Multiscale Characterization and Evaluation of Low-Energy Bird-Strike Damage in CFRP

Carbon fiber-reinforced polymer (CFRP) laminates are susceptible to barely visible impact damage (BVID) under low-energy bird-strike-like conditions. However, in previous studies, most damage evaluations for BVID were limited to a single scale. In this work, a multiscale characterization and evaluation method integrating the analytic hierarchy process (AHP) and the CRITIC weighting method was proposed to investigate the damage evolution of CFRP laminates under low-energy impacts (approximately 12-33 J). Delamination area (S_Da), indentation depth (P_D), surface crack aspect ratio (R_A), energy dissipation, and compression-after-impact (CAI) strength were analyzed based on phased-array ultrasonic C-scanning, 3D optical profilometry, and scanning electron microscopy. The results showed that P_D, S_Da, and energy dissipation increased from 108.73 μm to 213.93 μm, from 228.6 mm² to 695.8 mm², and from 5.96 J to 21.40 J, respectively, with increasing impact energy. Meanwhile, CAI strength decreased from 202.2 MPa to 118.9 MPa, with a maximum degradation rate of 41.16%. A critical transition was observed in the medium-to-high energy range, where delamination growth gradually plateaued, while intralaminar cracking and fiber fracture became increasingly dominant. The proposed framework enables quantitative grading of BVID severity and provides a practical basis for assessing residual damage in impacted CFRP laminates.