Optical stress sensing alumina nanocomposite coatings for aerospace structures

Alpha alumina (α-Al₂O₃) nanocomposites have a multi-functional stress sensing ability via their photoluminescence (PL) spectral peak shifts defined by Piezospectroscopic (PS) relationships. This has prompted the development of α-Al₂O₃ nanocomposite coatings to enable real-time stress measurements and damage assessment of structures with the potential for unparalleled spatial resolution and sensitivity. Here, two types of stress-sensing coatings were evaluated, including an atmospheric plasma spray (APS) coating on metallic substrates and an epoxy nanocomposite coating on a composite substrate. The stress sensitivity of the APS coating, represented by the slope of the peak shifts against stress or PS coefficient, varied inversely with the thickness of the substrates between 3.2 and 1.7 cm^-1=GPa. These values decreased by more than 50% after tensile cycling for all substrate thicknesses due to microstructural damage in the APS coating indicating the need for repeatability and durability studies. The epoxy nanocomposite coating successfully captured the stress gradients associated with an open hole tension (OHT) composite substrate revealing damage initiation at 77% of failure load, earlier than visual appearance of a surface crack (93%). The findings validate the successful development of quantitative and multiscale spatial resolution stress-sensing coatings, capable of detecting subsurface damage of composite structures, that will take structural testing and integrity monitoring to the next level.