Ultrasonic Guided Wave Evaluation of Sealing Interface Evolution in Flared Tube Fittings

The contact state of the sealing interface in flared tube fittings directly affects the reliability of tube connections, yet stable ultrasonic characterization of its evolution during loading remains challenging. This study proposes a shape-residual method based on the overall waveform mismatch of target echoes generated by local oblique-incidence guided waves. The method aims to characterize the relative evolution of the sealing-surface contact state during loading of flared tube fittings. Dispersion analysis, experimental group-velocity verification, and finite element simulation indicate that local oblique-incidence excitation generates a target wave packet dominated by flexural modal components in the flared tube. This wave packet exhibits good identifiability and continuity within the selected characteristic time window, making it suitable for subsequent state characterization. A reference-template-based shape-residual feature is then constructed using an early-load echo as the reference, so that the overall morphological deviation of the current echo from the early reference state can be quantified. Equivalent axial loading experiments show that the target echo becomes identifiable after the sealing interface enters a stable compressive contact state. As the axial load increases, the echo undergoes coupled changes in arrival time, amplitude, and local waveform profile. Across multiple independent repeated loading tests, the shape residual εinit exhibits a consistent overall increasing trend and captures the stage-dependent transition of interface evolution from rapid adjustment to gradual stabilization. Compared with normalized energy attenuation and peak amplitude attenuation, the proposed shape residual shows better trend consistency, repeatability, and load-range discrimination capability. These results demonstrate that the method can convert the evolution of the sealing interface state during loading into a quantifiable process parameter, providing a relative characterization approach for ultrasonic testing and state evaluation of flared connection structures.