Quantitative Assessment of Lightning Strike Damage in CFRP Composites Based on Real Ultrasonic Data and 2D Topological Skeleton Analysis

Lightning strikes induce complex internal delamination and matrix crack networks in Carbon Fiber Reinforced Polymer (CFRP) composites, critically threatening aerospace structural safety. While existing non-destructive testing (NDT) methods can locate defects, they lack the capability to precisely quantify the morphological severity of the damage. This study proposes a quantitative assessment framework based on 2D topological skeleton extraction, applied directly to real ultrasonic C-scan data of CFRP laminates before and after lightning strike. By applying binarization segmentation and morphological thinning, the single-pixel centerlines of the damage networks are extracted. Combined with Euclidean distance transform and 8-neighborhood topological analysis, the damage length, crack opening width, and topological branching points are precisely calculated. Experiments on the real Swinburne lightning strike ultrasonic dataset demonstrate that the topological branching points surged from 335 to 1719 (a 5.1-fold increase), and the damage length increased from 2189 to 5045 pixels (a 2.3-fold increase) after the strike. Threshold sensitivity scanning (from 0.45 to 0.65) confirmed that the fold increase robustly maintained between 4.6 and 17.0. This non-linear topological mutation is highly consistent with the inter-laminar fractures induced by lightning plasma explosion, providing direct topological evidence for the lightning strike damage mechanism. This framework offers an auditable and reproducible quantitative assessment tool for aerospace structural health monitoring.