Ultrasonic method for estimation the stress state in rails V. V. Muraviev, L. V. Volkova, A. V. Platunov, V. E. Gromov, and S. L. Korotkov Citation: AIP Conference Proceedings 1683, 020157 (2015); View online: https://doi.org/10.1063/1.4932847 View Table of Contents: http://aip.scitation.org/toc/apc/1683/1 Published by the American Institute of Physics Ultrasonic Method for Estimation the Stress State in Rails V. V. Muraviev1, a), L. V. Volkova1, A. V. Platunov1, V. E. Gromov2, and S. L. Korotkov3 1 Kalashnikov Izhevsk State Technical University, Izhevsk, Russia Siberian State Industrial University, Novokuznetsk, 654007 Russia 3 Izhevsk Railway Technical School, Branch of Samara State Transport University, Izhevsk, Russia 2 a) Corresponding author: pmkk@istu.ru Abstract. The article presents the experimental results of mechanical stress measurements in the R65 rail segment. The comparison with theoretical estimation the stresses arising from temperature impact on the fixed rail is conducted. The results of modeling the compressive stresses in the rail segment are presented. INTRODUCTION Residual stresses in railroad rails that occur in the manufacturing process and change later as a result of repeated exposure to train loads during operation. The most significant sources of residual stress in the manufacturing process are rolling operations, heat treatment and straightening of the rail. Subsequently, the residual stresses change during the rail operation as a result of elastic deformation of the rail and plastic deformation caused by contact interaction between wheel and rail. Also fixed rails are exposed to tensile stress in hot weather and compressive stresses in cold weather. Such temperature changes can lead to stability loss or fracture of rails and eventually lead to a train derailment. It is known that the temperature rise of 1qC results in an increase in stress about 2.5 MPa. In order to prevent the release or rupture of the rail it is necessary to timely determine the stress level. TERMAL STRESSES MODELING The calculated loads in rail, when the temperature influences on the railroad track are in range from 20qC to 38qC assuming an uniform heat distribution in the whip fixed at the temperature of 20qC, are in the range from 0 to í50 MPa (Fig. 1a). Longitudinal tensile strength at the specified interval varies from 0 to 400 kN (Fig. 1b), and the unrealized increase in length of 1000 m whip is about 200 mm [1]. The simulation in the ANSYS software is conducted to evaluate compressive stresses arising from the temperature increase with the application of longitudinal loads from 0 to 25 tons to the rail of R65 type and 250 mm in length (Fig. 2). This load corresponds to the stresses occurring in the fixed rail at a temperature of about 30qC. The graph shows that the change in load of 20 kN leads to change in stress of 4 MPa. ELECTROMAGNETIC-ACOUSTIC METHOD FOR RESIDUAL STRESS TESTING Among the known methods of residual stress evaluation the most promising one is the ultrasonic method based on acoustoelasticity effect which consists in the ultrasonic wave propagation velocity dependency on the stress state level of the metal. For the stresses evaluation in the rail, there are two transversal waves, polarized perpendicularly and along the rail axis and propagating between the head and foot of rail, used. Advanced Materials with Hierarchical Structure for New Technologies and Reliable Structures AIP Conf. Proc. 1683, 020157-1–020157-5; doi: 10.1063/1.4932847 © 2015 AIP Publishing LLC 978-0-7354-1330-6/$30.00 020157-1 ı, MPa F, kN T, °C (a) T, °C (b) FIGURE 1. Dependence of internal stresses (a) and longitudinal force (b) on the temperature of the rail The difference in arrival time of the two perpendicularly polarized waves pulses is proportional to the averaged stresses in the rail and the texture of the material [2]. Taking into account the anisotropy and initial residual stresses it is possible to calculate the stress state V according to the formula: §t · (1) V D ¨ 1 1¸ , © t2 ¹ where D is the elastic-acoustic coupling coefficient for the studied material; t1, t2—the time of two polarized waves propagation along the rail height. For rail segment stress state studies the electromagnetic acoustic structurescope SEMA, depicted in