Lamb Waves Topological Imaging of Multiple Blind Defects in an Isotropic Plate

Abstract

The study investigates the feasibility of the Lamb wave topological imaging method for detecting multiple blindholes in an isotropic plate. The topological imaging method is performed based on the computations of two wave fields, a forward and an adjoint, in the defect-free reference medium using different emitting sources. The image is computed by multiplying the forward and adjoint wave fields together and integrating them over time or frequency. The interferences of multimode aliasing and the scattering effect can thus be eliminated at the defectfree positions with an improved image resolution. To investigate the physical mechanism, the refocusing process of the multimode Lamb waves at the defect positions is presented by a face-to-face comparison between the snapshots of the forward and adjoint wave fields using the finite element simulation. The Lamb wave topological imaging method is numerically and experimentally verified to identify multiple blind-holes in an isotropic aluminium plate. The results demonstrate that the topological imaging method enables the suppression of the sartefacts resulting from the mode conversion and achieve high-resolution imaging of the blind defects

Novel method for detection of void defects under track slabs using air-coupled ultrasonic sensors

“…According to the dispersion equation of the multilayer slab, the group velocity and phase velocity dispersion curve of the guided wave propagation in the track slab can be obtained, as shown in Figure 8(a) and (b). The dispersion curve of the incident angle and frequency-thickness product are obtained according to equation (12), as shown in Figure 8(c). When the frequency-thickness product is 6 MHz mm, an A2 mode guided wave with a group velocity of 4000 m/s can be excited in the track slab at an incident angle of 3.8°.…”

“…where c 1 is the speed of sound in the air. According to equation (12), the phase velocity dispersion curve of a guided wave and the angle and frequency-thickness product dispersion curve of each mode can be obtained. As shown in Figure 7, when a guided wave propagates in a slab structure, it leaks into the air continuously, and the leaky guided wave formed in the air is received by the receiving sensor.…”

“…Ultrasonic guided waves have a large detection range, multimode characteristics, and strong detection ability and can detect structures from long distances and large scales with a small number of sensors [10][11][12] and can be used to select suitable single or multiple blues by adjusting excitation frequency. Ryden and Lowe 13 established the characteristic guided wave equation of a three-layer bonded concrete pavement structure using a global matrix and solved its dispersion curve, which provided theoretical guidance for the nondestructive testing of concrete highways using guided waves.…”

A SAFT Method for the Detection of Void Defect inside a Ballastless Track Structure Using Ultrasonic Array Sensors

“…The high-precision positioning, quantitative evaluation, and visual representation of void defects are crucial for the service performance and safety assessment of ballastless track structures. This context is also a hotspot for ultrasonic nondestructive testing [10]. The Synthetic Aperture Focusing Technique (SAFT) is a mature industrial ultrasonic imaging method that has been widely used in the detection of internal defects of concrete structures such as bridge decks [11], concrete columns [12], pavement [13], tunnels [14], etc.…”