Phase-Based Guided-Wave Imaging in Plates Using Band-Averaged Cross-Spectral Features and RAPID Mapping

Phase-based guided-wave features are less sensitive to amplitude fluctuation and time-of-flight or velocity-estimation uncertainty than conventional amplitude- or arrival-time-based features. However, their use in guided-wave structural health monitoring has largely remained limited to path-level damage indication, which does not directly provide scalable spatial localization for sparse, permanently installed sensor networks. This study addresses this gap by proposing a baseline-referenced Lamb-wave imaging framework that integrates a band-averaged cross-spectral phase (BACP) index with the Reconstruction Algorithm for Probabilistic Inspection of Damage (RAPID) for defect localization in plate-like structures. BACP quantifies defect-induced phase perturbations over the dominant excitation band while reducing sensitivity to amplitude scaling and unstable spectral components, whereas RAPID provides a transparent and computationally efficient means of fusing phase-sensitive transmitter–receiver paths into normalized probability maps. Three-dimensional finite element simulations and experimental validation on aluminum plates with circular PZT arrays were conducted to assess localization performance under variations in defect size, receiver count, sensor density, frequency band, excitation frequency, and thickness-loss severity. The proposed BACP–RAPID framework produced compact, high-contrast probability maps and achieved normalized localization errors below 3% in the experimental cases considered. Numerical studies showed that the framework remained effective under moderate sensing reductions, although severe reductions increased localization error. Benchmark comparisons with geometric phase change and amplitude-ratio methods demonstrated more accurate and consistent localization for the defect cases examined. The framework is mainly suited to single-defect scenarios, but the results indicate strong potential for scalable guided-wave structural health monitoring.