PSI - Issue 80

Seiji Mitsubayashi et al. / Procedia Structural Integrity 80 (2026) 423–430 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Thermoplastic resins are capable of being remelted by heating [Vogiantzi et al. (2025); Hou et al. (2025)], and therefore, they have attracted attention as a potential matrix material for CFRP from the perspective of the circular economy [Fedorenko et al. (2025)]. Nevertheless, carbon fiber reinforced thermoplastics (CFRTPs) generally exhibit lower mechanical strength than conventional CFRPs, which has prompted ongoing research aimed at improving their performance. One of the major challenges with CFRTPs is their poor interfacial adhesion, which can lead to delamination. Therefore, improving fracture toughness is critical [Sehrawat et al. (2025)]. A common strategy for enhancing the mechanical properties of composite materials is the addition of nanoscale fibers to the matrix [Chen et al. (2024)]. While fine glass fibers and carbon nanotubes (CNTs) have traditionally been used [Borowski et al. (2015)], plant derived cellulose nanofibers (CNFs) have recently gained attention due to their low environmental impact [Wu et al. (2025)]. CNFs not only offer sustainability advantages but also possess excellent mechanical properties [Jose et al. (2025)]. Previous studies have shown that the incorporation of CNFs into the matrix can improve the flexural strength and fatigue life of CFRP [Umeki et al. (2016)]. Furthermore, the authors previously showed that the addition of CNFs improved the impact resistance of CFRP after water absorption under elevated temperature and pressure conditions [Mitsubayashi et al. (2024)].Therefore, adding nanoscale fibers to the matrix is considered an effective method for improving the interlaminar fracture toughness of composite materials. However, the influence of CNF placement on interlaminar fracture toughness has not yet been fully elucidated. In this study, two types of CFRP specimens were prepared: one with CNFs applied to the carbon fiber surface, and another with CNFs incorporated into the resin. Fracture toughness tests were conducted to compare the effects of CNF added location on the interlaminar fracture toughness of CFRTP. 2. Experimental method 2.1. Materials The matrix material used was polypropylene (PP) (Prime Polypro ™ type J107G, Prime Polymer Co., Ltd., Japan), and the reinforcement was a plain-woven carbon fiber cloth (Torayca® CO6343, Toray Industries, Inc., Japan). The PP is a homopolymer with a melt flow rate of 30 g/10 min (measured at 230°C under a 2.16 kg load). Cellulose nanofiber (CNF) slurry (BiNFi-s WFo-10002, Sugino Machine Co., Ltd., Japan) was used as the reinforcing additive. 2.2. Method of adding cellulose nanofiber In this study, two types of CFRP specimens were prepared: one with CNFs incorporated into the resin, and another with CNFs applied to the surface of the carbon fiber fabric. To fabricate specimens with CNFs added to polypropylene (PP), either neat PP sheets or CNF-reinforced PP sheets were first prepared using a heat press (Mini Test Press MP-WCL, Toyo Seiki Seisaku-sho, Ltd., Japan). The CNF reinforced PP nanocomposite, with controlled CNF content by weight, was preheated at 180 °C under 0 MPa for 3 minutes. After preheating, the material was hot- pressed at 180 °C and 5 MPa for 3 minutes, followed by cooling to 30 °C under constant pressure using running water, resulting in the formation of CNF-added PP sheets. To fabricate carbon fiber fabrics with CNFs applied to their surfaces, a 2 wt% CNF slurry was diluted to concentrations of 0.025 wt%, 0.050 wt%, and 0.075 wt%. Each diluted slurry was stirred using a homogenizer at 3000 rpm for 10 minutes. The carbon fiber fabrics were then immersed in the slurry for 1 minute to allow CNFs to adhere to the fiber surfaces. Following immersion, the fabrics were dried in a constant- temperature oven at 100 °C for 24 hours, resulting in CNF-treated carbon fiber fabrics. 2.3. Evaluation of mode Ⅱ interlaminar fracture toughness of CFRTP To fabricate the test specimens, eight carbon fiber (CF) fabrics (180 mm × 180 mm) and ten PP or CNF/PP sheets (190 mm × 160 mm) were alternately stacked. A heat -resistant film was inserted at the mid-plane to serve as an initial crack, with PP sheets plac ed on both sides of the film. The stack was heated to 190 °C for 10 minutes using a heat