The increasing need for repairs of polygonized wheels on high-speed railways in China is becoming problematic. At high speeds, polygonized wheels cause abnormal vibrations at the wheel-rail interface that can be detected via axle-box accelerations. To investigate the quantitative relationship between axle-box acceleration and wheel polygonization in both the time and frequency domains and under high-speed conditions, a dynamics model was developed to simulate the vehicle-track coupling system and that considers both wheel and track flexibility. The calculated axle-box accelerations were analyzed by using the improved ensemble empirical mode decomposition and Wigner-Ville distribution time-frequency method. The numerical results show that the maximum axle-box accelerations and their frequencies are quantitatively related to the harmonic order and out-of-roundness amplitude of polygonized wheels. In addition, measuring the axle-box acceleration enables both the detection of wheel polygonization and the identification of the degree of damage.

A high resolution fiber-optic acoustic emission (AE) sensor using a random fiber laser (RFL) is proposed. The AE probe is undertaken by a random-gratings-based erbium-doped RFL. A narrow linewidth π-FBG is used as a wavelength locking and sensing element in the RFL. The random distributed feedback in RFL significantly extends the effective cavity length of the laser, thus reduces the thermal frequency noise of the laser and improves the resolution of AE signal. A narrow lasing operation with a 20 dB linewidth of ∼10.41 kHz and a frequency noise of ∼10 Hz/√Hz above 1 kHz is realized. The 3×3 coupler interrogation technique is used for signal demodulation. A high AE signal resolution of ∼280 fɛ/√Hz @ 1 kHz is obtained. To the best of our knowledge, this is the first time that RFL is used in the 3×3 coupler based AE demodulation scheme to improve the system resolution.

We proposed a high-resolution static strain sensor using random fiber lasers (RFLs) and the sweep-beat frequency interrogation technique. An RFL with 3-dB linewidth of 548 Hz and frequency jitter of ∼±70 kHz is established by random distributed gratings and a high-precision π-phase shifted fiber Bragg grating (π-FBG) ring mirror. Two RFLs act as probes and are interrogated by frequency-swept laser. One RFL is used for strain sensing, the other is used for compensating temperature and frequency shift of swept laser. By using sweep-beat frequency demodulation, a static strain resolution of 196 pico-strain (pε) is obtained in 160 s. To the best of our knowledge, this is the first time that RFLs have been used in static strain sensing based on sweep beat-frequency interrogation.

With increases in train speed and traffic density, problems due to wheel polygons and those caused by wheel–rail impacts will increase accordingly, which will affect train operational safety and passenger ride comfort. This paper investigates the effects of polygonal wheels on the dynamic performance of the track in a high-speed railway system. The wheel–rail interaction forces caused by wheel polygons are determined using a dynamic vehicle–track model, and the results are entered into a slab track finite element model. The influence of the harmonic order and out-of-roundness (OOR) amplitude of wheel polygons on the transient dynamic characteristics of the track(von Mises equivalent stress, displacement, and acceleration) is examined under high-speed conditions. The results indicate that the vibration acceleration and von Mises equivalent stress of the rail increase in proportion to the harmonic order and the OOR amplitude and velocity of a polygonized wheel. The vibration displacement of the rail first increases and then decreases with a change in the harmonic order, and reaches a maximum at the ninth order. The dynamic responses of the concrete slab layer, cement-asphalt layer, and support layer increase linearly with the harmonic order and amplitude of wheel polygons and decrease from top to bottom. Through a combination of numerical simulations and real-time monitoring of rail vibrations, this study provides guidance on potential sensor locations to identify polygonized wheels before they fail.