Research on an innovative nettle-based filler in the field of hybrid adhesive joints

This study examines the influence of micro-particles derived from Urtica dioica L. (nettle) fibres on the mechanical behaviour of structural epoxy adhesive joints. Short-fibre filler (diameter < 100 µm, length < 500 µm) characterised by noticeable porosity and morphological heterogeneity was added to the epoxy matrix in various concentrations (0.25–1.00 wt.%). Static tests revealed that the optimum strength, deformability, and cohesion of the joints were achieved at 0.50 wt.%, resulting in up to a 20% increase in strength compared to the reference epoxy. Higher filler contents led to poorer dispersion, increased heterogeneity, and reduced mechanical properties. Under cyclic loading (low-cycle fatigue), joints with higher filler content exhibited significantly shorter fatigue life, with premature failure often occurring early in testing—joints with pure epoxy or with 0.25 wt.% filler demonstrated better stability and fatigue resistance. Scanning electron microscopy analysis confirmed delamination at both the adhesive–adherent interface and within the adhesive layer under cyclic stress, for components subjected to cyclic loading, pure epoxy or epoxy modified with up to 0.25 wt.% natural nettle filler appears to be the most effective solution. The findings highlight the potential of bio-based fillers to enhance sustainable structural adhesive system.