Damage detection for aerospace composites using matrix resins functionalized with fluorescent probe molecules

Damage detection in aerospace composite parts is difficult because of their opacity and inhomogeneity. Low energy impact damage is especially troublesome because it often leaves no visible surface damage but can cause significant subsurface damage. Ultrasonic C-Scan can detect this damage but requires significant airplane downtime to conduct. This research proposes a method of damage detection based on fluorescent molecular probes with mechanochromic properties. Several molecules were designed and synthesized to be compatible with aerospace matrix resin and coating chemistry, and to exhibit fluorescent behavior that is dependent on local deformation and damage conditions. In a room temperature cured DGEBA-DETA solid resin, one probe showed strong fluorescent emission color change when samples were deformed in uniaxial compression or exposed to elevated temperatures. Relationships between the deformation mechanisms of epoxy and probe activation kinetics are explored using fluorescence imaging, reaction kinetics analysis, mechanokinetic, and hydrostatic pressure experiments. The results of testing support an activation mechanism for the probe that is dependent on intermolecular shear motion of the epoxy network. Copyright 2014 by Ryan E Toivola.