Reference-Based Layer Echo Suppression for Ultrasonic Evaluation of Thick Pultruded CFRP Composites

Pultruded carbon-fiber-reinforced polymer (CFRP) composites are increasingly used in large wind turbine blades, but ultrasonic evaluation of thick pultruded structures is complicated by repetitive layer reflections and prolonged transducer ringing. This study proposes a reference-based Layer Echo Suppression Method (LESM) to suppress deterministic structural responses that can mask internal defect echoes, thereby improving the visibility and identification of damage-related reflectors in thick CFRP components. Ultrasonic A-scan signals were acquired from sound and defect regions of a 100-mm-thick pultruded CFRP specimen containing an artificial defect at approximately 50 mm using a 0.5 MHz single-element contact transducer. LESM combines cross-correlation-based registration, independent wavelet denoising, reference-based Hilbert-envelope differencing, and positive-residual weighting of the denoised defect signal. The normalized cross-correlation coefficient between the reference and defect signals was 0.9941. DBR-based evaluation selected Sym5 at decomposition level 5. The final LESM signal yielded a defect-to-background ratio (DBR) of 34.82 dB, compared with 14.14 dB for the raw signal, an improvement of 20.68 dB. The defect-related waveform near 50 mm was retained while surrounding layer echoes and ringing were substantially reduced. By enhancing localized defect reflections without requiring additional ultrasonic hardware, LESM can support the identification of internal planar damage-like discontinuities in thick CFRP components using conventional pulse-echo measurements. These results demonstrate the proof-of-concept feasibility of the proposed method under the examined condition.