This study focuses on exploiting the non-propagating modes of the ultrasonic guided waves in structural components, such as plates, trusses, beams, pipelines, and rails, which are ubiquitous in today’s civil infrastructural systems. These long-neglected localized modes are not currently used by state-of-the-art engineering practice. A thorough understanding of them and their interactions with various structural defects will lead to a new paradigm of ultrasound technology that can support early warning of structural integrity conditions.
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.
Leveraging modal orthogonality, the normal mode expansion method provides an avenue to extract individual modes in a multi-modal guided wave propagation scenario. The team implements a mode separation technique based on the normal mode expansion and demonstrates its effectiveness in propagating guided waves in an aluminum bar with a rectangular cross-section. Instead of monitoring the wave profiles at the bar end, by the application of mapped field distribution, we can evaluate the amplitude of propagating guided waves at any location. The effectiveness of the proposed technique is verified through numerical models.
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.
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