We observe maxon-like dispersion of ultrasonic guided waves in elastic metamaterial consisting of pillars periodically bonded to one side of a beam. The pillars act as asymmetric resonators and induce strong hybridization between longitudinal and bending wave modes. This creates a local maximum (i.e., maxon) on the dispersion curve at the interior of the first Brillouin zone. We observe localized maxon mode with zero group velocity (ZGV) through numerical and experimental investigation. In contrast to the roton mode, our measurements also demonstrate a unique maxon feature of peak-frequency down-shift in space.
Most modern railways use continuous welded rail (CWR) because they support higher transport speeds, provide less friction, and generally require less maintenance. However, thermal buckling of CWR has been a long-standing challenge for the railroad industry. Rail neutral temperature (RNT) is the temperature at which the longitudinal stress of a rail is zero. Due to the lack of expansion joints, CWR develops internal tensile or compressive stresses when the rail temperature is below or above, respectively, the RNT. Therefore, thermal stress or RNT measurement and management of CWR become more important for railroad maintenance. In this work, the team proposes a practical and nondestructive method for RNT estimation exploiting local resonances in rails.
Local resonances, formed by zero-group velocity (ZGV) and cutoff frequency points, have been extensively studied using impulse-based approaches, such as pulse laser and impact echo. In this work, we showcase the electromechanical impedance (EMI) technique as an option to extract and promote zero-group velocity and cutoff frequency resonances in a waveguide structure. We identify the mechanisms of multiple resonances in the EMI spectra via a wave propagation perspective. Furthermore, we extract the dynamic response profiles at a cutoff frequency and a ZGV frequency to confirm the localized minimum frequency behavior within corresponding branches.
Local resonances formed by zero-group velocity (ZGV) and cutoff frequency points usually demonstrate sharp resonance peaks in frequency spectra, which can be utilized for nondestructive evaluation (NDE) and Structural Health Monitoring (SHM). The existence and application of those local resonances have been extensively reported in plate and pipe structures. However, local resonances in rails are rarely studied. The team recently reported that impulse dynamic tests can promote the local resonances in rails up to 40 kHz, and the results were verified using both semi-analytical finite element (SAFE) analysis and frequency-domain fully discretized finite element analysis. In this work, we present the discovery of ZGV modes and cutoff frequency resonances in free rails up to 80 kHz using piezoelectric elements. A miniature low-cost PZT patch works as a consistent excitation source compared with the impulse dynamic testing method. First, we implement the SAFE analysis to compute dispersion curves of a standard AREMA 115RE rail and to identify potential ZGV and cutoff frequency points up to 80 kHz. Then, to understand the existence and detectability of identified ZGV and cutoff points in a free rail, we install one PZT patch on the side of the rail head. A chirp signal covering 20 to 120 kHz is selected as the excitation to cover the desired frequency range. Finally, we perform a spatial sampling of wave propagation using three receivers along the wave propagation direction to calculate the dispersion relations experimentally via two-dimensional Fourier Transforms (2D-FFT). This study verifies the existence of ZGV modes in free rail up to 80 kHz and demonstrates the feasibility of using piezoelectric elements to generate local resonances.
In-situ thermal stress determination in structures is a challenging experimental mechanics task, especially if it requires a nondestructive approach. Thermal stress measurement and management of continuous welded rail (CWR) have become more important for railroad maintenance. Local resonances formed by zero-group velocity (ZGV) and cutoff frequency points usually demonstrate sharp resonance peaks in frequency spectra, which can be utilized for nondestructive evaluation (NDE) and Structural Health Monitoring (SHM). This paper examines the potential of the local resonances to provide an estimation of axial stress in rail structures. The local resonances are generated by bonding a piezoelectric element on the rail samples. A 610-mm rail sample was tested, and different axial stress levels were applied by an MTS tensile-compression machine and by measuring the local resonance signature in selected frequency bands to study sensitivity to axial stress of local resonances. The results show that appreciable sensitivities of the local resonances are found under varying stress levels and can be further utilized for in-situ thermal stress determination for rails.
Effective rail neutral temperature (RNT) management for continuous welded rail (CWR) is of great importance to the railway industry. RNT is the temperature at which the longitudinal stress of a rail is zero. Due to the natural axial constraint and lack of expansion joints in CWRs, rails can develop internal tensile stresses in cold weather or compressive stresses in warm weather, which can lead to rail fracture or buckling in extreme conditions. In this work, the team proposes a practical and effective method for RNT estimation. First, a contactless non-destructive and non-disrupting sensing technology was developed to collect real-world rail vibrational data, and a series of laboratory data collection is performed for verification. Second, the team established an instrumented field test site at a revenue-service line in the state of Illinois, and performed multi-day data collection to cover a wide range of temperature and thermal stress levels. Third, numerical models were developed to understand and predict the rail track vibration behavior under the influence of temperature and RNT. An excellent agreement (discrepancies less than 0.01%) between model and experimental results were obtained by using an optimization approach. Finally, a supervised machine learning algorithm was developed to estimate RNT using the field-collected rail vibration data. Furthermore, sensitivity studies and error analyses were included in this work. The system performance with field data indicates that the proposed framework can support reasonable RNT prediction accuracy when measurement/model noise is low.
In this paper, development of a nonlinear vibro-acoustic modulation technique based on non-contact piezoelectric sensors was investigated to detect the crack progression of concrete cracking caused by thermal treatments. Experimental results show that defined ultrasonic nonlinear parameter is in agreement with the accumulation of thermal crack. The phase velocity of Rayleigh wave and resonance frequency of vibrations were measured and compared with ultrasonic nonlinear parameter to validate the sensitivity of developed method. X-ray Computed Tomography (CT) technique is applied to visualize microstructure of thermal damage. The CT images show that proposed nonlinear parameter is reliable and well correlated with the microstructural defects of concrete specimen. Due to the advantage of removable characteristic of non-contact ultrasonic measurements, the developed non-contact nonlinear wave modulation method could be promising for quick and convenient damage assessment of concrete structures in engineering practice.
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