Dual-mode impact-wave interrogation of interfacial debonding in coated pipelines via baseline-normalized period–attenuation coupling

Interfacial debonding in coated pipelines alters local vibration and wave transmission, while conventional response features can depend on detection mode and defect medium. We propose a dual-mode impact-wave method combining equivalent dynamic identification with baseline-normalised period–attenuation coupling. Transmission detection captures responses transmitted behind the defect, whereas defect-top excitation probes local responses. The dominant frequency determines the characteristic period Td, and an equivalent attenuation coefficient α is estimated from the post-peak Hilbert envelope. The coupled parameter Kd=Td/α is referenced to the intact response using Kn=lnKd/K0. Finite-element simulations of air cavities and rubber surrogate defects reveal mode- and medium-dependent variations in frequency, envelope width, cumulative response and energy flow. Nevertheless, Kn maintains a consistent relative defect-size ranking across monitoring positions, with larger defects generally exhibiting lower attenuation coefficients and higher coupling factors. Impact experiments on coated steel-pipe specimens containing 10–30 mm rubber inserts confirm an increasing Kn with defect size under both detection modes. The proposed indicator therefore supports baseline-referenced relative defect-size characterisation, while energy-flow descriptors provide complementary insight into response persistence and transmitted-energy redistribution.