Temperature-dependent toughening of carbon/epoxy composites using rubbery NBR/PCL nanofibers: Mode I fracture and damage mechanisms

This study evaluates the temperature-dependent toughening effect of rubbery NBR/PCL nanofibers in carbon/epoxy composites under mode I loading. Double Cantilever Beam (DCB) tests at six temperatures (−30 °C to 120 °C) measured interlaminar fracture toughness in non-modified and nano-modified laminates. At 30 °C, modified specimens achieved 164 % higher initiation toughness and 394 % higher propagation toughness than non-modified ones, driven by nano-modified interactions that enhanced energy dissipation through plastic deformation and fiber bridging. However, at elevated temperatures (90 °C and 120 °C), toughness declined sharply due to modified matrix softening above the rubber’s glass transition temperature (Tg) and PCL melting point, impairing crack-bridging efficacy. Fractographic analysis revealed contrasting failure modes: non-modified specimens exhibited matrix cracking (cusps) and fiber–matrix debonding, with cusp height reduction at higher temperatures. Nano-modified specimens showed hole-rich fracture surfaces and robust fiber–matrix adhesion at lower temperatures, confirming nanofiber-mediated toughening. This effect diminished at higher temperatures as nanofibers softened, reducing energy absorption. The results underscore the temperature sensitivity of NBR/PCL nanofibers in enhancing fracture resistance, with peak performance at lower temperatures. These findings provide critical insights for optimizing composite designs for applications exposed to varying thermal conditions, balancing toughening benefits against temperature-induced limitations.