Epoxy-based nanocomposites containing sustainable fillers for the realization of speckle patterns for digital image correlation analysis.

The Digital Image Correlation (DIC) is a non-contact technique that analyses a sequence of images providing full field measurement of displacements and strains over time. The DIC experimental set-up requires a speckle pattern. In this work a new epoxy-based nanocomposite, containing TiO nanoparticles and coffee-derived biochar fillers, was developed to obtain an ecofriendly flame-retardant material with suitable mechanical and optical properties for its use as speckle pattern for DIC applications. The optimized formulation is characterized by a uniform distribution of TiO and biochar particles and can be employed as self-standing material during the manufacturing of composite reinforcements. The physicochemical properties, mechanical behaviour and fire performances of the nanocomposite are described. The incorporation of the additives in the epoxy resin increases the Young's modulus by around 30% and almost doubles the burn-through time with respect to pristine resin, suggesting a slight condensed phase mechanism arising from the synergistic action of TiO and biochar. The effectiveness for creating DIC speckle patterns is validated by comparing the experimental strains measured by means of DIC analysis with those obtained through a traditional technique. Finally, the durability of the speckle patterns was assessed by accelerated thermal aging tests, confirming the potential application of the material in structural health monitoring of composite structures.