1 Mechanical Testing Laboratory, 2 Igloi Street, Miskolc, 3519, Hungary, csaba.takacs@bayzoltan.hu 2 Mechanical Testing Laboratory, 2 Igloi Street, Miskolc, 3519, Hungary, reka.erdei@bayzoltan.hu

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MODELLING AND NON-DESTRUCTIVE TESTING OF HEAD CHECK DAMAGE OF RAILWAYS

Csaba TAKACS1 Reka ERDEI2

Abstract – Defects of rails might be formed during production and operation. During production rails have to be appropriate to strict standards, and various material testing have to be performed in order to minimize chance of defects in rails. On the other hand, load of rails significantly increased in the last years, as railway traffic, speed of trains and axle load increased. Effect of this growth in load of rails leads to raise the chance of forming and spreading defects during operation. In case of rails with high railway traffic, small, parallel cracks might appear on the running interface, and these small cracks are able to spread because of cyclic loading. This damage is called “rolling contact fatigue, RCF” or “head check” damage, what might cause complete break of rail. Detecting in early stage of head check damage is very important, but serious challenge for material testing experts. Non-destructive material testing and modelling were performed and results of modelling and testing were compared in order to determine optimal testing parameters for detecting damage of rails in early stage.

Keywords – Head check damage, maintenance, early stage, modelling, non-destructive testing.

1. INTRODUCTION

Rail transport is a significant proportion of transport, and ensuring safety is very important. Trains and rails have to be appropriate to strict standards, and non-destructive testing and maintenance plays an important role in safe operation. In the last decades, railway traffic, speed of trains, axle load of trains, and load of rails are significantly increased. Cycle load of rails cause small, parallel cracks on the running surface, what is called “head check” or “rolling contact fatigue, RCF” damage. Head check damage of rails might cause complete break of rails, and endanger rail transport, so detecting of head check damage of rails is important task in order to avoid accidents. Non-destructive test methods (penetration test, magnetic particle test, eddy current test and phased array ultrasonic technique with modelling) were performed on head check damaged rail samples, in order to determine, which non-destructive testing method is applicable to detect head check damage in early stage. After non-destructive tests, rail samples were cut, and embedded into resin, then microscopic measurement and hardness test were performed on rail samples, in order to validate results of performed non-destructive tests.

2. FORMING AND GROWING PROCESS OF HEAD CHECK DAMAGE

Head check damage of rails is caused by axle load of trains, and location of forming of small, paralell hairline cracks is the running surface of rails. These small hairline cracks are able to spread because of cycle load. In the early stage, spread of cracks is slow and angle of cracks are 20-35° with the running surface of rails. In the early stage, head check damage is not endangering safety of rail transport, but difficult to detect the damage with non-destructive testing methods.

After slow spreading stage of cracks, cracks might be branching and crack-growing direction is changing. Cracks in this stage are spreading towards the core of rails. Growing speed of cracks is increasing significantly, and might cause breakaway of portions and/or complete brake of rails [1]. Forming and growing process of head check damage can be seen in Fig. 1.

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Fig. 1. Forming and growing process of head

check damage [2]

3. RESULTS OF MODELLING AND NON-DESTRUCTIVE TESTS OF SAMPLES

3.1. Penetration and magnetic particle test

Aim of penetration and magnetic particle tests were to determine, is head check damage detectable by these methods in early stage. Result of penetration test can be seen in Fig. 2., and result of magnetic particle test can be seen in Fig. 3.

Fig. 2. Result of penetration test

Fig. 3. Result of magnetic particle test

Based on results of penetration and magnetic particle tests, these methods are applicable to detect head check damage in early stage. On the other hand, these non-destructive testing methods are not applicable to perform economically in case of long rails. Furthermore, depth of cracks can not be determined based on results of penetration and magnetic particle tests.

3.2. Eddy current test

Eddy current test was also performed in order to detect and sizing head check cracks of rail samples. Main test parameters of performed eddy current test can be seen in Tab. 1.

Tab. 1. Main parameters of eddy current test

Eddy current test equipment

Olympus Nortec 600

Test probe Olympus PL/500kHz-2MHz/Du

Test frequency 500 kHz

Horizontal gain 65,0 dB

Vertical gain 65,0 dB

Calibration of eddy current test equipment was

performed by steel reference specimen with 0,2; 0,5 and 1 mm depth notches. Eddy current indication of reference specimen can be seen in Fig. 4. and eddy current indication of rail sample can be seen in Fig. 5.

