Guided wave detection and localisation of weld defects in circumferentially periodic thin-walled structures

Weld regions in regenerative cooling thrust chambers are highly susceptible to cracking under thermo-mechanical cyclic loading, posing a risk of catastrophic rocket engine failure. While the inspection of these thin-walled, circumferentially periodic structures is critical, conventional non-destructive evaluation (NDE) methods are ill-suited for rapid, remote screening. This study introduces an axial-guided-wave-based technique for rapid detection and localisation of weld defects in complex cylindrical thin-walled structures. The core of this approach is a novel, unit-cell-based modelling strategy developed within the Semi-Analytical Finite Element (SAFE) framework, which incorporates pointwise constraints to accurately capture the modal behaviours and wave structures of existing guided wave modes in circumferentially periodic waveguides of arbitrary cross-section. From this model, we establish a systematic, mode-selection workflow for selecting guided wave modes with enhanced detection sensitivity to localised weld defects. This workflow, encompassing dispersion analysis, energy localisation, mode-shape evaluation, and excitability assessment, provides a physics-based alternative to traditional, ad-hoc mode selection. Through comprehensive three-dimensional finite element (FE) simulations and experimental validation, a specific longitudinal type L(0,1)-like guided wave mode is identified as optimal, exhibiting concentrated wave energy at the vicinity of the weld seam, minimal dispersion, and high excitability. Furthermore, we provide a direct basis for localising defects via time-of-flight and assessing their significance via reflection-coefficient analysis. The proposed methodology establishes a versatile new framework for applying axial guided waves, paving the way for quantitative integrity monitoring of complex, circumferentially periodic engineering components.