Experimental analysis of nanofiber embedding and layer thickness on fracture toughness in fiber–epoxy laminates

The application of Carbon fiber-reinforced polymer (CFRP) laminates is becoming more common due to their excellent in-plane properties and high strength-to-weight ratio. But interfacial strength is an obstacle that limits their structure reliability. Some common indicators which effects the interfacial strength are Mode-1 and Mode-II delamination, interlaminar and interfacial shear strength etc. Interlaminar delamination, subjected to out-of-plane stresses, is a common and critical failure in composite laminates because it propagates rapidly and can cause accidents. In this study, Zinc Oxide (ZnO) nanofibers were fabricated through the electrospinning (ES) process and deposited onto the surface of carbon fibers (CF)/epoxy laminate at varying electrospinning times to enhance their interlaminar fracture toughness. Different deposition levels were achieved by varying the ES durations of 30, 60, and 90 min and layer thicknesses of 28.86 ± 2.22 μm, 61.04 ± 2.76 μm, and 84.14 ± 4.83 μm, respectively. The influence of ZnO nanofiber layer thickness on composite performance was thoroughly investigated, and the most effective deposition level was identified within the investigated window. Mode-I fracture toughness \:G_Ic was calculated for all categories of specimens using standard ASTM-D5528. Compared to the untreated laminate, enhanced laminates with ZnO nanofibers exhibited significant improvements in fracture toughness. The value of \:G_Ic was increased by ZnO nanofiber depositions: 78% for category (30-min-28.86 ± 2.22 μm), 83% for (60-min-61.04 ± 2.76 μm), and 40% for (90-min- 84.14 ± 4.83 μm). Mass and atomic percentages of Zinc, Oxygen, and Carbon were identified using Energy-dispersive X-ray (EDX). To measure the ZnO nanofiber deposition layer thickness and to analyze the fractured surface, scanning electron microscopy (SEM) was used. Continuous interlocking of electrospun ZnO nanofibers with carbon fibers and epoxy matrix was observed in SEM observations, which contributed to enhanced delamination resistance by adding additional nanofiber bridging and pull-out mechanisms. These findings indicate a strong effect of the intensity of ZnO nanofiber deposition on the fracture toughness, but there is a limited amount of nanofibers that can be added. The study illustrates that the controlled reinforcement of ZnO nanofibers offers a reliable approach to enhance the delamination resistance of fiber-reinforced polymers.