Fig. 3, is used. It includes the electric pulse generator 2 that produces a powerful electrical probe impulse, the electromagnetic acoustic transducer (EMAT) for exciting and receiving short acoustic pulses of transversal waves, the amplifier 3 and the ADC board 4 built into the computer with a screen which reflects the waveform of reflected pulses (Fig. 4a). EMA transducer is mounted on an easily accessible surface of the rail web or head. Measurements can be performed on rails of different heights and different brands from R50 to R75 with the measurement error of pulses arrival time difference ǻt = r2 ns. According to the difference of arrival times t1 and t2 of transversal waves pulses (Fig. 4b), polarized in the transverse and longitudinal planes of one rail section, it is possible to calculate the stress ı values by the formula (1). FIGURE 2. Dependence of the calculated stress on the longitudinal load value 020157-2 (a) (b) FIGURE 3. Ultrasonic structurescope SEMA: general view (a); block diagram (b) (a) (b) FIGURE 4. Bottom pulses waveform (a), pulses arrival time difference (b) FIGURE 5. Electromagnetic-acoustic transducer 020157-3 (a) (b) (c) FIGURE 6. Scheme of the rail sounding: top view (a); front view (b); side view (c) The excitation and receiving of transversal waves is carried out using the contactless electromagnetic-acoustic (EMA) transducer [3] shown in Fig. 5 [10]. THE RESULTS OF RESIDUAL STRESS MEASUREMENTS BY EMA METHOD Experimental evaluation of mechanical stresses is conducted using the sample of the new R65 rail with length of 250 mm. The sample was subjected to compression with loads from 0 to 247 kN by the Instron 300DX machine. Evaluation of residual stresses is carried out using the SEMA structurescope placing the emitting and receiving transducers at the rail head center (Fig. 6). The measurement results are shown in Fig. 7. Initial stresses in the rail segment at zero load are from 34 to 37 MPa. With increasing compressive loads, there is a reduction of tensile stresses and growth of compressive ones in the rail. The load increase of 20 kN leads to an compressive stress increase of 4 MPa with the stress measurement error of 2 MPa. There is a satisfactory agreement between the results of numerical calculation and experiment. Some discrepancy of calculated and experimental dependencies can be explained by the stress heterogeneities along the rail cross section which are met on the way of ultrasonic beams, and a compensating effect of initial stress on the load. Thus, the total error can be estimated by the value of 5 MPa. Since the dependencies of stress values on applied external load magnitudes taken by the SEMA structurescope with specified error are linear, the internal stresses caused by temperature effects do not go out of linear elasticity, it can be argued about the possibility of using the electromagnetic-acoustic structurescope to measure the internal stresses in the rail whips when the object temperature varies from í50qC to +60qC. (a) (b) FIGURE 7. Dependence of mechanical stresses measured by the acoustic structuroscopes SEMA on the applied load magnitude: considering the initial residual stresses (a), with subtraction the initial stresses (b) 020157-4 CONCLUSION Thus, when using the electromagnetic-acoustic structurescope SEMA it is possible to track the changes of stresses in the rails. The use of EMA transducers allows tuning away from acoustic coupling, and repeated measurements can be taken in selected rail areas without additional time spent on surface preparation. Timely stress measurements in rails can prevent the deformation of the rails and their breakage because of temperature fluctuations and mechanical impact. ACKNOWLEDGMENTS The study performed by the grant of Russian Science Foundation (project No. 15-12-00010). REFERENCES 1. 2. 3. A. F. Lesun, The Kinetics of Fatigue Fracture Rail Lashes the Defects at the Base of the Rail, and Measures to Prevent, Ph.D. Thesis, Higher InterAcademy Attestation Committee, 2005. V. V. Muravyev, O. V. Muravyeva, V. A. Strizhak, A. V. Pryakhin, and E. N. Balobanov, RUS Patent No. 134658 (35 May 2013). N. E. Nikitina, Acoustoelasticity. Practical Experience (TALAM, Nizhniy Novgorod, 2005). 020157-5

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