New insights into topographically feature guided waves (FGW) propagation in non-uniform elastic waveguides

Ultrasonic feature guided waves (FGW) have been found to exist in a variety of topographic waveguides, such as welds, bends and stiffeners in plate-like structures. These FGWs are guided along and localised to the vicinity of the structural feature and exponentially decay with distance transverse to it, exhibiting enhanced long-range inspection capability of topographic defects. In the literature the trapping of elastic waves can be investigated via asymptotic approaches based on the long-wave theory, which are applicable to the slowly varying waveguides (of variation in thickness or curvature). The present paper proposes a unifying framework for explaining FGW phenomena in three-dimensional waveguides of arbitrary cross section. The possible existence of FGWs as well as their characteristics are revealed by combining the paired guided wave modes in different sub-waveguides with specified boundary conditions. The description and analysis of FGWs will sustain their NDT/SHM applications in a boarder range of engineering structures. We have identified four families of FGW modes in an example transversely unbounded welded plate via the SAFE-PML approach. A comparative study is performed between modal solutions for guided wave modes in sub-waveguides and the target FGW modes. The existence criteria are concluded for such topographically trapping effect, and the extent of mode confinement is quantitatively discussed. These numerical evaluations provide new physical insights into the FGW propagation and enable us to study trapped modes in general non-uniform elastic waveguides.