Smart Mechanoluminochromic Coating for Impact Damage Detection in Composite Structures

The susceptibility of composite materials to impact loads remains a critical challenge in aerospace applications. Conventional Structural Health Monitoring (SHM) systems employ embedded or adhesively bonded sensors, such as Bragg gratings or ceramic piezoelectrics, which compromise the mechanical integrity of the laminate and introduce complexities in signal acquisition due to optical, electrical, or wireless connections. Mechanoluminochromic (MLC) materials offer a transformative solution by exhibiting fluorescence emission spectrum changes under mechanical stimuli. This study presents a novel smart coating utilizing an MLC Cu-based Hybrid Coordination Polymer (HCP) dispersed into a thermoset epoxy-based paint. Applied via spray painting on a cross-ply carbon fiber reinforced plastic (CFRP) laminate, the coating enables full-field impact damage detection under UV illumination without altering the aesthetic appearance of the structure. Low-velocity impacts (LVI) were conducted at varying energy levels, and the resulting barely visible damage (BVD) was analyzed using smartphone-acquired images processed with Matlab algorithms. The green/blue fluorescence ratio demonstrated correlation with impact forces, while micrograph analysis of the laminate cross-section confirmed damage alignment with mechanoluminochromic responses. This innovative nondestructive inspection (NDI) technique offers cost-efficiency, reliability, and ease of implementation, establishing a promising avenue for SHM applications with potential real-time computer vision integration.