Fig. 4. Eddy current indication of 0,2; 0,5 and 1 mm depth notches

Fig. 5. Eddy current indication of rail sample

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Based on result of eddy current test, head check cracks can be detected by eddy current test method. In case of tested rail sample, depths of cracks are significantly greater than 1 mm, measured depths of cracks were 3-4 mm. Defects, caused by head check damage can be detected in early stage, eddy current tests can be performed economically in case of long rails, and depth of cracks can sized, but test parameters should be optimized before test.

3.3. Modelling and phased array ultrasonic testing

In order to determine optimal testing parameters of phased array ultrasonic testing, computer modelling was performed. Modelling was carried out by CIVA 2020 program, assuming a crack with 20 mm length and 3 mm depth, starting from the running surface. Result of CIVA 2020 modelling can be seen in Fig. 6.

Fig. 6. Result of CIVA 2020 modelling

Based on result of computer modelling, small portion of sound beam reflects back to the phased array ultrasonic probe, high gain needs to be chosen. High gain cause higher noise level, material testing experts needs to take into account this effect.

Based on result of modelling, chosen main parameters of phased array ultrasonic test can be seen in Tab. 2.

Tab. 2. Main parameters of phased array ultrasonic test

Ultrasonic test equipment

Olympus OmniScan MX2

Test probe Olympus 5L16

Wedge Olympus SA10-N55S IHC

Angle 40°-65°

TCG Ø=3mm side drilled holes, 27; 39; 51 and 63 mm depth

TCG Gain 21 dB

Coupling Sonotech Soundsafe NSN 6850-01-157-4338

Result of phased array ultrasonic test can be seen in

Fig. 7.

Fig. 7. Result of phased array ultrasonic test

Based on result of phased array ultrasonic test, defects caused by head check damage, can be detected in early stage. Depth of cracks of tested samples in some cases were over 3 mm. In case of long rails, phased array ultrasonic tests can be performed economically, and depth of cracks can be sized, but test parameters should be optimized before test, and continuously coupling is needed during the test.

4. MICROSCOPIC AND HARDNESS TESTS

After carried out and evaluated non-destructive tests, tested samples were cut and embedded into resin, in order to validate results of non-destructive tests.

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4.1. Microscopic measurements

Microscopic image of head check crack of tested rail sample and grain structure can be seen in Fig. 8.

Fig. 8. Microscopic image of tested sample, mag. 50X

Based on microscopic test, spread of cracks of tested samples are transcrystalline. Results of microscopic measurements of size of cracks can be seen in Tab. 3.

Tab. 3. Results of microscopic measurements

ID Depth (mm)

Length (mm)

Sample 1; crack 1 3,5 9,5

Sample 1; crack 2 2,0 5,8

Sample 1; crack 3 2,7 7,4

Sample 1; crack 4 2,4 6,4

Sample 2; crack 1 3,9 11,9

Sample 2; crack 2 2,2 7,5

Sample 2; crack 3 2,3 7,9

Sample 2; crack 4 0,8 4,6

Sample 2; crack 5 3,2 11,3

Sample 2; crack 6 3,9 11,5

Sample 3; crack 1 4,1 9,7

Sample 3; crack 2 1,7 3,9

Sample 3; crack 3 3,4 6,8

Sample 3; crack 4 3,6 6,7

Sample 3; crack 5 3,9 10,9

According to results of microscopic measurements,

depth of cracks were in some cases over 3 mm, along with results of eddy current and phased array ultrasonic tests.

4.2. Hardness test

After microscopic measurements, HV5 hardness tests were performed. Three series of hardness tests were carried out, measured hardness of samples, depending on distance from running surface can be seen in Fig. 9.

Fig. 9. Results of HV5 tests

Based on results of hardness tests, tested rail samples significantly hardened near the running surface, this hardening is caused by load of trains.

5. CONCLUSION

Head check damage might cause complete break of rails. Detecting and sizing head check cracks in early stage by non-destructive testing methods is very important in order to avoid accidents, but difficult task for material testing experts.

Cracks can be detected in early stage by penetration and magnetic particle tests, but can not be sized, and these tests can not be performed economically.

In case of eddy current and phased array ultrasonic tests, detecting and sizing head check cracks is available. Results of these tests showed great correlation with each other and microscopic measurements, but determine optimal test parameters before tests is very important.

ACKNOWLEDGEMENT

The research leading to these results is funded by Ministry for Innovation and Technology, No. NKFIH-8545-8/2019 project.

REFERENCES

[1] Girsch G.; Heyder R., Advanced pearlitic rail steels promice to improve rolling contact fatigue resistance. 7th WCRR, Montreal, Canada, June 2006.

[2] Janos Beli, Dr. Ferenc Horvat, Forming of head check damage measure and classification of defects, XIIIth Railway Running Technical Conference, Pécs, 2016.