Performance and failure mechanisms of interleaved E‐glass/epoxy composites using knitted fiber‐reinforced adhesive films

In this study, an adhesive film with embedded knitted fibers is utilized as the interleaved layer to evaluate the effect of carrier fibers on delamination resistance, crack migration, and R-curve behavior in glass fiber/epoxy composites. The interaction between the knitted fibers within the adhesive layer and their integration with the main laminate matrix is examined. Mode-I and mode-II R-curves are determined using double cantilever beam (DCB) and end-notch-flexure (ENF) samples, respectively, with fractography used to characterize damage mechanisms and crack paths. The low viscosity of the epoxy matrix during pressurized curing causes redistribution of the knitted fibers and variations in the adhesive layer thickness along and across fiber orientations. This results in localized shifts in fracture mechanisms, leading to variations in toughness values. The weak interface between knit fibers and the adhesive layer matrix induces crack paths that cause significant fluctuations in GIC values in Mode-I. The adhesive layer enhances fracture toughness by promoting a tortuous crack path, resulting in a 175.47% increase in GIC during propagation and a 171.56% increase in GIIC values during the initiation phase, respectively. The adhesive layer's thickness and fiber distribution, influenced by resin flow during manufacturing, played a critical role in fracture behavior. These findings provide insights into the mechanisms driving interlaminar toughening and highlight the potential of knitted fiber-reinforced adhesive films for improving delamination resistance in composite structures.