating Manual, Krautkramer , Germany ,2001

Tab. 1. The measured values of deformations and the calculated
values of the main surface stresses

[[epsilon].sub.a] [[epsilon].sub.b] [[epsilon].sub.c]
[[micro] m/m] [[micro] m/m] [[micro] m/m]

45 170 175
-288 -300 -250
-225 -35 -80
70 125 30
180 365 230
230 405 310
-190 220 -270
-495 -190 -255
150 170 75
-140 120 -40
-25 -40 45

[[epsilon].sub.a] [[sigma].sub.2] [[sigma].sub.1]
[[micro] m/m] [MPa] [MPa]

45 155 185
-288 215 230
-225 -15 100
70 -65 -55
180 -250 -165
230 -280 -210
-190 -175 -110
-495 -270 -255
150 -200 -50
-140 -80 -30
-25 -50 20