Enhanced damage imaging of BVID in CFRP using efficient local defect resonance identification

Nonlinear ultrasonic detection of carbon fibre reinforced polymers (CFRP) is recognised as effective for barely visible impact damage (BVID), but its sensitivity is often compromised by the masking of weak harmonic responses by noise. In this study, local defect resonance (LDR) spectroscopy is employed to improve BVID localisation under noise interference. A novel successive computing methodology (SCM) is proposed, integrating a validated 3D finite element (FE) model (calibrated via impact force-time history and delamination characteristics) with experimental scanning laser Doppler vibrometry. Critically, linear perturbation analysis performed directly on the impact-induced BVID model determines LDR frequency without requiring a priori defect location knowledge, simplified geometric assumptions, or cumbersome sweeping tests and signal reconstruction procedures. The identified LDR is validated using a proposed damage index, the maximum response amplitude (MRA) of harmonics, confirming that harmonic MRAs are amplified when the excitation frequency matches the LDR. The determined LDR frequency is then introduced into noisy nonlinear ultrasonic tests, resulting in significantly enhanced BVID imaging resolution despite noise disturbance. Quantitative assessment using the Michelson contrast metric confirms that high imaging contrast is maintained by LDR excitation even under significant noise interference, demonstrating its robustness for practical BVID detection.